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Home , Anthology 3, Audio power amplifier

itplc %.- the 3rd audio anthology

COMPILED FROM

AUDIO From August 1952 to June 1955

by C. G. McProud Editor

RADIO MAGAZINES, INC. MINEOLA, NEW YORK: 1 9 5 5 Copyright 1955. by Radio Magazines, Inc., 204 Front Street, Mineola, New York. Henry A. Schober, Presi- dent. Printed in the United States of America. All rights reserved. This book, or any part thereof,may not be reproduced in any form without permission of the publishers. CONTENTS Article Issue Page Simple High -Quality Phono Aug. 1954 5 Quality Amplifier For The Home May 1955 7 Transformerless 25 -Watt Amplifier For Conventional June 1954 10 The White Powrtron Amplifier Nov. 1953 13 The Maestro-A Power Amplifier Nov. 1952 16 Medium -Power Tetrode Amplifier With Stabilized Screen Supply Feb. 1954 20 Zero -Impedance Output Stage Jan. 1953 22 Dual -Channel Control Amplifier for Stereophonic Music Systems May 1954 24 Improved Compensation Circuits Nov. 1954 27 Note on Volume Controls Apr. 1953 28 The Selection of Tone -Control Parameters Mar. 1953 30 Flexible Tone -Control Circuit Sep. 1953 34 Three -Element Bass Control Oct. 1953 39 Hi -Fidelity Phonograph Design Apr. 1953 36 Versatile Control Unit For The Williamson Sep. 1954 42 Preamp with "Presence" Jan. 1954....45 Miniaturized "Preamp with Presence" May 1955 50 New Approach To Miniature Preamplifier Design Feb. 1955 54 Preamplifier Switching, and Equalizing Unit for Critical Listening Sep. 1952 57 Low - Tone -Control Amplifier Aug. 1954 61 Record Improvement with H -F Cut-off Filters Nov. 1952 63 Cathode -Follower Audio Control System Nov. 1952 66 Simplified Equalizer Design Dec. 1952 69 Pickup Loading and its Effect on Frequency Response July 1954 71 Motor Rumble Reduction in Wide Range Nov. 1954 72 Two -Channel Conversion for AM -FM Receivers Jan. 1953 74 Binaural or Stereophonic? Jan. 1953 77 Stereophonic Reproduction Oct. 1952 80 Discussion of Dividing Networks Dec. 1952 84 Phonograph Preamplifier for Magnetic Pickups Oct. 1954 86 D.C. Pack for Heaters and Bias Sep. 1953 90 Corner Horn For The Small Listening Room Feb. 1954.... 91 Tuned -Pipe Enclosure for Bass Enhancement May 1955 93 Evolution of "The Horn" Dec. 1954 96 Corner -Mounting Infinite Baffle Oct. 1952 99 Coupled Loudspeakers May 1953 101 The Concrete Monster July 1954 105 Real Theater Sound in a Small Package Feb, Mar, Apr. 1953....108 Vibration Reduction in Enclosures Dec. 1953 115 Feedback and Loudspeaker Damping Apr. 1953 118 Damping of Loudspeaker Cabinet Panels Sep. 1954 119 Speaker Treatment for Improved Bass Dec. 1952 121 Improving Loudspeaker Performance Sep. 1954 123 Foreword Continuing the practice of providing the reader with a single volume that contains most of the material from issues of the magazine of greatest interest to the serious audio experimenter, the 3rd audio anthology is offered in the sincere hope that its pages will con- tinue to provide a wealth of useful reference material without the necessary additional pages that go to make up a monthly magazine. The original audio anthology was created to bring the early articles that appeared in - ING into the hands of those who did not learn of the magazine's existence until some time after it was intro- duced. The articles that were in the first thirty issues of AUDIO ENGINEERING were among the most valu- able that had been published in magazines up to that time, and once the early copies of the magazine were exhausted, there was not any way in which the reader could secure them for his own use. Thus the audio anthology was born. As months went on, more and more readers-both of the magazine and of the anthology-came to ex- pect that there would be another compilation of the more popular articles, and so the 2nd audio anthology was introduced. With a precedent thus created, it was natural to expect that a third would be along soon, and here it is. We are grateful to our readers for their continued interest, and it is to them that this 3rd audio anthology is appreciatively dedicated. C. G. McPROUD, Editor AUDIO Mineola, N.Y. September, 1950 A Simple High -Quality Phono Amplifier

R. D. MIDDLEBROOK

An inexpensive audio amplifier with sufficient gain to drive a loudspeaker from a magnetic pickup and which uses an unusually large amount of negative feedback over the output tube and transformer.

THE BEST AUDIO AMPLIFIER IS wasted extended frequency response. The am-clipped. This effect is clearly visible on if the associated equipment can not plifier to be described uses a single 6V6an oscilloscope, not only for a sine wave take advantage of it. Many high -qual- beam tetrode tube, with an ordinary uni-but also for the complex waves of music ity are available commercially, versal output transformer. The perform-or speech. As mentioned previously, the all of them using push-pull output stages. ancefiguresshowthattheresults guiding principle in the design of the Their capabilities are impressive-andobtained are definitelyin the "high - amplifieriseconomy inconstruction so are their prices. Of the thousands ofquality" class: while maintaining adequate performance non -technical people who have bought for ordinary home listening: the acid these amplifiers, how many have spent Maximum output:3 watts test for success is therefore a listening Total distortion : 1 per cent at 3 test.Ifan oscilloscopeisconnected a proportionate amount of money on watts (1000 cps) their loudspeaker systems? across the loudspeaker, while reproduc- There is room for a cheaper and sim- Frequency range:3 db down at 20 ing heavily -recorded music,it will be pler audio amplifier which can still be cps and 25,000 cps seen that the gain can be increased until considered high -quality. It is not gener- Output resistance:0.1 ohm the volume is sufficient to drown all con- ally believed that a single -ended output The maximum power output of3 versation before the peaks start to clip. stage is capable of producing results in watts is obtained at middle frequencies; In other words, all activity must cease keeping with present-day standards ; yetthe maximum power obtainable at ex-while the music is at full volume-a con- it can be done. This article will describe treme low and high frequencies dependsdition which must surely warm the heart how excellent listening quality can be on the quality of the output transformer. of any hi-fi enthusiast. Need more be obtained from a circuit which, at first No amount of negative feedback can ef- said about the adequacy of 3 watts? fect any improvement in this respect. The complete amplifier has sufficient glance, doesn't look as though it would gain to produce full output from a mag- sound any better than a small table radio. Using the Thordarson Type T22S58 universal output transformer, the maxi-netic pickup, such as the General Elec- The secret of this transformation ismum available power output falls to 3tric variable reluctance type. Figure 1 negative feedback. It is the writer's be-db below 3 watts at 80 cps and at 8000 is a block diagram of the various stages, lief that most of the unpleasantness of cps. and itis seen that the circuit can be the reproduction from an ordinary sin- Since most present-day amplifiers pro-divided into two distinct parts, each em- gle tetrode or pentode output is due tofesing to be "" ace capablebodying heavy overall negative feedback. the low speaker damping, and not to theof at least 10 watts output, a word in de- harmonic distortion. Without enteringfense of 3 watts may not be out of place. Circuit Description into the fierce discussion on the merits 10 or 20 watts certainly give a consid- Figure 2 shows the complete circuit or otherwise of negative output impe- erable margin of safety, so that even thein detail. The grid resistor R, is chosen dance of amplifiers, it can be stated withpeaks of the signal give rise to distor- to be a suitable load for the pickup. The some confidence that an output resistance tion less than the figures quoted for max- first two sections form a straight somewhat less than the speaker resist-imum output. What theloudspeaker cascaded amplifier with a gain of 1400 ance is desirable in order to obtain sat-does to the transient is, of course, an-times, which is reduced to an over-all isfactory damping. By using enough neg- other story. In this amplifier, owing togain of 100 times by the negative feed- ative feedback this can be achieved withthe large amount of negative feedback,back from the plate of V, to the cathode the signal contains very small percent-of V,. This link contains the tone control a tetrode output tube, with all the attend-ages of distortion right up to the maxi-circuits. Various tone control arrange- ant advantages of reduced distortion andmum output of 3 watts, and if the signalments are possible, and can be chosen * 1928 Cooley Ave., East Palo Alto, Calif. isincreasedfurther,thepeaksaret(, suit the individual : however, two net- works which have been found adequate are described here. Figure 3 is the bass control, which 2 3db 36db gives a maximum of 12 db bass boost, NEGATIVE FEEDBACK NEGATIVE FEEDBACK the magnitude of the rise being con- VOLUME CONTROL trolled by the . The point BEAM PENTODE at which the rise begins is selected by TE TRODE II the switch. These turnover frequencies are 80, 140, 200, 300, 400, and 500 cps. DOUBLE TRIODE The bass boost is sufficient to compensate for the recording characteristic of LP records, and in practise it will be found TREBLE BASS TONE TONE that usually the maximum amount of CONTROL CONTROL boost is unnecessary. Figure 4 shows the treble control, Fig. 1. Block diagram of the amplifier, showiog the two negative feedback loops. which has one position giving 3 db rise,

5 one flat position, and four positions giv- plate current of V, decreases, then its ROM ing treble cut. The potentiometer deter- plate voltage increases, carrying the grid 001 001 001 001 001 001 01 IP mines the degree of cut, and the switchof V. with it. The plate current of V, selects the turnover frequency. Thesethen rises, and its cathode voltage in- 1-111-1-).1111-1-)1-1-)"177-.12 POLE mvis.2 MEG turnoverfrequencies are1500,2000, creases. This also raises the screen volt- 6 POSITION 4000, and 7000 cps. The 0.1 megohmage of Vs, tending to increase the plate SHORTING CLOCKWISE LOG SWITCH I resistor is merely to prevent a thump atcurrent of Vs and restore the operating TO TREBLE TONE CONTROL the speaker when switching from the point. Thus we have in effect an internal flat to the lift position. negative feedback loop which stabilizes Fig.3.A suggestedbass -boostcircuit.This Raised in level by 40 db by V, and the working points of V. and V.. This section corresponds to box "A" in Fig. 2. V., the signal is fed to the volume con-feedback loop is made inoperative for trol and then to the grid of VS. This tubesignal voltages by decoupling the screen has the unusually high plate load, R, of Vs. of 2.2 megohms, and is direct -coupled The resistor R. and capacitor C, be- to the grid of V1. Using a 2.2 megohmtween the plate of V. and the cathode of load enables a gain of 340 times to be V, are to suppress high -frequency oscil- realized ;if a.c. coupling were used, thelations. The capacitor should be a high - grid resistor of V. would have to be atquality mica type rated at 1000 volts. least 10 megohms to avoid shunting theIf different taps are used on the output load of Vs, and this exceeds the tubetransformer, or if a different transformer manufacturer'smaximumpermissible is used, the values of these two compo- Fig. 4. A suggested treble tone -control circuit. value of 1.0 megohm. Using direct -coup- nents may need to be adjusted. The This corresponds to box "B" inFig. 2. ling also has the advantage that noseries grid resistor of V, and the screen low -frequency phase shift is introduced. resistor of V. are included to guard For an 8 -ohm loudspeaker load, an against parasitic oscillations. tire amplifier can be built on a chassis output transformer turns ratio of 32 to The total current drain of the com-8 by 6 inches, and the quality of its re- 1 should be used. The voltage "gain"plete amplifier is only about 40 milli-production bears comparison with that from the grid of V. to the transformer amperes, and hence the power pack re- of amplifiers many times more expensive. secondary is approximately 0.76, thereby quirements are modest. Using a 350-0- giving a gain from the grid of VS to the 350 volt high voltage winding on the PARTS LIST-AMPLIFIER transformer secondary of 340 x 0.76 = power transformer, sufficient voltage is C, 0.1 id, 600 v, paper 260 times. Since the feedback voltageavailable to dispense with a choke in C, 50 µf, 6 v, electrolytic returned to the cathode of Vs is one favor of the much less expensive resistor C, .01 µf, 600 v, paper fourth of the output voltage, by virtue for smoothing purposes. The less efficient C, .001pif, SOO v, mica of the 470- and 1500 -ohm resistors form- smoothing thus obtained is nevertheless 0.5 µf, 200 v, paper ing a voltage divider, the over-all gain quite adequate, because the large amount C. 500 µ1, 50 v, electrolytic of negative feedback reduces the 120 - C, 100 µµf, 1000 v, mica is reduced from 260 times to 4 times: Csa, a10-10 µf, 450 v, electrolytic that is we have 2010g(260/4), or 36 dbcycle hum from the B+ line to complete C,, Cu 20 µf, 500 v, electrolytic of negative feedback. inaudibility. It is desirable to use one or R, pickup load resistor (see text) V, obtains its bias voltage from themore of the standard methods for re- R,, R, 0.12 meg, 1 watt very small grid current flowing through ducing 60 -cycle hum from the heater ofR,, R. 2200 ohms, /2 watt the 4.7 megohm grid resistor ;thisis VI, such as tapping a ground connection R,, R,, 0.47 meg, watt commonly described as "contact poten-across the heater winding, (as shown) R, 0.5 meg audio taper potentiometer tial." Bias amounts to about minus 0.75or raising the heater winding to some R. 4.7 meg, l/2 watt positive potential with respect to ground. R,, Rit 27,000 ohms, !A watt volt. The voltage developed across the R,. 2.2 meg, 1 watt 470 -ohm cathode resistor is only sonic Only one other point need be men- R,, 470 ohms, 3/2 watt 0.07 volt, and is therefore negligible.tioned in connection with practical con- R,. 56,000 ohms, V2 watt The reason for using this method ofstruction. The cathoderesistor,both 330 ohms, I/2 watt is thatit avoids the low -fre-grid resistors, and grid capacitor of Vs R,, 1250 ohms, 5 watts quency phase shift which would be in- should be mounted as close as possible R,. 1500 ohms, 1 watt troduced by the conventional cathodeto the tube socket. This is because the R,, 47,000 ohms, 1 watt resistor and bypass capacitor. input impedance to Vs is extremely high R,. 10,000 ohms, 1 watt One of the troubles inherent in direct - and the grid leads are very sensitive to Rlf 1000 ohms, 5 watts Ref 0.27 megs, TA watt coupled stages is the drift of operating hum pickup. T, Outputtransformer,"Universal" point. This is avoided here by deriving All figures quoted above are measured type, 32: 1ratio for 8 -ohm loud- the screen voltage of Vs from the cathode values, not calculated. Aural results are speaker of V., through the 0.47-meg resistor. excellent :the cleanness of the bass re- T, Power transformer. 3500-350 v at The operation is as follows : suppose thesponse is particularly pleasing. The en - 50 ma ; 6.3 v at 2.0 amps

Fig. 2. The complete schematic of the amplifier.

6 A Quality Amplifier For The Home ALBERT PREISMAN

Mothers and wives can operate this unit, which has a built-in preamplifier andan absolute minimum of confusing knobs and dials. Even the selector switch is cleverly eliminated; manufacturers take note!

THE HI-FI ENTHUSIAST hasbeen the grid of the other section is fed, throughamplifier employs two 1622 tubesin butt of innumerable jokes for manya pair of separate series 75,000 -ohm re- push-pull, with a power output of 16 years. The most fanatic have been sistors, from the FM and tape -recorderwatts. Such power output may not be accused of listening, not to music, but outputs. The two series resistors merely necessary for the home, but then one is only to white , and the somewhat serve to isolate the tuner and recorder under no compulsion to turn the gain up more reasonable members of this irasci- from one another to a satisfactory degree. to a maximum. And if it should be de- ble fraternity make a fetish of "systems" In other words, the stage following sired to employ this amplifier for a larger with a bewildering multiplicity of knobsthe phono preamplifier is an electronic room, the power is there to use. and switches, so as to be able to control mixer stage, and the grid of one section However, the amplifier described here every conceivable characteristic of theis in itself arranged to mix two inputs. is readily modified to suit the specific reproduced sound (except, apparently, Hence a total of three inputs can be ac- needs of the builder. The tone control its loudness). commodated, and any one, two, or all can be eliminated if not desired; the bass The writer was accused by his goodthree sources can be used to feed the boost can be made variable, and smaller frau of never having a radio or phono- loudspeaker.Itrequires no particular power tubes can be employed. Other pos- graph that was simple in operation, pre-technical savvy to operate: if you wantsible variations may occur to the reader sentable in appearance, and satisfying in to play the phonograph, you turn on the as he persues the description that fol- performance, and the thing that rankledamplifier, start the turnable, place the lows. most in his mind was, of course, the truth pickup on the record, and there you are. of the accusation. So he sat down toIf you want to play the FM tuner, turn The Phone Preamplifier figure out a circuit that could accommo- on the tuner as well as the main ampli- date a phonograph pickup, an FM tuner, The complete amplifier diagramis fier; if your wish is the tape recorder,shown inFig.1. The preamplifier con- and a magnetic tape recorder, to be turn it on as well as the main amplifier. sists of a 12AX7, with its two sections housed in a presentable cabinet, and ar- Should you-perish forbid-wish to play ranged to be operated with a minimum connectedincascade and employing two at once, or all three, merely turn on feedback. But first we note the R -C in- of switches and controls. those you want. This is all admittedlyput networks to the first grid; the use The pickup chosen was an ordinaryvery obvious, hence it is a desirable char- of these tends to cut down turntable GE variable -reluctance dual type havingacteristic for the family instrument. a 1 -mil diamond stylus and a 3 -mil sap- rumble, although the best way to mini- Now, as to tone controls : Women aremize this is to use a turntable as free of phire stylus. The necessary preamplifier not interested in engineering; that's why was built into the main amplifier rather this characteristic as possible, and then engineers have to work overtime design- toinstallitin a rock vault 10 feet than on a separate chassis. The powering engineering gadgets that do the underground. supply, however, was built on a separate technical work for them. Witness auto- Incidentally, the ordinary three -speed chassis to minimize hum pickup. matictransmissions,powersteering,changer that I am using had excessive The FM tuner happened to be a automatic washing machines, pre-cooked Meissner ; of course any other make can rumble until I slipped some rubber snub- oatmeal, etc. Hence no equalizer wasbers between the metal chassis and the be used. The important thing is that the employed in this system; it has a fixed output level is much higher than that of wooden frame on which it is supported amount of bass boost, and a flat high -by three conical springs. Itseems that the phonograph pickup, hence the tunerfrequency response. The only control is the springs are too free and permit the can be connected into the system at aa tone control of the "chopper -offer" whole assembly to quiver like pointimmediately following thepre- type; it provides a rather sharp cutoff a dowag- amplifier. er's third chin at a gabfest; the rubber at 10 kc, 8 kc, or 5 kc, as desired, and is snubbers hold it in check likea whale- The tape recorder happened to be a used (only by me, of course) to cut down bone corset. Magnacordette. This has its own pre- the excessive surface noise on some old The input resistor of10,000 ohms amplifier, which functions both for re- 78-r.p.m records that I have. seems to give the most satisfactory high cording and reproducing, and in the Perhaps the rest of the family will be - latter case has an output level compar- frequency response. An initial value of enticed in time to use it, but since they 20,000 ohms permitted the GE pickupto able to that of the FM tuner. Hence itplay only LP's and 45's, they appear to develop a double -humped peak at 6,000 would be only natural to connect the re- have little need for it. It is desirable toand 10,000 cps, with a minimum at 8,000; corder into the system at about the same have this control act on all three inputs, the present value producesa reasonably point as the tuner. hence it is arranged to follow the mixer smooth response. This, then, suggested the layout that stage, and so requires a separate tube Low -frequency boost is obtained by is actually employed. Following the pre- section for this purpose. amplifier is an amplifier stage consisting feeding back from the plate of the second The present horsepower race among tube to the cathode of the first viaa .04-gf of two sections of a 12AX7 whose plates manufacturers of the horseless carriage are connected in parallel to a common capacitor in series with a 47,000 -ohmre- had its origin in the power output racesistor. By varying the values of these plate load resistor. The grid of one sec- that began in the 20's, when the UX tion connects to the preamplifier, but the two components, the low -frequency peak 112-A supplanted WD -11 or UV 201-A may be varied to a certain extent, both tubes in push pull. There is a group ofin magnitude and frequency. However, Capitol RadioEngineeringInstitute, diehard moderates who claim one or two an additional boost is obtained by the 16th & Park Road, N. W., Washington watts output is sufficient. However, audio.011-tif capacitor and 270,000 -ohmre- 10, D. C. power is relatively cheap, and so thissistor shunting the 50,000 -ohm volume

7 control, together with the 75,000 -ohmuse the preamplifier in the Magnecor-with a 22 -ohm resistor and 25 -ohm po- resistor above this combination. dette for thispurpose, henceIhave tentiometer. This increases the voltage This latter network produces a reduc- omitted the switch. drop to a value adequate to serve as self tion in gain, but the gain is more than One further point to note is the .015-Pfbias for the push-pull output stage. The sufficient anyway, and the additional basscathode bypass capacitor employed in the25 -ohm potentiometer serves further to boost of 4.2 db raises the total to 19 db. second stage. It does not take effect until vary the bias of one 1622 tube relative Of course more can be had, but the re-about 4,000 cps, at which point it beginsto the other; in this way two 1622 tubes sponse from a test record using the RCA to decrease the inverse feedback in thiscan be equalized as to d.c. plate current New Orthophonic curve seems quitestage and thus raise the high -frequencyflow in the output stage. The 10,000 -ohm satisfactory for the values given. response. This appears necessary in theresistor is required to draw additional One advantage of employing two sepa- case of because of the appreciable bleedercurrentbecausethe12AX7 rate circuits for bass boost is that their input capacitance of a triode stage owing heater requires more d.c. than do the peaks may be staggered and thus used toto the Miller effect. The resulting attenu-plates of the 6L6 tubes at an operating shape the curve to a certain degree. Theation of the higher frequencies by severalpotential of 250 volts. The 10,000 -ohm curve as obtained for the values shownsuch stages is counteracted to an appre-resistor also serves as a bleeder for the is exhibited in Fig. 2; the curve marked ciable degree by this small bypass capaci- power supply. "Phono Input" shows a peak centered attor. By varying its value, the point at about 45 cps. This is to be compared with which compensation begins to take effect Mixer and Equaliser Stages the response marked "FM Input," which can be varied ; a smaller value causes the The 50.0J0 -ohm volume control fol- is that of the amplifier stages followingcompensation to set in at a higher fre-lowing the preamplifier stages is for the the preamplifier. The slight dip below 50 quency. phono pickup only, since itis assumed cps in the latter curve is mainly due to When the amplifier was completed, itthat the tuner and tape recorder have the output transformer. was found that the hum was appreciable,their own volume controls. In this way The 75,000 -ohm resistor in the bass -owing in part to the bass -boost. The so-each volume control can be preset to its boost network can be made variable inlution was to feed the heater of the firstaverage position, so that there is no sud- order to vary the amount of bass boost.12AX7 (whose two sections constituteden blare of sound as the systemis The 540 -ohm cathode resistor of the firstthe preamplifier) with the plate currentswitched from one source to another. stage may be changed to`a potentiometer, of the two 1622 tubes operating in push- The third stage employs the two sec- with the 47,000 ohm resistor connectedpull. Since these operate essentially classtions of a 12AX7 tube in the form of an to the arm. In this way the amount ofA, the even -harmonic components thatelectronic mixer, as mentioned previ- bass boost can be varied in this network are present in the combined cathode cur-ously. The common plateresistor of without varying the position of the peak rent are relatively small, and produce no39,000 ohms was found to be the desir- in the frequency spectrum. noticeable effects in the preamplifier. Theable value to use in conjunction with A switch in series with the feedback reduction in hum is very marked ; most the bridged -T network following it. The network can be used to disconnectit of the residual hum now present is verylatter employs a 2 -henry Hycor toroidal when a flat response is desired, such assmall, and is mainly due to stray mag-inductor, which can be switched by two when itis desired to connect a micro-netic fields acting on the phono pickup,sections of a 3 -gang switch to bridge phone. The concomitantincreasein FM tuner, or Magnecordette. The lattervarious combinations of series capacitors. gain is usually desirable in such a case is particularly sensitive to stray 60 -cps The bridged -T configuration gives a output of fields. sharp attenuation dip at the frequency the . However, I prefer to The 12AX7 heater is placed in seriesat which the two capacitors and the in -

Fig.1. Complete schematic of the amplifier. The input switch is eliminated by mixing all three sources so that itis only necessary to turn on tuner, phono, or tape machine as desired.

8 ductance are in series resonance. The value of R must be adjusted to the proper value to produce this sharp selective at- tenuation effect, but its value is not par- ticularly critical. The magnitude indi- t20 cated is120,000 ohms, but this value m a would differ with different inductors. . *10 Approximate values of the capacitors I- FM used are indicated, but exact values are a INPUT best determined experimentally by run- 0...... -''''.°= 1 ..,_,...... , 33 r ning a frequency response curve on the amplifier and adjusting the capacitor Fig.2. Amplifier re- W sponse from the 10 valuesuntilthe properresponseis pickup and tuner 1 ..1 'WON° obtained. Actually, odd values may be inputs to output. La INPUT cc required to hit the desired frequency, 20 and in this case a practical dodge is to use slightly unequal values of capacitors. MOIN A littlepatience and a number of a la IOR NESS capacitors willresultinthe response FREQUENCY INCYCLESPER SECOND dropping off fairly sharply at the de- sired cutoff frequency. series -resonant circuit; the rise in gain regulation of the output voltage as the However,thebridged -Tnetworkafter the frequency of maximum attenua- load resistance is varied indicate that attenuates only in the vicinity of onetion is due to the inability of the series - the internal output impedance, looking frequency;abovethatfrequencyits resonant circuit to keep the response into the 15 -ohm secondary terminals, is transmissionapproachesthenormaldown. The results are quite satisfactory reduced to 4 ohms. value once more. To hold down the re-despite these peaks, and in spite of the The split cathode resistor employed in sponse beyond the attenuation frequency, grave admonitions to avoid peaks, thethe phase-splitter stage affords the cor- the series -resonant network following"ringing" of the system is not noticeable rect grid bias for the stage and at the the bridged -Tisemployed. Thisin-to the listener. same time the correct drive voltage for volves a 0.5 -henry choke and suitable The main reason for employing such the connected 1622 grid. series capacitors, mounted on the thirda rather involved network is to obtain The output stage in itself is conven- section of the 3 -gang switch, to operateas wide a frequency response as possible tional. The 1622 tubes are operated at in conjunction with the bridged -T. consistent with the surface noise of the values recommended by RCA, 250 volts Approximate values are shown forrecord or other source. For LP's and on the plates and screen grids, and - 18 these capacitors also, but here again theeven good 78's I use the amplifier widevolts self bias. The output, as measured capacitors should be chosen experimen-open; I can take a certain amount ofat the secondary of the output trans- tally to resonate at a suitable higher hiss.Incidentally,I seemed to get a former, is 16 watts, and was determined frequency than the corresponding fre-sharperattenuationcurve using the by observing the output wave shape on quency of maximum attenuation of thebridged -T insead of a parallel twin -T,a scope, and noting where it just began bridged -T, and thus hold down the re-which otherwise would have been more to flatten. The exact percentage is not sponse to a suitably low value at theattractive because itrequires only re- important, as this is more power output higher frequencies. sistors and capacitors. than is required in the home. However, for the 8000- and 5000 -cps The maximum gain was found to be attenuation frequencies, the simple series about 102 db at 1000 cps. This is not only resonant circuits permit the response to Power Output Section very high, considering that no input rise again beyond its resonant frequency, The next two stages are sections of astep-up transformer is employed, but it because the amplifier still has adequate12AX7 tube, as indicated. The reason is also in excess of that required by any gain in this region. Hence, two addi-two sections are employed is to obtain phonograph pickup ;indeed, it reaches tional shunting capacitors are employedfeedback from the secondary of the out- down into the thermal and microphonic from the corresponding two switch con-put transformer to the cathode of thenoise levels of the first stage. This is tacts to ground, and these hold the re-first section, and then to use the second particularly the case since the gain at sponse down to an acceptably low value.section as the phase-splitter stage. low frequencies is an additional 18 db. The response curves of the filters are The amount of feedback that can be A 12AU7 can be successfully substituted shown in Fig. 3. The peaks shown areemployed is limited; for the UTC LS -55 for the first 12AX7, with about 7 db due to a tendency for the bridged -Toutput transformer the values of 13,000 reduction in gain. Hence, one might just choke to resonate with the followingohms for the feedback resistor, and 2200 as well use the 12AX7 and run the vol- ohms for the cathode resistor seem toume control alittlelower. permit the greatest allowable amount of The power supply is of conventional feedback. Tests made by measuring the (Concluded on bottom of next page)

+10

ca 0 1.10 co 8 o. 0 -.- 0 Fig.4.Over-allre- ' .6 0 1.- sponse from the 2 .4 grooves of a test /- 20 disc toa dummy op +2 loadresistorfollow- w 0 ing the output trans- > -30 ;.:- 2 formerissatisfac- 4 torily flat. W_I 4 a 6 10010 8 FREQUENCY IN CYCLES PER SECOND

/12 MUM IM 1000 140111 Fig.3. Thisgraphshowsthecharacteristic FREQUENCYINCYCLESPER SECOND with each of the three filters switched in. RE. --

9 A Transformerless25 -Watt Amplifier for ConventionalLoudspeakers D. P. DICKIE, JR.AND A. MACOVSKI'

A Low -Cost, High -Quality AmplifierUsing No Iron -Cored Components

DURI NG THE PAST SEVERAL YEARS region of most serious distortion audio amplifier design has in a The Transformerless Amplifier pro-transformer isin the low- andvery - gressed at a rate second only to that With these problems ofoutput trans- of the transducers associated with it inlow -frequency range. At these frequen- typical sound reproduction cies the magnetizing currentmay become formers in mind many have envisaged systems. Asufficiently high to produce saturationtransformerless amplifiers. While many decade ago one could justifiably point of the problems associated with trans- to phonograph pickups and loudspeakersflux densities in the core. Although in- verse feedback can substantially reduceformerless design seem overwhelming, as the quality -determining links in the at least one manufacturer has licked the average system. This dis-the distortion in the near -saturationre-biggest problem by winding a 500 -ohm parity has been steadily narrowedand gion, its application is dependentuponvoice coilfor his loudspeakers. Per- at the present time no one can generalizethe high- as well as the low -frequencyformance is almost unbelievable in those as to weak links. Suffice it to say thatcharacteristics of the transformer. Thereregions where transformer -type ampli- innovations in amplifiers have beenlessis no simple solution to the problem andfiers fall down. It was felt by the writers startling, since they hadan initial jumpcareful attention must be paid to the thatif outstanding performance could on the rest of the elements involved insometimes conflicting demands of goodbe obtained in a transformerless ampli- the sound reproductionprocess. Suchhigh- and low -frequency performance.fier which could drive loudspeakers of advances as have been madecenter pri-It should be pointed out here that theconventional impedances, a veryprac- marily about the output transformer. tical unit might be the result. With minor exceptions, problem is not merely confined to fre- no basic circuitquencyresponseofthetransformer. At the outset of study of the problem changes have found theirway into com-Most good output transformers exhibit it was determined that any design should mercially available amplifiers. a frequency response far wider thanbe a practical one. The use of trans- This attack on the problemwas a logi-that needed for sound reproduction. Themitting -type tubes or inordinate quanti- cal one since only the transformersandproblem of being able to transfer large tiesofreceiving -type tubes was not the vacuum tubes inan amplifier can operateina amounts of power without distortion,justifiable. Plate efficiencies comparable nonlinearfashionandparticularly at the low -frequency end ofto existing high -quality amplifiers should thereby produce harmonic andinter - the range, is another issue altogether. be achieved. Size, cost and weight should modulation distortion. Furthermore,the fundamental limitations Still another limitation imposed bynot exceed those of comparable ampli- on bandwidth most output transformers in high -qualityfiers. Furthermore, it was felt that for or frequency response have generallysystems is inability to operate well ina real contribution to be made, very been due to the output transformer.One of the most practical class -B and AB power -amplifier stages.exceptional operation should be the rule ways to minimizeUnless there is a very high co -efficientnot only in the usual respects but par- harmonic distortion is to employ inverse ticularly in feedback around those circuit elementsof coupling between the two halves of those respectswhere that are responsible for the generationthe primary, transient signals are gen-transformer -type operation has its weak- erated by thenonsinusoidal currentsnesses. Since most high -quality loud- of distortion products. Again, thestum- speakers are available in16 -ohm im- bling block has been theoutput trans- which flow in the half -primaries. Class -B former, for its high -frequency and AB operation can contribute greatlypedances, this amplifier was designed for attenua-to the power handling capabilities of an thatimpedance.Followingstandard tion and phase shift characteristicshave thus far limited the amount of amplifier stage, but unfortunately thesepractice, the entire unit was designed inverseclasses of operation have becomeas- forusewithpreamplifierssuitably feedback which could be stably employed. sociated with higher distortion. Whileequalized for the particular program This vicious circle has stimulatedmany inherent improvements in transformerthis is fundamentally true, the amountsource and capable of delivering 1 volt design, but the fundamental of distortion is not serious and if suffi-of signal. problems cient inverse feedback is employed the Preliminary study yielded some start- still exist, although mitigated inmagni- output signal will be a good replica ofling results. It seemed that the uncon- tude. It is unfortunate but truethat thethe input. Full realization of these moreventionality of the goal-that of driving Pacific Mercury Mfg. Corp.,efficient operating conditions must awaita low -impedance speaker directly-ac- 5955 Van Nuys Blvd., Van Nuys, Calif. thepracticalapplication of large + RCA Laboratories, 711Fifth Ave.,amounts of inverse feedback. Chai Yeh, "Analysis of a single -ended New York 22, N. Y. push-pull audio amplifier," Proc. IRE, June 1953.

QUALITY AMPLIFIER low values were employed rather than T.C1A speaker installed inan infinite electrolytics ; however, the writer isnot (Continued from previous page) baffle of about 15 cubic feet volume. The particularlyprejudicedagainstthem. results are generally approved by people Thereis,however, asatisfactionin who have listened to the system, and it design and needs little comment. Theknowing thatthese componentswill appears to be very suitable for the home. reason for the dropping resistor as astand up indefinitely, although, admit- As a matter of final interest, Fig. 4 second filteris that a spare televisiontedly, large electrolytic capacitorswere power transformer was used, which de- shows the response of the system from used for decoupling purposes in the vari- the G -E pickup to a 15 -ohm loadre- veloped an unexpectedly high voltage.ous stages, and hence in time trouble may sistor, employing a Dubbings D -101A This was reduced to meet the rated plateoccur at these points. Although smallerrecord for the RCA New Orthophonic and screen voltages of the 1622 tubes;decoupling capacitors could be used, thecurve. The results, it will be noted, are perhaps the transformer you may havevalues indicated do stabilize the ampli- fairlyflat, particularly at the low -fre- available will not require the same treat-fier to a large extent against linesurges ment. quency end of the spectrum, and confirm and similar "bumps." the satisfactory conclusions drawn from Oil -filled filter capacitors of relatively This amplifierisoperated into anlistening tests.

10 tually set off a chain reaction of furtherto ground. These use large capacitors, It should be noted that the 6082 is not innovations. While most of the featureswith no additional filtering which wouldintended for use with fixed bias unless a ultimately used are tried and true pro-raise the power -supply impedance. Tolimiting resistor is added in either the cedures in the electronic industry, thisget higher voltages for low-level stages,plate or the cathode circuit of the tube. particular combination of them is newadditional selenium rectifiers are usedAlthough this circuit does not use such to audio. The infusion of new bloodin voltage adding arrangements to ob-resistors, their omission is feasible be- seems to produce a healthy new approachtain 250 volts as shown in Fig. 1. cause the tubes are used under quiescent to an old problem. The absence of an output transformerconditions well below maximum ratings. allows 40 db of feedback to be applied by The voltage -amplifier stages are all Theory Of Operation connecting the voice coil directly to theoperated class A with conventional cir- The output stage of the amplifier iscathode of the phase splitter driver. Be-cuitry. A separate driver is needed for thesingle -ended,push-pulltypeas sides its distortion reduction character-each side of the output tubes, since in- shown in Fig. 1' The quiescent currentistic, the application of feedback servessufficient outputis obtained from the is equal in both sets of triodes, with noto reduce drastically the hum voltagephase splitter to drive the output tubes d.c. flowing through the load. The tubeswhich would otherwise be present. Sincedirectly. One side of the phase splitter are driven out of phase with the differ-the gain within the feedback loop is es-has a larger load than the other, since ence in current between the tubes flow-sentiallyunity, an additionalvoltagethe input to the lower group of output ing through the speaker load. amplifier is used, with separate feedback,tubes has the speaker impedance in the The most efficient method of utilizingto build up the input voltage to thecathode. This causes degeneration and this system is to bias the output tubesvoice -coil level. necessitates higher input than the upper close to cutoff with the operation ap- group.In thefirst voltage amplifier, prnaching that of class B. The usual Circuit Details bias isobtainedusingunbypassed injections to this mode of operation, Figure 1 shows the final circuit of thecathode resistors since the loss in gain such as switching transients and distor-amplifier. Three 6082 double triodes arecaneasilybe tolerated. The phase- tion, do not apply if no output trans-used. They are 26.5 -volt versions of thesplitter driver, however, has fixed bias former is used. Class -B operation pro-popular 6AS7G. These tubes are capableapplied to its grid by dividing down from videsmaximum poweroutputwithof 700 ma of peak current per triodethe - 140 -volt bus, since maximum gain minimum plate dissipation so that thesection at the plate voltages used. The is desired within the main feedback loop. peak current capabilites of tubes can6082's allow the use of a series heater The high -current power supplies use effectively be used. A low -impedancestring,eliminatingafilamenttrans-300-µf capacitors and 5 -ohm protective power supply is necessary with this ar-former. The tubes cut off at about 70resistors (each made up of paralleled rangement so that the supply voltagevolts, and a 60 -volt fixed bias is used10 -ohm units). A voltage adder circuit willnot dropexcessively whentheon each. The bias on one side is madeis used for the - 250 -volt supply, which maximum current is drawn and therebyadjustable so as to equalize the d.c. insuppliesonlybias.Anothervoltage reduce the maximum power output. the two sets of triodes and insure thatadder circuit is used to supply + 250 volts The power supplies used are half -no d.c. flows in the voice coil. To protectfor the driver stages. Additional R -C wave selenium rectifier circuits develop-the tubes, a low d.c. impedance of 56,-decoupling is used to minimize hum in ing + 140 v. and - 140 v. with respect000 ohms is used in the grid circuit. the low-level stages.

250V 140V 250V -100V 0 0 10,0000 0 0 4040,- 0 5..(t5OV 8 C,N7c,T 0 0.1 12AT7 18 000 INPUT

5/450v 0 40/150V

°140V

i5A5ov 75ma 56 250V

cx 0/50v o-ci"o T4T 0 140v SW 1--/s/VNt0/42w 117V -500ma 300 60 'LAC Zsov 0 * no 05 3 - 6082's 10/2w /isov 500mo

7 8 7 8 (V2w

12AT7 12AT7 6SN7 LOAD CORRECTION 4 5 54 (SEE TEXT) 7 8 BALANCE

- 15A5ov 75mo 6200

Fig. 1. Thisisthe complete schematic diagram of the transformerless amplifier. Its total cost is comparable to that of a single high -quality out- put transformer. For safety, use of a 1 -to -1isolation transformerisrecommended betweenthea c.lineandthe power inputterminals.

11 Operational Details with negligible distortion is practicallyoperating biasissufficientlylowto impossible in a transformer -type ampli- As withallpower-transforinerless ensure that the no -signal operating point fierofsimilarmid -frequencypoweris outside the curved region of the tube equipment, care must be used when con-rating. The low -frequency performance necting to other pieces of equipment to characteristic near plate -current cutoff. is directly attributable to the use of cir-The resultingefficiencyisabout the see that the cold side of the line is con-cuit components whose nonlinear nected to the chassis. Although this is prop-same asatransformer -typeclass -A readily achieved, the use of a smallerties are in no way dependentuponamplifier. power line isolation transformer would The square -wave performance of an eliminate the need for caution. amplifier is an indication of its ability As would be expected, 40 db of feed- to reproduce signals of a transient na- back can be applied only within a loop ture. The low -frequency square -wave having a minimum of phase shifts to response is a measure of the ability to avoid instability. It is therefore neces- reproduce extremely low frequencies. sary to modify the arrangement when The amount of "droop" in the square- using speakers which present other than wave response is the important feature a resistive load to the amplifier. This in this respect. A negligible droop at a situation is not generally encountered in particular square -wave frequency means transformer -typeamplifierssincethe that the amplifier will reproduce well output transformer itself becomes the down to frequencies which are only a main impedance at high frequencies. fraction of the fundamental. The loud- That is, the speaker high -frequency im - speaker damping, however, will prob- pedanceisnot reflected through the ably be the ultimate factor in low -fre- transformer. Figure 1 shows three alter-Fig. 2. Frequency response of this amplifier is quency transient performance. While it native ways to deal with instability dueflat at the low end and less than 3 db down at is impossible to secure unlimited amounts to an inductive speaker load, which not200,000 cps, making it one of the widest -band designsinuse today for audio. of damping through reduction of the out- only causes additional phase shift, but put impedance of the amplifier, it is im- causes higher amounts of feedback duefrequency. Thus the 40 (Ili of feedback to the increased load impedance. Theremains at that value over the entire 180 -ohm resistor merely limits the maxi-usable frequency range and satisfactorily mum impedance of the speaker and thusreduces the distortion without regard prevents excessive feedback. The 0.544to the frequencies involved. Even if the capacitor is a low impedance at highdistortion in the amplifier without feed- frequencies, shorting the inductive load.back were as high as 10 per cent, the The series 16 -ohm resistor and .01-pf40-db figure, corresponding to a voltage capacitor places a 16 -ohm resistor acrossratio of 100 to one, would reduce this the speaker at high frequencies and anto 0.1 per cent. open circuit at low frequencies. This While dealing with the tested results serves to provide constant impedanceit is worthwhile to mention the subject and feedback over the frequency range.of distortion. IM is a If any instability is noted with a givenvery good and rapid speaker, try one or the other for bestdistortion in an amplifier. It is, however, Fig. 3. Harmonic distortion is less than 0.4per operation. at its greatest value in testing systems cent at the related output of 20 watts into a The balance adjustment for zero d.c.where there is apt to be a marked dis- 16 -ohm speaker. Intermodulation measurements in the voice coil can be made with asimilarity in the nonlinear performance would mean little since there are no frequency- milliammeter in series with the voiceof the amplifier at the particular pair sensitive nonlinear elements in the design. coil(a closed-circuitjack might beof frequencies used in the test. In anportant that this internal impedancebe added for the purpose). It should beamplifier such as this where there is noat least several times lower than the repeated when tubes are changed orfrequency -sensitive distortion character-speaker's nominal impedance. Reductions after many months of operation. istic, IM testing would yield little toof amplifier internal impedance beyond the total fund of information. this point are not necessary but can do Performance no harm. The internal impedance of The efficiency with which a 16 -ohmthis amplifier, due to the large amount The operational results of the proto-loudspeaker may be driven directly toof feedback,isonly a fraction of an type model of this amplifier are shownproduce these large power outputs is dueohm. This provides excellent electrical by the curves and photographs. not only to the extremely high perveancedamping. The frequencyresponse,whichis of the output tubes but also to operation The high -frequency square -wave per- shown in Fig. 2, is flat over a very wideaproaching class B. Sufficient quiescentformance is a good indication, not only range. Since resistive -capacitance - current flows in each tube to ensureof transient response, but also of the coupled circuits are used throughout,good small -signal linearity. That is, thestabilityofthefeedbacksystem. A there is no serious limitation on response. To keep circuit complexity down, and to achieve the best feedback stability, a Fig.4. These photos reasonable amount of gain per stage is of oscilloscope traces desired. This, of course, will determine show square -wave re- sponse atfourfre- the high -frequency limitations, while the quencies-top row, interstage coupling networks determine left to right, 20 and the low -frequencies limitations. While 1000 cps; bottom the bandwidth without feedback would row, 10,000 and 50,- be wide in this design, the use of 40 db 000cps. Notethe of inverse feedback extends the ends of unusuallyshortrise the range manyfold. Most good ampli- time even at10,000 fiers exhibit a flat response well beyond cps and the complete absence of evidences the limits of audibilty, and this unit of instability despite probably ranks as one of the widest- a full 40 db of feed- band designs intended for audio use. back.Effectiveout- As shown in Fig. 3, the harmonic put impedanceis a distortion even at full rated output is fractionofan ohm, exceptionally low and virtuallyinde- eliminating any tend- pendent offrequency. The abilityto ency toward over- deliver 25 watts at 20 cycles and below shooton transients.

12 tendency towards instability (not neces-originally designed for a currently popu-signal may cause excessive cone ex- sarily oscillation)will manifest itselflar amplifier. cursions in speakers which are inade- as a decaying train of oscillations fol- The hum levelinthis amplifieris quately coupled to the air. If such is the lowing the rapid rise and fall of theabout .02 volt across the load. This iscase, the coupling capacitor between the ,square wave. Of course, if these oscil-60 db below the rated power output.first and second 12AT7's may be re- lating overshoots do not decay or dieThis value was found to be exceeded byduced to attenuate these effects. out, the system will oscillate continu-the extraneous hum ofthesupplied ously.Itis important to have amplesource material from the preamplifier, PARTS LIST stability in the feedback system to pre-which in itself was acceptably low. vent any form of "ringing" or overshoot -The qualitative results of an ampli- Capacitors on a rapidly rising square wave. Thefier are generally the results of listening 1 .05 p.f,600 v.paper actual rise time of the square wave itself 40.1 of,600 v.paper tests. While it is impossible to attribute 2 10 of,150 v.electrolytic in the output is largely a measure of thespecific attributes of good reproduction 2 15 of,150 v.electrolytic high -frequency response. to particular links in the system merely 340 of,150 v.electrolytic Several square -wave frequencies areby listening, some indication of per- 2 300of,150 v.electrolytic shown in Fig. 4. Although the highestformance may be had by comparative 340of,350 v.electrolytic frequency shown (50 kc)isonly ofmethods. As one would suspect from 2 5 of,450 v.electrolytic academic interest,it was included tothe features, the way in which this de-Resistors indicate that the performance was notsignexcelsiswhen handlinglarge 1 -watt greatly deteriorated at a high frequency amounts of power at low frequencies.6 100 ohms 1 39,000 ohms ordinarily used to test video amplifiers.Even at moderate listening levels, an 1 680 ohms 1 47,000 ohms The extremely light weight and smallexceptionalclearnessofreproduction 1 1000 ohms 4 56,000 ohms size as well as the low cost of this ampli-was noted on organ music. An RCA 1 1500 ohms 1 0.1meg fier stem principally from the absence ofLC -1A inits standard studio console 1 1800 ohms 1 0.15 meg heavy and costly components. The onlyphase -invertercabinetwas usedfor2 10,000 ohms 1 0.27 meg items of major expense are the tubes,listening tests.It was neither feasible 1 15,000 ohms 3 1.0meg the four selenium rectifiers, and the twonor desirable to employ anything near 2 18,000 ohms 1 12 meg largeelectrolyticcapacitors. The re-the full output of this amplifier with 1 33,000 ohms 2 1.5meg maining components are small resistorsthis loudspeaker, but at reasonably high 2 -watt and capacitors of the sort encounteredroom levels, the low -frequency reproduc- 4 10 ohms 1 12,000 ohms in most amplifiers. The three outputtion seemed exceptionally smooth and 1 560 ohms 1 27,000 ohms tubes are considerably more expensiverealistic. The use of a horn -type low - 2 8200 ohms than the normal receiving type, but arefrequencyloudspeaker,whichwould still reasonably priced. The other tubesmore efficiently load the cone, would are of widely used variety and inex- 10 -watt 20 -watt permit the use of higher and more real-1 2500 ohms 1 40 ohms pensive. Only two of the selenium recti-istic levels of, say, a pipe organ. While fiers are the 500 -ma type, while thethis has not been tried,itis thought 1 10,000 -ohm wirewound potentiometer other two are the small 75 -ma variety.that here is where the amplifier would2 75 -ma, 130 -volt selenium rectifiers The total cost, computed from the cata-excel. Since it is capable of delivering 2 500 -ma, 130 -volt selenium rectifiers log of a large parts supplier, is approxi-large amounts of low -frequency 1 6SN7 mately the same as that of a single high -low -frequency signals such as those de- 2 12AT7's quality output transformer of the typeveloped when tuning through an FM 3 6082!s

The White POWRTRONAmplifier STANLEY WHITE

A discussion of one possible cause of power distortion and a descriptionof a circuit developed to eliminate it. The author also describes his methodof dividing the frequency spectrum ahead of the power amplifier. This unit has been popular with listeners at recentdemonstrations.

MOST AMPLIFIERS are developed and amplifiers are to perform their basicspect to power to the extent of the tested using pure resistive loadfunction-that of presenting an electricaldeviation. impedances across the secondary power waveform to a speaker in such a From this definition, it can be seen of the output transformer. Determination manner that the acoustical wave radiated that any distortion measurement of an of intermodulation distortion, harmonic from the surface of the speaker is aaudio power amplifieris,infact,a distortion, and power performance are transformed replica of the voltage wave- measurement of power distortion. That based upon results obtained using theseform at the input to the audio amplifier.is, power distortionisa generalized resistive loads although it is well recog- It will be shown that present audio am-form covering intermodulation distor- nized that speakers do not present a plifiers create a power distortion of ation, harmonic distortion, and so on. constant load impedance over the entire magnitude of 6 to 8 db, and this type ofAny amplifier that changes its power frequency spectrum. However, for want distortion is not discernible by presentoutput with changing load impedance of a better method, resistive loads have day testing procedures. suffers from power distortionto the extent that the power output is altered. been retained as a standard procedure Definition1.Power distortion: A in determining the performance and op- power waveform generated by an audioItis recognized that the relationship erating characteristics of amplifiers. amplifier that deviates in any manner This paper proposes a basic change in whatsoever from the form of the input Stan White Inc., 727 S. La Salle St., amplifier circuitry that is inevitable ifvoltage waveform is distorted with re-Chicago 5,Ill.

13 between power, voltage, and impedance any speaker, and such factorsas theagreed that voltage feedback tends can be expressed by the formula resistance of the suspension to system, thehold the voltage constant regardlessof El resistance of the air load, thereactancethe load across the amplifier P =- (1) of the voice coil and terminals. cone, the reactanceThus any change in load impedancere- of the air load, and thereactance of where P= power output, the suspension system sults simultaneously in an inversepower E= voltage, and must be consid-change. If electrical impedance charac- ered as affecting the totalimpedance Zr load impedance. of the speaker, in addition teristics and acoustical output charac- to the pureteristics of a given speakerwere related In test procedures using resistive loads,electrical impedance of thevoice coil itself. in such a manner that electrical imped- it can be seen that if E remains constant, ance peaks occurred simultaneously with the power output will remain constant. acoustical peaks, the decrease inpower However, with variable load impedances Effect of Feedback response at the point of maximum acous- the power output will bearan inverse The majority of hi-fi amplifiers relationship to the impedance. em- tical output would be beneficial. How- ploy some form of voltage feedback,butever,inrealspeakers thiscondition From transducer theory, there area study of equation (1) will show thatseldom occurs. certain relationships between the elec-if voltage remains constant there trical and acoustical characteristics of will be The Powrtron circuit, Fig. 3, differs considerable power distortion, anditis from conventional amplifiers in that it adds a small amount of negative current feedback to a usual amount of negative voltage feedback, with the result that over a reasonable range of load varia- Fig. 1 (left). Top view of 10- tions the power distortionis held to and 20 -watt White amplifiers I db, whereas without the Powrtron fea- withfilternetwork plugged ture the same amplifier shows a distor- into the 10 -watt unit.Fig.3 tion of as much as 8 db. providesfornetworkto be Careful consideration of this will show plugged into the 20 -watt low- that it is useless to attempt to control the frequency amplifier. behavior of a loudspeaker by means of adevicethatwillsenseimpedance changes in the speaker, and this isex- actly what is done with voltage feedback. Many other effects of voltage feedback are definitely beneficial, as is well known. but the effect on power distortion is to increase instead of decrease it. Negative power feedback results in much less power change over a range of output loads than the other methods of Fig. 2 (right). Underside view of the operation. Positive current feedback re- 20 -watt amplifier. Large mica capaci- duces the internal impedance of power tor at lower leftis C. amplifiers to zero, but by so doing it increases power distortion.

FOR TREBLE 124u7 ----- VIA AVO

4IPuT

RIO 96V-41 1500- 6" 3V 20,000 SW 'OW 4

047 OHMS

SHORTING PLUG FOR SINGLE CHANNEL USE

* MATCHED PAIRS L2 LI R22 a. .5 a., V

20.000 - IOW 30H 60.48 OH, 220.. SWINGING Cl 3A

C7 V6 G43 05 544 -40/500 - 40/500 - 40/500

0 0

6 3. 14 r,

Fig.3. Complete schematic of the 20 -watt White amplifier arranged for plugging in the electronic filter network. 10-watt model isidentical except for output tubes, which are 6V6's, and theoutput transformer.

14 The Complete Circuit changesinthetonalquality of the advantages of the White system is that While the Powrtron circuitrefers output. the dividing network is used ahead of only to the addition of a single resistor Figure 2shows the underside of the the amplifiers, providing the advantage in the output circuit and the connection White amplifier. Note that most of the of low source impedance for the speak- back to a suitable point for the introduc-components ahead of the output stage ers. The principal disadvantage is the tion of feedback, there are some advan- are located on the terminal board, which need for two power amplifiers, it being tages to the complete White amplifier is laid out as inFig.4. The parts listquite usual to use the 20 -watt model and the method of introducing two sepa- indicates the wattages of the variousfor low frequencies and the10 -watt ratekindsoffeedbackissimplified resistors, as well as the types recom- model for high frequencies. greatly. InFig.3 it will be noted that mended. The shorting plug simply connects Rand C, constitute a usual form of the incoming signal to the input of the negative voltage feedback. The negative amplifier. However, when it is desired current feedback is obtained fromR,0in 0. Cs wis 0. Cs .0 to use two amplifiers, the shorting plug is removed and an electronic filter unit the return leg of the secondary of the 0--1R15 1-0 of R16 .0 output transformer. The cross -coupled is inserted in the socket.Figure5 is phase inverter, together with the direct - .0 C3) R theschematicofthefilternetwork. coupled driver stage make it possible to which consists of a dual triode connected introduce the two different types of feed- OH 422 1-0 0.1 R12 as two cathode followers. Each follower feeds a filter circuit-one of low-pass back with considerable ease. Further- O OH R14 I-0 more, if a direct A -B test is desired, it configuration, and one of high-pass con- is only necessary to short outR,.. 0-1R6 1-0 figuration. The low-pass output is fed Since the circuit is somewhat unique, 0--1RT 1-0 to the associated amplifier, and the other it may bear explanation. The input is output is fed to the treble amplifier. In RS the commercially available model, the fed into a level -adjusting potentiometer o4 10 and thence to the grid ofV,,,through treble output is fed through a pigtail C, and the grid resistorR,.(The use R9 cable, which is plugged into the second of the octal socket will be described Cl -0 amplifier. As shown inFig.3 the tpeble later.) C, and R, may appear unneces- output is channeled to another phono sary, but the slightest amount of d.c. jack, which is connected by a jumper to on the grid ofV,ais sufficient to unbal- Fig. 4. Arrangementofportson terminal thesecondamplifier.The terminals ance the operation of the entire system board shown inFig.2. shown are not those used in thecom- so C, is a mica capacitor-.033 of or In construction, it is suggested that the mercial version, but are indicated for larger-which has been found to be com- amplifier be assembled with semi -per-study of the circuit. pletely free from leakage. The cathode manent connections between the driver ofViais directly coupled to the grid ofstage and the output -tube grids, andFilter -Network Advantages V sb and a tap on the cathode resistorwith the negative -voltage feedback cir- The most recent trend in amplifier string ofV,.. R,provides for a balancecuit-R1,-C,-clisconnected. Then pass adesign has been toward increased neg- ofd.c.voltages throughout thefirstsignal through the amplifier and noteative feedback, using output transform- three tubes-the method of adjustmentwhether the signal increases or decreasesers of wider and wider range and placing being to setR, ata point where the whenR,0is shorted. If the signal de-more and more stages inside the feed- voltage between the plates of V., and creases, the leads to the two output gridsback loop. For optimum operation, all V sb is zero. The negative current feed-should be reversed, since the feedbackof the push-pull stages should be bal- back i3 connected to the grid of V,,- voltage developed acrossR,0should re-anced, and maximum phase shift must directly out of phase with the inputduce the gain, and shorting the resistor be kept to less than 180 deg. inside the section-and the output ofVfl,is fedeliminates the feedback. After the cor- feedback loopifoscillationisto be into the phase splitter in a manner sim-rect polarity is determined, the voltage - avoided. ilar to that fromV,..The direct coup-feedback circuitR -C,may be con- The two regions in which phase shift ling between the phase splitter section nected. will occur and oscillation becomes a and the driver is made possible by theThe Octal Socket problem are at the extreme ends of the use of a very large cathode resistor audio spectrum. The ideal way to de- forV,.It will be noted that these ca- The octal socket previously mentioned sign an amplifier is to keep the phase thodes are about 96 volts above ground,provides for the insertion of an elec-shift through the electronic section of resulting in a potential of approximatelytronic dividing network ahead of thethe amplifier limited to less than 5 deg. 90 volts on the plates of V,-this samepower amplifiers. With the shortingand allow the electrical characteristics voltage being applied to the grids ofplug in place, the amplifier functionsof the output transformer determine V,which results in a bias of around 6normally, and may be used to feed athe operating frequency of the amplifier. volts. single speaker, or to feed a two- orUnfortunatelythis ideal isseldom The output stage is the Ultra -Linear,three-way system with a conventional achieved. which has been described heretofore.'dividing network. However, one of the Itis well known that the reactive In the 20 -watt White amplifier, 5881's are used; in a very similar design for 10 watts output, 6V6's are used-this latter amplifier being used with the 20 - watt model to make the two-way am- plifier system to be described. The current feedbackisdeveloped 005 47,000 47,000 47,000 47,000 acrossR,shown as 0.47 ohms. In con- struction, it is suggested that this value be obtained by the use of a 1 -ohm 10 - watt adjustable resistor. Slight varia- tions in the power response character- istics may be had by changing the value ofthisresistor.withcorresponding

1 David Hafler and Herbert I. Keroes, "The Ultra -."AUDIO EN- Fig. S. Electronic filter network using a single 12AU7 as two cathode -follower sections to drive GINEERING,Nov. 1951. the R -C filters which comprise the dividing network ahead of the power amplifiers.

15 The Maestro-a POWERAmplifier

DAVID SARSER and MELVIN C.SPRINKLE A new version of the now famous Musician's Amplifier which should satisfyanyone's desires for more power-and which uses a newly developed tube type with modest plate supplyrequirements. N0All WEBSTER, in his book of words, defines maestro as "a master in any art,especially music." This name is particularly appropriate to this amplifier, shown in Fig. 1, for it com- bines the best properties of the now famous Musician'sAmplifierwitha prodigious increase in power output. It is truly the master of the art of recreat- ing music by electronic means. The success of the Musician's Ampli- fier'is too well known to require re- peating, but certain specialized appli- 10 cations have been encountered in which it did not fill the bill. We have in mind Fig. 1. The Maestro amplifier-a new contender forhigh -quality sound reproduction its power output, for its response, low home, or for disc -recording cutter driving, inthe or for any application where up to 90watts distortion, and low noise level leave little required. is to be desiredor home music listening. The development of FM broadcasting.Therefore it is necessary to increase the One application for which itis not modern LP records, and tape equipmentpower delivery of the amplifying system. entirely adequate is as a driver for a has set new standards for dynamic range disc recording head. The low distortion in reproduced music. It is now necessary * 548 Riverside Drive, New York 27, makes the Musician's Amplifier attrac- to re -appraise the power required forN. Y. tive, but it falls short on power, espe- critical listening. In the past, the pro- ** Sales Engineer, Ampex Corporation, cially when making LP discs where pre- gram material was compressed to a 35 - Silver Spring, Md. emphasis is required. The considerationsor 40-db range and maximum power Sarser and Sprinkle, "Musican's am- on power for disc recording arc wellcould be handled easily by the conven- plifier," AUDIO ENGINEERING, known and have been mentioned by these tional "15 -watt" amplifier. Today's trend 2 Sarser and Sprinkle, "Musician's am- writers previously.2 plifier senior," AUDIO ENGINEERING, Jan. is toward elimination of compression. 1951.

In the hi-fi field the final judgement carbon The POWRTRON is always that of the listening test. In R. 1.0 ohms,10 -watt, adjustable, (Continued froth previous page) the case of amplifiersitisdifficult to wirewound achieve a distinct improvement, but it RII 3000 ohms.1 -watt, wirewound isfelt that a listeningtestwith the 20,030ohms,10 -watt, wire - filter networks cause substantial distor-crossover amplifier will give thelis- wound tion in the process of sound reproduc- tener just such a distinct improvement. T, Powertransformer, White tion. However, the manner in which it Sound or Chicago PCR-200. 520-0-520 v at 200 ma ;5.0 v is caused is not nearly as well known. PARTS LIST (Fig. 3) The design of filter sections of constant- at 2.0 a; 6.3 v at 4.5 a ; potted. valued elements of resistance, capaci- C, .033 of, 1200 v. mica To Ultra -Linear output trans- C,, C, 0.5 of, 600 v. paper former,Acro TO -300,or tance, and inductance is standard en- C, 3.50 Aid, 500 v. mica White Sound gineering practice. However, the design C., C., C740 of, 500 v. elect. VI, V,, Vs12AU7 of filter sections capable of dealing with 8 11, 240 ma, swinging choke V,, Vs 5881 or KT66 the variable impedance presented by a 30 H, 60 ma, smoothing choke V. 5V4 speaker is a problem of serious magni- R, 0.25 meg potentiometer, audio tude. The use of a dual -channel ampli- taper fier using electronic filter sections at the 1.0 meg, 1/4 -watt, deposited car- PARTS LIST (Fig. 5) bon input of the system is deemed the best R,, R,, R. C,, C, .05 Id, 600 v. paper solution. R, 470 ohms,1 -watt, wirewound C 4 ,CS, CS The crossover filter is constructed in Rs 200 ohms, 4 -watt potentiometer, 750 µof, 500 v. mica a standard Vector CO -10-N turret can, linear CO, CM Rs, R. 50,000 CH .005 of, 500 v. mica making it readily interchangeable. Thus ohms,10 -watt, wire - R Rio 1.0 meg, 1/4 -watt, deposited car- the experimenter can construct several wound, matched pair bon different filter networks to determine the 1500 ohms, 5 -watt, wirewound R., R, 1500 ohms, 10 -watt, wirewound best operating crossover frequency for RJ11 20,000 ohms, 10 -watt, wirewound R,, R., R, 47,000 ohms, 1/4 -watt, deposited Rs., Rs, 50,000ohms, 10 -watt,wire - the speakers used, or by removing the wound, matched pair R, carbon network can restore the amplifiertoR,, R., R,, R,. 0.47 meg, I/2 -watt, deposited car - 500 ohms, 2 -watt, wirewound bon normal operation with a minimum of R,,, Rm 0.15 meg, 1 -watt, depositedcar- R effort. For the constants shown, the bon 12AU7 crossover frequencyisapproximately R17 350 ohms, 10 -watt, wirewound 560 cps. R,,, RA 1000 ohms,Y2 -watt, deposited

16 A typical example is in a recent re- cording of Ponchielli's "Dance of the Hours." In this selection, the pianissimo 'cello solo passage is repeatedly inter- tl rupted by a crashing chord played by the entire orchestra. With the usual 10 - to 15 -watt amplifier, the chord is heard, but without sufficient definition to suit Fig. 2. Comparative size of the new RCA the fastidious listener. In order to dis- 6146alongside the tinguish between the various choirs of 5881 and the KT -66. the orchestra playing this chord, which the trained ear can do in a concert hall, itis necessary that considerable power be available. A measurement of the peak produced by the chord shows around 22 db of change in instantaneous power. This is not, however, a true measure of the peak but is an integrated reading. to be a "jolly good bottle." ance, and will pass a 30,000 cps square This means that an amplifier of around wave with a vertical rise and a flat top. 100 watts is required. Since this chord Having found a satisfactory tube type, the next problem was to find a suitable containsfundamentalfrequenciesbe- output transformer. Search of trans- The Voltage Amplifier tween 30 and 4000 cps, it may be seen former catalogsfailedtoreveal one Large triodes like the 845 have a high that full power is required at these fre- which would meet all requirements, so bias, and transformer coupling is almost quencies. In addition to power over this a conference was held with E. B. Harri- a necessity. A power amplifier of some range, "clean" power is required up to son, of Peerless. On hearing the prob- sizeisalso required to produce the at least 15,000 cps for disc recording as lem he said, "I think I can do it." Sub- necessary voltage. The 6146, in common considered previously. Hence, we have sequently he has admitted it was a tough with other beam tubes, operates at a looked toward the development of an one. However, Harrison designed and reasonable bias of 50 volts. It requires amplifier which would combine the low built an output transformer for the 6146, around 35 volts r.m.s. per tube, or 70 distortion, low noise, and wide range of and although originally built especially volts for a push-pull pair for grid ex- the Musician's Amplifier, with substan- for this first amplifier, it is now in the citation, and this is quite in line with tially increased power output. Peerless line as type S -268-Q. When the 807 or 5881 drive requirements in tested in a matched network, the re- While the Musician's Amplifier theMusician'sAmplifier.Thus,the Senior2 was a step in the right direction, sponse is within1 db from 10 cps to 100,000 cps. Primary impedance is 8000 voltage amplifier of the earlier ampli- it had several shortcomings: it is large fier was adopted. without change, as is in size; it requires a power supply much ohms, and it will handle 50 watts at 20 cps, at least 80 watts mid -range. observed from the schematic, Fig. 3. like a transmitter, and which can be When usedina feedbackamplifier Design of the power supply proved to lethal; it requires a power amplifier as where the source impedance is10 per he a bigger job. In the Musician's Am- a driver; and it is like all Class A am- cent or less of the reflected primary im- plifierSenior,the power supplyre- plifiers-inefficient. And in high -power pedance, the transformer willdeliver sembled that of a small transmitter, and amplifiers, efficiency becomes important. close to 80 watts with no visible dis- the problem was current capacity and tortion at 20 cps. Primary inductance at high voltage. In the Maestro amplifier, New Tube Gives Clue 5 volts, 60 cps, is greater than 200 hen- the problem is regulation, since opera- ries, while at 80 watts the inductance is tion is Class AB. According to the data The recent announcement of the type aproximpately 800 henries, yet the leak- sheet, the plate current for a pair of 6146 by RCA pointed toward a solution age inductance referred to the primary tubes goes from a quiescent 57 ma to a of the need for more power with com- is around 7 mh. The d.c. resistance of peak of 227 ma, while the screen cur- paratively simple circuit design. This the primary is 115 ohms, and the inser- rent changes from 1 ma quiescent to 27 tube is a beam -power amplifier tube pri- tion loss around 7 per cent. Small in size ma at 120 watts. Another problem was marily intended for transmitter use. As forits power rating this transformer to obtain the 750 volts with the choke shown in Fig. 2 in comparison with the has proved to be excellent in perform- input that good regulation dictates. One 5881 and the KT -66, it is small in size, sturdily constructed; and it has a high power sensitivity. It can be used in a number of transmitter applications, but RCA's data sheet indicates that it will also serve as an amplifier or modulator, Class AB. This data sheet recommends-under ideal conditions such as perfectly regulated power sup- plies-that a pair of 6146's be operated with a plate voltage of 750 and a screen voltage of 200. This requires a fixed bias of 50 volts and a plate -to -plate load of .750 VDC 8000 ohms. Under these conditions, the 250 MA power output is approximately 120 watts 50.200 V into a plate -to -plate resistor. As a prac- REGULATED tical matter, we have departedslightly 10 75 VOC from these conditions and obtained,t 60.375 VOC sine -wave power output of 90 watts from 25 to 30,000 cps. All this and Class AB, too, with no driver and no grid -current 30 GROUND 13 - problems. The 6146 can he operated VI 2,3,4 1EAtkR3 le 6.3 VAG readily with resistance coupling from a voltage amplifier-and thus may be said Fig.3. Complete schematic of the Maestro. Note similarity to the Musician's amplifier.

17 solution was found by using two re- CONNECTOR TERMINALS ceiver -type transformers with the high - 0 REMOTE ON -OFF voltage windings inseries. The pri- 5 SWITCH maries are paralleled across 117 volts T 9 a.c. and the secondaries are phased so 6 3 VAC as to obtain 1600 volts r.m.s. from recti- 0 fier plate to plate. The two -transformer 63 VAC FOR VII HEATER 40.750590 250MA Fl SAW. Frwr scheme also provides the several 6.3 -volt L I ION JCI heater windings which are required. 60. 375 VDC The rectifiers employed are the high - 10 CT SW 1 se vacuum, high -voltage 5R4GY, ideal for NOT 0 heavy-duty use. Two are used in parallel. n USED sew CII VS 46T VG 46Y In preliminary work, a swinging choke T was used as input to the filter but it was os SPARE found thata conventional smoothing 6.3 VOLT choke works just as well. The require- ments of sufficient minimum inductance 60.200 Vet O and low d.c. resistance are met by the V9 REGULATED unit selected. The single high -voltage CT 65J7 Ir NOT filter capacitor is oil filled. a USED One of the important requirements in obtaining high quality from beam tubes 6.3 VAC FOR V9 HEATER is regulation of screen voltage. This is not always mentioned in connection with oSL I R? 4700 amplifier construction articles and so C)V10 VR75 GROUND does not receive the recognition it de- C2A 20 Cte 20 serves. In our preliminary work we used V575 0 75 VDC VR tubes to regulate the screen voltage but had poor luck. By the time the screen voltage was stable, the VR tubes Fig.4. Schematic of the No. 1 power supply, which employs two conventional receiver power were wellpast theirrated currents. transformers and the bias -supply transformer. Therefore the VR tubes were abandoned former and a 75 -ma selenium rectifier.trouble has been encountered in cabling and an electronically regulated supply Another VR-75 tube is used to stabilizethe 750 -volt plate supply with the other installed. A triode -connected 5881is thisvoltage, and enough currentis wiring, but care should be taken to place used as a pass tube, and a 6SJ7 is used drawn to make it steady. Two potenti-all live connections on female connectors. as the control tube, with a VR-75 sup- ometers, P, and P,, are used in the am- Bleeder plying the reference voltage. plifier to balance plate currents as well Performance current is passed through the VR-75 so as to set the bias. Note that the positive that changes in 6SJ7 current have no side of the bias supplyisgrounded; The performance of the Maestro am- effect. therefore, the anode of the VR-75 shouldplifier fully justifies the name. The gen- be grounded, and the cathode connectederalrequirementsforfrequencyre- Power Supply Circuits to the negative side of the bias supply.sponse, power output,distortion, and Referring to the schematics for the A 100 -watt, 10,000 -ohm bleeder re-noise have been stated, and the results power supplies-Figs. 4 and 5-it will sistor is used to supply the 400 -volt re-will be considered in that order. be noted that the screen supply circuits quirements of the regulated screen sup- The frequency response was measured aresimilar. During thedevelopment ply and the 375 -volt requirements for thewith a 1000 -ohm source resistance as program, two types of power supplies voltage amplifier. Details of the circuitthis is typical of the source impedance were constructed. The first type used are seen in the schematic, with the partsof cathode followers used in the better two receiver -type power transformers, listed at the end of the article. 10 -contact "front ends." Under these conditions, with the high -voltage windings series - Jones plugs are used to interconnect thethe response is flat with no perceptible connected. The second employed a stand- amplifier and the power supply. Novariation from 10 to 70,000 cps. There ard type of plate transformer which de- livers 900 volts each side of center tap. CONNECTOR TER...ALS 7 This latter unit has a streamlined ap- a REFINE ON -OFF 0 SWITCH pearance, and results in an attractive If T6 power supply, but a number of extra 6 5vAC FOR ye NEATER filament transformers must be employed. Figure 4 shows the schematic of the two -transformer supply, with a number 6.3 VAG of filament windings being available on 5 theexistingtransformers.Figure 4 O 750VOC 250111A shows the unit employing the single LI 10H jCI 60.375 VDC plate transformer with a .multiplicity of VS 1 0 5151 filament transformers. There are ad- T /co vantagestobotharrangements,but aside from thedifferencesintrans- V5 V former conections, the remainder of the e7 VAC power -supply circuit is essentially iden- 50.200VDC tical in both types of construction. SRI VII REGUL ATED Referring to the regulator circuit,it 1111.17 is seen that the potentiometer P,is CII used to set the output voltage to exactly 11.5VO't FOR VS NEATER 200 volts-although it may be set any- where in the range from 150 to 250 au volts. Changes in input voltage have no eta-Taw " 30 GROUND I V? effect.Itwill also be noted that the -10 75 VDC 6146's are operated with fixed bias. To

provide this, a separate circuit is em- Fig.5. Schematic of the No. 2 power supply, using a plate transformer and several filament ployed, using the 1 -to -1 isolation trans - transformers, in addition to the bias -supply unit.

18 is a 1.5-dh rise at 5 cps, and there is a droop of 0.6 db at 100,000 cps. These frequencies represent the limits of our present measuring equipment. From the smoothness and steepness of the square- Fig. 6. Power output wave transmission, it appears that the vs.frequencycurve response isbetter than the measured for the Maestro am- value. The completed amplifier passes plifier. square waves even better than the Musi- cian'sAmplifier, up to a10,000 -cps fundamental. At 30,000 cps the rise time isstill vertical while preserving a flat top. damping factor of 10.6, which is entirely The single -frequency power output at shown by the dotted curve inFig.7. Fig. This is close to the 90 watts -the powersatisfactory on any speaker. No traces 1000 cps is 902 watts, as shown in output at which a sine wave begins toof hangover have been detected on any 6. This is just before the sine wave be- of the speakers used. gins to be clipped, and when clipping be distorted. The amplifier can be con- does occur the clip is clean and sym- sidered to be almost distortionless below 75 watts on a sine -wave basis, since this Preliminary Output Transformer Tests metrical. There is no "fuzz" when the corresponds to around 50 watts on the amplifier overloads. Full 90 wattsis IM curve which is well below the knee During early development and before obtained at all frequencies from 25 to or break-point. the arrival of the output transformer 20.000 cps with a smooth decline begin- designed for this amplifier, tests were ning at 30.000 cps, the 3-db-down point The gain of the Maestro amplifier from a 1000 -ohm source impedance made using various other output trans- being at 40,000 cps. At low frequencies, - formers. Table 1 shows the results ob- the 3-db-down point is at 10 cps. The which is representative of cathode fol-tained with three transformers which low -frequency performance of the am- lowers -is 50 db. This is measured in were available: (A) Peerless S -265-Q, plifier when feeding a speaker load is accordance with methods described by with a primary impedance of10,000 superb. Haefner' and represents the power in- ohms, 40 watts; (B) Western Electric The low distortion of the Maestro crease that is obtained from a 1000 -ohm KS -9496 Beachmaster transformer, pri- makes it a worthy part of a high -quality source whose open -circuit voltage is 1.9 mary impedance 9000 ohms, 250 watts; music installation. Using the power out- volts. If this generator is terminated in and (C) Partridge deluxe type CFB, put as read on the IM set meter shows a 1000 -ohm load, the power in this load primary impedance of 10,000 ohms, 60 an IM distortion of 4 per cent only 1 db is .00091 watts. If the load be removed watts. below 90 watts; at 2 db below 90 watts, the IM distortion is only 2 per cent, as 14 shown graphically by the solid curve of Fig.7. An important consideration in la Fig.7.Intermodula- 1 MITEISKOULATIONINSTORT1011 MAESTROAMILWIER analyzing IM curves is the location of tion distortion curves the "break" from a low -distortion flat 10."--'POW1ENAS11E400/1 INSET 40CYCLES.SRC for the Maestro. The EQUAALENTSINEWW1POV/E11 .... portion of the curve to the upward bend. solid curve represents It The ideal curve as a function of power distortionfor power 14 would be horizontal up to the break output as read on the )1 point, then would rise sharply upward. IMsetmeter. The II This is the type of curve obtained from dotted curverepre- sents the same distor- ; 4 the Maestro. The break occurs at around tionplottedagainst 1 60 watts as read on the 1M meter. From equivalent sine -wave It ,/ power...... "... examination of the composite 1 M signal ...... as viewed on an oscilloscope. itis evi- Lo 10 0 100.0 dent that it is a complex wave and that POWER OUTPUT IN WATTS meters calibrated on sine waves will not give a true measure of the 1M signal output and hence the actual power out- and the amplifier connected. the power put of the amplifier. output will be 90 watts, or a 50-db in- Table 1 Thus it is desirable to find an equiva- crease. For those not familiar with this POWER OUTPUT IN WATTS lent sine wave which has the same peak gain concept, let us state that the Musi- value as the sum of the peak values of cian's amplifier has a gain of 44 db by Fr Transformer the low- and high -frequency components this method. For those not familiar with inthe IM signal. Aston" points out the gain requirements of their pream- cps A that adding a second tone to another plifiers, it may be said that if the pream- 20 50.7 11.0 42 tone causes less than 0.5 db increase in plifier will deliver 1.9 volts into a 0.5- 30 64 23.8 56 40 64 . U meter indication, but actually the meg load, the Maestro will put out the 36.0 56 100 64 75.0 peak amplitude of the combined signal full 90 watts. 56 is 1.25 times that of the low frequency. 1000 64 75.0 56 The noise level with the input shorted 5000 60 75.0 56 Aston also points out that IM meter is around 5 my across 16 ohms, which 10000 34.4 66.6 56 measurementscan beconvertedto comes out to -28 dbm. This is 77.5 db 20000 30.2 58.9 39 equivalent sine -wave power by multi- below 90 watts, whichisreasonably plying the IM meter power by1.47. good for an amplifier of this power. In Strictly speaking, the term "equivalent practice, it has been found that the noise In fairness to all concerned, it should sine wave" has no point inI M measure- is inaudible at 1 foot from efficient mod- be stated that only the Beachmaster was ments since the term IM presupposes ern speakers. intended for service such as imposed two frequencies. However, the concept The internal output impedance. or the by the 6146, but it had very poor low- is useful and does check with practice. sourceimpedancewhichfeeds the frequency power delivery, being intended Examination of the IM curve shows speaker, is 1.5 ohms on a 16 -ohm strap- forvoiceonly.However,theother the break at around 60 watts. On equiva- ping of the secondary. This gives a transformers are well known and were lent sine wave, thisis88.3 watts, as on hand so they could be tried readily. *Sylvester J. Haefner, "Amplifier gain Intermodulation measurements, using 3 R. H. Aston,Teclinicono,AUDIO EN- measurement."Proc. I. R. E.,July 1946, 60 and 3000 cps mixed at a 4 -to -1 ratio, GINEERING, Sept. 1948. p. 500. produced the results shown in Table2.

19 Table2 tried on speakersratedat10 watts 10,000 ohms, 100 watt, wire -wound, INTERMODULATION DISTORTION where it adds immeasurably to theper- adjustable, with two sliders formance. However, letus warn that R, 10,000 ohms, 10 watt, wirewound Power Output Transformer this amplifier must he used with caution. R, 100 ohms, 1 watt onIMset R, 2.2 megs, 1 watt Watts IC) In particular, great care must he taken to eliminate switching clicks Rs 68,000 ohms,2watts 56.2 23 10 21 in early R, 27,000 ohms,2watts 42 1.5 1.4 3.5 stages as well as pops from phonograph R, 4700 ohms, 1 watt 36 1.3 1.7 1.5 motor switches. With the gain well ad- R, 1.0 meg, l/2 watt 25 1.2 1.3 1.0 vanced, any such clickis almost cer- Rs 47.000 ohms, 1 watt 6.2 0.7 0.5 0.5 tain to damage the loudspeaker-par- R,. 0.47 meg, 1 watt ticularly the tweeter. if one is used -be- R,,, Ru22,000 ohms, 1 watt. matched Note that in the power output data cause of the high power capability of Ros 27,000 ohms, 1 watt thegreatest midrange power comes the amplifier. Remember that you can't R,1 22,000 ohms, 1 watt from the Beachmaster which has the crank up the gain on this amplifier R,,, R0.47 meg, 1/2 watt lowest d.c. resistance. but power at low Ros 390 ohms, 1 watt frequencies is poor. Note also that IM without considering the program ma- Ros47,000 ohms, 2 watts, matched terial any more than you can jam down RH, Roo0.1 meg, 1 watt measurements made at(A)cps for the low the gas pedal on a super -powered Roo 10,000 ohms, 1 watt, for 20-db feed- frequency do not show up the Beach- car master. Had 40 cps been used, the dif- without considering thetrafficcondi- back tions. A word to the wise is sufficient. RH 0.1 meg, 2 watts ference would have been apparent. T, Outputtransformer,PeerlessS- All tests were made under identical We have said that the Musician's 268 -Q ; 8000 -ohm primary to 16, 12, conditionsusingtheseries -connected amplifier is good, and that statement still 8, and 4 ohm secondary. 90 watts power supply and VR tubes for screen stands for 99 44/100 per cent of music power capacity regulation. 20 db of feedback was used. lovers, but for the 56/100 per cent who ** T. 900-0-900 vat 250 ma ;Chicago Under the same conditions, the Peerless want the last word in realism as of the P-67 S -268-Q developed 80 watts from 25 to present state of the art, here is the am- T, 125 v, 15 ma, half -wave; 63 v at 0.6 a; Stancor PS -8415 30,000 cps instead of 75 watts for the plifier. Just try it and see -butremem- 13eachmaster. ber the precautions. ** T4 Filament transformer, 5 v at 4 a ; Chicago FO -56, Peerless F -138-E, ListeningQuality or equivalent We have said many times that listen- Parts List **T, Filament transformer, 6.3 v at 3.0 ing quality in music is everything. In a ; Chicago FO -63, Peerless F -072- building a new amplifier, we naturally Amplifier and Power Supplies X, or equivalent try to get the widest range, least dis- CI 10uf, 1000 v, ** T, Filament transformer, 6.3 v at 1.5 tortion. and most power, butifthe Cl 20-20 La, 150 v, electrolytic a ;Chicago FO -615,Peerless F- finished product does not sound better, CS .05 µf, 400 v, paper 036 -X, or equivalent time is wasted. Through the courtesy of CS 20-20 if, 450 v, electrolytic *T,.T. 400 0 100 v at 300 ma; 6.3 v at 4.0 Walter Toscanini, we had at our dis- Cs 30 µf, 500 v, electrolytic a; 6.3 v at 5.0 a; 5 v at 4.0 a; posal some really fine tapes and CC 20 µf, 450 v, electrolytic Peerless R -800-A or equivalent a wide C,, CS .05 µf, 600 v, paper VI, V. 6SN7 variety of loudspeakers to check the CP, C,.0.25 µf, 600 v, paper Vi, V4 6146 listening quality of this new amplifier. Cu .05 ttf, 600 v, paper, metal -cased V,, V. 5R4GY It turned out to he truly the finestre- 5 -amp 3AG I.ittelfuse 117,V,. VR-75 produced music that we have ever heard. 10 H, 250 ma, filter choke ;Peerless C -455-A or equivalent V. 5881 It has been fedinto large two-way V, 6SJ7 speakers where the superiorityisim- Ps 25,000 -ohm potentiometer, linear mediately evident, and it haseven been Ps 50 -ohm potentiometer, wire -wound * Use only in power supply #1 P,,P,0.1-meg . linear ** Use only in power supply #2 A Medium -Power Tetrode Amplifier With Stabilized ScreenSupply

CULLEN H. MACPHERSON

Regulation of the final -stagescreen supply permits high negative feedback and excellent per- formance in an easy -to -buildamplifier complete with preamplifier and tone -controlstages.

MANY OF THE MEMBERSof the audio small home. The subject of this articlehigh dissipationcapabilities or large fraternity view with alarm the is such an amplifier. power supplies. (4) The unit should be increasing complexity of audio Before beginnipg the design several self contained and of small physical size. equipment. Although in most instances features were fixed on as being desir- A chassis 11x 6 x 2 inches with a the addendum is accompanied by "higher able. (1) The amplifier should have in- 200 -ma power transformer, output trans- fidelity," the price is sometimes higher puts for both a tuner and a phonograph, former, and 7 sockets mounted was avail- yet. The need has been expressed for awith appropriate for a re-able and design of the main amplifier fairly simple, self-contained audio am- luctance -type pickup. (2) It should havewas begun with this unit in mind. Figure plifier of conservative design and ac- a tone -control system furnishing both 1 shows the result. The output stage was ceptable performance which might be boost and attenuation of bass and treble. designed and constructed first, so that constructed for use in an apartment or (3) Adequate power at low distortion the tone compensation and preamplifier * Asst.Mgr.Reproducing components should be available from the power out- stages might be matched to it. Push-pull Div., Electro-Voice, Inc., Buchanan, Mich. put stage without requiring tubes of6V6's were selected as power output

20 tubes because of their relatively high 116.17 1/2 63017 6V6 efficiency and modest driver require- O. TO MEAN! SOJrtli SO.. ments. The 6V6's were fed from a catho- 0 dyne phase splitter which was coupled to ate., to a 6SJ7 voltage -amplifier stage. By 0 25 40, using direct coupling between voltage 1001, 200V amplifier and phase splitter the number 801,01, 11 500 of capacitors in the feed -back loop was SIGNAL FROM 0 0 reduced to one and the phase shift in- PREAMP 107,44,25 194v 2 PV troduced by it was partially compensated 0 for by the insertion of an R -C network 0001 3 between the grid of the phase splitter 0 and ground. P IV It will be noted in Fig. 1 that the 0 350V second triode of the 6SN7 is employed 6 V 6 as a voltage regulator for the screens of the 6V6's. Deriving the screen potential FILAMENT .1315 from a source of low impedance such as FROM PREAMP

045 MEG this results in lower distortion, greater 1/2 656,7 stability at peak power operation, and 50 allows the use of more feedback around 6116 3V AT IA the output stage. The residual ripple 5. AT3 3301, present at the cathode of the voltage regulator is effectively filtered by the ALL VOLTAGES 0 SUPS MEASURED WITH ry use of a 1-tif paper capacitor. 11 'EGON./V T V 1/ Sinceunbalancecurrentflowing - 267)-0mg0" through the output transformer would cause loss of power handling capability at low frequency, steps were taken to Fig.1. The main amplifier. Half of a 6SN7-GT isused asa screen -voltage regulator. The balance the last stage. Pairs of 6V6's 6SJ7 voltage amplifier and the phase splitter are direct -coupled. wereselectedwithtransconductance matched within 5 per cent. Their grids were then tied together and a 60 -cps signalintroduced to the grid circuit from the heater bus. Adjustment of the 150 -ohm balancing potentiometer to the point producing minimum 60 -cps output balances the stage dynamically. 3 and 4 show how the com- ponents were mounted on the existing chassis. This makes a compact and con- venient arrangement, but other construc- tors will, of course, do as well with other arrangements.

The Preamplifier The method of bass and treble con- trol employed in the preamplifier, Fig. 2, was selected because it required only two shielded leads from the amplifier proper to the treble, bass, andvolume controls. Such a configuration allows Fig. 2. The preamplifier provides for both magnetic phonograph pickup and tuner. The 65(7 simplicity of both electrical and physical is a phonograph preamplification stage with a simple feedback loop for bass equalization. The arrangement. Placement of a variable tone control is a single -shaft affair a center "flat" position and treble and bass boosts at the two ends of rotation.

I

Fig. 3 (left). The entire amplifier, including preamplifier, is on a single chassis with the power supply. Fig. 4 (right). The underside view shows that the space is well utilized

21 Zero -ImpedanceOutput Stage

RAYMOND G. ANTHES

Excellent transient and low -frequencyresponse and good loudspeaker damp- ing make this amplifier suitable for high -quality,low -powerapplications.

THE ZERO -IMPEDANCE STAGE to be de- eeptionally good because thestageiscannot correct for the fall -off in high- scribed was designed for home use.effectively acting as azero -impedancefrequency response in the transformer. along with its driver, to give goodsource feeding the primary of the outputThe writer prefers to quality performance at moderate cost.transformer.The output sacrifice some A series R -C circuit(R,, C, in Fig. 1) transformerhigh -frequency response for minimum used was of good quality and had 1-inchphase shift in the negative feedback cir- shunts the primary of the output trans-stack. A frequency response taken withcuit.This assuresthatthe feedback former so that the output tube worksthe loudspeaker connected, andmeasur- into almost unity power factor load. Thising output voltage across the works most effectively, reducing inter- secondarymodulation distortion to minimizes harmonic distortion and phaseof the output transformer indicatedthe a minimum, shift. The feedback circuits are direct3-db-down point was below 20cps at thegiving maximum reduction of harmonic coupled and the negative voltage feed- low end, and at 5000 cps at the highend,distortion and maintaininga low -im- back is taken from the primary of theand only 9 db down at 15,000 cyclesperpedance source feeding the outputtrans- output transformer rather than the sec-second. At 2% watts output intoa re-former, over and beyond the complete ondary in order toreduce undesiredsistance load at 400 cps, the totalr.m.s.audio frequency spectrum. It is possible phase shift to a minimum in this feed-distortion was under 5 to compensate for this loss in highs bya back loop. per cent. This is relatively high by most standards,butfixed equalizer in the preamplifier, but The low -frequency responseisex-quite low for a 6V6. this was not done because the high -fre- A disadvantage of taking the negativequency loss was not serious. Most pre- * Professorof ElectricalEngineering,voltage feedback from the primaryof theamplifiers incorporate some form oftone University of Manitoba, Winnipeg, Man. output transformer is that this feedbackcontrol circuit with treble boost which

FREQUENCY RESPONSE AT VARIOuS POWER LEVELS INTOEl OHM e TETRODE 2 RESISTIVE LOAD Oh 'I WATT 0 (from preceding page) 1amo.44.- 7 O NTERMODULATION DISTORTION AS 2 IC P MEASURED INTO A RESISTIVE LOAD 72 6 FREQUENCIES OF 60 AND 7000'1. IN resistance in the input circuit of a feed- 2 A RATIO OF4 1WERE USEDIN back amplifier is not the best design 00 S WATTS THE TEST 1.'° practice, but the only way to circumvent '41 6 5 such a situation is either to add another ti 20 50 100 5001000 5000 43,C00 4 stage or to place the control at an earlier 8 FREQUENCY RI CYCLES PER SECOND a point. No instability has been observed = it' 3 in the present arrangement, however, in g a year of satisfactory operation. The 6SN7 is arranged with triodes in Fig. 5 (right). Distortion curve shows thatin termodulation (60 and 7,000 cps, 4:1) just be- ''Y 2 cascade as a feedback pair. Use of feed- gins to rise above 1 per cent at 10 watts. back in this manner reduces tube noise, i1 Fig. 6 (above). Frequency response it lowers the output impedance to the tone- curve made with an 8 -ohm resistive load at 1- and 5-watt control circuit and also effectively re- power levels. 0 5 10 is duces stray signals from the tuner cir- WATTS OUTPUT cuit when the latter is not in use. In- jection of the tuner signal at pin 1 of some measure of hum reduction, and the second triode furnishesisolation biasing the heater supply above cathode quite smooth overload characteristics, from both the tone control and the pre- potential to prevent heater -cathode emis- and when deliberately overdriven to a amplifier. sion also contributes to reducing the out- level of 22 watts of sine wave output The preamplifierisquite a simple put hum to the point of 83 db below 10 into a speaker load at 2000 cps, round- affair by present standards and the only watts. ing, rather than clipping of the wave- defense offered, if one is needed, is that Figure5 shows that the distortion form was observed. it performs quite satisfactorily. The cir- level of the amplifier is acceptable by Itis the writer's earnest desire to cuit is similar to that of Williamson in modern standards, and may even be said steer away from the sea of superlatives. which bass equalization is accomplished to compare favorably with the hallowed At the present time the amplifier suits by a grid -plate feedback loop, the turn- Williamson. Such performance is attrib- him; undoubtedly this will not always over frequency being determined by the uted to the action of the screen voltage be the case. It is not to be heralded as R -C product and the amount of feed- regulator in holding the screen potential the latest answer to the audio man'scon- back by the ratio between R and theat a fixed percentage value below that quest of the 80 -meter band. Nor isit grid stopper resistor. The turnover point of the plate. Undoubtedly, the large re- necessarily "better than any triode am- is arbitrarily fixed at 300 cps which has serve of the power transformer, giving plifier." It does, however, have theap- been demonstrated in listening as a suit- excellent voltage regulation to the power pealingattributesof straightforward- able compromise. The 6S J7 has been supply, is also an assisting factor.Fig- ness, adequate performance with respect found to be more quiet and less micro- ure6 shows frequency response at 1 and to both power and frequency, and small phonic than triode pairs more conven- 5 watts output. physical size. It is felt that the circuit tionally used in . Undoubt- In conjunction with a modified Gately edly the metal shell may well be appealing where budgets is responsible for Superhorn transducer the amplifier has are modest and space limited.

22 can be used for this equalization. The value for Z used by the author The useofdirect -coupledvoltage The use of the series R -C networkwas low, being around 5,000 ohms ; feedback eliminates the need for a large across the primary of the output trans-was 15,000 ohms and C, was 0.01 pf.blocking capacitor and ensures proper former to provide virtually unity powerThese values are not critical. operation of the feedback circuit at the factor load to the tube is not new. The For R,C, time constant of 150 micro-lowest audio frequencies. The current theory of this is well known. If a seriesseconds, the reactance of Cs is equal to R -C circuit and a series R -L circuit areRs at approximately 1000 cps. Conse- connected in parallel as shown in Fig.quently the power loss in the resistor is 2, where the R's are equal,it can below below this frequency. In music, very proved that the impedance of the paral-few fundamental tones occur above this lel combination will be a pure resistancefrequency which correspondstotwo equal to R at allfrequencies,if R=octaves above middle "C" on the . JL/C. At frequencies above the re-Even though some available power is sonant frequency of the loudspeaker, thelost at higher frequencies,itis not a impedance measured across the primaryserious matter. of the output transformer with the loud- In the usual 6V6 class "A" power Fig. 2. SeriesR -C circuit and series R -Lcircuit speaker load on the secondary may be amplifier,the average screen current inparallel. roughly approximated by a series R -Land the average plate current both in- through the feedback resistor Rii must circuit. Consequently, within this fre-crease when a signal of sinusoidal or come through the output transformer, quency range, which extends from ap-symmetrical waveform is applied. If the which is a disadvantage. Resistors Rts proximately 125 cps to the highest audiosignal is keyed on and off and a d.c. and Rfs should preferably be wire wound. frequencies, the composite load imped-milliammeter placed in the plate circuit. Positive current feedback is obtained ance presented to the tube is very nearlythe meter fluctuates wildly.If instead from a portion of the cathode bias re- pure resistance with small variation inof maintaining a constant screen voltage, sistor Rks of the 6V6, as shown in Fig. 1. magnitude with frequency. a particular value of screen dropping Rke is a wire wound potentiometer, and If values of Rs and Cs are chosen toresistor R, is chosen, itis possible toserves as a control to adjust the output give the optimum composite load imped-minimize this fluctuation ind.c.plate impedance. This potentiometer may be ance, there will be appreciable reductioncurrent provided the screen bypass ca- replaced by a 200 -ohm resistor and the in available output power at the higherpacitor is removed. The action is sim-current feedback taken across the full frequencies where the reactance of Cs ple.In the presence of signal, screen 200 ohms. becomes small in comparison with thecurrentincreases.The resultingin- magnitude of Rs. This is a serious disad-creased voltage drop in R, causes a re- The positive feedback control is ad- vantage. A compromise between theseduction in screen voltage just sufficient justed in the following manner so that two factors was made in this design. to provide the required compensation the tube presents zero impedance to its In order to obtain instant action, theplate load. With the loudspeaker con- Adjustment of R and C screen -bypasscapacitormustbere- nected, an a.c. voltmeteris connected The effect of changing Rs and Cs can be observed readily on an oscilloscope by the simple circuit of Fig. 3, and values of Rs and Cs were finally selected in this way. The value of Rp used was 47,000 ohms, which approximates the Fig. 3. Circuit used to plate resistance of the 6V6. The phase determine optimum angle of the combination is determined valuesofR, and C1 from the ellipse appearing on the screen. ofFig.1. For the tube operating voltages used, the load impedance Z presented to the tube should be from 7000 to 10,000 ohms. The output transformer should match the loudspeaker to this load. A good compromise for R, is 2 to 4 times themoved. The removal of this capacitoracross the primary of the output trans- magnitude of Z and the time constant results in about 10 per cent reduction informer with moderate signal applied to RsCs should be approximately 150 micro-voltage gain, and a loss in screen filter-the amplifier. A resistorof5000 to seconds. i ng. 10,000 ohms (not critical as to value) is then shunted across the transformer primary. The feedback controlis ad- justed to a point where thereis no change in output voltage as this resistor is connected or removed. With full 200 ohms in the feedback control, the output voltage actually increases when the re- sistorisconnected across thetrans- former, indicating a negative -impedance source.It was found by measurement that the source impedance in the author's amplifier remains zero from 20 to 20,000 cps. It was not checked above this fre- quency. The drop off below 20 cps is due to the coupling capacitor C. It should be noted that a blocking ca- pacitor cannot be used in series with the negative feedback resistance Rft. If one were used, the negative feedback would become ineffectiveat some lowfre- quency, yet the positive feedback would stillbeeffective,and low -frequency Fig.1.Schematic of simple two -stage amplifier employing negative voltage feedback an oscillation or motorboating is likely to positive current feedback to obtain an output impedance approximating zero. occur.

23 A Dual-Channel Control Amplifier For Stereophonic Music Systems

WAYNE B. DENNY

This control center affords facilities for either genuine two -channel( binaural or stereophonic) use or for what the author terms pseudo-stereophony-meaning distribution of a single -channel source to several speakers through two channels.

READER S OF AUDIO ENGINEERING are tem. For them, the concept of the "holeas far as speakers and amplifying equip- probably well acquainted with thein the wall" point source does not pro- ment are concerned. principles and advantages of the vide the necessary realism. The pages of Thus, one of the major difficulties stereophonicreproductionofmusic.thispublication are probably not thewith either true or pseudo -stereophonic Those who had the opportunity to hearplace toindulgeinthe metaphysics reproduction is the multiplicity of equip- demonstrations of stereophonic repro-of space perception. Yet, in a matter ofment required. Two sources-phono- duction at the Audio Fair will testify tothis kind the reader is entitled to knowgraph pickups, radio receivers, or tape the enhanced realism of stereophonythe particular (and, perhaps peculiar) reproducers-are required for the for- over the usual single -channel reproduc- prejudices of the writer, so that duemer. In addition two complete ampli- tion. Recently a few stereophonic discs allowance can be made for them. Likefying systems and two complete loud- have appeared on the market and it ap- true stereophonic sound, pseudo -stereo-speaker systems are required. More than pears likely that more will follow. A few phonic sound has to be heard to be ap- two channels can be used, provided only radio broadcasters have employed theirpreciated. It is not the equal of stereo-that the required signals are available. AM and FM outlets to provide 2 -chan- phonic reproduction and it is manifestlySuch a system is expensive and itis nel pick-ups of studio programs, andinferior to true binaural reproduction.complicated to adjust. The multiplicity those listeners who have separate AM But in the opinion of many who haveof controls serves to scare off all but and FM receivers have been able to en- heard it, pseudo-sterophony is definitelythe most hardened . Such .a joy two -channel stereophonic reproduc-superior to single -channel point sourcesystem, though technically excellent, is tion.Response tothese experimental reproduction. hardly suitable for use by Aunt Minnie ! programs has been excellent. Early experiments with two or more These are some ofthedifficulties speakers operating from a single channel which confronted the writer. Was it Pseudo-Stereophony showed rather conclusively thatitis practical to install a system which would Long before the advent of practicalnot sufficient to connect the voice coils be suitable for either single- or double - stereophonic reproduction there existed in series or in parallel and hook themchannel operation? Would it be too ex- a group of listeners who preferred toto a single amplifier. The results ob-pensive? Could it be engineered to the hear their music reproduced via a multi-tained from such an arrangement arepoint where it was suitable for use by plicity of loud -speakers. This "multiplebetter than from a single speaker but the nontechnical listener ? source" school is to be contrasted withare not all they might be. What is needed There seem to be at least three solu- the "point source" school. The writer is separate control of volume and fre-tions. The simplest, though not the best belonged to the former group and, toquency response for each speaker. Re-nor the least expensive, is the purchase the extent that he must be content with sistance networks could, perhaps, ac-of two complete music systems. Each single channel audio, he still does. When complish the former but not the latter.system would require the usual com- high -quality program sources are avail-What is required can best be providedponents-a phonograph preamplifier, a able he uses as many as six loudspeakersby two complete amplifying channels, control amplifier with selector switches. situated at various points in the listen-each with its own volume control andvolume, and equalizer controls, a power ing room. Sometimes a single powerequalizer circuits. This is unfortunateamplifier, and suitable speaker with en- amplifier was used for all speakers. Atbecause such a system is, admittedly, aclosure. To this outlay must be added other times two and even three powercomplex one. In fact,itis essentiallya stereophonic arm with suitable car- amplifiers have been used to drive thethe same as a true stereophonic systemtridges, a good turntable, AM and FM various speakers When two or three amplifiers were used it was possible to tailorthesignalstotheindividual UTILITY JACK 1 speakers by changing the volume and P frequency response to provide what may CHANNEL A be termed "pseudo -stereophonic" repro- zo PHONO ??' CATHODE LOY/PASS 5 db HI -LO - OUTPUT A 2 v-o-4- AMP PRE -AMP SW 1 FOLLOWER FILTER.AT TEN duction.Such reproductiontakes on 4. EQUAL. some of the characteristics of genuine 5 6 VGA stereophonic reproduction but the two must not be confused. Pseudo -stereo- 3 o phonic systems are essentially single- channel systems despitetheir use of 4v o separate amplifiers and speakers. Gen- CHANNEL B A SW3 uine stereophony employs a multiplicity 6_0 CATHODE HIGH LOW .....0_,.,OUTPLIT B of channels which are completely sepa- 5v AMP rate from microphone to speaker. How- 40 SW2 FOLLOWER EQUALIZER I 516 VC. ever,many who haveemployeda v 0- pseudo -stereophonic system will testify that it seems to be far superior to the UTILITY JACK usual single -channel, single -speaker sys- Fig. 1.Block diagram shows what the dual -channel control amplifier containsThe switches *Grinnell College, Grinnell, Iowa. are shown inthe positions required for dual -channel stereophony from phonographrecords.

24 tuners. The worst feature of this first puts provided with equalized preampli-for each channel. Each utility jack is alternative is the complexity of connec-fiers suitable for use with variable -re-connected directly to the arm of its se- tions and controls. Itis likely to lackluctancephonographcartridges.The lector switch and ahead of the volume flexibility unless the user will toleratefour high-level inputs are suitable forcontrol. Any signal fed to either channel the use of temporary connecting cables.AM and FM tuners, television audio,is thus available for operation of a re- Aunt Minnie would hardly appreciatecrystal cartridges, and tape recorders.corder. Further, if the switch is turned this kind of installation.It looks too Two output channels, A and B, areto an unused position, the utility jack much like the pilot compartment of aprovided. Each output channel has itscan be used for temporary connection jet bomber. Its wires and knobs, com-own selector switch to connect it to any to an additional input source. pletely unintelligible to the uninitiated, one of the six possible inputs. Each Although only six input sources are do not favor relaxed listening. channel hasits own volume controlnormallyprovidedfor,theselector Another alternative would employ a (VCA and VCR in the figure), its own switches are actually 12 -position units. switching panel wherein eachsignal equalizer, and its own voltage amplifierOnly taps 1, 3, 5, 7, 9, and 11 are used source could be routed to one or more for raising the level to a value suitablebut in these positions are consecutively outputchannels.Each signalsource for driving a power amplifier. Channel Anumbered from 1to 6. The increased might terminateina key switch by contains a low-pass filter for suppression spacing between live contacts materially means of which connection to channel of needle scratch and intercarrier radio reducescrosstalk.Crosstalk may be A or channel B is made. Satisfactorynoise and it also contains a resistancefurther reduced by grounding the unused adjustment of such a system requiresnetwork providing about 5 db loss in- terminals. separate control of the volume of eachserted between the filter and equalizer. Another item of interest is the inclu- input source. But those readers familiarchannel B contains neitherfilter norsion of switch 3. When this switch is with the type of audio equipment usedfixed attenuator for reasons to be dis-thrown to the B position each channel in broadcasting will notice the similaritycussed later. In each channel the volumefunctions independently. However, when of this possible system to the standardcontrol is succeeded by a cathode fol- itisin the A position channelBis audio mixers which grace the controllower. The cathode follower in channelbridged across channel A. This isa rooms of radio stations. Clearly such a A isolates the volume control from theconvenience when the system is used mixer, while perhaps adequate for the filter and it also provides a low -imped- forpseudo -stereophonicreproduction. purpose at hand, is hardly a practicalance source for the filter. In channel B Under this condition the volume control solution to the problem as stated. the cathode follower isolates the volume VCs regulates the gain of both chan- control from the equalizer and providesnels. Further, switch 1 selects the single The Practical System a low -impedance source for the equal- signal source. During this type of op- A much better and thoroughly prac- izer. eration switch 2 serves to connect its tical solutionis shown in Fig.1. A In addition to the main features of utility jack with any one of the six in- glance at this functional diagram showsthe system there are two gimmicks which puts. This permits recording from one that this system will handle a total ofrequire explanation. The first of thesesource while listening to another, a use- six input sources. Two are low-level in- is the inclusion of two utility jacks, one ful feature.

V7 V8 R48 L4l trc 12r100C30 o 2 M R23 cc N^ 18 -20/450 N C11¢ 012 ® 0 C10 C1 C3 0 2 J9 Via V3o V 3 b 01 20 01 1 2 S L 7 '0 12SL7 01 12 SR 7 co 0800 v5 M co r- C4 - cc cv SW4 01 5 0

Cr-O 0 0 VIE 6002 0 O J1 0 0 01 R15 N 0 40 0 0 CC 0 0 CC aro I 01MEG C13 00015 c 02 C9 LOWPASS 50'25 FILTER cc 441 0 CA N fSC19 OC N C17 .01 V2a 0 R49 12 SL7 01 -"AAr--4, 17 22,000 J2 0 cc 027 V2b 0 Z, C28 0 coo0 C16 11 J10 C26 01 2 0 cc" V4a - 20/450 .4D 12SL7 01 125R7 C24 v6 SW1 cc 0 03 C20 0002 0 J 3 . 4 C21 0 0 C22002 50 01 0 Cr- 6 01 01 R40 cc cc 0 INPUT C23 0 1MEG asO g C29 re r- 0 SELECTOR 02 025 50/25 SWITCHES .00 3 2 0-Ere,A 5 SW3 POWER SOCKET VSV6 Vi V2 V3 V4 v 0 TO 7uNER, E 7C SW2 F.7/8111,2 0 SI_ J7 J8 .1 ALL RESISTORS 1WATT L.) > ALL CONDENSERS 600V > UNLESS OTHERWISE SPECIFED C1 10

Fig. 2. This is the complete schematic diagram. The separate elements of the systems are conventional, with the switching the unique feature.

25 Now comes the other gimmick. Ex- should be about 250 volts. V, and V, amination of Fig. 1 shows that the lower should be OB2's (108 volts) or 0C3's volume control VCR is included in chan- (105 volts). This arrangement permits nel B at all times, even when switch 3 about 210 volts for the preamplifiers. is thrown to the A position. It will be Resistor R,, should be rated at 10 watts seen that under this condition the gain and its resistance should be such that the of the upper channel is about five db current through itis at least 10 ma. lower than the gain of the lower ampli- A value of 4,000 ohms works out well fier channel as measured from switch 1, for two 0C3's and a 250 -volt supply. provided VCR is turned full on. Conse- Other combinations can be worked out quently, VCR acts as a balance control; for different supply voltages. However, it serves to adjust the relative signal make certain that the power -supply volt- levels of the two outputs and hasa age is at least 30 volts higher than the margin of gain adjustment from zero to *argot, sum of the nominal voltage ratings of the regulator tubes. Filters using volt- + 5 db. As stated earlier, some means of --t4sio; balance adjustment is a requirement for age -regulator tubes are superior to filters proper operation of a pseudo -stereo- using capacitors,particularlyat low Fig. 3. The control amplifier is mountedon a frequencies. phonic system. Note that the filter,if chassis 11 x 17 x 3 inches. Probably a smaller (Caution: Do not shunt used, operates on both channels in this one could be used. regulator tubes with a filter capacitor. type of operation. This causes oscillation.) Pseudo -stereophonicreproductionis The plate supply for the cathode fol- also possible with switch 3 thrown tohe can twiddle the knobs to his heart'slowers is obtained from the junction of the B position.Here, bothselector content while trying to squeeze the lastthe two regulator tubes. This isper- switches are connected to the same sig-decibel of "fidelity" out of the speakers. missible because the cathode followers nal source. Again, the maximum gain normally operate at very low signal volt- of channel B will be about 5 db higher The Circuit ages in this unit. Further, the inverse than the gain of channel A. However, The system whose functional diagram feedback inherent in the cathode follower operation of the equipment is simplified appears in Fig.1 has been engineered reducesdistortiontoextremely low when adjusted according to the pre- into practical form. The schematic dia-values. Incidentally, it will beseen that ceding paragraph because selection ofgram appears in Fig. 2. The individualeveryamplifyingtriodeisincluded the input signal can then be accom-elements of the system are entirely con- within a feedback loop. This feature plished entirely through adjustment ofventional. Only the inclusion of twocontributes to the very low distortion switch 1. In either case, channels A and of the amplifier which is a necessity if channels, the switching arrangements, the full benefits of available high -quality B are separately equalized. In this re-and the gimmicks mentionedearlier spect the system is different from a power amplifiers are to be realized. Com- make this system different from other parison of this unit with its predecessors, commercially available control amplifier more conventional systems, in principle designed for single- or 2 -channel use.at least. which did not employ as much feedback, But experience has shown that separate gave immediate evidence of the lowered Two power supplies (not shown) are distortion. equalization, together with balance con- required. One furnishes about 72 volts Intermodulationmeasure- trol, is desirable for best results. at 150 ma for six heaters connected inments using 60 and 7,000 cps,4: 1, It should be emphasized that when indicate that the distortion is nevermore series.Thispowersupplyis not than a small fraction of 1 per cent with switch 3 is thrown to the A position,grounded. The heater string is grounded normal signal the operation of the system is ratherbetween V: and Vs which are the two levels. similar to that of any other high -quality tubes operating at the lowest signal level. Design Details home music system. Even if Aunt Min-The other power supply furnishes about The equalizers are conventional in nie doesn't balance the two outputs cor- 250 volts d.c. for the plate supply. It isevery respect; the values of the compo- rectly the results can be no worse than entirely conventional but should includenents are similar to those used in com- those of an ordinary single -channel sys- plenty of filtering. The writer found amercial equipment of comparable quality. tem. When itis intelligently used, the 2 -section choke -inputfilterto be en-It will be noticed that only results can be very much superior. a single tirely satisfactory. transition frequency is used in thepre- This is a good place to take a tip More than ordinary attention to leadamplifiers. Previous preamplifiers built from one prominent manufacturer ofdress is necessary to guard against cross- by the writer provided for a choice of communication receivers. This manu-talk between channels.Inspectionof three or four transition frequencies but facturer was well aware that many ofFig. 3willindicate that no attempt his communication receivers-complete experience shows that only the onecor- was made to crowd apparatus. Theresponding to the AES curve wasever with selectivity controls, crystal filters,chassis is large; it measures 11 x 17 x 3used to any extent. Anyhow, ina unit beat -frequencyoscillators,bandspread inches and these dimensions are some-like the one described here the number dials, etc.-were often used by Mrs.what greater than those of the usual of controls should be reduced toa mini- Amateur for regular broadcast listening. control amplifier. mum. Of course, the purist may, if he To simplify the operation of the receiver It was found that the plate -supply in the hands of the lady of the house, so desires, introduce more flexibility if filtering shown is adequate to reducehe is willing to tolerate the addedcom- this manufacturer indicated thepropercrosstalk between channels provided the plexity of operation. settings of all controls by little coloredinput signal levels are all comparable. Ordinary volume controls are used in dots. When each control was set so thatThe writer's AM tuner produced signals preference to loudnesscontrols. The it pointed to the appropriate dot there- higher in level than any other signallatter could be used but the bass boost ceiver was correctly adjusted for broad- source (when measured at the selectoravailable from the equalizers issuffi- cast reception. There is no good reason switch)and there was some ridingcient to provide a close approach to the why this same technique cannot be used through of AM signals under certainFletcher -Munson curves at the volume for relatively complicated home musiccor.ditions. This was corrected by inclu- levels normally used. systems. Set each knob to point to the sion of an attenuator in the tuner itself. Examination colored dot and the system is correctly of thephotographs Another alternative is to provide levelshows that there are six variablere- adjusted for ordinary single -channel op-controls (screwdriver adjusted) in allsistors and two selector switches, all eration. In this way, Mrs. Audiophilesix input circuits as is done in someoperated by pointer knobs. Each channel can enjoy good music during the hours commercial units. requires a selector switch, a volume when her husband is away at work. The Two voltage -regulator tubes in series control, a bass control, and a treble system may not be in optimum adjust-are used instead of electrolytic capacitors control. Two toggle switches and two ment but it won't be too far off. Then, in one part of the plate -supply filter. The jacks also appear on the panel. One when Mr. Audiophile returns from work available plate -supply voltage under load s.p.d.t.switch provides for single-chan-

26 Improved Phonograph Compensation Circuits

R. H. BROWN

An up-to-date revision of a circuit presentedover a year ago provides for the presently used record- ing characteristics, including the RIAAcurve without which no modern amplifieriscomplete.

SINCE THE PREPARATION of the article With the exception of the 12-db-per- the characteristic as zero frequency is entitled"Hi -FidelityPhonographoctave rolloff on position1, this com- approached. For the LP and RIAA PreamplifierDesign'thenewpensator may be used in the grid circuit characteristics this zero -frequency boost RIAA characteristic has been announcedbetween two amplifier tubes which are is15.4 and 20.7 db, respectively. An- and there has been much clarification re-not connected by a feedback loop. When other method for selecting these resist- garding the recording characteristics ac-so used the 43,000 -ohm resistor between ors is to make the time constant of the tually used by different disc manufac-positions 6 and 7 of S.should be re- C, and Rs combination in Fig. 2 (Fig. 1 turers. Because of these developments itduced by an amount equal to the internal of the previous article), equal to the bass has become desirable to revise the com-resistance of the source of the signal pre -emphasis time constant of the re- pensation circuits in the triode preampli-supplied to the compensation network. cording characteristic. For the LP and fier design given in that article. For a simple plate -loaded voltage -ampli- Orthophonic characteristics this is 1590 The over-all revised circuit is shownfier stage without feedback this source and 3180 microseconds, respectively. The in Fig. 1. The 480-144 capacitor shownresistance is simply the parallel combi- method for selecting these resistors sug- in the diagram represents the total valuenation of the plate resistance of the am- gested in the previous article makes the of circuit capacitance at that point. Sinceplifier tube, the plate load resistance, and compensation correct for the extreme the stray capacitance present will dependthe following grid return resistance. The low frequencies at which the gain of the upon the construction of the preampli-grid return resistance should be con- basic amplifier begins to become a limit- fier, one should begin with a lumpednected between ground and position 7 ing factor, but it does not make the com- capacitance of around 400-420 Rti.f andof SS4 so that it will not interfere with pensation correct in the range covered add additional lumped capacitance asthe performance of the compensation by the majority of the bass -frequency may be required to obtain the correctcircuit. components. rolloffs. For convenience in making this Figure 1also gives the revised bass The method of connecting the switch- adjustment, the legend gives in paren-compensation circuit. The bass rolloff ing -transient -eliminating resistors as theses the proper relative output level ofresistors on positions 3 and 4 of SIR may shown in Fig. 1is preferable to that the amplifier in decibels at 10,000 cps forbe selected by neglecting the unity in the various treble settings. the denominator of Eq. (5)2 in the ar- A' * 302 S.E. Second, College Place, Wash-ticle referred to above and selecting Rs (5) to provide a maximum low -frequency 1ALP4 R1A' ington. 1 + ' I See page 36. gain which gives the boost required by R, + RI+ R,

to give "infinite" attenuation for inter - high intermodulation distortion at such channel whistles of 10 k.c. An audio high levels. For this reason the volume DUAL -CHANNEL oscillator and a v.t.v.m. (or oscilloscope) controls used in this unit are placed are helpful here but the adjustment can immediatelyfollowingtheselector AMPLIFIER be made by ear, if necessary, while the switches. The equalizersprecede the (Continued from previous page) signal source is an AM tuner. The value two -stage feedback pairs; the signal is of C can be determined so as to pro- reduced before amplification, not after it net or stereophonic operation. The othervide attenuation starting around 7,000 has reached the level of high distortion. .witch is a d.p.d.t. unit for placing theor 8,000 cps. The actual capacitance re- Sufficient feedbackisincorporated to low-pass filter in or out of the circuit.quired will depend on the Q of the coil reduce the generator impedance to a The two jacks are the utility jacks men-and the load into which the filter works. value of 2,000 or 3,000 ohms. More feed- tioned earlier. They are placed on theThe values of capacitance and load re- back could be used if desired because panel to permit ready access to connec-sistance given in the diagram worked the gain is higher than required for any tions to one or two tape recording chan-out well for the particular coil used by normal input signal. This same feedback nels. the writer. Some changes may be ex- also reduces distortion,as mentioned A word about the low-pass filter is inpected when using other coils. Unfor- earlier. Distortion is already low because order. You can calculate the constantstunately,the inductor shown inthe the signal never exceeds 1 volt anywhere for the filter from the equations for aphotographs is not commercially avail- in the entire unit. single m -derived section if you want to.able. Another feature of interest in connec- However, for the purpose at hand this The circuits used in the amplifiers tion with lowered output impedance is is neither the easiest nor the best methodwere chosen on the basis of low dis- the use of large (2-µf) oil capacitors for designing thefilter.Instead, thetortion. Some preamplifier circuits-in- for C and C,,. There is little point in writer installed a 150-tnd variable ca-cluding some designed earlier by the reducing the generator impedance of pacitor for C,4 across the inductor, L.writer-amplify the signal up to10 the amplifier to a low value if this ad- If L has an inductance on the order ofvolts or so and then the equalizer at- vantage is lost by high series reactance 1 henry this capacitance can be adjustedtenuates the signal. Most triodes show in the output coupling circuit.

27 TO POWER SUPPLY ojt 5 A C LINE Si

534va 3ZZO 15,000/1W 15,000/IW "NV' NAN 400-450Y AT 4 erna 201! 01 201 201 2121: 450 0J 450V 450i 450i 0 O1 0 25 S2a 12A27 12 Au7 TO PIN 7 OF SSN7

LEVEL

044 TO PIN 8 OF 65N7

TO CATHODES OF OUTPUT TUBES 501

6 N 0 S2 TREBLE ROLL -OFF S3 BASS TURNOVER 0 62 MEG S2o 1 -POOR 1- 300'\, 2- NAB (-16)* 2- AES oevwd..112 MEG of 3- RIAA (-13.7) 3- LP 4 - AES (-12) 4- RIAA 5 - LONDON (-105) 5- NAB 7;5 6- -3dIVOCTAVE FROM 3000(-5) 6- 6501.. 0 7 -FLAT ( 0) 7- 850'"U 9 RI 0 477/IEG FOR G E CARTRIDGE 47.000 FOR AuDAK CARTRIDGE NOTES 27.000 FOR PICKERING CARTRIDGE es3 ALL CAPACITANCES NI pt UNLESS SPECIFIED OTHERWISE R2 8200 FOR GE CARTRIDGE ALL RESISTANCES 210!,1/2 WATT UNLESS SPECIFIED OTHERWISE 5600 FOR AUDAK CARTRIDGE * It 5% ALL SWITCHES MUST BE SHORTNG TYPE 2200 FOR PICKERING CARTRIDGE SEE TEXT

Fig.1.Revised preamplifiercircuit now includes the new and almost universallyacceptedRIAA curve. given in the orginal articlein thatit earlier article to include tone controls. places only one of these resistors in the With a bypass on the first 12AU7 cath- feedback circuit at a time. ode a with about 10- Ideally, the 0.12-meg resistor between db insertion loss could be substituted for Su and the first 12AX7 cathode should the loudness control. With the compen- be shunted with a capacitance equal to sation adjustments provided by the re- 3.9/120 times the total capacitance to vised circuits presented herewith, plus ground shunting the 3900 -ohm cathode or minus 10 db of tone control should be resistor. This point is simply a technical more than adequate for all but the very purism for a preamplifier using a tube unusual situation. The output section of like the 12AX7, but there might be some the preamplifier should be redesigned if value in giving attention to it in a circuit a greater range of tone control is re- which used a larger feedback ratio; e.g., quired. a feedback bass -boost circuit in which The variety of recording character- the basic amplifier used low -mu tubes. istics in use is now sufficiently great that Fig. 2. Basic degenerative -bass -boost amplifier The author has received a number of one may dare to hope no new character- circuit. Vi and V. and associated components, inquiries regarding the possibilityof istic will appear which would make fur- excluding feedback elementsR,, R.1, and Cc. adapting the triode preamplifier in the ther revision necessary. comprise the basic amplifier. Triodes are shown for simplicity. A NoteonVolume Controls CHARLES P. BOEGLI* The author shows that it is not always satisfactory to place a high -resistance potentiometer just anywhere in an ampli- fier circuitif specified performance isto be obtained.

AN audio amplifier is generally madecan be made to yield output voltages of erationsthat appear to be generally up of several stages arranged inthe desired magnitudes. Thisis most overlookedinproviding a "volume" cascade so that the input signal isoften accomplished in high -impedance control of this type. fed through one after the other, beinglocations, by means of a simple potenti- Since hum and noise are most apt to amplified on the way, finally appearingometer; the output from the plate of one assume noticeable proportions in the in- as an output voltage at the loudspeakerstage is connected to the top of the po- put stages of the amplifier where signal terminals. Somewhere in this unit, it istentiometer, the other end of which is voltages are quite low, the volume con- usually necessary to provide a controlgrounded, and the grid of the following trol should generally be located after over the gain of the entire amplifier, sostage is fed from the slider. The purpose * CincinnatiResearchCompany,6431 that input signals of the given voltagesof this note is to mention some consid- Montgomery Road, Cincinnati 13, Ohio.

28 the first few stages. In this manner, the and the input capacitance of the next,ode stage with an output resistance of signalis maintained at the maximum as indicated in Fig. 1. When one stage 10,000 ohms connected to the top of a value compared to the noise through feeds directly into the next the problem 1-meg. volume control, there would be these criticalstages and becomes at- of attaining good high -frequencyre- some point at which the next stage tenuated only when it has reacheda sponse isfairly well defined, and thewould look back into as much as value quite large in comparison to noise solution consists simply in keeping the 10,000 + 1,000,000 voltages. On the other hand, should the output resistance of each stage as low - 252,500 ohms volume control be located too near the as possible. Low -mu triodes with plate 4 output tubes, the possibility is increased resistances of the order of 10,000 ohms and if the input capacitance of the fol- that the signal will attain sufficientmag- are excellent in this respect and may be lowing stag were sufficient to cause a nituie before reaching the control to used with plate -load resistances of al- cause unnecessary distortion in previous most any size without adversely affect- stages. Placement of the control at a ing high -frequency response. With high - point where the maximum signal level mu triodes and pentodes the plate -load is from 2 to 4 volts will usually lead to resistors, as well as the grid resistors of a fairly satisfactory design, suffering the following stages, must be kept small neither from distortion nor excessive in order to realize good high -frequency noise. It goes without saying that the response. But what occurs upon the in- control must not be placed inside the troduction of a volume control? feedback loop, if any are employed in Obviously, when the slider is at the the circuit. top of the control, the situation is the Fig. 2. Equivalent circuit of a stage followed by The only disadvantage to locating the same asif no control were present. a volume control. control otherwise than at the input to When the slider is near the bottom, also,3-db drop at 1000 kc without the volume the following stage is fed from a verycontrol, there would actually be a drop low impedance so that the high -fre- of 3 db at about 4000 cps at this position R EOUVALENT OUTPUT quency responseiseven better than RESISTANCE OF STAGE 1 of the volume control. when the slider is at the top. In less ex- C TOTAL INPUT CAPACITANCE We find, consequently, that the re- treme positions, however, if the volume Of STAGE 2 sistance of the volume control is rather control resistanceisquite high, therestringently limited by the capacitance may be a substantial loss of high fre- into which it must work. As an example, quencies-and it is precisely these posi- suppose we had a 12AY7 stage with an Fig.1. Low-pass filter existing between two tions that are most important. output resistance of 20,000 ohms work- stages of an audio amplifier. Reference to Fig. 2 will make this ing into a 6SL7GT with an input ca- clear. R. is the output resistance of the pacitance of 100 µµf. At best, there will the amplifier is that the voltages of the preceding stage and R, is the volume - be a drop of 3 db at 80 kc. Suppose fur- signal sources used with the amplifier control resistance. The symbol a repre- ther that we cannot tolerate a drop of must-to a degree, at least-be con- sents the positionoftheslider and more than 3 db at 50 kc in this particu- trolled. Thus, with the volume control measures the fraction of the voltage ap- lar stage. Then the maximum output re- at the input we need concern ourselves pearing at the top of the control that sistance at any position of the control only with the minimum input required is applied to the following grid. Now must be 32,000 ohms. To find the maxi- for full output, but if the control is situ- Ro, the resistance into which the follow- mum permissible volume controlre- ated later, the maximum input also be- ing grid looks is aR, in parallel with sistance we use Eq. (4) with 20,000 comes important. An input stage han- R.+ (1 -.a)121, which is found to be ohms for R., 32,000 for Rom, and solving dling a minimum of 0.5 volt may well aR,Rs + (a- ce)R,' for R, we obtain Rs= 108,000 ohms, and (1) a 100,000 -ohm control would be used. be designed to carry a maximum of the Ro - R. +R, order of 1.5 volts ; obviously, if a tuner Suppose we are more exacting and supplying 6 volts is connected to such We are interestedinthe manner in state that the high -frequency response an input, overloading willresult. The which Ro varies with a. Hence, differ- with the volume control must never be best arrangement would therefore seem entiation of the above expression with any poorer than without it. This is the to be one in which the volume control respect to a yields same as saying that Roos in Eq. (4) must is placed near enough to the output be the same as Rs: dRo12,R. + R. - 2a12,1 stages to maximize the signal-to-noise (2) R1+ Rs ratio and close enough to the input da Rs +12, -R. stages to prevent serious distortion, and The resistance R. is a maximum at the in which individual pads are used on value of a found by setting the right = 3R, (5) the various signal sources to bring their side of Eq. (2) equal to zero and solving To realize this performance in the case outputstoapproximatelyauniform for a: just described we should have to utilize level, preferably slightly over the mini- a 60,000 -ohm control. That these values mum required to drive the amplifier R,R2+ Rit - 2aR,' =0 are much lower than those commonly in fully. + R, use will readily be recognized. a=2R, (3) The installation of a volume control Effect on High -Frequency Response thus appears to be somewhat more in- The high -frequency response of an When a assumes this value, themaxi- volved than simply inserting a 1-meg. amplifier (utilizing a good output trans- mum output resistance potentiometer at the amplifier input. In former)islargely determined by the Rs particular,theuse ofrelatively low Rom - cutoff frequencies of the series of low- 4 (4) values is indicated in cases where the pass filters incorporated into it and made control is working into appreciable ca- up of the output resistance of one stage For example, if we had a low -mutri- pacitance.

29 The Selection of Tone -Control Parameters

EDGAR M. VI LLCHUR*

Proper design of tone controls requiresa study of the conditions which must be cor- rected. The author delineates these conditions, and explains the requirements for each.

VARIOUS ToNE-coNTsol. circuits with given transition frequencies' and 15 1 i 1I TM I I I rates of boost or cut have been ex- --- SPIJUCER RESPONSE IPRIESSUIN) - - TONE CONTROL RESPONSE NOWISE)

tensively discussed in technical litera- 10.---- RESULTANT WAXER OUTPUT ' , ture. Little has been written, however, (PREMIX) about considerationsinvolvedinthe selection of the response -curve param- I 5 1- eters,whichsometimesseemtobe Fig. 3. Resultant fre- S quencyresponse _ chosen more or less arbitrarily. While of 1 - -3- ,A- on -./ "flat" power amplifier stages are care- audio system us- V0 Aq'' ing a commercial . ... ft fully designed for an audio -frequency speakerinthe $50 I response which is kept to a maximum class and the treble V Atj k i 5 I 4 random deviation of a fraction of a boost of (A), Fig.1. decibel, S I impropertonecompensation ri (either manual or automatic) gio lo}I in the 141 same amplifier may introduce, or leave t' t1 11 uncorrected, very large inaccuracies of -15 n nnw frequency distribution relative to the in lom novo perceived original. These inaccuracies FREQUENCY IN CYCLES PER SECOND haveonoccasioncompletelyover- shadowed the benefits of the above -men- tioned careful design, and have become a primary factor in determining over-all volumelevel.The inclusionof suchwhich allows for equalization of quality. circuits does not make variable toneany frequency conditions, or he can Fixed tone equalization is used when controlsuperfluous,sincetherearereduce the flexibility of control, choos- the frequency characteristics for which manyunpredictableconditions for ing compromise response curve param- compensation is being made are con-which it may be desirable to adjust theeters which are capable of producing stant. Modern home reproducing sys-frequency response of the reproducingapproximatecompensationforthose tems containseveralfixedorauto-equipment. conditions most likely to be encountered. maticallyvariableaudioequalization The designer of fixed equalizers doesThe degree of tone -control complexity circuitsassociated with FM pre -em-notordinarily have toworry aboutaccepted in non-professional equipment phasis, recording characteristics, pick-what parameters to use; with certainhas increased quite a bit since the day up frequency distortion, or changes of exceptionshistransitionfrequencies,of the single treble -cut switch, and the and the required rate of boost or cut,most common arrangement in current * Woodstock, New York. have been exactly determined by hisaudio amplifiers is a two -control, con- ' The transition frequency is the inter-problem. In contrast a tone -control de-tinuously variable system. This allows section of the theoretical linear slope of the signermustfurnishthemeansfor progressive cut or boost of either bass response curve (a slope which the actual correction, by the same circuit, of allor treble, but no independent control curve only approaches) with the frequency sortsofsignalaberrations. He canof the reference frequencies at which axis-see Fig. 5. either provide a complex control systemthe response curves begin their slopes.

t5 445 _.---- __- .20 *20 . ,re , .15 4.15 3 .90 a0 g 3. 5 3 0 11e....".245' le0 or ...

E-6 , ... 935 ;-r

L+-15 Wis 1,tOv° ft 61- -25 Z"'"'..°...... IS WOW W . )0000 IMO KO WA IDWO FREQUENCYKWFREQUENCY IN CYCLES PER SECOND FREQUENCY IN CYCLES PER SECOND

Fig. 1. (A), Voltage response curves of a typical commercial tone control circuit of the R -C type, maximum positions.(B), Voltage response curve of a commercial tone control circuit of the L -R -C type, maximum positions.

30 A circuit which introduces a progres-operator's adjustment of tone controlsThis factor, referred to as the Fletcher - sive response slope rather than uniformis in the nature of a search for maxi-Munson effect. will be discussed in the elevation or depression of a whole bandmum fidelity to the perceived original, paragraphs devoted to bass boost. offrequencies,besides beingoffarthe psychological mid -point ceasesto Equipmenttrebledeficienciesare simplerdesign,providesthedesiredhave much significance. Tone controlusually most severe in electro-acoustic form of compensation in most applica- becomes tone compensation,and thedevices such as loudspeakers, pickups, tions. Itis common although not uni- problem of response -curve parametersand recording heads, but also occur in versal practice for dual tone controlsrevolves about the question of what thecouplingcircuitsinaudioorinter- to use approximately the same tran- mediate -frequency amplifiers. sition frequency region, usually at orcontroller must be equipped to compen- The worst offender in a given system below Loco cps,forbothbass andsate for. In the following discussion itis ordinarily the loudspeaker. In Fig. 2 will be assumed that the amplifieris treble variation. Figure 1 shows the manufacturers'publishedon -axisre- response curves of two such tone con-being designed as an independent unit,sponse curves for six speakers in dif- trol circuits employed commercially. and that the brands of components with ferentpricerangesareplotted.Al- The justificationforselectingthis which itis to be used are not known.though the performance represented by these curveswillvary greatly under differentacousticalconditionst h e graphs may be taken as indicators of a general trend. There is one feature 111111 IIII REFERENCE,LEVELS III 00;s,viresto° which all may be seen to have in com- ream EQUAL"'"ass mon, and which is characteristic of the s2 Kant' BillFC`ct-TTG:41..."1"4 greatmajorityofconeloudspeakers

q 1111E1 11111 withhigh -frequencydroop;thefre- u111111 111111 quency region of the first two octaves 5§MIMI Fig.4. The 50- and above 1,000 cps, far from being attenu- 1111111i13 111111 1111h 80-phon equal loud- nesscontours(from ated,isaccentuated(because ofthe I,; .M9.11111 1011111111M IPA FletcherandMun- newresonancesintroducedbycone :'- 01111111111ia:MLIZIM11111111 ZIIII son) superimposed. break-up), and treble droop does not begin before 4.000 cps or higher. This ::a 111111 11111111110. .11111 fact has a significance beyond the ob- it 111111 111111 11111111 vious implication concerning compen- "Iv 111111 111111 111111 sationforspeakerdeficiencies.Any 111111 111111 111111 losses in thefirst two treble octaves M. 100 8100 ,.... which are likely to be met with from FREQUENCY IN CYCLES PER SECOND other causes will probably he compen- satedorevenover -compensatedby speaker characteristics. reference frequency region is that it is Treble Boost Crystal phonograph pickups have a considered psychologically "neutral" in The most common purposes for whichtypical velocity response above pitch. It has been pointed out that 800treble boost will be needed are: hundred cps which decreases with fre- cps is the geometric mean between 40 quency quite regularly up to a rather and 16,000 cps, frequencies which may 1. Compensation for treble deficiencies in associated reproducing equipment. sharp cut-off somewhere between 4,000 be taken as nominal limits of hearing 2. Compensation for treble deficiencies and 10.000 cps. When this regular droop underaverageconditions.Sincethe in program material. does not conform to the desired record- perceptionoffrequency,likethatof 3. Compensationfor discriminatory ing characteristic further compensation amplitude, closely follows the Weber- acousticabsorptionwithafrequency is properly provided by a fixed R -C net- Fechner law(inthat the degree of characteristic different from that of the work rather than by the tone control. sensationvarieslogarithmicallywith hall or studio in which the sound origi- The comparatively smooth and accu- the stimulus)the geometric and not nated. rately predictable slope lends itself to the arithmetic mean of the audible fre- An additionalpurposeforwhich fixed equalization, and the recommended quency spectrumisitspsychological treble boost has on occasion been con-circuits or necessary data for their de- mid -point. Thus there are about foursidered necessary, but isnot,is com-sign are usually readily available from and one-half octaves ofuseful audiopensation for the variation in hearingthe manufacturer. But whether or not frequencies on each side of 800 cps. frequency characteristics associated such a fixed network is provided, the Ifwe assume,however,thatthe with changes of sound intensity level.treble boost control cannot be designed

siAi i_efl_ III/i a A l 1 ka I 04" I C it A /I '%".1 4"... I t a 1 t/ I .... _.I/ a -- z 11 A s i lot..., 1---1 . .. oak0 1 red I / i 111 V s A I I li1II fir a 4 I i ---S.SPEAKER SPEAKER --- -10.SPEAKER ' --- 12 I 1 . 1 / r if leSPEAKER I --- 5-1S. SPEAKERS 1 I - - ill.r. It- s-- 1 vis i II 1 I 1 se moo so ,_ ,- I. ICMoom 100°D FREQUENCYIN CYCLES PER SECOND FREQUENCY IN CYCLES PER SECOND

Fig. 2, (A) and (B). Pressure response curves of three commercial speakers in a price range of $20 to $150.

31 for correct crystal pickup equalizationthe compensation will not be used. This except for the second droop, because theunhappy effect is illustrated by Fig. 3, required transition frequency wpuld bein which the tone -control and speaker, well into the bass region. response curves of two high-grade com- The contributionof high -frequencymercial units are combined. losses by other components cannot, of Determination of an optimum transi- course, be predicted, except as to onetion frequency that will best suit the factor ;it may be expected that lossesvarious requirements of treble boost can- will be confined to frequencies above thenot be made with precision, but it would second treble octave. A survey of cir-seem that present commercial practice cuit components, recording heads, etc.,makes use of a frequency which is at will indicate that itis rare for trebleleast two octaves too low. The operator droop to set in before 4,000 cps or so.of the set may be impressed with the Even AM broa'1cast bandi.f.trans-dramatic power of his treble boost but formers provide,atworst,relatively may stillbe loath to useit.(Some even coupling to 3,000 cps. Britishmanufacturersleantowards Treble deficienciesin program ma-higher transition frequencies for treble Fig. 6.Circuitvaluestoapproximatethe terial may result from low-grade studioemphasis-one manufacturer uses 3,500 curves of Fig.5. As many of the values of the equipment, from transmission circuits,cps.) original/fcircuit as possiblehave been and from old records. Such losses are A single R -C network can approach retained. almostalwaysassociatedexclusively 6 db per octave in rate of boost. The with the third and/or fourth treble oc-simplicity of the single network is one taves. companiessubscribe.Table 1lists some good reason for accepting this slope asof the different ways in which the treble The writer has not found a quantita-the maximum, and it will prove amplespectrum is or has been treated by rec- tive study which compares the frequencyfor most purposes, particularly in vieword manufacturers. transmission of typical living rooms with of the fact that treble emphasis brings Although treble recording character- that of halls or studios, although meth-to the fore harmonic distortion in theistics are ideally equalized at the pickup, ods for room transmission measurementshigher ranges.Ifthe transitionfre-variable pickup equalizers are still the have been outlined.2 Therefore no com-quency is chosen as 3,200 cps-two oc-exception rather than the rule, and the ment will be made on the subject othertaves above 800 cps-an insertion loss oftone -control designer must assume that thantomention the fact that room20 db will produce something less thanat least some of the burden will fall on acoustics-notablyt h ereverberation12 db of maximum boost at 13,000 cps,the treble -cut control. If the setting for time versus frequency relationship-mayincluding about 3 db of boost at theminimum treble furnishes close to 6 db be a factor requiring tone compensation. reference frequency itself. These char-of attenuation per octave from a transi- Reference is made to this factor underacteristics seem to represent a reason-tion frequency of 2,000 cps, approximate the heading of treble boost merely onable compromise between the require-compensation for any of the recording the basis of subjective experience, butments of all factors. The use of an even pre -emphasis curves can be achieved. undoubtedlyothercompensationsarehigher transition frequency might be de- also involved. sirable, but would place a greater limita- When treble boost is needed, then, ation on the maximum boost of useful TABLE 1 high transition frequency is usually de-frequencies that could be achieved with Typical Transition Frequencies sirable. The use of a lower transition a single R -C network. Transition Rate of frequency for the sake of increasing the Records Frequency Boost ( cps) ( db octave amount of boost available at the upper Treble Attenuation end might prove satisfactory ifit were AES Standard Curve 2500 not for the marked tendency of loud- Treble attenuation is required for: Columbia 78 Cr 33 1/3'1590 6 speakers to emphasize the frequency re- 1. Compensationforvaryingtreble RCA 78' 1000 2.5 gion of the first few thousand cps. A pre -emphasis in recording. Londonf frr78' 3000 3 transition frequency of1,000 cps or 2. Reductionofrecordsufacenoise London 33 1/3' 3000 6 lower may cause treble boost to accentu- and high -frequency distortion. EMI 78' none 3. Compensation for rising treble re- Older records none ateashrillnesstowards whichthe sponse of associated reproducing or stu- speaker performance is already inclined, dio equipment. and needed boost at frequencies above 4.Tonal balance against a thin bass. Settings ofthe treble control which yield 4,000 cps may carry such a penalty that less than maximum attenuation will also, Treblepre -emphasis inrecordingin most circuits, automatically shift the 2E. C. Wente, "The characteristics ofvaries considerably, and thereisno transition frequency higher, correcting sound transmission inrooms," J. Aeons. standard, either of transition frequencyequalization for some of the records Soc. Am., 7, p. 134, Oct., 1935. or rate of boost, to which recordingwith more gradual pre -emphasis slopes. Thedesirabilityofequalizationfor treble recording characteristics prior to the tone control is emphasized by the differences between the transition fre- quencies of some ofthese recording .to-N4Nooo, curves and the frequencies at which the ._ I,:k .. equipment deficiencies previouslydis- 15-- _ cussed introduce treble losses. For ex- el° 10 ample,animproperlyequalizeddisc _._ Fig. 5. Suggested which has been recorded with a given C*5 N... tone control fre- 0 p..- treble characteristic can present an un- FT', FT

311 NO iN0 'mm aomo 4 D. T. N. Williamson, "High -quality FREQUENCY IN CYCLES PER SECOND amplifier modifications,"Wireless World, 58, p. 173, May, 1952.

32 control may not be able to compensatesirable than unequal frequency distor-at the 50-db level, bass boost at slightly for the characteristics of both the recordtion, but that there is considerable lati-more than 6 db per octave, with a and the reproducing equipment. tude in design for balanced response.transition frequency of 400 cps, is re- It has been established by several in-The parameters of treble attenuation forquired. This is, of course, for the ex- vestigators that record -surface noise isachieving careful balance would be de- treme case of a 30-db difference between fairly evenly distributed over the fre- termined by a study of bass deficiencies the original and the reproduced level. quency spectrum on the basis of energy At this 30-db difference the bass boost likely to be met with. The treble attenu- required during orchestral peaks, when content per cycle. Since each arithmetic ation discussed in previous paragraphsboth levels may be increased by 20 db, frequency interval has a similar amountis fairly symmetrical to most of theseis much less; during very soft passages of noise energy, the noise content will low -frequency deficiencies. the boost required will be more. We increase with each successively higher It will be seen that the transition fre-must work on the basis of the average octave.5 It is therefore correct to thinkquency suggested for treble attenuation levels. of perceived surface noise as increasing is different from that for treble boost, The 50- and 80-phon curves, and the with frequency, and compensation forand conditions are likelyto existin 60- and 70-phon curves in between, are treble pre -emphasis in recording is verywhich both are required simultaneously. practically identical in shape and slope effectiveinreducing scratch,asthe Such simultaneous compensation, how- from 500 cps up, meaning that a re- system intends it to be. ever is inherently barred when a singleduction of intensity level from 80 db There is however, an advantage incontrol is used for both boost and cut. to 50 db will produce no significant being able to introduce sharp cut-offs at change in the apparent frequency dis- given points of the frequency band, since tribution in the treble region. Even if it records which have littleor no fre- Bass Boost were desired to compensate for the 2- quency content above the cut-off point The conditions requiring bass boost db maximum loss in treble, the ordinary are : R -C network could not produce the can then have their surface noise re- necessary duced without the signal being severely 1. Decreasedhearingsensitivityto shapeofresponsecurve, bass frequencies at low sound intensities which may be read from Fig. 5 as a penalized. Some types of distoration are uniformly elevated plateau over most of also most pronounced in the range above (Fletcher -Munson effect). 2. Recording characteristics whose thetreblespectrum. Correct volume 5,000 cps, and sharp cut-off helps reduce bass turnover frequency is higher than compensation should therefore involve such effects with least change of the sig- the one for which the repoducing equip- no adjustment of treble frequencies. nal. Sharp cut-off at high frequencies, ment is designed. If the record reproducing equipment however, is incompatible with the other 3. Bass deficiencies in records. has only a single turnover frequency a duties of the treble tone control, and 4. Bass deficiencies in reproducing or compromise value of 500 to 600 cps is should be accomplished by separate net- studio equipment. usually chosen. Some records have been works. The general tone control must be The well known family of equal loud- made with higher turnover frequencies limited in its scratch reducing role to aness contours published by Fletcher and as high as 800 cps. Thus a moderate more gradual attenuation of the treble Munson makes it evident that the appar- amount of boost to add to that of the band. fixed equalization will on occasion be ent frequency distribution of energy in called for. Such equalization, however, is Certain ,ifincorrectly given program material will vary greatly equalized at the studio, have a rising required by a relatively small number at different intensity levels. If the ampli- of records. treble response above one or two thou- fier has a correctly designed compensat- sand cps. Attention is occasionally called Acoustically recorded discs, and some tothischaracteristic when broadcasting network associated with its volumeelectrically recorded ones, have a thin studio use of an incorrectly compensated control this effect will be counteracted. bass because of weaknesses in recording microphoneproducesover -crispand but the volume control setting required equipment and techniques. These ordi- "hissy" speech. The compensation re-for a desired intensity level is not neces-narily require boost below two or three quired for this type of emphasis is com- sarily an accurate index of the intensity hundred cps, although the drop in bass patible with that already planned forlevel of the original program, and fur- responseisin general too sharp for records. ther adjustment may be necessary. The adequate compensation. Tonal balance of bass and treble for electrical level of an input signal does Bass deficiencies in reproducing equip- the most satisfactory over-all result is not have a constant relationship, in dif- ment,liketrebledeficiencies,tend an inherently subjective problem. Like ferent program material, to the soundto occur towards the extreme of the compensation for the Fletcher -Munson level which it represents. frequency scale. Most moderate quality effect, it involves a decrease of objective The purpose of volume compensation loudspeakers (see Fig. 2), pickups, etc.. fidelity for the sake of an increase inis not, of course, to straighten out thedo not show a significant drop in bass apparent realism. The fact that the prob- curve of frequency perception,(that's response until frequencies below 100 cps, lem is one of perception rather than of the way the music sounds in the con- and then with sharply dropping curves. reality indicates that investigation re-cert hall) but to shape this curve at theHere too compensation cannot be ade- quires a statistical technique.° reproduced intensity level so that the quate. perceived frequency distribution is simi- Discussions of aural balance' have lar to the perceived distribution at the Adjustment for the Fletcher -Munson indicated that approximately equal fre- original intensity. We may take 80 db°effect is probably the most frequent func- quency distortion, geometrically relative as the average level of a 75 -piece or- tion of bass boost. The transition fre- to the frequency mid -point, is more de - chestra heard from a good seat, and 50 quency required for this equalization lies db9 as the lowest level at which thisbetween that for a high turnover fre- 3 B. B. Bauer, "Crystal pickup compen- music is likely to be reproduced, withquency and for record and equipment sationcircuits,"Electronics, 17,p.128, deficiencies, and the compromise para- Nov., 1945. any concern for quality, in the living room. Superimposing the two appro-meters that appear most reasonable to 6 Itis suggested that a useful approachpriate curves on the same horizontalthe writer include a transition frequency would beto compile judgment datain one octave down from the spectrum mid- which the jury compares various conditionsaxis (Fig. 4), it will be seen that in point, or 400 cps, and the standard maxi- oftonalbalance,bothsymmetrical andorder to achieve the original perceived mum boost rate of 6 db per octave. asymmetricalto 800 cps,withthefull frequency distributioninreproduction range of sound, indicating which condition seems more like the undistorted one. (Ask- "Frequency Range and Power Consid- ing the jury to indicate preference between Hugh S. Knowles, "Loudspeakers anderations in Music Reproduction," Jensen balanced and unbalanced conditions, with-room acoustics," The Radio Engineering Technical Monograph No. 3, Jensen Manu- out an ever-present standard, involves theHandbook, Keith Henney, editor, p.:.:1, facturing Company. Note that a level of danger of measuring factors other than ap- 3rd edition, 1941. 50 db is less than 10 db above the average parent frequency distortion.) 8 Ibid. random noise level in a city apartment.

33 Flexible Tone Control Circuit

BASIL T. BARBER'

The advantages of variable inflection pointsin tone controls are well recognized, but the circuit complications and additionalexpense does not always justify their use. The author Americanizesa previously published circuit which is relatively simple.

tinuously variable transition frequency and a number of other advantages. The N ICI,' original article was written by P.J. \CA) T Baxandall2 and for a detailed explana- MOW. tion of the circuit the reader is referred to his article. (e, One way of looking at this circuit is to analyze it in terms of Laplace Trans- il forms.Oversimplifyingtheoriginal schematic, the circuit can be broken T.. ,,..,... c 7., down into two lead and two lag net- Fig.1. Simplified block diagram of the tone works, as shown in Fig. 1. The lead net- Fig. 2. Frequency response of the actual lead controlwhich employs negative feedback to work (C), being inside a negative feed- and lag networks ofFig. 1.B' and C' are providevariableinflectionpoints.Idealized back loop, becomes a lag network and curves B and C after feedback. lead and lag networks are shown. similarly the lag network (B) becomesshould therefore prove to be effective in RECENT INVESTIGATIONS on the sub- a lead network.' The net curve willcompensating certain deficiencies of an ject of tone controls indicate thetherefore be either a boost or an attenua-audio system explainedinVillchur's desirability of having the transi-tion, as shown in Fig. 2, dependingon article.Specifically,"partial"speaker the relative position of the time con- compensation can be accomplished with- tionfrequenciesvariable,withthestants To, Te, To and Te. When Ta = Te amount and direction of variation deter- out any boominess (especially on male and To= Tot, the curves cancel each other voices),andwithouthigh -frequency mined by the degree of boost or attenua-producing a flat frequency response. tion required. distortion. Figure 3 presents the tone -control cir- The circuit presented has, in addition, The analysis and determination of thecuit in detail. It is similar to the originala number of other advantages which are varying parameters and their range are circuit,4but with parameters slightlyequally important. In most tone -control ably covered in Mr. Villchur's article.'modified to make the circuit adaptablecircuits at present, the apparent boost is Most tone -control circuits available atto American tubes and components. attained at the expense of an equal at- present have asingle transitionfre- Figure 4 shows the actual frequencytenuation at the center frequency. An quency between 800 and 1000 cps, which response of the schematic of Fig. 3. Theadditional gain equal to the boost attain- remains essentially constant at any posi- low -frequency turnover is made variableable must therefore be provided. Ina tion of the controls and their flexibility from 800 to about 100 cps, depending on"flat" position, there is unity gain plus the degree of bass boost or attenuationall the distortion generated by the stage and effectiveness is therefore limited. required. The high -frequency turnover A method of tone compensationis of amplification. In the proposed circuit, presented in this article offering a con-varies similarly from 1000 to 8000 cps,the loss is in the form of negative feed- depending on the degree of treble boostback with its associated advantages of * Research Engineer, Mass. Inst. of Tech-or attenuation required. These curvesreduction of distortion, impedance, and nology, Cambridge 39, Mass. 2 Wireless World, October, 1952. noise, and the increase of the linearity 1 "The selection of tone -control param- 3See Appendix. and frequency response of the circuit. eters," AUDIO ENGINEERING, March, 1953. 4 Ref. 2, p. 404, Fig. 6. For instance, if we assume that a boost

slope from a lower transition frequency A reference frequency and slope to of 500 cps. The second method ismorecompromise between the variousre- SELECTION consistent with the other requirementsquirements for which bass attenuation of bass cut. (from preceding page) will be needed are: transition frequency Bassaccentuationinreproducing500 cps, maximum slope, 6 db per octave. Ban Attenuation equipment is most often associated withConclusion acoustical resonance of an open speaker Bass attenuation may be required for enclosure at some frequency below 200 Figure 5is a graph of tone -control the following conditions: cps and with mechanical -acoustical reso-frequency characteristics chosen on the 1. Recordingcharacteristicswitha nance of the speaker system below 100basisofthe abovediscussion. The bass turnover frequency lower than that cps. The effects of tone -arm resonanceparameters vary from many of those in of the reproducing equipment. and turntable rumble usually occur be-common commercial use in that the ref- 2. Accentuation and distortion of bass low 50 cps. As in the case of low -fre-erencefrequenciesareshiftedaway frequencies by reproducing equipment. from the spectrum mid -point, about one 3. A weak treble which creates tonal quency boost, we cannot furnish accu- imbalance. rate compensation for equipment frail-octave down for the bass, and two oc- ties at the extreme low end, butwe cantaves up for treble boost. As a conse- The required compensation for equip- alleviate the condition in some quence the total amount of controlis ment designed with a 500 -cps turnover, measure. Aural balance of a weak trebleap-somewhat less than is often provided. and playing a record with a turnover pears to demand a low transition fre- Figure 6 is a tone control circuit with frequency of 250, 300 or 400 cps (all of quency. Probable treble droop occurs,values assigned to approximate these which values have been used) may be as we have seen, at least two octavescurves. Some allowance has been made approximately achieved by a gentle at- above 800 cps, and geometrically for the fact that the transition frequen- tenuation of about 2 db per octave from sym- metricalbasslosseswould beginatcies are affected by the degree of boost 750 cps down, or by a sharper downward about 200 cps. or cut used by the operator.

14. 12 527

rs _ Ii E0 1 I is 10 5

1-1010

-10 -15

-20

(TREBLE) 10 HMO 100 1000 10020 FREQUENCY IN CYCLES PER SECOND

Fig. 3 (left). Over-oll schematic of the circuit as modified for American tube types. Fig.4(right). Responsecurves obtainablewiththefeedback type tone control. of 10 db is required at both extremes of db/octave,hardlyadequateforany 1.Transformation of a Lag into a Lead the audio spectrum, then 7 db of nega- effective suppression without sacrificing Network (Bass).

tive feedback willstill remain at both the musical content of the program. Referring to the upper section of Fig.1 extremes, and17 db at the mid -fre- Loudness Control we have, quencies. At "flat" position, the circuit Compensation for the Fletcher -Mun- re, Kee. 1 has a loss of 1 db5 which is negligible. son curves with the tone controls would TS + 1 The circuit can therefore be inserted seem to make them perform a function anywhere with no complications. actually belonging to a loudness control. The only disadvantage is that the in-In addition, once the tone controlsare set on the reciprocal of the Fletcher- put to the tone control must come from Munson curves, their usefulness forany a low -impedance source but not neces- other function will be restricted, if not sarily from a cathode follower as shown nullified. in Fig. 3. A source impedance of 10,000 Speaker to15,000 ohms will have no adverse The most serious misunderstanding effect, and this impedance can be readily seems to be on the question of speaker attained from a low -mu tube such as acompensation. It is well known bynow 6SN7 or 12AU7. that phase as well as amplituderesponse Because to the large amount of nega-of a speaker playsan important role on tive feedback employed, the output source the quality of music reproduction.A impedance is low, giving the circuit allspeaker, being essentiallya non-linear the advantagesofa cathode -follower electromechanicaltransducer,hasa output stage. phase response which includesseveral Fig.5. Suggested wiring arrangement for the reversals, rendering tone control. The two potentiometers R, and Use and Abuse of the Tone Controls any effects for ef-R, could easily be combined as fective compensation ratherfutile, espe- o.concentric While the writer is not naive to thecially control. with simple networks.Circuitsfrom which point of believing in "ideal curves," flathave been designed whicheffectively speakers, and acoustical heavens in gen- 1C, (T,+1) compensate for some speaker deficien- e. Ks(TS +1) 1+ Ki1C5 eral,itis believed that in a reasonablycies, using variable slope as wellas vari- high -qualitypreamplifier -equalizer et TIS +1+1CRICs theable turnover frequency, buttheir cost T,S4 -1 tone controls should not be called uponis usually several times the price of the 1+1C,K, to perform functions which they wereaverage speaker, making any impendingThe original lag (B), has been therefore not originally designed to perform. "Fire Sale" on high -quality loudspeakerstransformed into a lead network (B') hav- Following are the areas where abusehighly improbable. ing two break frequencies, 1+ K,K, apart. is most likely to occur: Why use tone controlsat all then ? 2.Transformation of a Lcad info a Lag Preamplifier They filla definite need and most of Network (Treble) There are scores of well designed cir-their functions are covered inMr. Vill- Referring to the lower section of Fig. 1 cuits which offer adequate flexibility andchur's article and can be summarizedaswe have, adaptability to meet any record charac-providing: le,'- 11C4(T4S +Med/Cs= ei teristics and their deviations without re- 1. Over-all tonal balance of the com- fi-oin which sortingto tone -controlcompensation.plete system. Since the low -frequency turnovers and 2. Compensation for any acoustical e. K. thehigh -frequencypre -emphasisfall such as resonance, for example. K21C,T4S +1+ ICJC, well below and above the "center" fre- 3. Compensation for any deficiencies quency of the tone controls, no adequateof studio equipment, means of transmis- 1+1C4lis compensation can be attained. sion, or for faulty records. As for reducing any record noise, hiss, 4. Bass reduction to eliminate boomi- KiKs ) 7,s 1 or rumble, the tone controls are notness of male voices, especially on low 1+ IC,K. especially effective since, at best, mostsettings of the loudness control. The original lead network (C) has been of them have an attenuation rate of 6 APPENDIX transformed into a lag network (C'). The method of combining the two original 5Since the total gain in a negative -feed- Negative feedback can be employed tonetworks (A) and (D) and the two modi- back circuit is equal to the forward gaintransform a network into its reciprocal, asfied networks (B') and (C') to obtain ail divided by one plus the loop gain. In ourshown below. Although the following der-overall response(boost in this case)is case, this ratio is less than one. In addition,ivations are for ideal lead and lag net-shown in Fig. 2. there . is a slight reduction in gain due toworks, they apply equally on actual net- WhenTb >T., and Ta> Tr, then an the cathode -follower input, but the circuitworks having two break frequencies, bothattenuation of both extremes isattained, parameters have been selected to keep thebeing important in determining the over-allwhile at Ta =TBand Tr= Te, we have a total loss less than 1 db. frequency response of the circuit. completely flat frequency response.

35 Hi -FidelityPhonograph PreamplifierDesign R. H. BROWN

A comprehensive discussion ofthe principles of determiningcomponent values to provide the compensation required, combined with practical pointerson construction. SEVERAL EXCELLENT phonograph pre- amplifier circuits have appeared in gain provided by Vs will be the literature but it is difficult for the individual who is nota full-fledged As= AI audio engineer to find the information 1 + (4).4.'' (1) he may need for designinga high -qual- ity preamplifier to meet his individual where A; is the gain provided byVs preferences,or to makedesirable with a completely bypassedcathode. changes intelligentlyin equipment al- Cr Cr Ck and screen bypass capaci- ready on hand. This article aims toset tors if pentodes are used, must bese- forth in a summary fashion basicma- lected to provide essentially uniform terial which a technically minded audio gain down to the lowest frequency ft. hobbyist or an amateur engineer would need in the design or revision of quality for which full compensation is desired. To accomplish thisitis preamplifying and compensating equip- Fig.I.Basic degenerative -bass -boost amplifier desirable to ment. Rather than attempt a compre- circuit. Vand V. and associated components, choose the value of Cc (in farads)so hensive survey of the problems and excluding feedback elementsR., R,,and C, that the product fireRg is betweenone theory of preamplifier and compensation comprise the basic amplifier. Triodes are shown and two, and to choose Ck,so that the circuit design, attention will becon- for simplicity. product fLa, is between one andtwo times the mutual conductance of fined to straightforward basic circuits.quencies as high as 900 cps. As a 900- V.. In many cases adequate gainmay be Basic Amplifier With Degenerative cps turnover requires approximately 29 Bass Boost (lb boost at 30 cps, the preamplifier gainobtained without use of a,. Datare- at 30 cps must be the antilog of 29/20quired for the selection Figure 1 showsthebasiccircuitor 28.2 times as great as in the high- screen bypass capacitorisordinarily around which most high -quality phono-frequency range if bass compensation isnot available, but one can usually make graph preamplifiers are designed. Re- to be provided down to 30 cps fora a satisfactory choice by requiring that sistor R, provides degenerative feed-900 -cps turnover. Since due to theac- the product of the bypass capacitance back to set the amplifier gain to thetion of Cs the gain only approaches the (in farads), the lowest frequencyof desired value for high frequencies,re- full basic amplifier gainas the frequency interest, and the plate resistance of the duce amplifier distortioninthe high -is reduced, in order to obtain goodcom- tube when triode connected havea value frequency range. and extend the high- pensation down to a given frequency itbetween one and two. If it isnot con- frequencyrange beyondthatwhich is necessary to design fora frequency venient to use capacitancesas large as could be obtained from the basic ampli-one octave lower. Thus for good com-those required by the foregoingcondi- fier without feedback. R, providesde- pensation down to 30 cps fora 900 -cps tions, the situation may be relievedby generative feedback to set the maximum turnover one must have a basic ampli-providing around10 db of feedback value of bass boost. reduce amplifier fier with a gain at least 56 timesgreater through R3. distortionattheextreme lowfre-than that required in the high -frequency Once the basic amplifier has beende- quencies, and extend the low -frequency range. signed to provide adequate gainfor the range below that which could be ob- A preamplifier designed foruse with maximum bass boost desired,the next tained from the basic amplifier withouta "Williamson" type power amplifierstep is to select R, so that the amplifier feedback. R, may be omitted( made and a G.E. magnetic cartridgeshould will give the right amountof gain for infinite) when compensation is desired provide between one andtwo volts out-the high frequencies. At highfrequen- down to a frequency requiring the full put for a 10 -millivolt input at 1000cps. cies (5000 to 10,000 cps) thegain is gain of the basic amplifier. Ct providesIf this preamplifier is to providegood given by 6-db-per-octave bass boostas figuredcompensation down to 30cps for turn- from the turnover frequency forwhich overs up to 900 cps it must havea basic A°111, Ct is selected. Aur amplifier gain of between5600 and 1 + (2) The first step in the design ofa pre- 11.200, with Rs selected to obtaina gain Rs + R, amplifier is to design a basic amplifierof between 100 and 200 inthe high -where BF which has isthe gain of the basic sufficient gain to providefrequency range. Ifno turnovers aboveamplifier whena resistance equal to down to the lowest frequency of interest500 cps are to be used, goodcompensa- RsR, is placed in parallel compensation for the highest turnovertion to 30 cps may be secured with Rt.,. with aThe gain provided byV. under these frequency to be used. Informationon basic amplifier gain of between3100 circumstances electrically correct turnover frequenciesand 6300. High-level cartridges, isequal to the mutual such conductanceofV.multipliedby a may be found in the literature, and it asClarkstan andPickering,require resistance equal to the will be observed that thereare combina- minimum preamplifier gains parallel combi- approxi- nation of the plateresistance ofV., ations of listening conditions,listener mately only one -sixthas great. Rs + R., Rt.:, and Rul. To preference,andrecording The selection of V., Vs, and obtain accurate character- associated 6-db-per-octave bass boostitis neces- istics which may requireturnover fre-components is best made from referencesary for A.11F to be large enough to the resistance -coupled amplifier to * Walla Walla College, College data make negligible theunity in the denomi- Place, supplied by tube manufacturers.Because Wash. of the unhypassed cathode nator of Eq. (2)-i.e., forA'Isr to be resistor thegreater than /0/6/(Rs+ R:). Whenthis

36 condition issatisfied Eq. (2) becomes R, + Rs AMY - (3) 35S, R, '"%.,. Since RI will usually be chosen from 30I.. resistance -coupledamplifierdatafor a25 proper biasing of V,, one may write the V following equation for Rs, Z20.- z 900%, Rs= R,(Agr - 1), (4) g15 where All, is now the required high - >W p 10 frequency gain-i.e., the gain without 4 500.-.., bass boost. If the loading effect of R, + Rs -I keeps A.IIP from being large enough to CC 5 renderinsignificant the unityinthe 0 INm7...0 denominator of Eq. (2), the situation 5 20 30 50 70 100 200 500 ,000 2,000 5,000 may be remedied by connecting R3 and FREQUENCYIN CYCLES PER SECOND Ct to an unbypassed cathode of a stage following Vs (possibly a cathode fol- lower output for the preamplifier). Fig. 2. Bass performance of pentode amplifier example discussed in text. Dotted lines indicate The maximum gain available from idealized 6-db-per-octave slope. the amplifier may be designated ALF It will now be instructive to see how 0.241 meg. for R. This provides at high and is given by these principles are applied in selecting frequencies a 45 db feedback in addition the components for a practical high-to the 6.8 db of current feedback on V, Al, = RIA' (5) fidelity preamplifier for use with a GE due to its unbypassed cathode resistor. 1 + cartridgeanda R, + + "Williamson"type For 900 and 500 cps turnovers Eq. (6) poweramplifier.On consultingthe requires values for C, of 740 and 1310 where A' refers to the normal gain ofresistance -coupled amplifier charts in aµµf respectively. the basic amplifier without feedback. In receiving tube manual one finds that practice the coupling and bypass ca-a basic amplifier gain A' of approxi- The actual performance of this pre- pacitors often prevent realization of themately 18,000 may he obtained by usingamplifier is shown in Fig. 2. The value full value of ALF. for V, a 6SJ7 with 180-v. Ebb, 0.5-meg. chosen for ALFwas 35.1, db above that From the fact that a simple resistance - Rt.,, 2.4-meg. screen resistor, 2.0-meg.chosen for an amount just equal to capacitancearrangementwhichwill Rg,, and 2410 -ohm R, to obtain a gain the boost required at 15 cps by a 900 provide 6-db-per-octave boost computed of 95; and for V, a 6SJ7 with 180-v.cps turnover, The necessity for design- from a turnover frequency ft must giveEbb, 0.5-meg. Re 2.2-meg. screen re-ing to a frequency one octave below the a 3 db boost at ft,' one obtains the fol- sistor, 1.0-meg. R/4, and 2180 -ohm Rktlowest frequency to which full compen- lowing relation for the selection of Cr. sationis desired isillustrated by the to obtain a gain of 192. 900 cps turnover curve in Fig. 2. 1 For 10-db minimum feedback ALF= Cr - (6) 1800/(antilog 10/20) which is5700. In a preamplifier designed for use 2.ref t (R, + Rs)* From Eq. (5) one finds that this re- with a high-level magnetic cartridge, a 1 The boost will be very close to 1 db at quires 20 megs. for Rs, R, + Rs being value of but 15 for AB. would be ade- 2 ft, 3 db at ft, 7 db at V2 ft, and 12 db atnegligible in comparison with R. Forquate. One could use the basic 6SJ7 f t Ato.= 100, Eq. (4) requires a value of amplifier discussed above with 33,700

TO Pd/EM ,41PPL

400-4300 AT 4.511&

TO PIN 7 Of GSM/

TO PINS OF GSM/

TO CATHODES 0, 00 TINT TUNES

I°E-°-°'

1( L°1 - le - 2 - . 32- TREBLE MOLL -OFF 53- SASS TuRNOvER

E5?,,0 I. FLAT E 300 L 3 lb/OCTAVE FR051.000 2. 400% 4 21/2 0dOCTAvE FOES 003. 1 SOO WE 0005T 001 47.000 FON A01500 CIATIDDGE 4.II AS/ OCTAVE TOON 2000. 4 S00% /1,27,000 FON INCKEROKI CATITROGE CAPACITOR VALUES IN LAILF.S S S IS/OCTAVE FNON 1.530. 5 115C OTNEG fON (001ELEC. CANTR001 SPECIFIED 000E50 SE G. IS 45, OCTAVE F0010 2.500. tO50 ALL SWITCHES SOS it SHORTING TYPE. 5000 TOR Au0A5 CANTRIDOE 3% TOLE RANGE. 2.200 Foll ftotRING CARTRIDGE 5E5,31055 ARE E .7%.."2 WATT UNLESS 5.200 TOO GEN ELEC. COO TWINE INECNIED 0 TNE5110150.

Fig. 3. Complete schematic of author's preamplifier with six bass turnover conditions and six roll -off positions.

37 ohms for Rs and 4 megs. for Rs to obtain maximum gain desired at the lowest good compensation down to 20 cps fora frequency of interest. The requirements 900 -cps turnover. Actually the 6SJ7 is on Cc and Ck are the same as discussed a poor choice for Vt, for unless d.c. is for the basic amplifier in the degenera- used on the heaters one is likely toex- tive -bass -boostcircuit.The high -fre- perience hum difficulties. The 6J7 would quency gain is given by the product of be more suitable in this respect,even the mutual conductance of the tube and though it will not provide quite as much a resistance equal to the parallel combi- gain. The high-level cartridge makes nation of the plate resistance of the possible good bass compensation with tube, Re., R., and R5. The required value a simple preamplifier using a twin triode for Cr may be computed from Eq. (6) such as the 12AX7, 6SL7, or 7F7. by substituting R. for RI +R,. Themax- Triodes may be used in a preamplifier imum gain should be at least 6 db above, designed for use with a low levelcar- or two times greater than, that required tridge by following the basic amplifier for the greatest bass boost desired, if with an additional stage of amplification a 6 db per octave slope is to be followed as shown in Fig. 3. With a high-level to the lowest frequency of interest. A Fig. 6. Circuits for commonly used roll -off les cartridge the amplifier noise and hum 6SJ7 operated to obtaina maximum than 6 db per octave.(Alis accurate within level will be about 16 db lower with gain of 200-46 db-will provide fora -±14 db to 15.000 cps.(B)is accurate within ± respect to the signal level than would be high -frequency gain of about three and db to 10,000 cps and gives 1 db less than the case with the same amplifier anda ideal attenuation at 15,000 cps. low-level cartridge. If the preamplifier is to provide for is shown in Fig. 5. Rd is usually selected switching between various turnovers for cartridge damping inaccordance with the recommendations of thecar- tridge manufacturer. If Rt, iszero, an attenuation at 6 db per octave will be obtained as figured froma frequency equal to the reciprocal of the quantity 6.28 times the product of Ra in ohms and C in farads. Ra should be large enough so that its parallel combination with Ra acting in connection with the Fig. 5. Treble roll -off circuit.R., selected for cartridge inductance will notcause ap- cartridgedamping and/ortoobtainroll -off preciable additional high -frequencyat- rates between 6 and 12 db per octave.IL, tenuation, unless a roll -off rate greater selected to obtainroll -off ratesless than 6 than 6 db per octave is desired. To Fig. 4. Basic variable -load -impedance bass -boost db per octave. ac- amplifier circuit. Triode shown for simplicity. complish this the resistance of Ra and full bass compensation down to 30cps Ra in parallel should he at least 125.000 for a 900 -cps turnover when usedin times the cartridge inductance in hen- the selector switch must be of the short- this simple circuit. There will be, of ries. Figure 6 gives data for selection of ing typeto avoid seriousswitching transients high -quality capacitors should course, no degenerative feedback to pro- components for trebleroll -offatless vide for low distortion obtainablewith than 6 db per octave as required by be used for Cr, and a 10- to 20-meg. the previously discussed resistor must be placed in parallel with system. For a some RCA Victor and some British 78 the 500 -cps turnover compensationmay be rpm recordings. selectorswitchfor each switch carried about one octave lower. position. These precautions are required or about because of the d.c. potential difference 6 db more gain may be provided forthe Noise and Hum Reduction high frequencies. Ifthe preamplifier across Ct. The value of the switch- Byfollowing islaid out and shunting resistorsis not critical, but adegenerative -bass- wired carefully, hum and noisemay be boost amplifier witha stage of amplifi- reduced to a negligible level by simple the combined resistance of all of them in cation containing variable load parallel should be no less than about 40 imped- means. Resistor noise will ordinarily be times R. ance bass boost with a very low turn-unnoticeable; the ultra -perfectionist may TheColumbia over frequency -50 cps, for example-practically eliminateit by using wire long-playing recording extra boost for the low bassmay be characteristic requires a 500 -cps turn- obtained if desired. wound resistors in the plate circuit and overwitha maximum bass boost of any unbypassed portion of the cathode about 14 db. For this compensation the Treble Compensation circuit of the first stage. Most of the capacitor used for Cr should have shunt- hiss noise from an amplifier usually ing it a resistor of such value as to pro- For a 6 db per octave roll -off,the comes from the first tube. Tube noise vide in parallel with the resistor used simplest treble compensation is obtained varies greatly between tube types. and for Rs an effective resistance for the Rsby selection of the resistanceshuntingbetween individual specimens of a given term in Eq. (5) that will give a value themagneticreproducercartridge.2 type. Unless a selected low -noise tube for ALP which is five times the value of Neglecting the effects of cartridgein- is used, tube noise will usually be less AM' for the amplifier. To avoid serious ternal capacitance (usually pegligiblein in a preamplifier employing a triode switching transients a blocking capaci- the audio range), 6-db-per-octaveat- in the first stage than in one using a tor should be placed in series with this tenuation twill be obtainedas figuredpentode. If the first stage is to use a resistor. This blocking capacitor should from a frequency equalto 6.28 times pentode itis well worth while to use be chosen so that the product ofits the cartridge inductance in henriesdi- the 1620 or the 5879 fortheir lov capacitance (in farads) and the lowest vided into the sum of the internalre-microphonism, hiss, and hum. Where a frequency of interest isat least equal sistance of the cartridge andthe ef- low -noise triode is desired one can use tothe reciprocal of three times thefective external resistance shuntingit. the 12AY7 or a triode -connected 5879. resistance of the shunting resistorin One can select the roll -off frequencyby The most obvious and straightforward series with it. adjustment of the preamplifierinput way of eliminating hum arising in the resistance with eithera fixed positioncathode circuits is to use direct current Variable -Load -Impedance Bass Boost switching arrangementor a continu-heater power. D.c. for heaters may be ously adjustable potentiometercontrol. obtained from afull -wave drydisc A simple. commonly -used type of bass A more flexible treble roll -off boost circuit is shown in Fig. 4. Rk, RI, circuit bridge rectifier with a simple capaci- and Ru may be selected from resistance- 2 Norman Pickering, "Effect of load im-tance filter of between 1000 and 5000 coupled amplifier data to provide the pedance onmagneticpickupresponse." microfarads. The rectifier will require AUDIO ENGINEERING, Mar. 1953. an r.m.s. input voltage about 50 per

38 A Three -Element BassControl GLEN SOUTHWORTH

One solution to the problem of providing realismin music reproduction is to introduce a controlled amount of "hangover" bymeans of the circuit described which permits boosting bass at threeseparate frequencies independently.

system, room acoustics, and individual musical tastes.

The bass -boost circuitry described in this article is an attempt to provide a musical form of bass emphasis similar The three -element to the kind generated naturally by good bass tone -control acoustics. To do this, particular atten- unit designed to ob- tion has been paid to the transient char- tain power supply acter of most bass sounds as well as from main amplifier.some of the significant factors of human hearing. The results obtainable should he highly valued by the listener who desires good, full, audible and "feelable" bass together with a minimum of inter- action or apparent distortion of high frequencies. Figure 2 shows three of the most commonly used methods of bass boost. These are the R -C network, the R -C -L IN THE DESIGN of conventional tonecibels boost or cut in reference to 1000 or resonant circuit, and the inverse - controls a cut and dried procedure iscps, typical steady state response curves feedback method of obtaining bass em- very often followed. A sine -wave testbeing shown in Fig. 1. While very use- phasis. A fourthtypeoffrequency signal is used and the circuitry adjustedful in many cases, this type of tone con- emphasisisobtained by altering the to provide the desired number of de-trolis not very likely to provide ade- feedback path of a triode or pentode in quate compensation for some programsuch a manner that the circuit simulates * Box 2S Idaho sources as well as the listener's speaker a resonant inductance -capacitance com-

to a.c. heater operation may usually be num box. In constructing the preampli- PREAMPLIFIER DESIGN eliminated by the simple arrangement fier it is important to keep to a minimum of Fig. 7. The value of the bypass ca- the stray capacitances in the treble com- (from preceding page) pacitance is not critical (values less than pensation circuits and in the input con- 1.0ttfhave been used successfully). nection to the 12AX7. If the preampli- Satisfactory hum reduction may often fieris to be used only with a Pickering cent greater than the required filament be accomplished by eliminating the bal- voltage. One should provide a variable cartridge, or any other make of similarly ancing potentiometer and connecting the low inductance,itwould be wellto series resistance between the rectifier positive bias to the center tap of the fila- and the capacitor so that the filament double the compensation capacitor con- ment transformer. When the balanciug nected to the first 12AX7 grid to 1000 voltage may be adjusted to the proper potentiometer is used it is well to locate value. With this arrangement a standard µla and reduce to one-half the values it and the associated bypass capacitor given for all the compensation resistors 12.5 -volt transformer may he conven- near the first stage of the preamplifier iently used for a 6.3 -voltd.c.heater connected to the treble switch points 2, as it takes the place of the usual ground- 3. 4, and 5, thus reducing stray and input supply. ing of one side of the heater line. If one is not so unfortunate as to get capacitance problems. a poor tube in the input stage. hum due In the author's opinion, any adjust- Illustrative Preamplifier Circuit ment of the system frequency response As an illustration of the principles beyond the simplest that will compen- discussed in this article, Fig. 3 gives the sate for the recording characteristics of schematic of an easily constructed pre- the disc being played will deteriorate the amplifier designed for use with the au- transient response. As a concession to thor's Ultra -Linear Williamson power situations in which simulated live pro- amplifier. This preamplifier has ample gram loudness is undesirable or unper- gain for use Nvith a low-level GE reluc- missable, the circuit of Fig. 3 has been tance cartridge and carries compensa- designed to imkorporate an IRC loudness tion fully down to 30 cps for the highest control. For thiscontrol to function turnover frequency. At maximum gain properly the level control must be set settings approximately one-third volt in- to give simulated live program loudness Fig.7. Hum reduction circuit. Neither of the put on the high-level inputs 3 and 4 will with the loudness control at its maxi- heater lines may be grounded, and a center tap drive the power amplifier to full output. mum setting. Much of the criticism of onthe transformer should not be used.Po- The author's unitiscompletely con- loudness controls has come from failure tentiometer is adjusted for minimum hum. tained within a 10 x 4 x 2% inch alumi- to do this.

39 bination.Thoughmorecomplicated 3 shows a series of scope photos illus-acteristics, the resonant circuit has an- than the LC circuit of (B) in Fig. 2,trating the effects of various types ofother aspect that has suffered consid- This circuit has a number of distinctbass circuits on transient pulses. In the erable misunderstanding. Thisisthe advantages.Itrequires no expensivecase of the synthetic resonant circuit, fact that a high -Q circuit will generate high -Q inductances, and is therefore not nearly pure bass fundamentals are gen-"hangover" when shock excited. Un- erated from the transient pulses not only fortunately, hangover has come to he at the main resonance frequency, butassociated with a host of undesirable for an appreciable range on either side. in bass reproduction, and as The synthetic construction of funda- aresult,resonant circuits have been mental tones from transient pulses may little used in recent years. Actually, a be consideredtogivethefollowing thumpy or boomy reproducing system is benefits.Ittends toduplicate effects apt to be suffering from too little hang- produced naturally by good acoustics, over rather than too much. The reason it tends to provide a signal more easily for thisis that rapidly damped wave Fig. 1. Comparative steady -rate response reproduced by presentdayspeakers, trains produce strong "side -bands," or curves of conventional bass -boost circuit(A) and-rather paradoxically-it can result adjacent frequencies, and the ear tends and resonant bass boost (B). in better apparent highs. This last char- to hear and be irritated by the spurious acteristic is a psycho -acoustic effect re- high -frequency components of atoo - susceptible to hum pickup from straysulting from the fact that the masking rapidly damped bassnote.Similarly, magnetic fieldsLikewise, it is a high -characteristics of the low -frequency re- adjacentresonancesinthespeaker, impedance device with essentially zeroproduction are radically changed. As a baffle, or acoustics tend to be strongly insertionloss and as suchiseasily result, more bass boost can be usedstimulated and consequently may pro- adaptedto,or combined with,otherwithout drowning out the highs with duce disagreeable beats. audio circuitry. In addition, control ofstrongly masking pulses or noise com- The introduction of artificial hang- the circuit is very flexible and apparentponents. At the same time it may be over before the loudspeaker means sev- Q's from zero to near infinity may befound thatcertaintransients become eral things. Short duration wave trains obtained by varying the setting of asemi -audible that were not reproduced are lengthened and non -symmetrical com- single potentiometer. at all previously and these give a soft- ponents are largely eliminated, with the The steady-state response curves of a resultthatanobjectionablypitched simulated resonant circuit are shown in speaker or cabinet resonance is much Fig. 1 in comparison with those of a lessaptto be stimulated.Similarly. conventional tone control. Two factors fewer spurious sidebands are produced, are worth noting: First, that the curve with the result that the listener hears obtained is such that added accentua- deeper and clearer bass. This last may tion is obtained in the low -bass region be consideredofdefiniteimportance where itis frequently needed to com- when listening at low levels due to non - pensate for poor speaker and baffle ef- linearity of the ear which gives the ficiency, inferior acoustics, and possible effect of heavily damped wave trains. defectsinoriginal program material. However, certain precautions should be Secondly, the response can be made to taken to secure optimum results with drop off quite sharply below a chosen resonant bass boost circuits. In the av- frequency in order to reduce the pos- Fig. 3. Oscilloscope photos showing pulse series erage phonograph, there are apt to be sibility of amplifier or speaker overload with repetition rate of 100 cps. (A) shows ini- three major sources of mechanical res- by very -low -frequency components, or tial pulse shape; (B) shows output of R -C bass- onance in the bass region. These are the in some cases to attenuate serious hum boost network such as that of (A)inFig.2; (C)illustrates output from feedback type of loudspeaker, the speaker cabinet, and in the program material. boost circuit; and (D) is 'scope trace of output the phonograph pickup arm, and some- Steady-state measurementstellonly ofthethree -elementbass -boostcircuit. times the proper combination of these part of the story, and in many cases the elements can result in a system with transient response characteristicswill ening and more musical effect to what warm, vibrant, bass without electrical be more important in determining whatmight otherwise be strident highs. boost. However, if the electronic reson- the reproduced bass sounds like. Figure In addition to its pulse -forming char - ance is peaked at or near the frequency of one ofthe mechanical resonances verydisproportionateresponsemay occur. Practical Circuit Figure 5 shows the schematic of a bass -boost circuit which employs three 0 different resonant elements. By using two or more resonant elements in the bass region the over-all response may be more closely compensated for and tendencies toward "one -note" reproduc- , a tion are greatly reduced. In the circuit

5 -Shown, each control serves a dual pur- pose, acting as a means of controlling the "Q" of the resonant element as well 0,J as an attenuator of that particular chan- nel. Thus, when the three bass controls are turned all of the way down, no bass boost occurs. When the controls are turned part way up, a low "Q" reso- nance is simulated and pulse forming occurs. When the controls are turned : all the way up, a high "Q" resonance is 9J , simulated and artificial hangover oc- curs. The fourth control shown is the Fig. 2. Bass -boost circuits-(A) is the resistance -capacitance circuit; (B) is theresonant circuit, input level to what iscalled a "side usually low7 impedance; (C) is the feedback type of bass equalization; and (D)is the simulated resonant circuit, with frequency determining elements shown in amplifier," a channel which introduces dotted box. no frequency discrimination, and which

40 is used to set the level of the high -fre- quency response in comparison to the bass. In some respects, this part of the circuit is worthy of special attention in itself,inasmuch astheslopeofthe high -frequency response curveisnot changed, but merely the relative am- plitude of all of the highs. This means that it is possible to achieve emphasis of claritygivingmiddlehighswithout adding excessive high -frequency noise to the reproduction. Fig. 4. Three -element While the unit shown in Fig. 5 ob- bass -boost circuit tains power from the main amplifier, with self-contained the system shown inFig. 4 is com- power supply. pletely self contained, and is designed foreasyinsertion between a crystal pickup or a preamplifier and the main poweramplifier.Theself-contained power supply, shown in Fig. 6, furnishes a B+ voltage of about 150, which is adequate for most purposes. This sup- ply can be incorporated into a single chassis, as in Fig. 4. and will mount readily in a x 6 x 2 in. chassis. In a former and moving the exploring coilteristics. For example, the substitution of about ituntil the region of minimumpentodes for the triodes shown in the field is established. schematicmay producesignificant. thoughnotnecessarilyundesirable. Performance changes in tone color. Likewise, when using larger values of capacitancein The frequency of each resonant cir-the feedback loops, such as .01 pi or cuitis determined by the values of thegreater, the effect of dielectric hysteresis threeresistors and capacitorsinthe feedback loop. Increasing the value ofmay alter the resultant sound, but in any of these components will lower thesome cases this may be beneficial. resonantfrequency,whiledecreasing Aside from the benefits of deeper and Fig. 6. Power supply incorporated in unit shownthe values will raise the point of reso-more musical bass, the use of the three - nance. Suggested values for specific fre- element bass control seems almost to compact,selfcontained,unit,carequencies are shown on the diagram, butadd a new dimension to radio broadcasts should be taken that the two dual triodessuperior performance with a given audioand the various elements of a station's are located in the minimum field frominstallation may likely be obtained withtechnical personality seem to stand out the power transformer. This positiona different group of frequencies selected more clearly due to the superior repro- can usually be located by means of ato match the resonances of the system. duction of low -frequency transients. For small pickup coil-such as a magnetic Bass tone is likewise strongly influ-example a control room microphone phono cartridge-and a high -gain am-enced by the amount of low -frequencythat is poorly shock mounted may ex- plifier, the best procedure being to de-distortion present in the signal sourcehibit an interesting serious of thumps termine the point of minimum field in-or reproducing equipment. In the con- and bumps while the operator is reading tensityfromthepowertransformerstruction of the three-way bass circuit, news or changing records in the middle beforeanyofthecomponentsarethe use of different components tend to of an announcement. Similarly, matters mounted. This may be done by con-alter the character of the resonant decay like buns in different audio channels, microphonics, turntable or record rum- necting the primary leads of the trans-curve due to changed linearity charac- ble, air conditioner noises, and so on, tend to show up more clearly. Of course, if these sounds become too distracting they can easily be eliminated by proper setting of the bass controls, a procedure somewhat easier than getting out the hammer and saw to modify the acoustic resonances of the room or cabinet. Inconclusion,the use ofmultiple resonance bass circuits should appeal to many listeners who desire fullness in the lower registers. To date, the only detri- mentaleffectsthat have been noted when using circuits of this nature are found when the electronic resonance is set too closely to one of the mechanical resonances of the system, or when ex- cessive distortion elsewhere has tended to counteract the benefits derived. Par- ticular attention might be paid to the elimination of cabinet or room rattles which may be generated by the funda- Fig.5. Schematic of tone control system using three different simulated resonant elements, mental bass tones. Similarly, amplifiers a side amplier, and a cathode -follower output. Note that no over-all volume control is included. such as the "Ultra -Linear" are recom- Resonant frequencies are 45, 80, and 120 cps. mended.

41 C22o _ C2213 _ C22c _ OUT

20 2(57 20-7.

iv I 2 0/200V R33 R34 R35 33,000 18,000 t3,70 VS 3C C 0 12 AT 7 BANDPASS FILTER VOLUME

OTHER INPUTS BASS 4(CLOCKWISE R21 R19 1 OMEG

0 VI V2 C9 1?Ax7 12AX7 025

017 4 2 C15 TO 25 100milf R24 018 47,000 p- 10 MEG 47,000 111CLOOK WISE) 50/25V TREBLE /77 RI R2 CHASSIS 011MEG/2W 4 3 G

.- 1220muf C2K004

Fig.2. Complete schematic diagram except for d.c. heater supply. Versatile Control Unit For The Williamson

CHARLES R. MILLER

A combination preamplifier, tone controls, and bandpass filter giving minimum noise and distortion and maximum convenience.

IN THE FIELD OF HOME MUSICreproduc- by using all the heaters in series as the The complete circuit appears inFig.2. tion, the and its cathode -bias resistor of the power tubes, The design of the preamplifier 17,-V, is modifications have come to enioy pre- using a method to be described later. It fairly conventional, with the equalizer eminence due to outstanding perform- was felt, further, that the second could values16, Rt, CI,and C. being obtained ance, but there has been no similar stand- be met only by using negative feedback by appropriate transformation ofthe ardization in tone -control and preampli-on each stage. While it may be argued networks given by Boegli" for record fier units intended to complement the that the signal in low-level stages is so Williamson. The unit described in this smallthatnegativefeedbackisnot C. P. Boegli, "A preamplifier for mag- article is an attempt to meet that need. needed, it has been shown that this is netic and crystal pickups,"Radio and Tele- not the case.' vision News,July 1950. Functionally, the unit is divided into * C. P. Boegli, "An improved equalizer- five sections as shown inFig.1. First, it ' W. B. Bernard, "Distortion in voltage preamp," Radioand Television News,April was decided that to preserve a high sig- amplifiers,"AUDIO ENGINEERING,Feb. 1953. 1951. nal-to-noise ratio, all tubes were to be operated with direct current on the heat- ers, and secondly that all the sections must be capable of low -distortion per- Fig. 1. Block diagram formance matching the Williamson. The CATHODE TONE C AT HOOE of the controlunit. P"4°. OUTPUT first specification can be met most easily FILTER PRE AMP FOLLOWER CONTROL FOLLOWER * The Graduate House, Mass. Inst. of Technology, Cambridge 39, Mass.

42 compensation. The gain -frequency char-inatingswitchingtransientsdueto section and treble through the lower. acteristics of the preamplifier for various changes in d.c. level. With either control arm nearer the in- equalizer values is as shown in Fig. 3. If The input cathode follower V, and theput, the corresponding output signal is simplicity is desired, the network shownoutput cathode follower V. are perfectlyboosted, and with the arm nearer the as the compromise network can be usedconventional, serving the purpose of iso-output, the output is attenuated. The re- and any additionalcorrection needed lation and impedance transformation. A sulting frequency response is as shown obtained through the tone controls.Itdual -potentiometer gain controlgives in Fig. 4. One advantage of this system should be noted that in no event are half - best compromise between minimization is that at boost positions, there is still watt resistors used, these generating aof distortion and noise. If only an input enough negative feedback to insure very much higher noise level than would begain control were used, noise generated low distortion. One disadvantage if the predicted on the basis of thermal noise. in the following stages would reduce sig-circuit were to be used with a.c. heater Furthermore, cathode bias is used in thenal-to-noise ratio and if only an outputoperation is that the actual grid signal is second stage to reduce distortion' gain control were used, distortion mightvery low and consequently hum trouble There may be some question as to thebe caused in the preceding stages due tomight be expected. Note one important use of the 12AX7 as the preamplifieroverloading. It may be asked why elec-precaution-the potentiometers used are tube for low -noise service instead of thetrolytic capacitors were not used in thelinear -taper units and are not the usual 12AY7. Fairly extensive tests were madecathode followers. First, it is the writer's logarithmic controls used in conventional on the noise and hum level to be expected experience that such capacitors are atone circuits. from various tubes. In particular, aboutsource of noise and distortion and should The bandpass filter is of conventional two dozen 12AX7's and 12AY7's werebe avoided whenever possible.5 Secondly, design with a 12-db-per-octave slope at checked for noise equivalent input at theadefinitelow -frequencycutoffwas the high -frequency end. To preserve bal- grid. The equivalent hum and noise inputwanted to insure stability. Wow in aanced response Cr, was selected to give were compared, both for a.c. and d.c.turntable can easily overload a poorlylow -frequency cutoff matching that at heater operation, with the only point ofdesigned system in which the amplifierthe high -frequency end, asit was felt superiority of the 12AY7 being in theis very nearly at the motorboating point. that an additional 12-db-per-octave high- matter of hum with a.c. heater operation.It is idle to talk about flat response downpass filter at the low end was not justi- With d.c. heater operation and the usualto subaudible frequency unless the lis-fied. The Chicago Transformer NSI-1 gain vs. frequency characteristics fortener is prepared to "listen" with air- unit was specified since tests showed this phonograph equalization, the most im-tight seals between his ear drums and the choke to have the highest self -resonant portant source of noise is flicker noise, loudspeaker cone. frequency of any of the commercially presumably caused by cathode poison- available units. Note that if the self - ing.* Filter and Equalizer capacitance of the choke is too high, the Flicker noise varies inversely as the filter is converted from constant -k to tn- square of the frequency and hence the The tone control stage V. perhapsderived operation, with consequently less output due to flicker noise varies in-needs explanation. Thisissimply an satisfactorytransientresponse.The versely as the cube of the frequency. Foranode follower° with feedback voltage valuesofresistance and capacitance the tests mentioned, there was more va-determined by the impedances between were chosen for critical damping and not riation within a given tube type thaninput, control gritl, and plate. If the in- for maximally flat response. The result- from one type to another. In any event,ternal gain of the stage is infinite anding behavior is shown in Fig. 5. the average 12AX7 was better than thethe input impedance at the grid infinite, The power supply for the control unit average 12AY7 for flicker noise. Repre-the over-all gain can be shown to be inis somewhat unconventional. The heater sentative 12AX7's in the preamplifierthe ratio Zia,, where Z, is the imped-current is taken from the main amplifier gave an unweighted signal-to-noise ratioance from plate to grid and Z. the im-by using the cathode currents of the out- of 68 db for a nominal 1 volt output (10pedance from input to grid. More simply, put tubes, about 15 ma drain from B+ as my input at 1 kc). As predicted, the out-the actual input signal is determined byshown in Fig. 6. Previous tests on the put noise spectrum had the sharply ris-the setting of the two controls, Rs: andWilliamson have shown an almost con- ing low -frequency characteristic typicalR. Bass signals go through the upperstant current taken by the output tubes of flicker noise. Hum level was so far for any power level up to the. overload below this noise as to be unmeasurable. point. To prevent a burnout of one of the s J. E. Lilienfeld and C. R. Miller, "Dis- heaters from pulling the cathode voltage In the complete schematic (Fig. 2) tribution of conductivity within dielectric 12,,, and R,, are for the purpose of elim-films on aluminum," Jour.Electrochem. up to B+, a neon lamp is inserted as Soc., May 1953. shown. In normal operation the lamp is ' Valley and Wallman, " ° P.J.Baxandall, "Negative feedbackbelow 50 volts and thus will not fire or Amplifiers." Radiation Lab Series, Vol. 18, tone control,"Wireless World, Octoberaffect operation. Burnout of a heater is McGraw-Hill, New York, 1948. 1952. signified by the neon lamp glowing, at

BASS CONTROL TREBLE CONTROL

I III 1 I 11 11 1 11 R C - -- FULL CLOCKWISE - - AES Ali COL 79,nu 420 COMPROMISE ONE-HALFCLOCKWISE L _ +to - ..COL.LP1 - I: hh. - - -- _ -' vA 50 5-- ill z 0 411.111 cr r 19.IIII.. 111 F. _ 0 - -5 - - - --""4111.9. - A11111111111111111 --.4:1111110.a... ' _ONE-NALFCOUNTERCLOCKWISEISE 30.---4-- -15 '---- - IfrrCA - COMPROMISE 20 r RgAES FULLCOUNTERCLOCKWISE 78 __ - -

I IIIII 1 11111111 a . . MOO /0 .00 1000 , cp. 100 100.. "°W FREOUENCY IN CYCLES PER SECOND FREOUENCY IN CYCLES PER SECOND

Fig.3.(left). These curves show the equolizotion afforded by the various combinations of C-C,R1-16. Fig. 4 (right). The range of tone control available from the circuit.

43 0111111.'"01.11"W"-

-.".\DE

0

A

'7., 20 II

Fig.5.(left). The five positions of the bandpass filter switch give these results me 403 ,004 FREQUENCY IN CYCLES PER SECOND Fig.7.(above). Test setupfor distortionmeasurements. which time the system should of coursetrots or the filter. For 10 volts output pilot light are external to the main chas- be turned off and the trouble fixed. This and maximum bandpass, the distortionsis as shown in the photograph. There is scheme has the further advantage of pre-was about 0.1 per cent. With 10 voltsthus no 60 -cps source within the chassis. venting B+ from rising to a value de-output and minimum bandpass, the dis-Magnetic coupling is prevented by the structive to the electrolytics if one of thetortion increased to 0.3 per cent. How-usual wiring practice of a ground bus heaters should burn out. The second fea-ever, since 10 volts represents a value 20connected to the chassis only at the in- ture of the power supply is that B+ for dh above overload for the main amplifier.put. The shorting switches used for the the control unit is taken from voltage - it is felt that for normal operation, bothsignal circuits are the new Centralab regulator tubes. This simple precautionharmonic and intermodulation distortion PA2000 series, which both are smaller will save a good deal of grief in tryingwill be negligible. Since most of the dis-and give better performance than the to design for decoupling adequate to pre-tortion is caused by the filter loading theusual wafer type. The additional tube vent motorboating.Itisalso usuallytone control output, this could be madesocket is for a proposed microphone pre- more economical than the electrolytic ca- negligible by insertion of a third cathodeamplifier. When and if this is used, the pacitors needed to give the same low -fre- follower to drive the filter. For the 1 - 12AU6 tone -control tube will be replaced quency stability. volt operating level, this was not deemedby a 6BH6, with a second 6BH6 for the necessary. microphone preamplifier. In any event, Final Results The mechanical layout is quite normal.the total heater voltage will be 36 volts To test the total distortion generated The entire control unit is built within anfor the series string. The electrolytic ca- by the unit, an intermodulation analyzer8 x 12 x 3 -inch chassis (seeFig.8), with pacitor used for filtering B+ within the would be best. Without access to such athe tubes being arranged on a terminal control unit is mounted on a Mallory unit, the following test procedure wasboard, which itself is shock mounted by PS -6 socket for easy replacement. Ven- adopted. Equipment layout was as shownmeans of grommets. To keep electro-tilation holes are drilled in the bottom in Fig. 7. With the tone controls set atstatic coupling from getting hum into plate. flat position and the gain control at max- the signal circuits, the power switch and (Concluded at bottom of next page) imum, the signal from the GR Micro- volter was raised until the output was at the desired level. At the design output level of 1 volt, the distortion was unmea- surable for any setting of the tone con-

440V FINAL POWER AMPLIF ER STAGE

Fig.8.Photo of the 12AU6 underside shows terminal -board c o n - 12AT 7 struction.

12 AX 7

300V 4 2 TO PLATES OF CONTROL UNIT

Fig.6.D.c.for the heaters is takeninthis manner from the cathode circuit of the final stage of the Williamson.

44 Preamp with "Presence" C. G. McPROUD

Introducing a circuit which departs from "flat"to pro- vide the elusive quality calledpresence, and which incor- porates most of the features desired by the critical listener.

WRILE THE GENERAL TREND in am- The Complete Circuit magnetic pickups began to flatten off too plifier and equipment construction Every so often, a confirmed experi-soon, resulting in insufficient bass in the has usually been in the direction menter feels the urge to make a change -lowest range -thatis,below 50 cps. of completely "flat" response curves, it usually having in mind the desire to addSome amplifiers have insufficient com- is well established that certain types of one or more operating features whichpensation characteristics for the most correction are useful to compensate for appear to be desirable. Reviewing thecritical listener, and while the trend for microphone placement, for deficiencies in real or fancied faults in existing cir-the newcomers to hi-fi seems to be to equipment, and for personal preference. cuitry, the writer planned a preamplifier reduce the number of available curves - However, the use of a separate control and tone -control amplifier which wouldlargely because those who are entering for increasing "presence" has not been incorporate all of the desired featuies.the hi-fi field "fresh" will generally build introduced in home equipment to date, Taking the steps one by one, they addtheir libraries exclusively from the LP although it has been used in professional up about as follows : and 45 catalogs -there is some advan- equipment for many years -although not tage to having complete flexibility. necessarily by that name. Some preamplifiers do not operate as The tone -control circuit first described In the early 30's, certain types of loud- quietly as would be desired -having aby BaxendalP and more recently for use speaker systems in theatres were in need hiss or hum level too high for completewith American tubes by Barber2 offers of some form of correction in the mid- satisfaction. Furthermore, it was felt thatsome interesting possibilities. Principal the low -frequency boost requiredfor range to increase the illusion of realism. among these is the variation in the in- It is probable that the deficiency was due flection point as the amount of boost or to the speakers themselves, but in any cut is changed. The advantages of this case a correction in the form of a small show up when using a wide -range loud- boost somewhere between 2000 and 3000 speaker system with usual source mate- cps was introduced to improve presence. rial -both records and radio programs. The boost was of the order of 4 dh, and In addition to the presence control, it was usually located at 2700 cps. also seemed desirable to incorporate a Following some experimentation along low-pass filter system into the amplifier, these lines, it was found that the addi- and the L -C network described by Mar- tion of a boost ranging from 4 to 6 db at kow3 was investigated for the purpose. 2700 cps gave the illusion of moving a solo violinist out in front of the loud- I Wireless 1Forld. October 1952. speaker, or of causing the singer with a 2 Basil T. Barber. "Flexible tone control popular hand to step out in front. Ad- circuit." See page 34. mittedly the effect is slight, but it is suf- Fig. 1. (Left),Centralob Couplateforthe ficiently noticeable to warrant its inclu-tone -control circuit. (Right), the Couplate as- 3Elliott W.Markow, "Record improve- sinn in the amplifier to be described. sembled on a dual -concentric control. ment with 11.f. cutoff filters." See page 63.

TABLE I L, 0.8 Hy VERSATILE CONTROL UNIT R, 0.11 meg. 2 watts Component ValuesforDifferent R, See Table I EqualizationCurves (from preceding page) R, 15.000 ohms. 1 watt, 5% Curve C2 R, R, 22.000 ohms, 1 watt, 5% .0035 R, 36.0110 ohms. 1 Modifications RIAA 470 tstif 1.0 meg watt, 5% ffrr 220 .004 4.3 R 62,000 ohms, 1 watt, 5% The schematic of the amplifier -Fig.1 Col 78 1000 .1104 4.3 R, See text -shows fixed values for the frequency - RCA 78 470 .003 1.8 1500 ohms, deposited car- determining elements, C, C., and R,, and for Old AES 620 .004 1.6 bon. 1% the values shown, the turnover would be at Col LP 1000 .1105 0.56 0.22 meg. 2 watts approximately 375 cps, and the high -fre- Compromise 470 .005 1.2 R,, 1.0 meg, 1 watt quency response would be down approxi- R,, 0.15 meg, 1 watt mately 6 dh at 10,000 cps, which closely fits RH 2400 ohms, 1 watt the fin- curve. These values can be changed Rif, R 10.0 megs, 1 watt tosuit the individual preference, or the The value for R7 isthat specified by R,,, I?,, 1.0 meg, 1 watt circuit can be arranged sothat various the pickup manufacturer as optimum. A R,., R31 1600 ohms, 1 watt values could be switched in at will. Cs con- 2 -watt unit is recommended to ensure low - noise operation. Rs, 15,000 ohms, 2 watts trols the turnover frequency and R, controls Dual25,000 -ohmpotenti- the amount of flattening or deemphasis at PARTS LIST ometer, linear taper the low end. The amount of rolloff is con- C,, C, Sec Table I R,, 0.1 meg, 1 watt trolled by the capacitor C,. C, 2400 µµf, mica, 5% Rn, Res 1.0-meg potentiometer, lin- For equalization tofit the RIAA curve C. 1200 itia, mica, 5% ear taper -which is becoming more and more the Cs 560 µµf, mica, 5% R:f 0.47 meg, 1 watt standard in the industry -it is advisable to C. 270 µµf, mica, 5% R1, 47,000 ohms, 1 watt decrease the value of C, to .0035 µf, chance C7, C7, C,:, C17, 0.25 gf, 600 v, paper 68,000 ohms, 2 watts R, to 1.0 megs, and increase C, to 470 µµf. C.. C.,,. 50 µf, 25 v, electrolytic R.: 560 ohms, 1 watt This should give suitable results with almost C., C,,.C:e 0.1gf, 600v, paper R 0.22 meg. 2 watts any LP record, provided sonic adjusting of C11 1.0 tsf, 600v, paper R.11. R57 33,000 ohms, 2 watts the tone controls could be resorted to. For C,,, CI, 4700 ggf, 400v, paper RSI 18,000 ohms, 2 watts other equalization curves, the values shown C..1, 100µid. mica V I.V, Two halves of 12AX7 in Table I could be substituted for those C,. 0.15lif, (100v, paper V,. V; Two halves of 12AT7 in Fig. 1. C.. 2 u f. 200 v. paper V. 12AU6

45 in the low -frequency range. When tubes with less gain than the 12AX7 are employed, the difference is even more noticeable, which may account for a deficiency in low -frequency boost when a 12AY7 is used in a circuit de- signed for the 12AX7. To correct fully for the necessary low- frequency boost, it is apparent thatmore gain is required from each of the tubes (or tube sections). The logicalanswer is to use two pentodes. Some pentodes are too noisy for this service, consider- ing that the input signal is in therange of 30 my. The circuit was first builtup with 5879's, but these tubes-while of excellent low -noise characteristics-have Fig. 2. Resistor mounting card assembled in small chassis with four sockets attached. Ter- hardly sufficient gain to give the desired minal lugs may be staked into any desired holes in the board, and thus fitany circuit. Grid low-trequency boost. The Genalex Z729 and input circuit components are on the top side;plate and decoupling components are recently introduced in this country, is located on the bottom. claimed to have even lower noise and \ nd as this writer has long been a pro-crafted units because of the difficulty of hum than the 5879, but it has an avail- ponent of the loudness control, this tooobtaining these strips with terminals lo- able gain about equal to that of a 6AU6. had to be included-but with some formcated ideally for the particular circuit The latter tube is likely to be somewhat of decompensation which could easily bebeing built. However, with the Alden noisy in low-level applications, particu- an uncompensated volume control. terminal cards, any desired number of larly if a.c. is used on the heater. With Adding these up, one arrives at theterminals may be installed exactly where the Z729, however, suitable performance astonishing figure of eight as the re-the user wishes. Figure 2 shows the main with respect both to gain and noise has quired number of controls, nearly as badterminal board with the four sockets at- been achieved. as the early TV sets before the trendtached to it and with most of the ter- The circuit is relatively conventional. toward simplificationresultedinsetsminals inplace.Detailed information A feedback circuit from the plate of the with only two knobs. Follownig the sameabout the terminal boards and the method second tube returns to the cathode of the procedure, the amplifier was planned toof using them is included with the con- first, and frequency corrective circuits use only four "apparent" knobs-eachstructional data. are introducedinthe feedbackloop. being a dual concentric control. ThusPreamplifier Details Series capacitors control the turnover the phonograph preamplifier becomes a frequency, with the New Orthophonic dual switch each with five positions ; the The amount of low -frequency boost available from the conventional equalized rolloff being obtained by connecting a tone control becomes a dual -concentricfeedback circuit depends upon the gain shunting resistor across one of the ca- assembly of two potentiometers;theavailable from the two tubes. Consider, pacitors. The turnover frequencies avail- presence control and the low-pass filterfor example, the use of two sections of able are 800. 5001.1)( for New Ortho- switch are combined into a third unit ;a 12AX7 with gain of around 40 for each phonic), 500, 400 (AES), and 300. and the fourth consists of another dual - The high -frequency rolloff is provided concentric control-one section beingsection, or a total gain of 1600. This by connecting suitable capacitors and re- the loudness control and the other beingcorresponds to approximately 64 db. For sistors across the feedback resistor to a volume control. an output of 1 volt and an input of 10 my give rolloff of 4. 8. 12, and 16 db in addi- Obviously, few oftheseunitsareat 1000 cps, a gain of 100 is required, readily available from jobber stocks, sowhich is 40 db. This gain must be avail- they were described to Centralab, andable at 1000 cps. However, to provide the four complete units were soon forth-the low -frequency boostrequiredfor A 'max coming. Centralab engineers had alreadyproper equalization of magnetic pickups, \ designed the Senior Compentrol, whichit is necessary that an additional gain of combines the loudness control with a vol-20 db be available at 50 cps ( for a 500 - umecontrol,and whichemploysa cps turnover). Since the maximum gain printed circuit "Couplate" with all theavailable from the tube is around 64 db compensating elements. A study of theand 40 is required at 1000 cps, the re- Baxendall circuit indicated that anothermaining 24 is all that is available for Couplate could be built with three ca-boost of the low frequencies. Now the pacitors and four resistors in a singlefrequency -response curve does not con- 20 00 '00 soo '000 unit, as shown at the left in Fig. 1, andsist of two straight lines, but is composed FREQUENCY N CYCLES PEA SECOPO of the flat portion above turnover, plus assembled onto the dual potentiometer at Fig. 3. Construction of the curve obtained from the right. Thus the construction of thea portion which increases with lowering feedback network around two tubes. Curve is entire tone -control circuit is consider-frequency at the rate of 6 db per octave. asymptotictotwo straightlines. ably simplified. A standard line of Cen-plus a portion which is again flat, but tralabunits-availableprimarilytowhich represents the total gain of the manufacturers-included dual concentrictwo sections of the tube without feed- tion toa flat position. All of the switch- switches and dual units with a switchback. These threestraightlinesare ingisaccomplished by aCentralab and a potentiometer. These were kept injoined with curving sections, as shown Series 30 switch-the panel section con- mind during the layout of the amplifierin Fig. 3. sisting of a single deck with two 5 -posi- circuit, and when the requirements were From this,itisseen that thereis tion switches and the rear section con- settled, the problem of making the con-hardlysufficientlow -frequencyboost sisting of another single deck with one trols was left to Centralab. All four unitsavailable from a 12AX7 to provide full 5 -position switch. All of the resistors are available for anyone who wishes toequalization for the AES curve nor for and capacitors are mounted on the re- duplicate the amplifier. the 800 -cps turnover, if the compensa- sistor board, with flexible leads connect- One of the features of the constructiontion is to extend fully to the lowest fre- ing to the switch. The schematic of the is the use of "custom-built" mountingquencies to be passed. This discussion preamplifier is shown in the left portion boards for many of the components.does not apply to the New Orthophonic of the over-all schematic, Fig. 4. The While resistor -board construction is notcurve, sinceitrolls off with a maxi- response curves of the preamplifier sec- uncommon in factory -built equipment,mum required boost of 18.6 db at 30 cps. tion are shown in Fig. 5. It will he noted itisless often encountered in hand-The LP and NARTB curves are similar from the schematic that no provision has

46 C30. 20 R34 27,000 835 10,000 R36 5600/2 R37 3900/2 O.

0 SPECIAL TONE CONTROL 7Dot C* I019E6104E6 0- RAM - v1 2729

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NOTEt R21* ISLOUDNESS CONTROL B-0 SECTION OF CENTRAL AB

NOTE 2 R15 ANO 3*, c ARE ONE UNIT 093 F113 13,000 0 200vA SWI' 0 1tV 0 0 VI

V2 41tV

Fig. 4. Overall schematic of the preamplifier and controlsections. been made for switching inputs. The pre-is used as a cathode follower. Further- tube section, Vie, is a cathode follower amplifier has input and output jacks-more, thereisrather more high -fre- which feeds the low-pass filter circuit. theinput coming directlyfrom thequency equalization in the tone -control The filter circuit is an adaptation of pickup and the output being fed to acircuitthanisconsidereddesirable. the network described by Markow,3 and selector switch on the tuner. The outputSince additional gain was necessary, the is connected at the output of the cathode of the tuner switch if fed back to thefirst section is used as an amplifier with follower. This circuit requires the use of input of the tone -control section, and itsimproved performance throughout.In another inductance, but the performance output is fed in turn to the power ampli-the final form the control section requires meets the specification laid down before fier. There is no reason why the switch-an input of .08 volts for an output of 1.0 the construction was begun. The control ing should not be incorporated in thisvolts which gives some leeway for opera- is a Centralab 30a unit, and consists of chassis if it is desired, but for the writ tion of the volume and loudness controls. four 5 -position switches mounted on two er's application the tuner switch was With the Couplate and the dual pot, it decks and operated by the outer shaft, mostconvenient.Furthermore,thisis only necessary to connect four wires together with the presence control-a method provides complete flexibility andbetween the tube chassis and the control 50,000 -ohm linear pot operated by the permits easy interconnection of otherto have a complete low- and high -fre- inner shaft. Three of the switch sections preamplifiers or tone -control amplifiersquency tone -controlcircuit.There- are used in the filter circuit, while the for comparative tests-a condition whichsponse curves obtained are plottedin terminals of the fourth serve astie is often required in this particular in-Fig. 6 for the center or flat position and points for the matching resistors, R., stallation. for two degrees of both boost and cut. and R0. The eight capacitors The second section of V, feeds the vol- and the four terminating resistors are Tone -Control Section ume control and the first section of V, is mounted on another Alden terminal card, The remainder of the circuit comeslocated between the volume control and as shown in Fig. 7. The frequency -re- under this heading, although there arethe loudness control, giving complete sponse curves for the filter section are several other functions incorporated inisolation between the two controls and shown in Fig. 8. this section of the amplifier. The firsteliminating any interaction. The final it may be argued that the presence of step consists of the "presence" circuit, which includes RI!, RII, RI., Cu, Co, and BOO This is a simple loss network with a 500 400 tuned circuit in the shunt leg. The tuned 300 circuit is shunted by a 50,009 -ohm linear 500LP pot which varies the amount of boost 15 -,- from 0 to 6 db. The boost is approxi- mately linear with respect to the amount 10 of rotation of the pot. The L -C circuit Fig. 5. Measured 5 is tuned to 2700 cps, and listening tests curves obtained from 0 - , ...04111 have shown that the L -C ratio prescribed the preamplifier sec- gives about the best effect. A smaller in- tion. ductance and larger capacitor could be 10 tuned to the same frequency, but the 15 "bump" in the response curve is narrow, 20 and the subjective effect is not as de- sirable.

30 The Baxendall circuit provides no NO ION IONS 71111110 gain from the first tube section since it FREQUENCY IN CYCLES PER SECOND

47 screen resistors, together with the de - couplingresistorsandcertainother components can be placed on the other. 1 """:null 111111E1119inn This requires that the bottom of the '''' mum! chassis be cut out to provide access to pion inuipp. the "bottom" of the resistor board, since a 11..vglimmum Ine.O. Fig. 6. Curves show- changes may be required during the impERoviii ing the limits of tone construction or-possibly-afterwards poulii....;:s7,uhum c controlaction, . __...... mid to- when service becomes necessary. i.- 1-t -pi-_- 1 _-_----mmulial gether with an inter- The mica capacitors used in the equal- mediate curve for izer circuits may best be Silver Micas, ° iiiiiii0 iiillilbillgiillw both boost and cut. since they may be obtained with the '''Fran iiiiii iiihim closest tolerance. The small amount of E 1111111 111111 IIIIIIIMIE shift in values can readily be tolerated 2° 111111 without apparent effect, but most con- 111111 111111 structors will wish to take advantage of 111111 1.11111 . I. 111111 the 10 per cent tolerance of this type of . component. The two inductances listed FREQUENCY IN CYCLES PEN SECOND may be any type of toroid available. If the entire installation is to be used at the filter network after the output of the flexibility for the most critical listener,some distance from any hum -producing cathode follower detracts from the ad-yetfor the other members of the user'sfields-such as a phonograph motor- vantage of the follower as an outputfamily-those who would normally be source-usually considered desirable inconfused by eight controls-the pream- case the power amplifier is to be located plifier switch may simply be left in the at some distance from the control sec- 500LP and -12 position, and the tone, tion. However, for this particular appli-presence, and filter controls can be left cation the two units are to be located in the "flat" position. Either of the level- close enough that a three-foot cable willcontrolling knobs may be used to set vol- he used for making the connection, andume. The choice of knobs and switch the impedance of the output under theposition arrangement on the preamplifier worst condition-that is, at "flat"-is ofcontrol is such that the 500LP and the the order of 11.000 ohms, and a shunt- 12 position are coincident, and could capacitance of 300 ilt4f could be tolerated easily be arranged to be "straight up" in with a droop of only 3 db at 50.000cps. the normal playing position for LPrec- Under these conditions, therefore, itwasords. Similarly, the two indicating points considered preferable to locate the filteron the tone -control are arranged to be at the output of the control -unit rather"straight -up" when in the flat position, than at some earlier section of the cir-and the control tapers are designed cuit. toFig. 7. Mounting of the components for the low- have a "flat" range of at least 15 deg. passfiltercircuit.The input impedance -ad- The non -indoctrinated user justing resistors ft, to It, are mounted on the Volume and Loudness Controls can be told to leave the filter and presence control switch. In order to provide loudness -controlalone-if that would solve the problem.less expensive units may be used. But action in the amplifier, and yet to be ableConstruction for complete freedom from the effect of to use as much or as little compensation any externalfields,the toroidisthe as might be desired, both types of con- The construction of this unit followsanswer. trols are provided. A 0.25-meg volumefairly simple lines, with the possibleex- The original unit built by the author control is located between Vie and V4a,ception of the resistor board which wastakes the place of one which has been and the compensated control is locatedused in the prototype. While there is no inservice forseveral years, andis between V4a and 174e. The Senior Corn-particular need for this type of con-powered by the same supply that has pentrol employed here consists of twostruction, it does simplify the work tobeen in service since it was originally controls with concentric shafts-the vol-some extent. described in the series on "Residence ume control being operated by the outer Figure2 shows the resistor mountingRadio Systems," published in 1948. This shaft and the loudness control beingcard used in the original model. Thesupply was designed to furnish 12 volts operated by the inner shaft. Any degreeresistor card, together with the tubed.c. at a maximum drain of 1.0 amps, of compensation can he obtained quitesockets which are mounted on the card,and with a relatively low ripple. The readily. The response curves of the loud-was attached to a channel designed forunit has been tested with a 6.3 -volt a.c. ness -control section,Fig. 9,are shownmounting onto the chassis with rubbersupply and has been found entirely sat- for the maximum -volume condition, and grommets. With the Genalex Z729 tubes,isfactory, provided the latter is suitably for conditions where the 1000 -cps level however, the extra care in reducingcenter -tapped to ground the circuit as is10, 20, and 30 db below maximum,microphonics has turned out to be un-near to the zero -potential point as pos- respectively. necessary, and the sockets may just as sible. Circuit values have been adjusted forwell be mounted on the chassis. Ade- While the circuit shown makes no minimum IM distortion at normal out-quate room is available on a 3 x 5 x 13provision for switching between phono. put, resulting in a figure of 0.7 per cent chassis, although the original was maderadio, TV, tape recorder, or any other at 2 volts. There is adequate gain fromon a 3 x 6 x 14 base-a size which ispossible source, there is no reason why any tuner with which the amplifier hasrather difficult to obtain. Layout of thethe builder could notplace another been used, and the output from typicalterminal card is not difficult, but sinceswitch in the circuit and arrange to ac- LP records is within 6 db of that fromevery constructor has his own ideas ofcommodate any desired number of sig- the AM and FM tuners being used. just how he wishes to do the job, spe- nalsources. However, most modern Considered from the standpoint of thecific layout is not indicated. With thetuners provide switching facilities, and number of controls in the amplifier, this Alden terminals, however, it is possiblethis unit is designed to furnish only the unitseemstohave overstepped theto arrange the terminals so that all ofbasic requirements of a sound system- boundaries of the current trend towardthe grid -circuit components-as well asthe preamp, the tone -control facilities, simplification. However, viewed from thethe cathode resistors and their by-passand some means forcontrollingthe front panel, there are only four knobs-capacitors, and the compensating resis- volume. apparently-and these are arranged in a tors and capacitors-may he placed on Reference toFig. 7will show the convenient fashion. There is sufficient one side of the board, and the plate andarrangement of the components-except

48 the inductance -used inthe low-passformance it requires only a small change R0 50,000 ohms, linear put, (com- filter circuit. The four capacitors at theto outwit the law of diminished returns. bined with Sw,) left are shown with their shunting re-If we can "cheat" the response curve R17 1800 ohms, 1/2 watt sistors, and are -from left to right -just enough to make the change reason- R,. 18,000 ohms, 1/2 watt C to Cu respectively. The next fourably perceptible, we have accomplished Rim, 6 Centralab Senior Compentrol RD 1000 ohms, 1 watt capacitors are Cu to Cu respectively. R., 6800 ohms, 1 watt One end of each of the capacitors and Ri. 13,000 ohms, 1/2 watt resistors is grounded, and leads connect 0 R,,, R,. 11,000ohms, 1/2 watt from the other terminals to the two arms R.. 8200 ohms, 1/2 watt of Sw,. Constructionisnotcritical, RD 12,000 ohms, 1/2 watt since the impedances are low at this 10 RSI 10,000 ohms, Y. watt point in the circuit. Rft 7500 ohms, 1/2 watt R,, 5600 ohms,1/2watt Special Components 20 RN 10,000 ohms. 1 watt R,. 5600 ohms, 2 watt Special units are listed for several of Rs. 3900 ohms, 2 watt the components, but all are available. 50 R,. 1.8 meg, 1 watt The preamplifier switch -consisting of SuP,, Sw, Preamplifier switch, Centralab two 5 -position decks, one with one arm 30c type, special (see text) and one with two, and both being ac- ,000 5000 5000 10000 20000 Sw, Centralab 30a type, combined tuated by a dual -concentric assembly - FREQUENCY IN CYCLES PEP SECOND with R,s, special. islisted as part number SPY -3002, a .1 ,, 1., 1. Amphenol 80-PC2F receptacle Centralab Type 30c switch. The Bax- Fig.8.Curves of low-pass filter action. ./. Amphenol 80-C receptacle Tone Con-Centralab Couplate No. 12614, endall tone control unit is available com- something worthwhile, and thetime pletelyassembledasCentralabpart trol with 1.0-meg front section and money have not been spent in vain. and 0.5 meg tapped rear sec- number C3-300, and consists of the two tion dual -concentric controls pots assembled as a dual -concentric unit PARTS LIST together with the Couplate. The com- V,. Vt Genalex Z729 .05 of 600 v., Aerolite vvs, 12AX7 binationpresencecontrolandfilter C Cl .002 pi, molded paper 12AU7 switchisavailable as Centralab part Cs, C. 0.5 of, 600 v., paper number SPB-3001. The Senior Corn- C,,CH, C0.1 of, 600 v., Aerolite pentrol is already available as Centralab CIS, CD part number C2-100. All units have Cl, C., CD 2000 p.1.4f, mica standard bushings (-32) in. long, C, 1200 opi, mica Notes on the with the inner shaft1 9/16 in. long, CO 3300 p.of, mica CIO 200 oof, mica measured from mounting surface, and 620 oof, mica Preamp with Presence the outer shaft 1 1/16 in. long, measured 1000 oof, mica from the mounting surface. This pro- Numerous readers have indicatedthat C14 3500 Rid, mica vides sufficient room for any panel up C,. 150 Rid, mica the 0.45 H choke limed in the original data to in. thick. was unobtainable. Since toroids with in- CD, C,. 50 of. 6 v.. electrolytic ductances of 0.4, 0.5, and 0.6 H are regu- Several types of knobs are obtaina- 0.25 of, 600 v., Aerolite larly available, the following table indicates ble for these controls, since the dual - CD, C,. 1100 w.f. mica values for the low-pass filtersection for concentriccontrolhas become quite C,., CD 1600 oof, mica use with inductances of these values: common on TV sets. The writer's TV C,. 3000 oof, mica set is a "home-made" Techmaster 630 C,., Cn 5600 oof, mica INDUCTANCE CIO, CV 20-20 of, 350v..electrolytic mountedinableachedoakcorner Comp 0.4 0.5 0.6 li speaker cabinet, and the knobs used are (Mallory FP -227) 1.0 H, toroid inductance Cn, Cu .0012 .001 .00082 those normally used on the RCA 8TS30 L. 0.45 H, toroid inductance Cu, Cn .0018 .0015 .0012 receivers. To have matching knobs, the C51, C1. .0033 .0027 .0022 R, 47.000 ohms, 1/2 watt C25. Cos .006 .005 .004 same type were used. Tan knobs are R, 3900 ohms, '/2 watt Ru 11,000 15,000 18,000 R28 10,000 12,000 15.000 obtainable as stock numbers 73227 and R, 0.5 meg, 1 watt, low noise Rn 7,500 9,100 11,000 73231; the dark brown knobs are stock R, 1.5 meg, 1 watt, low noise Roo 5.600 6,800 8,200 numbers 73226 and 73230. Four of each R. 1.2 meg, 1 watt Ru 10,000 13,000 16,000 R. Rso 10,000 12,000 15,000 are required. Other types of knobs may 1200 ohms, 1/2 watt RII 9,100 11,000 13,000 be obtained from various sources, but R,,, R,.. .0.1 meg, 1 watt, low noise Ru 6,800 8,200 10,000 R,,, RD Rs' 5,100 6,200 7,500 may entail some searching. All of the R. 0.47 meg, 1 watt Centralab units may be obtained on R. 0.1 meg, V2 watt The design voltage for the preamplifier order from any Centralab distributor. R,., R,, 33.000 ohms, 1/2 watt was 275 volts, but satisfactory performance 1.0meg, 1/2 watt should be obtained with any voltage be- Performance R,,,RS, 27,000 ohms, 1 watt tween 250 and 300. The performance curves of the several functions of this amplifier have been shown in Figs. 5, 6, 8, and 9. The pres- ence control may appear to be another 111111111111MIN11111111111:!111111111

example of painting the lily, and in this 0IPTIETPMERPHI 1,15 era of attempting to obtain a response M!!!!!!!!!11111111111111111111111111111M curve which is completely flat from zero to infinity may appear to be unneces- 40 sary, butitis only suggested that a Fig.9.Responseof 111::::!!111104111111M011111111M1 quick trial be made of the idea before theSenior Compen- trol for maximum 20 passing judgment. Like many circuit andfor- 10, - 20, 111111111=1\511111111.11111111W arrangements, it is not always possible and -30 db output to pass upon their effectivenessuntil at 1000 cps. 30MI I1111011111i... one has an opportunity to listen to the 1111111111111MM11111111111111111M results, and itis this writer's opinion MII1111111111111111111111=.4M11111111111111 that "somthing new has been added" when the presence controlisturned from off to on. The effect is neither 11111111111111111116111111111: ear shattering nor eye opening, but as lo. woo IMO one approaches the optimum inper- FREQUENCY IN CYCLES PER SECOND

49 Miniaturized "Preampwith Presence" C. G. McPROUD

Simplifying the construction of this popularcircuit by eliminating the low-pass filter section, reducing the phonopositions from five to three, and employing a printed circuit panel forthe major part of the wiring.

THE "PRESENCE" CONTROL, first intro- Circuit Requirements duced to the home music system in adjunct to a modern music system. Since these pages over a year ago, has Before designing an amplifier circuit, a combined volume and loudness control it is first necessary to consider what facil- is available at jobbers, this unitwas proved to be quite popular with thoseities are to be required. A phono selected as being simple and effective. who appreciate an adequacy of control preamp is a necessity in a "front end," ofcourse, The tone control system is thesame as facilities-perhaps just that little shade so that is included. In the light of modern that used in the original "Preamp with of boost in the midfrequencies tomove LP records, however, it is not feltneces- Presence"-a circuit first described by the principal instruments or singersasary to provide as much flexibility today Baxendall, and adapted to a dualcon- little further toward the spotlight. as it was two years ago, for most recordscentric control and a single printed cir- Popular as the circuit was-and stillwill play quite well on the RIAA or New cuit unit with most of the components is, for that matter-there isno denyingAES curve. For the older 78's,a higher combined into a package not much larger that it was fairly complicated, that the turnover frequency is considered desira-than a postage stamp. two toroids used in the original modelble, together with less rolloff. In this The output section should be a cathode are expensive and not readily available instance a compromise was made-a 650- follower to provide for those installations where the front end would be located on jobbers' shelves, and itisrather cps turnover and a rolloff of 8 db at 10,- at 000 cps-which is apparentlya satisfac-some distance from the power amplifier. larger than usual for most control units In addition, it was desired that the of the present day. Coupled to that istory choice judging from listening tests. se- A much lower turnover frequency isre- lector switch should silence all inputsex- an increasing interest in printed circuits,quired for the early foreign records, socept the one in use to avoid crosstalk which are admirably adapted to produc-this curve was adjusted for a turnoverunder all conditions; and an additional tion quantities but just short of impossi-frequency of 300 cps, and the high endoutput was desired to permit feedinga ble for the average experimenter. Any-was left flat-any touching up in thattape recorder without any effect from one can design the circuit panel, but region can be done by the tone control.volume, loudness, or tone controls. getting it built at a reasonable cost is The presence control was considered Figure 1 shows the top and panel something else again. a necessity. It does not have a largeview of a completed unit arrangedto However, two of the problems haveeffect on the reproduction, but it doesplug into a National Horizon 20 ampli- been eliminated in this design. In theprovide the midrange boost that aids infier, drawing all operating power from first place, the printed circuit panelsare makingtheinstrumentsstandoutthat source, and feeding signal to the already built, and are available.' Thusslightly. This control gives a bumpon power section through an Amphenol the construction the response curve of only 5 db maxi-plug on the rear panel. Figure la shows is reduced to one of the suggested construction, a design that assembly and soldering, withreasona-mum, with some effect extending about ble assurance that the unit will workone octave each side of the resonantpermits placing the amplifier ina con- frequency. ventional cabinet, supported only by the as described when itis completed. control bushings. In this form the entire Volume and tone controls are alwaysunit is 2 in. high by 744 in. long by 4% required, and the writer believes firmly ' See last paragraph of article. in. deep. While itis probable that the in the loudness control asa necessaryunit could have been made somewhat

NAM) 5" 711 0 WIXOM/ 01/14,04 fO

Fig. la (left). The completed preamp arranged for use with National Horizon 20power amplifier. mended arrangement for use with Fig. lb (right).Finished preamp inrecom- any power amplifier, and suitable for mounting bymeans of control bushings.

50 R5 0.47tA C575C

5*100 4 PAA101:11111 DECIMAL CAPACITOR VALUES IN MP 0 CONNECT ION PO.NT ON IS 9" 033M C7 2'0 OTHERS IN IMF UNLESS MARKED PRINTED CIRCUIT PANEL RESISTOR VALUES IN OHMS,N MEG *, * * NOT DESIGNATED CS 56 DREG V3 V4 VS 99 0.66M C9 12 A X 7 12ALj7 '2 12W7

V011 P2 R17 0 IM 150., 0.1 P1 C4 CI BASS C1 0.1 3.! 6 119v PS 1,11 fl6r '000, P30 Z729 C 77,* 2 CI..'0, 53, 1c 14.004 7 KS K2 K3 104 NT J5 93 66 a. R3 L1 rile 10M :10 N § 022 C2 C C10 .virw74CLA A St NC3 3 a, '5C 0.1 SK 015 pri cc t 012 1340 cc Y-E*0r3G OGND , RA tEl 56000 192 RI9 22,000 SV/ *NNW 0**Ci6o 13°6060 10uf 10 ie

Fig. 3. Schematicofminiaturizedpreampwithpresence. smaller, it must be remembered that thePreamp with Presence. At normal op-loudnesscontrolsectionofthe dual tubes and a decoupling capacitor musterating settings of the volume and loud-volume -loudness control unit, Pie. Tape be accommodated, and that the controlsness controls the hum -and -noise level is recorder feed is connected at this point take up some space. In fact, it is recom- better than 60 db below the 1 -volt nomi-through the 0.1-meg resistor Rs. to pro- mended that the components be placednal output with the heaters operated onvide isolation. The arm of the loudness on the printed circuit panel before thea.c. With d.c. operation, the improve-control feeds the grid of the tone -control tubes and the capacitor are put in place,ment is less than 3 db, which speaks well input stage, one half of a 12AX7. The and before the controls are mounted. for the hum -free operation of the Z-729.cathode resistor is bypassed by C1. suffi- Figure 2 shows the unit of Fig. laCertain other types have been tested inciently to give flat response over the last in the construction stage. the circuit, and while the 5879 has afour stages. The plate is coupled to the low hum -and -noiselevel,itsgainis tone -control network, and this in turn is lower, so the usable range is some 6 dbcoupled to the tone -control output stage, Circuit Description less. With d.c. on the heaters, selectedthe second half of the 12AX7, which The circuit arrangement is fairly con-6AU6's perform satisfactorily, but thefeeds the volume control, Pie, and its ventional. The phono pickup is fed intotrouble of selection eliminates them from arm is connected to the grid of the out- the grid of the first tube which servesconsideration forreliable useinthe put driver, one half of a 12AU7. The only as a preamplifier. Equalization is home. presence -control action derives from the introduced by means of feedback around The phono preamp is followed by thepartial bypass of this tube through the the first tube, much in the same mannerselector switch-a four -gang, five -posi-L -C network composed of Ci and Li as used by Leak in the TL/10 amplifiertion rotary switch. Two of the sectionsand the 50,000 -ohm potentiometer Ps. control unit, which is simple, effective,are used for shorting out the unusedWhen the resistance is maximum, the and easy of adjustment. In Fig. 3, feed-inputs, the third section for changing cathoderesistoriseffectivelyunby- back is shown from the right side of C theequalization network around thepassed. As the control is turned, the re- to the grid of V:, withRsand C, servingfirst tube, and the fourth section as thesistor is bypassed more and more, but as the stabilizing load into which theactual input selector. The arm of thisonly at the resonant frequency of the feedback voltage is fed. This practicallysection connects to the top end of thetuned circuit, which is approximately eliminates the impedance of the pickup as part of the feedback network, and by varying the size of C the response may be kept flat up to 20,000 cps, if desired. Three feedback networks are shown- C5,Rs,C., andReprovide the RIAA or New AES curve;C7, R7,C., and Rs pro- vide the 78 curve and C. and R. provide the FOREIGN curve. Referring only to the AES curve, Cs and RI control the turnover frequency. while R. adjusts the low -frequency roll - off, and C. adjusts the high -frequency rolloff. While there is some interaction between these components, it is not diffi- cult to arrange practically any type of curvedesired. With the components shown, the New AES curve is accom- modated within 0.2 db. No rolloff-either high- or low-end-is provided for the FOREIGNcurve, which accounts for the use of only one pair of components. The preamplifier tube is the low -noise, high -gain Z-729 usedinthe original Fig.2. Unit of Fig.lain construction.

51 usual construction where itis always recommended that a strong mechanical joint be made before solder is applied. The components are inserted from the R15 side opposite the one on which the wir- C /0 ing is, although leads can be connected 4- from the wiring side with equal facility. 4-R13 --010 In this unit, all components are located K3 on the clear side, and are thus protected. Soldering presents only a slight problem -the principal caution being that a low - wattage iron be used. A person thor- oughly experienced in soldering should have no trouble even with a 100 -watt iron, but if too much heat is applied the copper is likely to lift from the phenolic Fig. 4. Location of ports on printed wiring panel. Unmarked panel or even to burn entirely off. With square between 1 and 2 connects a 25 -watt iron and a good multicore to grounded arms of SW,. solder, the operation is comparatively slow, but it proceeds without any possi- bility of lifting the pattern. The copper 3000 cps. The cathode follower is di-since this unit will not be available fromcoating on the panel furnished is tinned, rectly coupled to V, and feeds the out-normal outlets,itisto be furnishedand the usual procedure is to apply the put line. R. and1?,,,together withalong with the printed circuit and the Cm, and C lob serve as decoupling net- tip of the iron to the lead and the wiring sheetmetalcomponents.Anyde- pattern simultaneously. The solderis works. sired arrangement may be employed, ofthen applied and the iron rotated around It will be noted that all of the controlscourse, and the circuit may be built up the lead slightly until the solder flows are the same as those used in the originalin a conventional form. However, it is all around the lead, making a neat con- Preamp with Presence, and supplied bymuch easier to build on the printed cir- nection. Centralab. The selector switch iscom- cuit panel and is likely to be morecom- On the panel furnished, components bined with the presence control, andwas pact. Furthermore, the arrangement of are located as shown, the leads soldered used originally as the low-passfilter parts has been tested, and the unit isinto the small circles shown in Ftg. 4. switch. Both volume -loudness and bass- almost certain to work immediatelyon Connections from wires-such as those treble controls are the sameas those completion. used in the original model. from power, input, and output connec- The presence control is located rela- tions-are indicated by small squares, Construction and these are numbered. The schematic tively far back in the amplifier, and is shows thesesquares.Connectionsto consequently at a higher -level point than The construction of electronic appara- tubes,thedecouplingcapacitor,and previously, which reduces the suscepti- tus using printed circuits is very simple. other components are all indicated by bility to hum pickup and thus eliminates In most instances the tube sockets be- the small squares. the need for a costly toroid coil. The come a part of the circuit panel itself, Figure 5 shows the appearance of the coilspecified for thiscircuitispartbut for this unit it was desired that the underside of the panel after all the parts numberCS -1051,manufactured by tubes lay parallel to the panel so as to are soldered in place, while Fig. 6 shows Aladdin Radio Industries, Inc., Nash- reduce the height. The process of build- the top of the panel with the components ville, Tenn. The coilis wound on a ing is simply that of inserting the com- in place. The tube brackets are shown powdered iron core, and the entire unit ponent leads through holes in the panel, in Fig. 7, although they are supplied is encased in a powdered iron shell, with clipping them off 1/16 in. from the sur- along with the circuit panel and Li, and resulting low hum pickup anda Q offace, and soldering them in place. This the front and back panels.The order the order of 15 at 1000 cps. However,method is a complete reversal from the of construction recommended is to mount

11111B11

4,1

,r

1.1 V II 6. ynV

Z.,

_ 41111MMINE"11 1' IP- f 01111

Fig. 5 (left). Underside view of completed amplifier to show soldered component leads.Fig. 6 (right). Top viewof amplifierbefore attaching front and rear panels.

52 0.22µf,400v,molded paper CIO 10-10/450, electrolytic, Sprague TVL2750 .0211111MIIIIIPPLIIII Single81A phono jack, Cinch 1111- CONTROL III I., I., I.Double phono jacks, Cinch Iiiihn:t l 81B (2 req.) -0 nvirli n 1.0 Hy, Aladdin CS -1051 II 0111i111=Ilat`..t F. Fig.7. Phono equal- Volume -loudness control, ization and presence Centralab C2-100 control curves. I'vl, a Tone Control unit, Centra- 0 Mil 11111111111"1111M lab C3-300 -0 mini mo mow 50.000 -ohm potentiometer, 20 111111 ..111111 1111111Mas linear(combined with Swi)CentralabSPB- 3001 IONIIIIIIIMEll R. 47,000 ohms, watt N CYCLES PER SECOND R,, R,. 0.1 meg, watt FREOuENC R., R.., Ris1500 ohms, Y2 watt R, 1.0 meg, 1 watt the tube sockets and the capacitor wafertrol. The maximum range of the loudness R. 0.22 meg, 1 watt on the two tube brackets, and then as-control on the Centralab unit is 35 db, Rs 0.47 meg, watt semble them to the wiring panel. Thenwhich prevents the user from turning R. 2.7 meg, Y2 watt insert all the components and solder inthe level down to an absolute minimum R. 0.33 meg, watt with this section. In listening tests it is R. 6.8 meg, Y2 watt place, followed by the wire connections 0.68 meg, Y2 watt to the sockets. Leads should be connectedfound that with the volume control set R. to the controls and the latter mountedatitsmidposition,theoutputlevel R.., R.., R.0.1 meg, 1 watt onto the front plate before attaching thethrough a usual amplifier is about normal R 6800 ohms, Y2 watt plate to the brackets. The wires are thenfor average use, and the loudness con- R..,R.. 56,000 ohms, 1 watt simply inserted in the proper holes andtrol operates to give suitable control overR., 22,000 ohms, 1 watt soldered in place. Small holes along the the listening range. Sw,.. b.c, aSee P, Figure 8 shows the range of the tone V. Z729 high -gain, low -noise ends of the tube brackets are for leads pentode Genalex running from one "compartment" to thecontrols, together with the effect ob- VS -S 12AX7 other. tained with the Baxendall circuit when 11.-S 12AU7 The backplateisnext assembledusing the Centralab components indi- with the two dual phono jacks and thecated. The loudness compensationis 1 -9 -pinsocket,molded, single phono jack and the leads areshown in dotted lines. withshield connected in place before attaching to 2 -9 -pin wafer sockets the tube brackets; the connections are PARTS LIST then made and soldered. Because of the "Kit" Availability small space, the Z729 must be inserted C, 100 µAC silver mica,El Menco type CM -15 This circuit can be constructed in in its socket and the shield applied be- is fore inserting the 12AX7 opposite it. C, 50µf,6v,electrolytic, conventional form, if desired, but it The voltages at the various points in Sprague TVA -1100 somewhat simpler and more compact the circuit are shown in italics in Fig.C., C., C.', Cu when assembled on the printed circuit 0.1 lAf,600 v,metallized panel with the chassis parts shown. The 3 for a supply voltage of 250. The circuit four chassis parts are of 18-ga. cadmium workssatisfactorilywith any supply paper voltage from 225 to 275, and if a higher C. 750 µµf,silver mica, Elplated steel and are drilled and punched voltage were to be used a series resistor Menco CM -19 tofitthe tube sockets and capacitor should be connected between the source C. 200 µgf, silver mica, CM -15 mounting plate, as well as for all wiring and the amplifier unit to reduce the Cr 270 µµf, silver mica, CM -19and mounting holes. The printed circuit voltage to a nominal value of 250. Total C. 56 µµf, silver mica, CM -15 panelis1/16 -in. phenolic. Also furn- current drain is approximately 6.5 ma C, 1000 µµf, silver mica, CM - ished is the 1.0 -Hy. choke, which fits in at a 250 -volt supply. Filament require- 19 place on the printed circuit panel, as ments are 0.8 amps at 6.3 volts. .015 µf, 400 v, molded paper indicated.Thesefiveitems,together 300 µµf, silver mica, CM -15 with specific instructions, are now avail- Performance .0047 0, 400v, moldedable and the author will be pleased to interested readers thesources. A 1 -volt output is obtained with both paper advise volume and loudness controls at maxi- mum-bass and treble controls flat-at 420 an input of 0.09 volts from thehigh-level --.- - inputs, and from an input of 1.6 my on - phono at the AES position. The other 00 -.4.14 two phono positions give a 1 -volt output . -IIINIII...... from an input of only1 my. This is somewhat more sensitivity than usual, ob...r but with two volume controls in series 8 :gailleill...... --,.:VW11111111101.--.111bn -one the volume control and the other Fig.8. Tone control 10 the loudness control-some additional and loudness contour gainisrequired to provide sufficient curves. -r.:,...` ...... --..,, 20,'" .. ... -20_--"-' range of control. IM distortion at the `s. ______1.....__--- 1 -volt output is approximately 0.4 per ''--v-... cent, rising to 1.5 per cent at an output 's. ....102 ...... --...-`' -...... --. of 5 volts, which is more than is likely ...--''' to be used with modern power amplifiers. _t------....Qt.....

Response curves are shown in Figs. IS ICS ION tom ''''' 7 and 8, the former being a composite of FREOUENCy iN CYCLES PER SECOND the phono curves and the presence con -

53 New ApproachTo Miniature PreamplifierDesign

ARTHUR J. ROSE

Simplification of design requirementsis said to yielda small and inexpensive unit with performance justas satisfactory to the ear asmore standard products.

ASTUDY OFmanufacturers'specifi- benefit from the simplification is another, cations and of the technical litera- particular curve. That initselfsug- more difficult, matter. gests that equalizers need ture will reveal that the major In accord with this thinking-and in more flexi- differencesbetween phonograph pre- bility. flagrantviolationofconvention-a Fortunately, the type of correction amplifier designs is mainlya matter ofpint-sized preamplifier was developed styling. The general preamplifier design by the writer. It fulfills admirably theafforded by certain bass and trebletone recipe as usually followed calls for the controlscloselyparallelsthat of the purpose generally served by larger, more record equalizer. It appears that following: complex units. The reduction equal- inthe izers that do not have the flexibilityof 1.Equalizers for Recording Charac-number of parts brings the costto ancontinuous variation arounda particular teristics. uncommonly lowfigure. 2. Means for switching inputs. characteristic might just as well bere- Frequency Correction placed by a fixed referencecurve and 3. Gain control. then adjusted to suit the circumstances 4.Frequencycorrectionforroom Bass and treble controlsare usually with a properly designedseparate set acoustics,speakersystemvari- thoughtnecessarybecauseofroom of bass and treble controls. ations, and human hearing dis-acoustic and speaker -system variations. This seemingly unscientific facthas crepancies. These are unchanging once thesystem met some stiffresistance from those 5.Meansforovercomingsystem has been installed in a particular lo- who claim they must havesome place in flaws,e.g. cation. Why, then,is turntable rumble and it not possiblethe preamplifier where thesignalis record noise. merely to fix the positions of thesecon- exactly the same as that entering trolsafterinstallation and then rely the A restudy of the manner in which solelyupon recording microphone. Their pointis these goals are reached has shown that therecordequalizerto admittedly only an academicone, since much can be done toward simplification match each record type to the correctedthey also admit that this conditionhas system? Overlooking the loudnessver- little bearing on the correctness of of circuits, based on eliminating whatsus hearing difficulties for the moment, re- isreallysuperfluouseven production-particularlyiftonecon- though the answer is rather obvious: equalizerstrols are not constantly adjusted. "necessary" from the standpoint ofcon- do not adequately correct from record The vention. Once the absolute minimumto record. solution to the matter really lies with requirements have been established,it the problem of "a properly designed What record manufacturers do toset of bass and treble controls." is only a matter of fulfilling them withtheir productsis as little extravagance as possible. The purely a matter of Another point-morepracticalbut problem of educating those who willspeculation. All the user has isa re-also more subjective-lies in thefact commended playback curve. Before thethat most records worth playing through average record sounds right the listener a high -quality system in the first place * 300 Broadway, Dobbs Ferry, N. Y. must do more than just switch in thisare of recent vintage. The majority of

V2-6AV6 0 250V,SMA .

InPHONO-MAX BASS 111111 'S,..711,4 11111Ini CEI 02 '"'"'""'"''''''''' MIC MAX BASS.1111 111111 ' 4 NomRANGE'-4..SI:: PHONO .. OVERLAP AREAII . _. 111111 MICROPHONE . S§, Vi C C12 zo I JI --__...;;.-, PHONO MIN 100-500 '''' ...... ` BASS 111111 .1),/fii..!.*:fe.-s.,.'''''''Siii:V.REFERENCEEQUALIZATIONCURVE "if W t.4 sow R18-0 1MEG W' 'irliii0ii...711111111 MAXPRESETRESPONSE . .4.. OUT- t': :,.....: . _ _ 41 VIMl'a n r 4 Ell] TM PUT sg IF 4( / a'd ii ; orimmommones ---:-..1- --": ....1111PMR152allIZE TOLERANCEAREA! -- -POSSIBLE tna' IiiMill&LsRANGE OF\ 211 cc -4,1111111112N111._.CONTROL - .002 ,0 11111 11111 C13- 01 8- END . 11111 ii 111111FAMCIIIIII2MINPRESETRESPONSEIllm 1 RESISTORS 110%, 1/2 W 11111 11 EXCEPT WHERE INDICATED v. 2 CAPACITORS10%, 400W V 63V 0 11111 111111 111111 '4S EXCEPT WHERE INDICATED i 0 2 0 0 0 0 1 0 0 1000 10000 Sib4_ SHORTING TYPE MALLORY FREQUENCY IN CYCLES PER SECOND

Fig. 1. (left) Schematic diagram of the 2 -tube miniaturepreamplifier. Bass and treble controls ore controlled by concentric rangement of Fig. 3 indicates. Fig. 2. (right) These shafts as the knob ar- curves show the maximum equalization ranges of the preamplifier and the referencecurve dis- cussed in the text.

54 the current recordings are very simi- lar and there is promise that the RIAA "standard" may really become a standard! Most recordings are made with a 500 -cps bass turnover. In some cases there is bass pre -emphasis that requires a flattening of the playback curve below 50 or 100 cps. Treble pre -emphasis is usually at a fixed rate approaching 6 dbperoctave,withtimeconstants ranging from 50 to 100 microseconds. For reasons made evidentlater,the writer's miniature preamplifier uses a straight 500 -cps turnover without de - emphasis as a fixed reference curve.

To reduce the circuit to a minimum, Fig.3. As constructed by the author, the preamplifier is built in an aluminum box with this only those functions that can be justified aluminum panel on the front. The outer shaft of the concentric bass -treble control is operated as essential are included. There is only by a Plexiglas disc. a rare need for bass rolloff, especially at low and medium volume levels. Awith sufficient low bass to rival much 10 kc. Why de-emphasize itto aflat fortuitous circuit arrangement permitsmore formidable systems. In itself, this response and then boost or droopit some bass droop at higher volume set-fact shows that the needis only for ± 15 db ? The same effect can be had tings, but requires only the components improvement in circuit efficiency in most without initial de -emphasis if the sig- for boost. cases where bass is lacking, rather than nal is fed into a system with a + 5-db Need for large amounts of bass boostfor additional circuits. boost. Then, application of only rolloff at low volume levels to produce proper In accord with this trend of thought, to a maximum of 30 db will produce scaling of sounds that were originallyall functions related to the bass region the desired result. loud has given rise to loudness controls. are combined in the miniature preampli- That sounds simple enough but there Although their use has been justified fier in one circuit, with a single control are some apparent drawbacks. De-em- more by the whims of bass -crazed audio governing whatever adjustment is neces- phasizing to the extent of 30 db in a hobbyists, it is desirable to have sufficientsary. Figure 2 shows its range. simple circuit involves reducing ampli- boost available to cover extreme condi- tudes below the midfrequency region. tions. The actualloudness correction Treble Correction This occurs because a single R -C net- to be used is subject to variation depend- work has a maximum slope of 6 db per ing upon the relationship between the Treble compensation can be simplified octave. Another seeming drawbackis playback and the original levels, with in like fashion. It has been general prac- that recordings vary in the amount of due consideration to individual hearing. tice to apply 50 to 100 microseconds de - pre -emphasis, and radio inputs and some It is evident that there are a great num- emphasis in the equalizer, and then by recordings have no pre -emphasis at all. ber of possible circumstances and anyuse of the treble control apply up to 75 To justify intuition on the part of compensation fixed solely as a functionmicroseconds further de -emphasis or em- the writer,listeningtestswere con- of playback level would prove to be phasis. It is easy to see that some saving ducted using a wide variety of record- incorrect in most of them. can be made by not including de -empha- ings and program material with a con- Usingterminologywhichdivides sis in the equalizer. This entails applying siderable variety of associated equip- hearing and acoustic correctionsinto a variable de -emphasis to a response ment. In no case was there need to two functions is paradoxical. It wouldwith fixed treble boost. vary the treble response more than 15 be interesting to learn how someone in For example, a recording with50 db !It appears that all that is- required averagecircumstancescanmakea microseconds pre -emphasis initially has for most purposes is a certain maximum purely acoustic correction without bene- treble boost to the extent of 10 (lb at response depending upon the associated fit of hearing. Even sound -level meters are fitted with frequency -weighting net- works to simulate the response of the human ear. Both acoustic and hearing correction can be and are made simul- taneously, provided the maximum bass response is adequate. The similarity of characteristics used as a basis for lumping corrective meas- ures is more than just fortunate. It sug- gests to the writer that some types of correction may be imaginary and other types need stronger emphasis. Or view- ing from another angle, there can be only one ideal resultant correction curve for a given situation, and a much smaller range of variation is needed to cover most situations than would be supposed. Obtaining a large amount ofbase, boost can prove to he difficultif one wishes to reduce circuitry to a very simple form. Making allowance for nor- mal boost requirements plus that needed for low levels has been solved by the writer with a simple circuit that applies a sort of automatic correction so that theoretical limits of boost are actually Fig. 4. Interior of the preamplifier. Size can be judged by the tubes. Even though the panelis attained. This circuit has been used with just about large enough to accommodate the controls, there isplenty of space inside for com- success in supplying an 8 -in.speaker ponents and wiring.

55 equipment. Therefore,itbecomes en- inductance of the pickup. tion below 20 cps, providing a good tirely feasible to use only a treble rolloff Although one is inclined to shuddermeasure of rumble and flutter elimina- control with a maximum droop of 15 at the prospect of a 4-kc limit to re-tion. At higher volume -control settings, to 20 db in a system with an adjustable sponse, the effect is not the same unde-the balance between the two branches boost. Once set for a given equipment sirable one that would be obtained withof the feedback bass -control network is lineup, the boost adjustment need not a filter of much higher slope-say 30upset and bass droop is effected at the be altered. The range of net adjustment db per octave. Using 4 and 6 kc withextreme lower settings of the bass con- can be seen in Fig. 2. the simple, well known filter arrange-trol. Should additional droop be needed ment shown in the schematic affords fora phonographinput,theselector The Miniature Preamplifier most effective removal of record noiseswitch can be setto the microphone and distortion with a minimum apparent position. To place FM reception on a The simplifications made thus far can loss of highs. be carried out in a spectacular manner. par with recordedmaterial,the75 - To illustrate, a very satisfactory pream- Position 4 retains only the 200 -times,microsecondde -emphasisnetworkat plifier was constructed with a single flat -frequency -response amplification ofthe output of most receivers can be tube! However, two tubes are used in the first stage. The remaining two posi- bypassed. tions,5 and 6, switch the radio and the unit shown in Figs. 3 and 4 to bring Since the input stage is non -frequency- the midband gain up to a more accept- auxiliary inputs directly to the second stage. The reference equalizerisnot sensitive, maximumsignal -to -humis able level for general use. Output with effective in these last three positions. realized.With thebypassed cathode the two -tube arrangement is 2 to 3 volts arrangement, humisextremelylow. with a 10-mv input, as compared to 0.5 Bass Circuit ithcaret',wiring and appropriate volt output with a single tube (triode - regard to grounds placement (mainly pentode). Most of the burden is carried by thea matter of returning the heaters to a Atfirstglance,theschematicof second stage.Variable bass boostis separate and lower -than -signal ground) Fig. 1 may be somewhat confusing. achieved by selective feedback from plate hum can be reduced to 60 or 70 db Aside from the obvious functions that to grid. The plate circuit is loaded withbelow signal level. a closer study will reveal, there are the series arrangement of R1., RIB, and Pentode operation in thefirst stage several hidden effects derived from care- Cu. Output is taken through the treble -favors signal-to-noise ratio as compared ful component arrangement. Perhaps the control potentiometer. This configura-to an equivalent triode because of the best manner of description would be an tion forms the backbone of the circuit. low input signal. Distortion is somewhat over-all picture and then a clarification In the phonograph positions, Cu ishigher. Even so. it is a matter of arguing of the details. effective. In other positions, it is shortedtrifles. Distortion is not discernible at Three input jacks are provided. One to ground. While in the circuit, it pro-anylevel.Measurements indicatethe is for a magnetic cartridge or micro- vides the reference equalization curve.order of 0.25 per cent at the overload phone and the other two accommodate Not so readily apparent is the fact thatpoint and.05percentwith normal normal high-level, high -impedance de- this capacitor in conjunction with Ratsettings. vices. The 6 -positionselectorswitch andR1,formsafrequency -sensitive Much remains towards simplification performs the following : load on the second stage. This causesof the means of controlling sound repro- the amplification to rise somewhat withduction-particularly for home use.It Pos. Name Function lowered frequencies. Therefore, the feed- ishoped that future trendsin audio back bass control is exceptionally effi-equipment design will be to accomplish cient and gives about 6 db more boost 1 Phono 1 Normalphonograph the necessary results in a more concise position than would normally be expected. Asmanner than has been observed to date. an added result, the equalization curve approaches the theoreticalideal. This 2 Phono 2 Phonographposition PARTS LIST for moderately noisy gives additional boost with a straight - records lined 6-db-per-octave slope instead of C, .003 jcf, 400 v, paper the usual rounded and depressed curve.C. CO. CIS.01 µf, 400 v, paper There isfurther advantage to this CO, CO 25 uf, 6 v, electrolytic 3 Phono 3 Phonographposition C,. C,. 0.1 µf, 400 v, paper for very noisy records circuit arrangement. Treble rolloffisCf. C .002 uf. 400 v, paper easily accomplished, but more significant,C. 200 ma; 400 v, ceramic cable capacitance can be fully offset! 4 Microphone Position for using a C. .02 µf, 400 v, paper low-level microphone This eliminates the need for a low -impe- C 100-500 µid. adjustable inphonographjack dance output such as a cathode follower. C 400 Auf, 400 v, ceramic Theoretically, the upper response can phono pin jacks be in the megacycles if desired. R,. R,1 27.000 ohms,l/2 watt 5 Radio Radio tuner With anadditionalstep,completeR, 6800 ohms, 1/2 watt treble control is obtained. By overcom-R. 4700 ohms, 1/2 watt 6 Auxiliary Televisionsoundor R,.R. 0.1 meg, 1 watt tape recorder pensatingthecablecapacitance,the RS,R,.. upper frequency response can be boosted R,,.R,. 560 ohms, 'A watt to any acquired amount. An adjustableR., R 0.33 meg, watt Positions 1, 2, and 3 retain the ampli- trimmer Cu performsthisoperation. R 0.1 meg, 1/2watt fication of the first stage, keep a refer- The rolloff circuit is designed to giveR. 1.0 meg, Y2 watt ence equalizer in the circuit and control a maximum attenuation of about 20 db R,. 0.5-meg potentiometer,linear the upper limit of frequency response. at 10 kc. The range of control thus af- R,7 1.0-meg potentiometer,linear Position 1 yields a flat response, while R,. 0.1-meg potentiometer,linear forded is complete for most imaginable R,7 and R,. assembled from IRC 2 and 3 furnish cutoffs at 12 db per circumstances. Concentrikit components) octave starting at about 6 kc and 4 kc There are several other design fea-Sw, 2 pole 6 pos. switch. Mallory respectively, by resonating the capacitor tures worth noting. Coupling constants 3126J switched across the input grid with the are chosen to give a fairly steep attenua-V,. V, 6AU6

56 A Preamplifier Switching, and Equalizing Unit for Critical Listening

M. V. KIEBERT, Jr.

Given a "cleaned -up" output amplifier, the next step is to follow the same steps in optimizing the preampli- fier and control unit section of the complete system.

FOR THE ENGINEER and/or golden ear 1 2 3 331/3 who enjoys good music and must 70 P. U. DISC SYSTEM SWITCHING AMPUFIER and live with the distaff side of a family EQUALIZATIONE AMPLIFIER EQUALIZER LOUD notinclinedtowardstheintricacies ofequalizers,equalizersettings,and U SPEAIIER similar gadgetry, it is necessary to de- DISC EQUIPMENT sign a high-fidelity system to take into CONTROL account this point of view and enable Fig. 1. Block sche- tolerable program enjoyment without maticoftheentire personal supervision or the teaching of a system. course in the fundamentals of aural per- MIC PRE.AMP ception and audio circuits.

From this writer's point of view, a typical system is conveniently broken RADIO TAPE down into four sets of basic elements for RECEIVER RECORDER thepurposeofdesignconsideration. These are, namely, the disc preamplifier and recording characteristic equalizingtheirconsequentsensitivitytonoise istics of a combined preamplifier and assembly ; the system switching and en-and hum pick-up. Both of these latter recording characteristic equalizing as- vironmentalequalizingfacilities;the functions are logically located in thissembly which is combined with switching power amplifier; and the loud speaker control unit, "nerve center" position. and enviroment equalizer facilities. The and associated acoustical system. These The loudspeaker and its enclosure-final unit was engineered for installation are shown in the block diagram of Fig. 1.which constitute a complete acousticalin a coffee table, along with the radio Of these four elements listed above,system-together with the power am-receiver and both33-1/3and 45 r.p.m. onlythepreamplifierandrecordingplifier. which in this particular applica-turntables. The power amplifier, micro- characteristic equalizing assembly andtion was a modified Williamson, are ofphone preamplifier, and magnetic record- the switching and environmental equal- essentially straightforward design or maying equipment, aswellas the loud- izing facilities pose a major audio de-be purchased units of basically linearspeaker assembly, are separately located sign engineering job which is ordinarilycharacteristics and accordingly are as-for convenience. under the control of the designer. This sumed to require no additional equaliza- A separate power supply forthis design problem is exacting due to thetion, nor to require other than an essen- "nerve center" and the use of an output tiallyflatfrequency characteristicof transformer provide for circuit isolation, low levels involved, and to the fact thatgood transient and intermodulation per-thus avoiding stray circulating ground the switching controls and equalizing formanceinordertoprovidesatis- currents in the audio system. Residual functions are most easily accomplished factoryreproductionofthe complexhum and noise are therefore consider- at relatively high impedance levels with fed into these two elementsably below the threshold noise level of a of the system. quiet room, even when listening within This article outlines the design, con-one foot of the speaker. * Applied Research Inc., Chicago, 111. structionand performance character- The final unit more than meets the

24 20 20 tri. Bo 16 , COL 78 16 ",*..A "AL(HIES - +12 .., 12 RCA 78 rie I;...."'"."...... 8 *8 .ot nolooN .04 *4 .....c..2:..,..e DECA 76 .64 ...... * ..---,.' db0 ."'--,"fr. ....Dm OD 0 .

....

e ., 2 1E . i,./... Oil -16' /1/ 16 --....ifr...... NAB ciecti4' e 20 el AES1' -2° IEc .4 20 r My, 100 1077 IMOD 10000 low i0.20l0FREQUENCY IN CYCLES PER SECOND FREQUENCY IN CYCLES PER SECOND Fig. 2. Recording characteristics for which the preamplifier was designed. (A), left, those used by leading manufacturers of 78-r.p.m. records, and (B), right, those used for transcriptions and LP records, including the AES curve.

57 poorer IM performance, which is prob- ably a result of the greater non -linearity oftheirbias system. Construction Figure 3 shows the completed unit. The controls from left to right are as follows: Input selector Sw, (magnetic Fig. 3. The complete recorder, , 45-r.p.m. rec- chassisofthepre- ord player, and 33-1/3-78 transcription amplifier and equal- table) ;record -equalization (record - izer control unit. type) control; Sw,, loudness control ; low -frequency environmentequaliza- tion; and high -frequency environmental

equalization. The power supply for this unit, as VI( shown in Fig. 4, has been built on a at MI separate chassis in order to permit its M 11111111 location separate from the other unit. In assembling components for use in original design requirement of a systemof these types of discs are shown at (A) thepreamplifierassembly, shownin amenable to technically correct operation and (B) in Fig. 2. It is to be noted thatschematic form in Fig. 5,it was found by unskilled personnel, while still pro-while specifications and much of thenecessary to utilize a bridge in order to viding adequateflexibilitysuchthat literatureindicate that the transitionselect the correct values of capacitance additional and adequate high- and low -from a constant amplitude to a constantfor the particular value of equalizing frequencyenvironmentalequalizationvelocity curve is a discontinuous func-feedback resistance utilized. Ordinary, was readily available to satisfyindi-tion,actualrecordingcharacteristicsstock type capacitors run approximately viduallistenerpreference and/ortogenerally indicate coupled systems with± 20 per cent tolerance, while ± 10 per compensate generally for various roomonly one degree of freedom with the at-cent units appear to invariably run right acoustic anomalies. tendant curves as shown at (A). Obvi-to the limits of the tolerance and ac- cordingly, the importance of utilizing Design Criteria the correct values of capacitance anu In considering this system,itwas resistance in the equalizing circuits can- believed to be desirable that only one not be too greatly emphasized if reason- switch setting should automatically pro- ably good compensationisto be ob- vide the proper and optimum low- and tained. Mica units of ± 5 per cent tol- high -frequency compensationrequired erance have, however, been generally for most discs, and that the midpoint found to be very close to the correct setting of the environmental equalizer value. controls should provide a system which The other components of the system would have a "flat" over-all frequency are not critical except that special atten- characteristicatmaximumvolume .1.46;060 tion should be given to the plate resist- setting with automatic compensation for ors in thefirst stages to insure that the Fletcher -Munson curves at lower Fig. 4. Power supply used for the preamplifier these units are of the "low -noise" type volume levels.It was desired that the unit. and that the resistors used in the feed - environmental equalization be capable hack equalizing circuits have a negli- of either boost or attenuation of at leastously and fortuitously, such curves aregible voltage -resistance coefficient. 20 db at both the high and low ends ofmuch easier to compensate than those Relativetolow -noise -leveland/or the audio spectrum in order to provide which are discontinuous. low -voltage -coefficientresistors,there forpersonalpreference and/or area In order to secure optimum signal-to- areseveralpointsalltoo frequently balance. Due tothefactthat mostnoise ratios with low-level pickupsit overlooked by the casual audio experi- naturalphenomenadonotchangewas decidedtoutilize approximatelymenter-points which might wellbe abruptly, and based upon many listening20 db of gain prior to any recordingoutlined at this time. tests of various types and slopes of net- characteristicequalization.This was Forminimumnoise,wire -wound works, the environmental equalizationdone in the first stage which, however,units are always quieter than composi- was based upon 6 db per octave (singledoes have an output selector (Sw,) intion units, but before using wire -wound degree of freedom) network swingingorder that higher level pickups, such asunits in feedback circuits, make certain around a mid -frequencyofapproxi-the Pickering and Clarkstan units, maythat they are non -inductively wound and mately 1000 cps and more than criticallybe used through the same equalizingin fact have a negligible reactance at damped in order to avoid difficulties. stages. Use of a pre -preamplifier withfrequencies up to 100 or 200 kc-other- Inordertoprovide the optimuma bypassed cathode resistor also helps wiseexcessivehigh -frequencyphase recording characteristic equalization, it the hum and noise problems which ariseshift will cause trouble. was necessary to survey the recording duetoheater -cathodecoupling.An In the present assembly a wire -wound, industry both here and abroad, andequalizing preamplifier frequently rec-1 -watt unit used in the plate circuit of secure the recording characteristics ofommended for this application gives risethe pre -preamplifier improved the sig- each of the most widely used types ofto spurious hum and noise due to thenal-to-noise ratio by 18 (lb when this records. Accordingly, an investigationunbypassed resistor in the input stage.unit was substituted for a TA -watt com- was madeandreasonablyaccurateWhen using the RPX040, RPX041 orposition resistor-yet the composition recording curves were made availableRPX046 pickups, G.E. recommends theresistor was being operated at only 20 from the major recording studios asuse of a 6SC7 with grounded cathodesper cent of its rated dissipation. follows: and work -function bias, with equaliza- Composition resistors used incur- EMI 78 (British Parlophone, Colum-tion accomplished by means of a RCrent -carrying loads such as plate cir- bia, Brunswick, HMV, etc.) losser network between the preamplifiercuits or unbypassed cathodes should be Columbia 78 (U. S.) stages. This recommendation was prob-carefully suspect of being noise sources RCA 78 ably based on their findings relative toin low-level circuits. Where space and London, Decca 78 unbypassed cathode resistors. However,miniaturization appear todictate the Columbia LP, N.A.B., and the rec-test of the G.E. configuration as com-use of composition units, use 1- or 2 - ommended A.E.S. curve pared to the present unit or other unitswatt units, even though the dissipation The recording characteristics for eachof more conventional design, does showrequirement is only 0.1 watt or even

58 0 RCA 45 0 REK -0 -KU 33 93 - 76

VI & V2 12AY7 vs 1/2 12AY7 SHIELD 01 0.15 Moo - LOUDNESS CONTROL VS I RC orLIVINGSTCN " 1/2 I2AY 7 0

UTC A-26 7500n. TO 500'

1.O T

150

AIR DESIGN 00800 25 1.000 FROM Mit. 060594 GO MA 2%o PRE AMP LI 4--(2 4 40 0A2

0 1 3 2A IA Ne4 ;:k PILOT LAMP AA DESIGN TI 50104 VI-2 POWER CONTROLLED 00V1ROUED SWITCH INPUT OUTLET OUTLET CKT FOR 0.P.S.T. 115V FOR RADIO FOR MONO MAIN AIVPL. SWITCH SYGIO SW4

Fig. 5. Over-all schematic of the preamplifier and control unit together with the power supply shown in Fig. 4. less. With these units used in place ofdividing potentiometer and while themust likewise be carefully selected. Mica the usual 1/3- or 1/2 -watt units, it isassembly may not have too good a volt-units or other high -quality types should frequently possible to pick up an extraage -coefficient characteristic, no appre-be used. The miniature, metalized paper 10 to12 db of signal-to-noiseratio. ciable difficulty will be experienced be-types are frequently found to be quite Where appreciable d.c. currents are notcause they have a relatively high valuenoisy when used in low level circuits. normally present as in the case of theof minimum bridging impedance-about It will be noted that the output of the grid resistor and where the a.c. signals105,000 ohms as compared to the ap-recording characteristic equalizing am- are not large, then the smaller IA -watt proximately 10,000 ohm output imped- plifiercontains an auxiliarylow-fre- composition units are satisfactory. ance (not considering equivalent outputqency equalizing network to give a last Infeedbackcircuitssuchasthatimpedance duetofeedback)ofthefine touch to the low end of the record- foundinthepreamplifierrecording 12AY7 output stage, across which thising characteristic compensation curve, characteristicequalizingstage,care divider is placed. Were the 4700 -ohma point which is all too often overlooked. should be taken to see that the feedbackcathode resistor a wire -wound unit and The output cathode follower circuit resistor and the associated cathode re- the feedback resistor a composition unit,employs a bridging and isolating trans- sistor are of the same type of composi-transient signals would be badly dis-former in order to avoid the circulating tionmaterialifspace or componenttorted-a point easily established whenground currents normally encountered availability limits their items to compo-intermodulation tests are made on suchin single -ended outputs which usually sition elements. Under such a conditiona system. give trouble in providing hum -free in- the two elements will act as a voltage Thelow-levelcouplingcapacitors put to the power amplifier when this ,..... unit is located an appreciable distance

away from the preamplifier. A bridging +24 / N type of transformer is used in order MAX.. N.[ AeV4.'"x" +20, BASS 7 ' TREBLE _ that it may be terminated in a 600 -ohm BOOST or BOOST load without adversely affecting the dis- +16 s tortioncharacteristics of the cathode

+12 Ar------;*1- follower. It is interesting to note that +e the 300 -ohm output impedance of the '\ cathode follower can deliver 4 volts +4 Fig.6.Curves show- r.m.s. into a bridging load with the IM _ _ _ 0 .4e 4."64111._ ing limits of equaliza- kept to below measurable level. When tionof the bass and this same stage is loaded by 300 ohms, -4-- -/ trebletonecontrols. then the output at 1 volt r.m.s. was 8 found to have over 5 per cent IM dis- MAX. . / -12 __MAX.TREBLE tortion. BASS 7/-* -16 ATTER ATTER. . The pre -preamplifier and recording characteristic equalizer amplifier, along -20 /10' -24,,, with all associated coupling capacitors . and resistors have been mounted in a no 1000 .160 N." FREQUENCYIN CYCLESPER SECOND sub -assembly and this assembly shock -

59 mounted from the chassis with flexible leads for vibration isolation. This unit will be noted in the upper leftcorner of Fig. 3. All the recording characteristic equal- Fig. 7. Response izing resistors and condensers have been through equalizer mounted on Sw, with flexible leadsat section with controls B and C extended to the shock -mounted centered and with pre -preamplifier and equalizing ampli- loudnesscontrolat fier. at maximum level The "touch-up" low -frequency equal- position. izing network between points Dand E has been placed on the bottom of the chassis under the pre -amplifier sub-as- sembly. Care was taken in wiring the unitto insureasingle -pointground which would be located at the lowest signalnoise level more than 80 db below thisthere is also ample margin between op- level point. All input circuitswere care-level. The overload point with the pre- erating point and the overload point of fully isolated from ground with insu-preamplifierswitchinthe maximumthe environmental equalizer section of lating washers used on the unbalancedgain position was at 80 millivolts r.m.s., this unit when it is noted that the max- input circuits. The actual ground pointwhich indicates that there is more thanimum level feed into this section will employed the chassis negative electro-an ample overload margin when a G.E.normally be only 1.215 volts r.m.s. lytic bypass capacitor point whichwaspick-up is used under this condition of Under normal conditions of operation, located immediately and convenientlyoperation. The overload point in the below the pre -preamplifier and record- a 15 -millivolt input signal as typical of minimum gain position,isof course, ingcharacteristicequalizer -amplifier a G.E. pickup, will provide a signal of sub -assembly. approximately 0.8 volt r.m.s. under theapproximately 0.55 volts r.m.s. across Care was also taken to insure mini-same conditions of operation. the 600 -ohm output of the preamplifier, The environmental equalizer section mum capacitance to ground of the inter- under which condition of operation a stage coupling capacitors betweensec-of the circuit was similarly tested, both signal-to-noise ratio of better than 80 db tions of V, and V, by mounting theseat maximum boost andattenuation. is readily obtained. This level appears two units on insulating stand-off pillarsAgain by careful parts layout it wasto be more than adequate as an input just back of the low- and high -fre-found that the noise level was more thanfor a magnetic recorder and most con- quency environmental equalization con-80 db below the normal signal level. Theventional power amplifiers. trols. specific value was not measurable due From past experience, it was believed to the fact that the vacuum tube volt-to be essential that the excellent system Performance meter available for these tests could not read a lower level. performance should be obtained regard- The environmental equalizer sectionless of tube selection. Twenty 12AY7's Figures 6, 7, 8, and 9 show the actualwas checked for overload characteris-in various combinations and of various reproduction characteristics, and con-tics and output voltage, both under theprevious operating histories were eval- firm the fact that the recording charac- uated. Thanks to the type of circuitry, teristic conditions of a terminating load and a equalizingcircuitsprovidebridging load.In thefirstcase,15.2 neither the gain, levels, nor frequency complementaryequalizationforthevolts r.m.s. input appeared to be theresponse changed more than 0.3 db re- curves of Fig. 2. overload point, which gave 5.0 volts gardless of their combination. Each element of this system was care-r.m.s. into a terminating load, or 6.7 The writer wishes to acknowledge fully investigated as to signal-to-noisevolts r.m.s. into a bridging load. With ratios and overload characteristics. and credit George Beggs, Jack Shoup, maximum boost and utilizing a 100 cps Sherman Fairchild and Paul Landaman The pre -preamplifier and associatedsignal as a reference, the overload point for many of the ideas and suggestions disc -equalizing amplifier, for example,was 10 volts r.m.s. input, which gave with a 15 millivolt embodied in this unit and for suggesting (r.m.s.)input at2.55 volts r.m.s. into a terminating load,the rather wordy descriptions of several 1000 cps provides 1.215 volts r.m.s. out-and 4.5 volts r.m.s. into a bridging load. design points often presented in the lit- put to the loudness control with the From the above, itis apparent that erature of this new field.

+24 +20 - +20t.i.1

+12 +12 '... +8 +8 :I".

+4 II +4 4.,.. 0 0 EMI 4 _____ ...... 4

12 -12 4..N...... , ...AM 16 _ __ -16 4., -COL. 20 -20 - 24 TO , . . im now TO ISO i one 10o FREQUENCY IN CYCLES PER SECOND WOO i0000 100°° FREOUENCV IN CYCLES PER SECOND Fig. 8 (left). Measured playback characteristic for LP records. Fig. 9(right).Measuredplaybackcharacteristicatvarioussettingsofthe controlsfor78-r.p.m.records.

60 External and internal views of the author's self -powered control unit. A Low -Distortion Tone -Control Amplifier

W. B. BERNARD, Cdr., USN*

A well -designed "front end" which should provide sufficient flexibility for anyone, since the curves can be tailored to fit by judicious selection of capacitors and resistors.

OVER TIlE PAST FEW YEARS the author amplifier of (D) resulted. As the sliderwhere a tube is required to operate at has been working to develop an in-on the treble control is moved to the left20 db above the desired output level in put amplifier and tone control cir- the amount of high -frequency feedback is order to allow for a 20 db boost at high cuit with fewer than the average number reduced and therefore the gain of theor low frequencies. This excess level of faults. This article is a story of theamplifier is increased at these frequen-adds considerable unnecessary distortion evolution of such a circuit and Fig. 1 iscies. If the slider is moved to the right so the next step was to eliminate the nec- the family tree of the circuit. the amount of high -frequency feedbackessity for operation at such a high level. The circuit shown at (A) in Fig. 1 is increased thus reducing the treble gainThe circuit shown at (E) in Fig. 1 was of the amplifier. designed to produce all compensation but is a widely used circuit which provides All the foregoing circuits suffer from for boost and cut for both bass and treble. bass cut by variation of the feedback cir- There is no interaction between the two one fault. At the midfrequencies therecuit. Because of the practical difficulties tone controls but the circuit requires thatare one or more points in the circuitof obtaining bass cut by variation of three grids be operated at low signal levels with the attendant danger of hum and at high levels the three triodes with- out feedback are likely to produce more distortion than the output amplifier that follows them. The circuit of (B) is a simplification using one high -mu triode. Its only ad- vantage is simplification since the dis- IP; tortionproduced was quite highas shown in the author's article on distor- tion in voltage amplifiers1 and the low - (A) (B) (C) and high -frequencycontrolssuffered from interaction. BASSCUT To alleviate these difficulties the cir- cuit of (C) in Fig.1 was developed. 1II The inverse feedback reduced the dis- i tortion to a reasonable level and the re- sistor between the sliders of the bass and treble controls minimized the interaction of these controls. In general the circuit of (C) was a great improvement over anything previ- tilij killig ously tried but it was considered desir- I tag able to separate completely the bass and treble controls so the treble control was moved to the feedback network and the TREBLE TREBLE TREBLE (0) (E) (F) * 4420 Narragansett Ave., San Diego 7, Calif. Fig. 1. Six steps in the development of the control unit described by the author. Each of the ' AUDIO ENGINEERING, Feb., 1953. circuits from (A) to (E) had some faults; all are presumed to be eliminated in(F).

61 (A)

f-

B )

Fig.2.Using a variableresistor and fixed capacitors in a tone control gives curves like (A). By changing capacitors, leaving the re- sistor fixed, variable -turnover curves may be achieved, asin(B). feedback itis accomplished by an RC circuit connected to the output of the amplifier. For the sake of illustration separate bass boost and cut controls are shown but itis of course desirable to combine these on one shaft. The circuit of (E) was a step forward but it still did not seem to be the opti- Fig. 4. Over-all schematic of the tone -control amplifier, combined with ordinarypreamplifier. mum design. The requirements for a voltage gain of 10, tone compensation of put impedance of the low -mu triode is have the same turnover frequency and 20 db, and about 20 db of negative feed-quite low and it is further reduced by the initial slope as shown at (A) in Fig. 2. back, were not compatible with the oper- inverse feedback so any length of output By changing the capacitor and leaving ation of the high -mu triodes with reason- cable within reason will have no appre-the resistor fixed we get curves which able loads. These difficulties were over-ciable effect on the high -frequency re-have the same slope but different turn- come in the circuit of (F). Here a pen-sponse. A calculation shows that underover frequencies as shown at (B). Only tode voltage amplifier is used for the first the worst condition (maximum trebleby varying both R and C canwe obtain tube and a low -mu triode is used for theboost of 20 db) the output impedance of the curves with different slopes which is second tube. the amplifier is about 600 ohms. This the result usually desired. This makes it This circuit fulfills the requirementsgoes down to less than 100 ohms if flatdesirable to use a step -type control for listed in the paragraph above and hasresponse or treble cut is used. These im- treble boost and cut although in thiscase additional advantages. The pentode gridpedances are as good as or better thana single potentiometer would serve. By which is fed from the volume control haswould be realized by a cathode followera special arrangement of switch wafers a very low input capacitance when com-using the same tube. the resulting assemblies are quitecom- pared to a triode where the Miller effect The fact that a single potentiometerpact. The maximum compensation and has to be considered and the applicationcan not be used for both bass boost andone intermediate curve are shown in of the inverse feedback further reducescut may be considered to be a disadvan-Fig. 3. this input capacitance. As a result, the tage but the use of a step -type control also Figure 4 shows the diagram of the major portion of the capacitance load on has some advantages in that each step cancomplete unit which was constructed. the volume control comes from the wir-be designed to give any desired curve up The phonograph preamplifier section is ing and consequently there is no problem to the maximum of 6 db per octave cor- conventional and will not be described in of loss of high -frequency response at mid- rection. A variable resistor with onedetail. In addition to the three choices of settings of the volume control. The out- fixed capacitor gives curves which all low -frequency turnover compensation,a sharp -cutoff filter as described by Pick- ering' is incorporated at the input of the preamplifier. The values shown are for a GE cartridge. A 10 -cps high-pass filter is incorporated at the output of theam- 20 plifier to reduce turntable rumble and to prevent low -frequency transients from jo unduly disturbing the power amplifieror Fig.3.Limit curves causing speaker cone excursions ofun- from the author's desirable amplitude. Such transientsare la 0 unit, with one inter- ID prone to occur if a radio tuner which is mediate curve shown operated into the system is tuned from to indicate trend of station to station with the volume control $0 -:- ----r----"-".- control operation. turned up to a reasonable level. It may be stated as a general rule that there is no 20 reason to feed into the main amplifier any (Concluded at bottom of next page)

PO )0000 iao ION I 00OP 'N. C. Pickering, "Effect of load imped- FREQUENCYIN CYCLES PER SECOND ance on magnetic pickup response," AUDIO ENGINEERING, March, 1953.

62 Record Improvement with H -F Cut-off Filters ELLIOTT W. MARKOW

Simplifying the design of practical low-pass filters for use with sound reproduc- ing systems to eliminate unwanted high -frequency distortion and record scratch -with a few hints as to the constructionof asimple and usefulunit.

CONSIDERABLE EMPHASIShasbeen treble boost difficult for the ear to take

placed on the design ofsuitable even though the brilliance and "pres- 5 circuits and equalizers to compen-ence" of the recorded reproduction has 0 sate for the original amplitude levels ofbeen improved and the recording ob- CURVE El recorded music. One very important viously has highs which could profitably :A 5 factor which has received comparatively be used if distortion and noise factors 10 little emphasis is the advantage of being were removed. 15 able to cut off at a certain frequency. The solution to this apparent dilemma z Infact,inabilityto do this actually is obvious and is well known :use a 2° nullifies to a great extent the benefits sharp cut-off filter to eliminate all fre- which derive from equalization. Com- quencies above a desired cut-off fre- ES 25 mercial equalizers and preamplifiers are quency-usually determined by the ac- IOW 10010 available which provide quite adequate tual content of the record or program. FREQUENCY IN CYCLES PER SECOND bass and treble equalization, but few This is rarely provided in commercial which provide the additional refinement equalizer -preamplifiers designedfor Fig.1. Cut-off characteristic of a single -sec- of sharp cut-off. home use but is very necessary to get tionfilter designed for cut-off at 5000 cps. It is commonly known that one of the the utmost from recordings. An elabo- (Airepresentstheresponseofthefilter most important factors in the pleasing rate well equalized system that lacks using values of R and L given by Fig. 3;I BI reproduction ofallreproduced sound this necessary feature is still incomplete. responseforvalues ofR reduced to 2/3 of is the lack of distortion in the extreme the Fig. 3 values or L increased by a factor of It is quite important to emphasize the 1.5. upper and lower ends oftheaudio words "sharp cut-off." If sharp cut-off spectrum. This distortion is particularly tent. None of the mathematics or theory is not provided the circuit provides es-involved will be discussed; just presen- noticeable and most common in the up- sentially only roll -off, which does not per end ofthe spectrum, andit tation of design curves and information. is approach the effectiveness of sharp cut-Filtersof thistype do not come as usually here that the present day repro- off. Most so called "scratch filters" pro- duced music contains most distortion cheaply or as easily as simple resistance- vide only a 6 db per octave roll off andcapacitancenetworks. The necessary and noise. This is particularly true of are not recommended for a high -quality recordings parts will cost somewhere around $15, ( and broadcasts of record- system. but this seems small,in the writer's ings) because of the high noise factors It is the purpose of this article to em- and harmonic opinion, compared tothe results ob- distortionswhichare phasize thedesirabilityofadding atained. overemphasized when any attempt at sharp cut-off filter to an otherwise well trebleboost Proper cut-off makes many recordings ismade. Thesefactors, equalized system and discuss the designof normally poor quality sound surpris- unfortunately, combine to make adequate factors involved so that those interestedingly good and quite enjoyable. It will can add a suitable filter to their presentnot remove distortion occurring in the * P. 0. Box 145, Silver .Spring. lld. equipment or builditinto new equip -tran,,nii..ion hand hut itwill remove a

The photos show the appearance of the The IM distortion vs. volts output is finished unit, which is built in a 7 x 12shown in Fig. 5. This curve was taken LOW -DISTORTION x 3 chassis. A sub -chassis of 3/32 al- with 5 feet of shielded cable connected to uminum is fastened inside the chassis tothe output. Although a very long cable AMPLIFIER hold the tube sockets and power supply may be used without affecting the high - components. frequency response the plate current re- (from preceding page) 3.0 quired from the output tube to produce a given high -frequency voltage isin- 20 creased and with the increase in current comes an increase in distortion. Since low -frequency signal that the speaker 10 the power of high frequencies in the cannot handle. 608 usual program material is quite low the In many cases the low -frequency coup- 06 greatest difficulty in the use of a very ling afforded by supplying the preampli- 05 long output cable will likely be hum volt- fier plate power from the output amplifier 04 ages. is sufficient to make the whole system 03 This article is intended to be a general just marginally stable, or even unstable, 02 discussion of the principles behind the so the power supply for this unit is self design of a class of circuits and not a contained. This independent power sup- step by step constructional article so no ply also makes the whole unit much more 01 dimensional drawings are included. It is versatile. The power transformer is a 40 - 5 10 15 20 25 30 thought that the amplifier pictured will or 50 -ma shielded unit designed for use RMS OUTPUT VOLTS fill the usual requirements' found in high- in TV boosters. This transformer is op- Fig. 5. IM distortion curve for outputs in excess fidelity home music systems and that the erated into a voltage -doubler rectifier of 8 volts rms. Distortion below this valueis general principles outlined will be useful and an RC filter. almost unmeasuroble. in a wider field.

63 lot of noise without removing a notice- medium- or low -gain triode amplifier or able amount of music. Hi-fi extremists FILTER PROPER -.I cathode follower and still have reason- will argue that this is not high fidelity, INPUT. R Cheractensfic I L R FL able values of inductance and capacit- but it is far better that the sounds please Impedancercify, f ance if certain factors are considered. the esthetic sense than that a few extra Because of the loading effects of the cycles, with attendant noise and distor- filteritis almost impractical to insert tion, be reproduced to satisfy a theoreti- OUTPUT the filter in the plate circuit unless the cal curve. The pros and cons of this LOAD, Charoctensfic load resistance of the tubes is very low, subject have been well covered in nu- Impedonce so if at all possible it is much more pre- merous other articles and need not be ferable to operate the filter from a cath- elaborated upon here. Let us simply say that personal preference dictates your ode -follower stage where loading effects Other relation hips can be practically eliminated. Where course of action.In every case your this filter can logically be inserted in R Fc A I 5 non -technically minded listener-those an existing system is a matter of avail- listening just for the sake of the music- able gain, tube line-up, and physical will invariably prefer a reasonable cut- t 2.RYLE s 2 layout. Itis best designed into a new off on imperfect sources. 0 of inductance L must scceod 20 of 5.000 cps preamplifier and fed from a cathode Hereisanimportantpoint :AN follower, but it can be added to most EQUALIZED SYSTEM WHICH CUTS OFF AT Fig.2.Single -section low-passfilterwhich existing amplifiers particularlyifthe 5000 CPS AND WHICH DOES NOT OTHER- provides a slope of 22.6 db per octave attenua- preamplifier and power amplifier are WISE INTRODUCE DISTORTION WILL SOUND tion beyond cut-off. connected through a cable. There are ad- OF SURPRISINGLY HIGH QUALITY AND strict relationship between these valuesvantages in operating at as low an im- WILL BE ESTHETICALLY VERY SATISFYING. and the value of inductance required. A 7000 -cps system will sound somewhat The input and output resistances arepedance level as practical and this usu- better ;afull10,000 -cps system will ally means operating from a cathode commonly known as the characteris-follower output. Hum pick-up and prac- sound uncannily realistic and will ap- tic impedance of the filter. It is around proximate the "presence" of a live per- tical values of capacitance are the major these values and the desired values ofreasons for using a low impedance. With formance. Anything above this comes cut-off frequency that thefilter com- dearlyand,withnormallyavailable a toroid coil the problem of hum pickup ponents are determined. Fortunately theis avoided. Itisquite important that music sources, very infrequently. components need not be of close toler- ance. the filter characteristic impedance pre- Filter Design sent a negligible load on the driving Without going into a lot of mathe-source, and as a rule of thumb the char- The amount of cut-off provided by a matics, one very practical circuit fromacteristic impedance should be a mini- single -sectionfilterdescribed hereis thestandpointoftransientresponse, mum of about 10,000 ohms. shown inFig.1.The same general sharpness of cut-off, and reasonableness There are several approaches to the of component values is that shown inselection of component values. Exami- shape and amount of cut-off per octave Fig. 2. With correct selection of com- prevails regardless of design cut-off fre- ponents this will provide about 22 dbnation of Fig. 3 will show the relation- quency. Experience seems to indicateof attentuation per octave with only 3 ship between inductance L and charac- that cut-off frequencies of 5000, 7000, teristic impedance R. In the usual case db of attenuation at the design cut-offthe filter is designed around a commer- and 9000 cps are adequate for any home frequency. The precise sharpness of cut- system, and for practically all available off is, to a slight degree, a function ofcially available value of L, chosen to recordings.Itis operate from some arbitrarily chosen unfortunately much the "Q" of the inductance L but for characteristic impedance. Values of in- easier to design a filter on paper than values of Q above 20 there is very littleput and output resistance and capacit- to execute it in terms of actual equip- increase in cut-off sharpness. Morecom- ance are switched as necessary to change ment. A filter is not a complicated de- plicated circuits will give sharper cut-offthe cut-off frequency and the character- vice, but in order for it to work properly but require additional components andistic impedance. If we restrict ourselves itis quite fussy as to operating condi- usually have undesirable transient char- to reasonable values of characteristic tions-in particular the input and out- acteristics. The design formulas for thisimpedance and tuning capacitance, as put impedances seen by the filter section filter are also given in Fig. 2. Figures 3can be determined from Figs. 3 and 4, and the relationship between thein- and 4 show the relationships between it will be seen that the useful range of ductancein the filter and these imped- characteristic impedance R, inductanceinductance valuesat5000 -cpscut-off ances. The input and output resistances L, and capacitance C. varies between about 200 millihenries "seen" by the filter section should be This filter can be put in either the and 3 henries, all commercially available equal ;furthermore thereisa rather plate circuit or the cathode circuit ofany values. Characteristic impedance varies

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Fig.3. (left). Chart showing relation between inductance Land characteristic impedance R.Fig. 4) right). Chart showing values of capacitance C for indicated values of inductance L inFig. 3.

64 between about 10,000 and 100,000 ohms. is used in a moderately low impedancetance; however at these high impedance Suitable inductors are available fromcircuit, therefore its capicitance mustlevels the capacitance of the circuit and several manufacturers in a largerangebe large if the bass response is to beinductance itself becomes an appreciable of values. One value of inductanceLadequate. For a 3-db loss at 50 cpspercentage of the tuning capacitor and chosen to operate at the desired imped-20,000 -ohm circuit, C,. must be 0.17 itf.must be considered, particularly for the ance level can serve for all three fre-By the selection of the correct value for9000 -cps filter. quencies by merely switching resistorsC,, bass response correction to corre- Because of the filter, there is an in- and capacitors as illustrated in Fig. 5.spond to high -frequency cut-off can besertion loss which becomes noticeable The inductor should havea Q of at accomplished. Itis well known that aas a difference in volume level as the least 20 at 5000 cps. It willcost some- system sounds best when the upper andfilter is switched out of the circuit. To where between $4 to $10 dependingupon lower frequencylimitsare bothre-compensate for this difference in level value, type of winding, and shielding. stricted so that the products of the limitsthe use of resistors R and R,. are sug- Toroidsare recommended,especially equals approximately 400,000. As angested. The insertion loss will be about for high -impedance circuits. example, if the treble response extends6 db so that ratio of R. and R, should to 10,000 cps the bass response shouldbe about 1to 1.If R, is chosen to be Practical Filter Example extend down to 40 cps for the systemabout 33,000 ohms, R. will also be 33,000 to sound right. If bass is restricted theohms. Since this is the value for R the A typical circuit is illustrated in Fig.system sounds "tinny.-If treble cuts 5. Values of capacitor C switch is wired so as to connect to R, may be taken off at 5,000 cps and bass continues flatfor positions a and b. This amount of directly from Fig. 4. Values of input to 40 cps the system sounds "boomy."loss may need some adjustment depend- resistance are not critical but selection The rule is that the product of bass anding upon your personal preference re- should be made on the low side of the treble cut-off frequencies should equalgarding relative loudnesses with and design value rather than exceed the de- 400.000 approximately. Therefore, for awithout filters. sign value. As an example of how this system which cuts off at 5000 cps the filter was calculated, suppose a filter is bass should also cut off at 80 cycles. One other point of importance: with required to operate in the cathode cir- The way to accomplish this partiallythe input and output impedances given cuit of a triode, as in Fig. 5. The input is to select C, so as to be just largeby Fig. 3, a treble boost of about 2 db impedance of the cathode circuit is usu- enough to pass 40 cps (with only a 3-db is introduced to give slightly flatter over- ally given approximately by the simple loss) with the values of characteristicall response up to the cut-off frequency. equation Z =1/G.; however, it should impedance chosen for 9000 -cps cut-off. It is fairly important that some form of be remembered that this conditionis Using the example of Fig. 5,if C isroll -off equalizer precede thefilter to true for an unbypassed cathode resistor chosen so thatits reactance at 40 cpsreduce this slight peak. If no equalizer and any capacitance inparallel with equals 33.000 ohms the attenuation willis used the values of inductance as speci- and approximating the value of the be 3 db. This same value of Ce will how- fied by Fig. 3 can be increased by a cathode resistor will change this rela-ever provide an attenuation of 3 db atfactor of 1.5 (or the characteristic im- tion. The characteristic impedance of the 100 cps (reactance equals 18.000 ohmspedance reduced) to give a somewhat filter should therefore be higher than at100 cps)ifthe characteristic im-smoother roll -off at frequencies before the cathode resistor to avoid by-passing. pedance is 18.000 ohms as for the 5000 -cut-off. In either case the cut-off is of A good working value is to choose thecps filter, giving a product of upper andthe order of 22 db per octave for a set characteristic lower limits of 500.000 which will sound of values correctly selected as described, ohms, as has been done in the example.quite properly balanced. Similarly, bal-as is illustrated in Fig. 1. If this value of characteristic impedance ance is maintained for the case of the No other single factor will improve a is chosen the input impedance of the 7000 -cps filter. cathode follower remains approximately high -quality,well -equalized system as If plate coupling is selected, the selec- much as a well -designedfilter.Itis 1/G, (about 500 ohms for most tri-tion of components is made in the same odes)and no consideration need be difficultto appreciate just how good way.Inthiscasethecharacteristic some of your older records can sound given to either the actual cathode re- impedance of the filter should le high sistor or input impedance. High values (and some radio broadcasts improved) of cathode resistance are usually used to compared to the load resistor of the tube. when all the noise and distortion above keep the gain close to unity. A characteristic impedance as high as a reasonable cut-off frequency are elimi- Having arbitrarilyselected an ap-75.000 ohms at 5000 cps still gives prac-nated. Install one in your equipment and proximate value of 20,000 ohms for thetical values for inductance and capaci-listen for yourself. characteristic impedance, refer to Fig. 3 and select the nearest commercially available value of inductance which cor- responds to a characteristic impedance of approximately 20,000 ohms, which in this caseis 750 millihenries for the 5000 -cps cut-off frequency. Selection of the inductance should be done on the basis of the lowest desired frequency of cut-off so that all succeeding impedances will be larger than that at 5000 cps. Values of characteristic impedance for frequencies of 7000 and 9000 cps are read directly from Fig. 3 and are seen to be 24,000 ohms and 33,000 ohms re- spectively. Similarly values of capacit- ance required for this value of induct- CALCULATION FOR SNITCHPOSITIONS ance and for the chosen cut-offfre- COUPLING CAPACITOR a OPEN. NO FILTER quencies are read directly from Fig. 4, 9,000cps CUT OFF 2ft 40 . 33.000 7,000cps CUT-OFF

and are found to be .0027 pf, .0014 itf, 0,1 of a 5 000cps-CUT-OFF and 850 µILL Let us examine further the selection Fig.5. Schematic of suggested filter to be connected between a cathode follower and thesuc- of coupling capacitor Ce. This capacitor ceeding tube to provide cut-offs at 5000. 7000, and 9000 cpstogether with aflat position,

65 Cathode -Follower Audio Control System

G. B. HOUCK

The connection of existing equipment to a typical high -quality amplifier and speaker system often poses difficultiesinswitching. The author outlines a method which makes it possible for anyone to operate the "system" without excessive indoctrination.

IN THE CONSTRUCTION of many home control simplicity. Whatever the psycho- careful choice of control types, clear audio systems, basic design funda-logical implications may be, the fact re- controlidentification, proper arrange- mentals are often overlooked. Notmains that simplicity of operation is a ment, and most important, convenient only are some of these installations verymost important consideration in audio operating location. expensive, but frequently difficult to op-system design. This may be qualified by erate and inflexible in use. Several de- stating that simplicity must be achieved Design Considerations signshavebeenillustratedrecently provideditisnot at the expense of When theactualconstruction and which feature a confusing profusion ofquality. The qualification immediately installation of an audio system is con- controls, located either where they arereveals the weak point of most ready-templated the usual thoughts are di- not easily seen, or where their manipula-made units, particularly in the case ofrected to a design which presupposes tion is awkward. tone -compensationcontrol(adesign starting from scratch. This may be the In the past, when audio systems weredrawback, not necessarily an engineer- wrong approach. It effectively tags the designed for hobbyists and technicians, ing one). owner's present equipment for disuse or there was little objection to laboratory - One other point regarding the rela- even the junk -yard. It is also somewhat typeinstallation's.' When non -techni-tive merits of ready-made and custom cally trained persons such as profes-built systems should be brought out at misleading in that it attempts to con- sional musicians came in contact withthis point in the discussion. Too many vince the owner that there is nothing they usually found it necessary tocustom designers have the notion thatworth retaining in his present equipment which may include a TV receiver, FM- learn enough electronic and acoustic the only obstacle in the way of increased AM radio, phonograph etc.,of fairly fundamentals to control the medium ef-sales arfd installations is a mere prob- goodquality.These units may also fectively. lem of packaging improvement. Per- representaconsiderableinvestment. Today the audio field has expanded,haps this idea is prevalent for the rea-Furthermore, the custom-built system, largely because of the availability of son that the expansion of the home audio when the costs of cabinetry are con- high -quality, low-cost components, andbusiness has coincided with an increas-sidered, is often prohibitively expensive, an increasing awareness among con- ingproficiencyinthetechniquesof and usually cumbersome or simply im- sumers that "hi-fi" systems can be much installation both infurniture andin movable." As suchit may become a more satisfying than ready-made units. structures. Indeed, the ability of some source of frustrationto some home- Apparently, however, the demand fordesigners in rendering electronic gearmakers, forever obsessed with the urge such ready-made units has not dimin-elegantly inconspicuous is legerdemain. for periodic rearrangement of furnish- ished to the point where manufacturers Unfortunately, many installationsare ings. are unduly concerned. On the contrary,perpetrated by persons not primarily The approach to this problem pre- the present market forconsoles andconcerned with audio. If the decorator sented here in its broadest scope is one combinations suggests possible advan-and cabinetmaker have no understand- which offers the builder the double ad- tages over those of custom-built systems. ing of the operating requirements ofvantage of retaining not only the op- Of course there are many factorssound systems, orifthe professional erating features of his present equip- influencing salesof ready-made unitsdesigner considers them less important ment, but the actual equipment as well. and audiocomponents.Perhapsthe than novel cabinet design, it can be ex-At the same time, an increase in audio features dwelt upon in this discussionpected that the results will be anything qualityis made possible. These three are not those immediately apparent tobut satisfactory. Controls will be located featuresrepresentedtheinitialgoal the prospective customer. Nevertheless, close to the floor,2 labels will be non-when development of the cathode -fol- as factors of good system design, theyexistent, record changers will be inac-lower control system was first under- cannot be disregarded. cessable, etc. There is always room fortaken. Other advantages, made apparent The designer of audio systems mustimprovement in packaging design when with the final perfection of the tech- realize that simplicityisnot only ait meets the requirements of human en- nique, will be discussed and evaluated virtue of sound design, but a publicgineering. The term in its truest senselater. demand. This facthas been demon-implies an approach in which basic de- With the decision made to construct strated repeatedly in the sales of com-sign fundamentals are applied in sucha high-fidelity audio systemutilizing petitive merchandise. For example, aa manner as to cause response and con- existing receivers, the task became to TV receiver was put on the market a trolby the instrumental and human while back which featured "one knobmechanisms to be smoothly coordinated. investigate the quality of the receiver control." Regardless of its engineeringThe designer should understand thatunits. Measurements of a typical AM merits or deficiencies it enjoyed a good achievement of this goal is possible onlytable model radio indicated an audio sales record, primarily on account of its when all the factors are carefully con-frequency response of 20 to 7000 cps sidered. The best designs will featureexcluding the output transformer. Sim- * 21 Barton Place, Port Chester, N. Y. ilar measurements with TV receivers 1 M.P. Johnson, "Multipurpose audio 2 Jeff Markell, "Cabinet design for Hi-Fi amplifier,"AUDIO ENGINEERING, August systems," Fig. 4, AUDIO ENGINEERING, May 3WilliamC.Shrader, "Audiointhe 1947. 1952. home," AUDIO ENGINEERING, May1952.

66 and FM tuners led to the assumption was added to an electronic organ for an OUTPUT STAGE that except for their audio output sec- HIGFI7DELITTAITIP7 echo amplifier and speaker arrangement. leY6,6K4,501.6,ett.1 Miniature tube types such as the 12AU7, tions, most of these receivers could be TO classed as "hi-fi." The next step, then, OTHER PROGRAMI 12AT7, etc., result in a space saving. SOURCES I was to devise a method of bypassing the Figure 4 shows the schematic of such audio output sections, apparently the a unit. The assembly is simply mounted principal sources of poor frequency re- by fastening it to the chassis or securing sponse and distortion. This would be 4 it to the cabinet. The only caution to be S t4 accomplished with an absolute minimum 51 observed is the need for an adequately It !FRNei filtered plate supply, as any hum present of circuit changes in the receivers. : ANY LENGTH wasalsoobviousthatsome means CABLE TO AMP. 5441 DC will be fed into the audio output. It is should be available for using one high - RELAY alsorecommendedthatanisolating qualityaudiopoweramplifierand PLUG INTO SOCKET L. transformer be installed in a.c. trans- speaker system for all of the units. The former -lesssetsto eliminate possible final step involved design of an adapter shock hazard. POWER device for linking the receiver front - SWITCH Construction of the adapter plugs is ends with the amplifier. With the com- not difficult. The socket should be simi- pletion of these design steps, several TO POWER lar to the type illustrated, in Fig. 5. The unique advantages became immediately TRANSFORM plugs, if unavailable from parts distrib- apparent. There would be no modifica- TO LINE uters, can be fabricated from old tube tion of existing unit controls nor any bases(containing a sufficient number change in operating precedures. No re- Fig.I.Circuit schematic tor the cathode -fol- of pins). The resistor and pigtails are building of receivers nor extensive cir- lower control system. soldered to the socket as shown in the cuit changes would be necessary.' The photo and drawn through the plug pins various units could be located wher- along with the output leads which are ever desired. Total cost would be no passed through a small hole drilled in more than necessaryto purchase or the side of the plug. construct the power amplifier, speaker Installation of the complete system and several adapters. is simplicity itself. Since the line im- Four steps are involved in the reali- pedances are low, the interconnection zation of such a hi-fi system. The en- may be made using ordinary zip -cord or gineering and craftsmanship required bell wire. For under -the -carpet wiring, arc a minimum. The first step involves TV antenna lead-in ribbonis recom- the addition of a few components to the mended, since itisflat and causes no power amplifier, as shown schematically lumps or bulges. at (A) in Fig. 1 and pictorially in Fig. 2. The second step is the "adaption" of Phonographs the units. The third step is the construc- Fig.3.Adapter chassis used with electronic So far, nothing has been said about tion of the simple adapter devices. The organ,showing simple"add-on"unitwhich the use of a phonograph with this sys- fourth step of the com- may be employed with tuneror any other tem. There are few hi-fi record playing plete system. soundsource. devices these days which do not require pedance sources. Figure 2 shows a Wil-preamplifiers and some means of equal- Circuit liamson type amplifier with relayin- ization. In the case where the phono- Before describing these four steps in stalled. graph player has its own preamp and detail,itisdesirable to consider the As outlined a few paragraphs above,audio output, any ofthepreviously circuitryinvolved,includingpurely no modification of the receiver units ismentioned adaption techniques can be electronic functions. The essence of thenecessary in most installations. Theresimilarly employed. If the builder has system is the connecting link betweenare, however, a few cases where a simple nothing more to start with than the the receiver unit and the hi-fi amplifier.addition to a receiver may be needed.basic player mechanism and pickup, he Basically. it is a cathode -follower deviceIf the tuner has no audio output section or terminates, for example, in a detector is advised to construct a preamp-equal- which effectively shorts out the regular izer, as shown in Figs. 6 and 7. The output transformer and provides specialor discriminator output, a unit similar low -impedance output from the cath-to the one shown in Fig. 3 may be con-three requirements for such a device ode.5. 6 The pin connections shown are structedeasily.The 6SN7 assembly are proper load for the pickup, suffi- for output tubes similar to those shownillustrated is one particular type whichcient gain, and cathode -follower output at (B) in Fig. 1. Other tube types may be used with appropriate connections. In the power amplifier the d.c. com- ponent of the signal is used to energize a 5000 -ohm d.c.relay which controls primary power for the amplifier. The audio signal passes through a blocking capacitor and a potentiometer adjusted for the proper operating level. The usual power switch remains in parallel with the relay contacts, providing manual Fig. 2.Williamson - typeamplifierwith power control in the event high -imped- cathode -follower con- ance sources are connected to the ampli- trolsystem installed. fier. The input terminal jack is wired to accommodate either high or low-im-

4 Ulric J.Childs, "Why not use your present tuner?" AUDIO ENGINEERING, April 1952. 5 G. B. Houck, "Gain chart for cathode followers," Telc-Tech, August 1947. 6 Audio Design Notes-"The cathode fol- lower," AUDIO ENGINEERING, June 1947.

67 impedance. The design illustrated pro- vides a minimum ofcontrol.Ifthe builder desires a more elaborate equal- izer,severalexcellentdesignshave been presented in previous issues.? Location of Units Fig.6.Compact There are practically no restrictions self -powered pre- on the location of the units of the com- amplifier -equalizer plete audio system. Because of the low - designedtofeed a impedances and the need for only two phonograph signal to conductors, the units may be placed as thepower amplifier far apart as desired, without undesirable inthe same manner effects such as high -frequency attenua- asdescribed for tion or hum -pickup. In general, the TV other sound sources. receiver should be located where it can be viewed easily. In the case of the speaker, a corner locationisusually preferred because loading is improved. There is certainly no reason why a TV receiver cannot be located directly above or even near the center of the sound dissipationproblems aresolved, andobvious that no loss of quality is ob- source.° One serious objection to thethere is little chance of hum ptoducedserved. Although the output lines of by power transformer interference. every unit are wired inparallel, no harm is done if more than one unit is Performance turned on at a time. In this case, the In discussing the performance of thebias on the cathode -follower may be system it is important to emphasize theupset and the resulting sound is some- functional aspects as well as electronic. what disagreeable, if not confusing. Since the designcallsfor bypassing The distinct advantages of this sys- those portions of the receivers whichtem are revealed when functional per- represent the chief source of poor qual-formance is evaluated. The control fea- ity, it is obvious that better performanceturesoftheseparatereceiversare is achievable.It should be reasonable retained, control of hi-fi audio selection Fig.4. Schematic of adapter chassis shown in to expect the audio quality to be limitedis completely automatic, and thereis Fig.3. only by the capabilities of the power very little chance of control confusion. amplifier, provided there are no lossesSince most persons are familiar with in the connecting link. If an amplifierthe control arrangement of conventional comparable in quality to the Williamson receivers, the builder need not consider preparing an instruction manual for the is employed, it seems safe to assert thatsystem. Whereas there are some audio high-fidelity will be obtained. As forsystems which can be operated only by the connecting link,itsimpedance istheirdesigners,thecathode -follower generally less than 500 ohms. If theaudio control system installedin the average levelis approximately 1 volt author's home and in several others is and the hum and noise level as much used with equal facility by visitors and as 100 microvolts, or 80 db down, it isfamily, including his five -year -old son.

C7 390uK

Fig.5.Assembly suggestedfor adapter plug C6 SW2 used with typical output tubes in AM. FM, or 200 Pi, C7 0.1 TV receivers. tt2 12AT7 C2 05, 1/2 12AT7 12 AT7 K 0.27 Mop location of any units close to the speaker CI DS cabinet is that it results in poor con- trol. If the operator is forced to stand less than a foot or so from the speaker when adjusting the volume control,it is quite likely that the level will be un- C3 009

satisfactory at other distances. As a RII tug CE/ R I2 matter of fact,itis most desirable to CAA 22,000 224we WWY have the controls as near the listening R16 R15 R14 RIO R 7 position as possible. This, of course, is 100 2.500 1000 56,000 one of the principal features of the cathode -followercontrolsystem.It should not be difficult to place the FM SELENIUM or AM radio next to a comfortable oNRECTIFIERS chair, or the phonograph near the rec- ord storagespace. With the power 1.4 amplifier and its power supply located in a closet, in the cellar or attic, heat SW 1 POSITIONS SW 2 POSITIONS POS CROSSOVER POS ROLL -OFF 7 St.George andDrisko,"Versatile BOO cps FLAT 2 400 CP 3,0C0cov phonographpreamplifier,"AUDIO ENGI- 3 250 cps I ,500elm NEERING, March 1949. 8 C. G. McProud, "A new corner speaker design," AUDIO ENGINEERING, Jan. 1949. Fig. 7. Circuit of preamplifier pictured inFig. 6.

68 Simplified Equalizer Design

GEORGE A. DOUGLAS

Charts and tables to reduce complication-and construction hints to ease building. IN applications requiring exact modi-more than 20 db of attenuationin athis requirement is usually met by ap- fications of the audio spectrum, con- single equalizer, it will be seen that theplying (c) or (d), in which case itis stant -resistance equalizers are capa-shelf suppressors serve where a gradualpossible to control the slope. ble of doing the job with maximumcurve is required not exceeding 6 db The first step in the design of an efficiency and convenience. The cor-per octave. Networks may be cascadedequalizer is to draw on logarithmic pa- rection of cutter peaks and tape droopto obtain a steeper characteristic. butper the actual and desired response of at the high end, the incorporation of pre -emphasis, and numerous other alter- ations of response can readily be accom- plished with the design data that follows, Fig.2. Methodof and which involves only simple arithme- addingpre -emphasis tic to calculate values of components; to recording charac- teristictoeliminate or, with the use of a reactance chart, a I0 a cutterpeak,for B few steps of multiplication. example. Curve A Figure 1 shows the four configura- represents normal r e - 0A) tions with their transmission charac- sponse, while curve B teristics used to cope with the equalizer represents desired re- sponse. The required problems commonly met in audio work. A - NORMAL RESPONSE equalization is repre- II - EQUALIZED RESPONSE The circuits of (a) and (b) are shelf sented by the differ- suppressors,orconversely,low and ence between the two high boost; (c) and (d) are peaking curves. equalizers. As itis inadvisable to use 10 lINK MO MAO MOO * 51 E. 42nd St., New York 17, N. Y. FREOUENCY IN CYCLES PER SECOND

lel b (C) la)

1 I I

1 I I

1

I

I

1

1

I

I I I, f fr

CI L3 Li LS C

RI R1 RI Rt 1 RI R1 R1 R2 R2 R2 R2

LO C2 L2 L6 C6 C4

K -1 K-1 92-1 K-1 I LI -L L2 . L L3 0 - --. L LS ---.-L ir( K-1 IT. 0' rk- keI

1K ITC 0-1 C2 11----i- C C 1 -C L4 ff- L 1.6 - -. L Y-1( K-1 K-1b"-1 K-1 It."

1V 1 C5 Kiir--r92-1 C - -K -11771 - -1 b I Fro. 1/2 Pad Lose I" R° MA fr.Freq. of Resononce f c4.11 91-1 C6 K-1 t*C 4r, K Alltintication Factor 159,000 C MN 20109 K Pod Loss db b. rI' flo Line Impdonc

Fig.1. Chart showing the configurations of four different types of equalizers, together with formulas For determining the reactive elements.

69 the equipment requiring modification or 159 x 600 J -= 13.62 Mh correction. This will furnish informa- -7000 tion to construct the transmission char- 159,000 acteristicoftheequalizer. On this C = = .038 Rf curve note the design frequency on the 7000 x 600 transmission characteristic at the line Now,since of half the total attenuation. With the K-1 1 design frequency f and the impedance 1.5= L of the line in which the equalizer is VK b'-1 to work, calculate L and C, or take their values directly from a reactance chart. b2 -1 L, to L, and C, to C may now be as- -K -1 certained by substituting in the equa- tions figures taken from the accompa- CS=C b`-1 nying tables. b1 K- 1 Fig. 4. Two equalizersconstructedbythe For the shelf suppressors only Table author -left, a plug-in unit employing Ce=CK -1 1 a fixed K-1 VK amount of equalization, and right, a unitpro- 1 is used. The values for- and - VK b2 -1 viding step -switch control of attenuation. K-1 From the tables : and for R, and R, for an impedance of for to some extent by including itin 1 ohm, are located opposite the degree /- the pad. Thus, whenan inductance ap- =1.213 10 pears in the shunt arm, its d.c. resist- of attenuation desired. For other val- VK ues of R, and R multiply by selected ance should be deducted from R,, and K - I a resistor selected which has an actual Ro. The data in Table 2 determines the = .824 b2f11; slope of equalizers (c) and (d). The = .512 (Concluded at bottom of next page) VK b ratio in column 1 corresponds to fr/f TABLE 2 (the resonant frequency divided by the R,= .520 1 =.952 EQUALIZERS frequency of one half the attenuation). R5= .704 17j Opposite this are found the solutions of Substituting, wenow have: bs -1 1 1 and bi b-1 -I L,-= 13.62 x 1.213 x .952= 15.7 Mh b2 -I b -1 L5= 13.62 x .824 x .512= 5.8 Mh 10 .187 4.348 Example: C, = .038 x .824 x .512= 0.16 Rf C, = .038 x 1.213 x .952 Curve (A) of Fig. 2 depicts a typi- = .043 Rf 10 R,= .520 x 600 = 312 ohms .359 1.785 cal cutter response above 1000 cps. It 8 is desired to design a 600 -ohm equal- R5 = .704 x 600 = 423 ohms 10 izer for a recording characteristic with For many applications in the 512 .952 10 db of pre -emphasis at 10,000 cps. upper 7 frequency range,r.f.chokes can be Ordinarilythelow -frequencyshelf 10 suppressor could be used, but in this utilized, providing the d.c. resistanceis .637 .568 case the cutter peak would be undesir-not excessive. The photographs, Figs. 4 and 5, show a completed 10 ably increased. It is apparent that the equalizer .750 .333 rising slope of the peak can be con-with 5-mh. coils of about 15 ohms.To 5 tinued to the 10,000 -cps,10-db point; provide proper inductance,selectca- 10 therefore the obvious solution is to de-pacitor(s) of correct value for C in .840 .190 sign the equalizer with the configura- Fig. 3. Set the oscillator to the desired 10 tion of (d) with a resonant frequencyresonant frequency, with more induct- .910 .099 of 10,000 cps, and an attenuation' ofance in the circuit than is needed, and 10 10 db. From the curve, the point ofremove turns until maximum attenua- .960 .042 half the attenuation, or 5 db, is foundtion occurs. 2 at 7000 cps, the design frequency. L and C can now be ascertained : Coilresistance can be compen,a:«1

TABLE 1 EQUALIZERS

L OSCILLATOR K -I VIC R. R3 IT db VK K -I 1 .113 8.833 057 8.68 2 .232 4.307 114 4.32 3 .345 2.902 171 2.84 4 .468 2.135 226 2.20 Fig. 3. Circuit arrangementfordetermining 5 .586 1.705 280 1.646 resonant frequency of LC combination. 6 .709 1.410 332 1.34 1 While the term "attenuation" usually 7 .826 1.193 382 1.118 refers to a loss,it has a slightly different 8 .949 1.053 .430 .946 meaning when used with an equalizer of 9 1.082 .923 .476 .812 ally of these types. In the case of equal- 10 1.213 .824 520 .704 izers(c) and (d), attenuation refers to 11 1.356 .737 .560 .602 the difference in the transmission at the 12 1.500 .667 598 .536 resonant frequency to that at frequencies 13 1.644 .608 .634 .472 remote from the resonant frequency. For 14 1.790 .558 .668 .416 those equalizers of types (a) and (b), at- 15 1.949 .513 .700 .368 tenuation refers to the difference in trans- 16 2.116 .453 .728 .310 mission at the two extremes of the fre- 17 2.285 .437 .752 .268 quency spectrum. In both cases, this "at- 18 2.461 .406 .776 246 tenuation" is determined by the loss in the 19 2.654 .376 .798 .224 resistive network, which is normally re- 20 2.848 .351 .818 202 ferred to as an attenuator or pad.

70 Pickup Loading and its Effect onFrequency Response

W. A. FITZMAURICE and W. JOSEPH

If you notice a difference in quality when you move your phono equipment ten feet or so from the ampli- fier, do not be surprised-there's a good reason for it. The authors explain why and tell how to avoid it.

HOME AUDIO installations employing 6/Ai magnetic pickups, the cartridgeis . IN Cs 540110 usually located somewhat remote from .10 Rs.CD the preamplifier by distances ranging up- C. 140 µsit I iii ward from one foot. Because of the ca- Rs c° pacitance associated with the intercon- cs . 1W necting cables, the question is raised as 4 Rs a'0 - 1 to the effect of this shunt capacitive load- Fig.2. Curve show- C. 540 pill1 I ing on the response of the cartridge. In ing changeincar- 2 R. 50,000 z .°,041°1 addition, there seems to be considerable tridge response with 0 Ifthili..-41111 change in load resist- cs 14Kid ftliSII doubt as to the exact effect of resistive -2 ii I. ance. z Rs ' 50P00 loadings as recommended by various 1411111116- I IN sources. a. 4 Cs 40µµf Re 50,000 I I OM! To answer these questions the authors z -6 I 0, 40,140,orS40µµ/ I carried out an investigation to determine R,15,000 the response of variable reluctance car- 6, 40,140,or 540140 10 1 II tridges under varying conditions of load- Ri 5000 I I a% ing. . . NM INN FREQUENCY IN CYCLES PER SECOND

the input capacitance of the Ballantinc 19-db peak at 7500 cps with Ce of 540 µpf Model 300 a.c. voltmeter, and stray wir-and RI of infinity, to a 3-db peak with ing capacitance. The audio oscillator out- the same Cs and Re of 50,000 ohms. With put was maintained constant at all fre- Cs of 40 pof and 140 isof and R. of 50,000 quencies at 0.01 volts. ohms, the resonant effect holds the re- Fig. 1. Equipment set-up for measuring cartridge The results obtained were plotted and sponse level to approximately 10,000 cps. response. are shown in the curves of Fig. 2. It will The "water fall" drop thereafter is a The measuring arrangement used isbe noted that the effect of shunt capici-characteristic of the effect of resonance. shown in Fig. 1. The output of an audio tance is quite marked when the shunt re- As R. is decreased, the effect of C. be- oscillator was connected in series withsistance is in the neighborhood of 50,000 an inductance of 520 millihenrys and a comes progressively less, and drooping ohms or greater With no load resistorresponse is obtained. With R. of 15,000 resistance of 340 ohms. This simulates (infinite Ra), pronounced resonant peaks the General Electric variable reluctance ohms, the response is down 3 db at 5000 are obtained for all values of Cs meas-cps and 8 db at 10,000 cps. With Rs of cartridge, type RPX-040, RPX-041, orured. With Ca of only 140 ti4sf, a 15-db RPX-050. Various combinations of C 5000 ohms, the response is down 3 db at peak is encountered at about 16,000 cps. 1500 cps and down 8 db at 4000 cps, in- shunt and R shunt were used. The shuntSuch peaks tend to accentuate noise and capacitance consisted of mica capacitors, dependent of all values of C. used. produce "dirty" highs. Belowresonance,theperformance * 316 Milford Ave., New Milford, N.J. The damping effect of R. (decreasedcharacteristics may be analyzed by refer- ** 52-18 19th Ave., Brooklyn 4, N.1'. Q), may be seen by the reduction of aring to to Fig. 1 and noting that the out- put of the audio oscillator (equivalent to the internally generated voltage of the EQUALIZER cartridge) is impressed across a voltage divider consisting of L, R, and the par- (from preceding page) allel combination of Rs and Cs, all in series. The butput is taken across the value equal to R. minus the coil resist- parallel combination of RI and Cs. Since ance. the impedance of L increases and the im- Asageneral observation, r.f. chokes pedance of Cs decreases with an increase can be used in applications requiring in frequency, a voltage divider exists inductances of less than 15 mh. While a whose output is a function of, and de- high Q is desirable, it can nevertheless creases with, an increase in frequency. be discounted to the extent thatitis The data given applies specifically only possible to attain the necessary objec- to the cartridge simulated. However, tive.For extremely"sharp"curves. from Table 1 listing the impedance of the best results can be obtained by the use more popular types of variable reluctance of toroids, but for most applications it cartridges, it can be seen that the same will be found that r.f. chokes can be Fig. 5. Internalconstruction of the problem exists to some extent in all cases. used, and at considerably less expense. equalizer shown at the rightinFig.4. A survey of the types of cable com- (Concluded at bottom of next page)

71 Motor RumbleReduction in WideRange Phonographs

JOHN R CATER

A negative -feedback-type R -C filter, the author finds,is the only way to banish table rumble permanently andcompletely, short of acquiring a broadcast -studio unit. MUSICLOVERS, who have striven to $100 have very annoying rumble. approach perfection in theirre- The nature of rumble suggests frequencies will wipe out the rumble producing systems and have lav- a solu-without affecting the usefuloutput of tion. Many phonographmotors havethe vast majority of records. ished much money and mental anguishfour poles. These motorsrotate at 1800 in the effort may wonderto what end rpm if they are synchronous, and around Figure 1 shows such a filter. Item - they have achieved excellent bassre- 1725 rpm if they plays a cascode amplifier Vs witha paral- are induction motors.lel -T network in a feedback loop. It is sponse when it brings up motor rumbleRumble results from mechanicaland to an intolerable level. Ofcourse, there electrical imbalances in the absolutely essential that the components are turntables available with extremely motor whichof this twin -T be both accurate and cause a variation in speed onceper rev-stable. It is well to measure them rather low rumble, but they are bulky andthey olution, or 30 and 28.7 timesper second. cost several hundreds of dollars. EvenIt is a rare record indeed (not than trust the marked or coded values. countingSince the ganged potentiometers Ri have some turntables selling for more than test records) which has such lowmusi- cal only 10 per cent of thevalues of the 108 Warren St., Nutley 10, N. I. notes. A narrow -band frequency- rejection filter centeredon the rumblefixed resistors with which theyare in series, they may be considerably lessac -

PICKUP LOADING the same results may be obtained with (Irvin preceding page) (%) 12AU7, (%) 6SN7, 6J5or numer- ous other triodes. Different values of plate voltage have little effecton the re- monly employed shows theminimum ca- sults. As careful attention must be paid pacitance per foot of high-graderubber to the filtering of the cathode follower microphone cable to be about 40 Rtf.and B+ supply voltage as for a preamplifier. for the smaller phono cables,about 60 µO. Coaxial cables When a follower is employed, it isrec- are available with ommended that de -emphasis be capacitance per foot as lowas 13 wif. accom- Of course, there is always the plished after the output of the follower. foot or so An excellent example of sucha system of high -capacitance phono cableincor- may be seen in an article by P. W. St. porated in the record playerarm to beFig. 3. Equipment set-up for measuring contended with. cartridge George and B. B. Drisko entitled"A response when feeding a cathode follower. Versatile Preamplifier" in the first Audio noted. The effect of various values of Anthology. Cathode Follower Helps coupling capacitors, Ce, on the low fre- When cable capacitance isadded toquency response was explored. A Cc of the input capacitance of the 0.05 µf with an Rk of 5000or 10,000 ohms TABLE 1 preamplifier, produces a droop of only 1 db at 20 it is unusual to find Ca lowerthan 100 cps. Manufacturer Cartridge Approx. L Approx. R gµf. From the foregoing itmay be seen A 0.2-meg Ra was employed to elimi- Model mh ohms that with such capacitance itis possiblenate any resonant effect arising from General ElectricAll home 520 340 to obtain very satisfactoryresponse from phono arm cable capacitanceas repre- types the cartridge up to 10,000 sented by C8 of 40 144. Audak L-6 750 600 cps, and de - When operating with 300 volts Pickering 120} emphasis may also be providedto com- on the 165 600 pensate for today's recording practices.plate and a 5000 -ohm, Y2-watt cathode 140 resistor, a 6C4 draws 3.5 260 130 750 This, however, meansvery careful jug- ma. Virtually gling of Ca and Ra and requiresmeasur- ing equipment not availableto most ex- perimenters. In addition, it is often de- i sirable to incorporate variable de-empha- I sis networks into the system with c,11000util the Rk 5000 or 10,000 preamplifier remotely located from the i record player with a consequent intro- C .1'-' Ca 2500put duction of a Ca of 500 µµf or more. Fig.4. Curve show- ingchangeincar- Rk 5000or 10,000 Cc 1 0 Ft \ With these factors in mind, the authors tridge response with z believe that higher quality installations 0 ' change in a load iso- 1 should incorporate a cathode follower latedby a cathode N Ce 1 Opt z r immediately adjacent to the cartridge. follower. ci ill I li c, 250000 0a 1 / The simple cathode follower shown w Cc 0.25 Ft Rk 5000 in cc Fig. 3 was set up and tested. Theeffec- tiveness of this arrangement isbrought cc 0.05 pf C.5 2500F10 out by the response curves of Fig.4. Rk =10,000 Even with a Ca of 2500 tigf, a negligible 111111 effect on high -frequency I responseis 1000 70000 10000 10100FREQUENCY IN CYCLES PER SECOND

72 Using again the response in the flat part +250V of the range as zero level, the output was down approximately 1 db at 36 cps and Cl,.01 C2 1C2___ 6 db at 32 cps when the rejection fre- I quency was adjusted to 30 cps. N Higher -Qfiltersemploycathode - followersinthe feedback loop were Vio .41 tried, but were too sharp in cutoff unless two or more were stagger -tuned. With 12AT7 just one such filter, the rumble frequency 0 t R2 would vary enough to rear its ugly head VIE R1 periodically on each side of the rejection 0.5A1M Rot Rod 05MEG. cote 1% frequency. C301 40 :A" A twin -T used as a brute force filter T2. is unsatisfactory because it has an effec- tive Q of onlyY2and affects the re- sponse up to several hundreds cps. The circuit presented here, inciden- tally, permits one to use test records for 12VDC ALL CAPACITANCES IN 250V ALL RESISTORS 1/2 WATT UNLESS INDICATED adjusting equalization without having 1W 40 HEATER CIRCUIT R4a-d GANGED 50PO00HM LINEAR POTENTIOMETERS MATCHED FOR 5 PER CENT TRACKING BASED UPON rumble render the results unintelligible. 0 FULL RESISTANCE VALUE. It is fortunate that the major compon- ent of rumble, for the motors considered, is so low, because any attempt to "poke Fig.1. This is the rumble filter, turnable to between 28.9 and 32 cps. Note carefully that the a hole" in a pass band does give transi- ganged potentiometers must be matched for tracking. R. -R_ and C: -C_ should also be checked, ent troubles. As has been pointed out, preferably with a bridge. very few records (or for that matter, curate and stable without causing trouble.grid of lid), the gain is approximatelyvery little music) have frequencies down equal to GmRL and is much larger in to 32 cps. So if we eliminate a few cycles From a group including one IRC Q11- starting at this point, we are not tamper- 123potentiometer, and sixM11-123value than for Via. Since the signal ap- "multisections," the writer was able toplied to Vie is degenerative, there is a ing with the really usable band. select a set with a maximum trackingvery appreciable loss in signal level at error of 5 per cent (based on full resist-the output of the filter, but the effective ance value). Two 50,000- and one 25,-Q of the twin T is greatly increased. 000 -ohm potentiometers arerequired. Because of the loss (approximately 30 Less tracking trouble will be experi-db), the signal from a low-level pickup enced if two 50,000 -ohm units are con-must be passed through a preamplifier nected in parallel to obtain the 25,000 -first, though the output from high-level ohm value. RI, Rs and Rs are 1 per centunits such as the Weathers FM pickup tolerance deposited -carbon -film resistors.oscillator may be passed through the Most composition resistors are likely tofilter without further amplification. age too much to be satisfactory. Cs, Cs, Itis usually desirable to bring the Cs and Cs are silver mica capacitors cho-level up again immediately. One triode Fig. 2. The parallel -T network. sen for 2 per cent tolerance. section is sufficient ( Yrs) and if double The values chosen permit the centertriode is used, there is one section avail- Though some purists may insist that frequency to be adjusted sufficiently inable ( Vso) for a cathode -follower so thatbecause they can hear down to 15 cps most cases by simply setting Rs. Refer-output cable capacitance may be neg-and their speaker has useful output this ring to Fig. 2, the null frequency for thislected. low, they want to equalize correctly to network is 1/2xR, and for the values Cascodes and cathode -followers placesuch a point, the simple fact is that they selected, ranges from 32 to 28.9 cps. Ifan undue burden upon heater insulationwill probably have to use signal genera- it desired to lower this range of frequen-unless appropriate steps are taken. The cies, it may be most conveniently doneheater -cathode rating of most tubes istors for such dubious aesthetic flights. by adding parallel capacitors across Cs90 volts. If the heater supply is biasedFor those whose turntables have two and Cs in Fig. 1. Since the parallel ca-90 volts above ground, grounded cath-pole motors (thus placing the rumble in pacitors will be small in comparison toodes are still within rating, and cathodesthe neighborhood of 57 cps) we can only C, they need not be silver micas. If theoperated at high voltages may reachoffer sympathy. components give too low a frequency+ 180 volts without exceeding the tube's when properly matched, the null fre-rating.Suchbiasedheatersupplies quency may conveniently be raised byshould be heavily bypassed to ground. REFERENCE shunting R., R. and Rs (Fig. 1) by ap- With theequipmentavailablethe Valley and Waltman, "Vacuum Tube propriate resistors. writer was not able to measure attenua- Amplifiers," New York:McGraw-Hill In the cascode amplifier V., the gaintion at the rejection frequency because of Book Co., Chap. 10. for signals into the grid of Via is equaldistortion in the audio oscillator used. pRz. It was estimated to be in excess of 25 to For signals into thedb below the region of flat response. RL + [(A + 2)Rp].

73 Two -Channel Conversion for AM -FM Receivers

C. G. McPROUD

Details of simple changes which give the advantage of two separate receivers to tuner chassis which employ independent AM and FM channels but with coupled tuning capacitors. Ideal for binaural or stereophonic broadcastprograms.

SOME TIME AGO, the writer made a This entire system was described in anchannels. In the system for which this few changes to a standard AM -FMearlier series of articles.2 modification was made, the switching tuner chassis in order to be able to The next step in the program was tobetween AM, FM, and Phono is accom- listen to one program on FM while re- secure a combination AM -FM tuner ofplished in the control amplifier. There- cording another on AM, or vice versa,good quality, with the idea of separating fore, since the phono and the TV inputs all without realizing how advantageous the sections so as to have the advantagesto the tuner were never used,it was this modification would be when radioof two -channel reception if desired. Be- preferredtoeliminate thisswitching stations started to use their two trans-cause of its physical construction, thefacility from the tuner chassis. However, mitters to radiate the two -channel sig- Browning RJ-12B was selected. In this to reduce current drain and consequent nals for binaural or stereophonic pro-receiver-as in several other types-theheating, a selector switch was used to grams.' tuning is accomplished by two entirelypermit operation of either channel sepa- The whole ideaisso simple andindependent variable capacitors, coupled rately or both together. This switch has straightforward that anyone faced withtogether only by means of a dial cable.threepositions-the centerproviding the problem of providing both AM andThe modifications are now reasonablyfor both AM and FM operation, the left FM programs simultaneously from aobvious. With the advent of two -channelposition energizing only the AM tuner, single tuner should come up with thebroadcastingforbinauralorstereo-and the right position energizing only same solution. However, since such aphonic programs, the receiver becomesthe FM tuner. solution has not yet appeared in print, doubly useful. In addition, neither the volume con- this one is offered as one way to accom- Before discussing the actual modifica-trol nor the power switch were needed plish the desired end. tions to the R J -12B, let it be said thaton the tuner chassis, both being pro- At the time the modification wasthe general idea of these changes canvided elsewhere in the system. There- planned, there were no stations regularlybe adapted to any AM -FM tuner whichfore both of these were removed. As a broadcasting two -channel programs, butuses separate devices to tune the twomatter of fact, when the R J-12 is to be it must be admitted that there are timessections. The Browning RJ-20 is similar used in a typical modern system which when two programs of interest are onto the R J-12, and the Meissner 9-1091incorporates a control amplifier, both the air at the same time, and the humanmay he converted in a similar manner.of these components may be removed, hearing and interpreting mechanism isThe details may differ, but the principleleaving only the two central knobs on unable to cope with both at once. Forremains the same. For simplicity, onlythe tuner chassis-one for tuning and this problem, the simplest solution is tothe changes to the RJ-12 will be de-the other as the selector. However, for record one of the programs. As onescribed, and they are divided into two the modification, two tuning shafts were element of the writer's home system, aparts-electrical and mechanical. needed in addition to the selector switch, wire recorder was employed for just so in order to preserve symmetry, the this purpose. The quality left something a.f.c.switch was wired to the front to be desired, but it was at least good Electrical Changes panel, which is somewhat more con- enough to convey the information con- The electrical changes required arevenient than its normal back -apron lo- tained in the program-probably as wellsimple, and are made primarily to per-cation. Thus we have four control posi- as any of several million table -model setsmit the simultaneous operation of bothtions on the front of the chassis-a.f.c. thatserve asthesoleentertainment switch, tuning, selector, and blank. The medium in many households. At one 2 C.G. McProud, "Elements of residence original controls were, in the same or- time, this system employed two separate radio systems," AUDIO ENGINEERING, Sept. - der, the power switch, tuning control, tuners-originallyawide -ranget.r.f. Dec. 1948. Reprinted in AUDIO ANTuri.m.y. selector, and volume. Figure 1 shows the tuner of the Miller type, with modifica- tions of course, and the other an in- expensive FM tuner with a ratio detec- tor. The next step in the program of im- provement included the design and con- struction of separate AM and FM tuner chassis, both tuned by means of push Fig. 1.External ap- buttons. The FM tuner used relays to pearance of the con- switch adjustable capacitors in the oscil- verter Browning All511 6100 /11 miti;00160111-) Ti11-0-' AI lator circuit, while the other used relays RJ-12Btuner, show fil_11_1_111111 .J 111 11 1 L 11_1_ 1_ __Jim to switch the plate supply to any one ing the two dial 90 92 996 90 100 102 101 106 MS 109 of four single -channel fixed -tune chassis. pointers.

e 1With no intention of getting into this controversy as to the correct name-at least not in a constructional article-the writer respectfully refers the reader to discussions on this subject by Tinkham on page 77

74 Installa two -pole three -position the same type as the present one in the TO TO hole from which the volume control was F-1,1 OSC. switch(Centralab 1462)inthe hole OSC. from which the selector switch was re-removed, and mount two3/4 -in.idler moved, and connect the new switch aspulleys as shown in Fig. 3. These pul- 0 0 shown in Fig. 2. The remainder of theleys must be free to turn easily, and leads should be connected so that the in-eyelets for mounting them are usually dicator tube works on both sections ofsupplied with the pulleys. Note that the *et bottom of the left idler is on a line with AYO the receiver at the same time. To accom- °^',0 plish this, connect the lead from pin 5the top of the right one, and that this 0 of the 6AL7 socket to the AM a -v -c bus; line is slightly above the bottom edge connect pin 4 to the ground end of theof the glass dial plate. A third idler pul- discriminator network ; and connect pinley should be reamed out to run freely Fig.2. Schematic of the switch connections to 6 to the "high" side of the discriminatoron the A -in. selector switch shaft. be mode inmaking the conversion. output, through a 1-meg. resistor. Thus Following the diagram of Fig. 4, care- front of the modified chassis, which isthe plate supply is connected to the AM fully restring the AM dial cord. This but slightly different from a normaloscillator in the left and center positions cord is shown in solid lines and should chassis. It will be noted that there areof the switch, and to the FM oscillatorbe drawn up so as to be quite tight, with two dial pointers-the one at the top ex-in the center and right positions. the spring extended to about 1/ times tends only to the AM dial scale, while its normal length. Where the dial cord the new one at the bottom extends up- New A.F.C. Switch wraps around the knob shaft, it is sug- wards to the FM dial scale. The normala.f.c.switchinthisgested that three turns be taken. Test the Taking the changes one by one, thechassisisa single -pole double -throwstringingbeforeproceeding,making following procedure should be followed.slide switch mounted on the rear apron.sure that the knob shaft turns freely and Remove the power switch and its asso-By prying up the tabs on the sides ofthat the dial cord behaves properly in ciated wiring. Remove the volume con-this switch, the slider element may bethe shaft depression. trol, leaving the wires disconnected butremoved and upon replacing the rear Following a similar procedure, string at the same location temporarily. One ofbakelite plate of the switch, the wiringthe FM cord, using the dotted lines of ther.f.coilswill have to be pushedis not disturbed and the termnials serveFig. 4 as a guide, and again test the slightly backward to permit the controlmerely as tie points. Install a single -poleoperation thoroughly before proceeding to be removed. If done carefully, nodouble -throw rotary switch on the frontfurther. damage should result. Now, remove the apron where the power switch was, and It is suggested that two similar dial selector switch, leaving allits wiringconnect three wires from the old switchpointers be obtained-the writer prefers intact. This will save a lot of tracingto the new one, making sure to maintainthe type which are made of a fluorescent when connecting the new switch. All the same operating arrangement. plastic as they are easily seen. Since one the shielded wires from the switch to After cutting the shafts to the sameis to slide along the top of the dial plate the jacks on the rear apron may be re-length as the original tuning shaft, theand the other along the bottom, try them moved, makingiteasiertoget theelectrical changes are completed. Oneout first, and cut off short enough that switch out of the way. Next, remove thehole remains unoccupied on the frontthey clear, yet long enough to reach the tip jacks marked AUDIO OUTPUT andapron-that from which the volume con-dial calibrations. Then remount the dial RECORDER INPUT-thefirstand thirdtrol was removed. plate and attach the top pointer, using a from the left end of the rear apron when few drops of radio cement in addition to viewed from the back of the chassis. In Mechanical Changes crimping the back of the slider. The ver- the two holes, mount two 1.0-meg audio - The first step is to remove the dialtical line at the left end of the dial scale taper potentiometers, wiring the armsand all the dial stringing. Then removeindicates the position of full meshing of to the adjacent tip jacks, and the groundthe large pulley from the AM tuningthe capacitor plates for both sections. end to a chassis ground lead. Connect acapacitor shaft, and remove the smaller Therefore, the pointer should be secured .05-4f capacitor from the junction of R. pulleysfrombothcapacitorshafts. to the dial cord at this end of the dial and Ru (in the Browning diagram) Mount a A -to -h shaft extension on theand with the capacitors fully meshed. -from which a shielded lead to theFM capacitor shaft, and firmly attach a The FM pointer is similarly mounted switch was removed-to the high end new 4 -in. pulley to this extension, mak-to slide along the bottom of the dial of the potentiometer nearest theAM ing. sure that the dial cable opening inplate. This may take some careful ad- end ofthechassis.Connect anotherthe pulley is at the bottom when the justment to make sure that the pointer .05 -pi capacitor from the junction of Rucapacitor is half meshed. Remount thepasses the other cords without catching, and C44 to the high end of the other po-4 -in. pulley on the AM capacitor shaft,but it can be done with a little care. It tentiometer. These two potentiometerswith the groove of the FM pulley aboutis suggested that the sliders be given a permit adjustment ofthe outputs to1/16 -in. further from the panel thanlight coating of Lubricate or some simi- equal levels. the AM pulley. Mount a tuning shaft oflar lubricant.

IDLER

A-14 TUNING SELECTOR F-1141 TUNING

The solid line Fig. 3 (left). The tuner chassis with the dial plate removed toshow the dial stringing. Fig. 4 (right). diagram of the dial stringing. indicates the path of the AM cord and the dotted line showsthe path of the FM cord.

75 predominantly on the left, it is probabletion. The output impedance is sufficiently that FM would be the left channel. low that amplifiers may be located up to The balancing control provides a loss20 feet from the control unit without of about 3 db in each channel in thecen- appreciable frequency discrimination. ter position, with a variation of 3 db in the level of either channel as the control is rotated from one end to the other. If the control unit is to be used with ampli- PARTS LIST fiersat any appreciabledistance,it ReceiverModifications would be desirable to add two cathode 1 2 -pole, 3 -position switch, Centralab 1462 followers, as shown in Fig. 7. There is 1 SPDT rotary switch some additional gain in this circuit so2 1.0-meg audio -taper potentiometers, small the balancing control has a mid -position size loss of 6 db in each channel, with a range Fig. 5. Schematic of simple controlforstereo- 2 .05-µf capacitors, 400 v:, paper phonic programs. of 6 db front one end of the control to1 Dial drive shaft with panel bearing the other. The output from thisarrange- 3 44 -in. idler pulleys Output Circuits ment is approximately the same for all I 4 -in. dial pulley, with tension spring positions of the switch, and one addi- 1 Shaft extension, 54 -in. hole,0 -ill.shaft For use as a tuner capable of receiv-tional position has been added to permit 2 Slide -rule dial pointers ing two separate channels,reversal of the sides in the stereo posi- Nylon dial cord nothing further is required, since the two output jacks on the rear apron pro- Fo Figure 5 vide the AM and FM signals independ- 1 1.0-1.0 meg. dual potentiometer,alidu, ent of each other. However, for binaural taper use, some means of controlling the vol- 1 2.5-meg. potentiometer, linear or,if con- ume of both channels simultaneously is centric control is used an advantage. Therefore itis recom- 1 IRC K-2 Concentrikit mended that a separate control unit be 1 IRC KS -2 Universal Shaft Kit provided. This unit can be as simpleas 1 IRC B11-239 base element 1 IRC B13-137 base element possible, containing only a dual potenti- 1 IRC M13-137 base element ometer, relying upon the two output 4 Input jacks, RCA Phono type controls on the tuner chassis for bal- 2 0.22-meg. resistors,1/2 -watt ancing levels between the two sections 2 0.47-meg. resistors,1/2 -watt of the receiver. If a more elaborate unit 2 is desired, so that either channelcan be ForFigure 6 connected to either power amplifieror to both at will, it is equally possible to Same parts as for Fig. 5, with the addi- make such an arrangement. In any case, tion of 1 Centralab 1414 switch if the unit is to be used primarily for SWITCHES GANGED POSIT 1 - INPUT 1 ON BOTH OUTPUTS stereo programs, the balancing control POSIT 2 - INPUT 2 ON BOTH OUTPUTS is desirable. Figure 5 shows a simple POSIT5 BOTH INPUTS THROUGH TO BOTH OUTPUTS, For Figure 7 NO CONTROLS control,withoutanyprovisionfor POSIT STEREOPHONIC, THROUGH CONTROLS Same parts as for Fig. 6, except for the switching, yet equipped with the bal- substitution of the following for the re- ancing feature. This entire unit can be sistors listed: mounted on a small bracket or onan Fig. 6. The addition of a switch provides greater 2 0.27-meg resistors, 1/2 -watt unused space on the front panel of the flexibility in the operation of the converted2 0.39-meg resistors, 1/2 -watt installation. To reduce the number of tuner. With the switch at A, input 1is con- 4 0.47-meg resistors,1/2 -watt nected to both outputs; at B, input 2 is con- 2 0.1-meg resistors, 1 -watt apparent controls, itis suggested that nected to both outputs; at C, each input is the complete unit be assembled using 2 10,000 -ohm resistors, 2 -watt an connected to a separate output, without the 2 3900 -ohm resistors,1 -watt IRC Concentrikit, with the balancing volume and balancing controls in the circuit; 2 Noval sockets control operated by the outside knob, and at I); the inputs are connected through the 2 12AU7 tubes the dual potentiometers operated by the controls for binaural programs. 2 0.1-µf capacitors, 600 v., paper. small inside knob. The parts list shows the components for either type ofcon- struction. A more flexiblecontrol systemis shown in Fig. 6. The selector switch has five positions-(1) both AM and FM channels feeding separate output cir- cuits, with AM on output 1 and FMon output 2; (2) same as (1) except that AM is on output 2 and FM on output 1; (3) AM feeding both output circuits; (4) FM feeding both output circuits; and (5) stereophonic, with the dual vol- ume control and the balancing control in the circuit. The only eventuality not 01 OUTPUT 2 provided for is the reversal of the stereo- phonicsources,which could become necessary in certain instances. This was not included because of the impracti- cality of locating a four -pole six -position switch of reasonable dimensions. How- THREE SWITCHES GANGED POSIT 1- INPUT 1ON BOTH OUTPUTS ever, it is probable that some standard- POSIT 2- INPUT 2ON BOTH OUTPUTS ization in channel usage will occur, so POSIT 5 ` INPUTS152 SEPARATE ON OUTPUTS 152 RESPECTIVELY, NO CONTROLS POSIT - STEREOPHONIC, WITH CONTROLS that the FM channel is always on the left POSITS - STEREOPHONIC, WITH CONTROLS, REVERSED and AM on the right. or vice versa. Since the normal orchestra arrangement Fig. 7. Similar switching plus the addition of two cathode followersmay be preferred because places the high -frequency instruments of the lower output impedance offered with this arrangement.

76 BinauralorStereophonic?

R.J. TINKHAM*

The author clarifies the meanings of the two words which have been used to describe this latest of interesting innovations in the field of sound reproduction.

SOME CONFUSION apparently still ex- Microphone Placement ists in the minds of many regarding Apparatus intended for one field often the definition and connotation of the ends up in another. Gene Carrington, words binaural and stereophonic.Bi- educational director and lecturer for a naural means, literally, "two -eared." It large midwestern organization, learned refers to the fact that the sense of hear- of the foregoing and made orchestral ing in both ears, plus the brain, makes it recordings at Interlochen (Michigan) possible for us to analyze sound, to dis- Summer Music Camp under the direc- criminate against unwanted sounds and, tion of Dr. Joseph Maddy. Mr. Carring- to a large extent, to localize the source ton conducted experiments with respect of a sound. Stereophonic, on the other to the placement of the two mikes with hand, means "three-dimensional sound," separations varying from many feet to or a sound source "in the round," much about the distance of a human's two as we perceive it in actual experience. ears. Headphone listening showed that It can be stated, then, that one uses his the sound was most realistic when the binaural sense to perceive stereophonic mikes were about 8 or 9 inches apart. impressions. This is much like saying Fig. 1. The elements of a true binaural system. And the demonstration was astonishing that one uses his binocular sense of in its realism; one seemed to be stand- vision to achieve stereoscopic or three-like a male singer with a cracked voiceing right in the midst of the orchestra. dimensional impressions. If we stop uphumming in the bathroom instead of an During the past year, several demon- one ear with a finger when listening toauto on cobblestones?" Money had been strations of "binaural" sound have been someone in a noisy place, we will ob- invested in supposedly good equipment, presented, using two loudspeakers-one serve how the ability to discriminatebut the reproduced sounds were not con- for each channel-in a large room. The against the background noise disappearssidered natural or normal. Some formresults were puzzling to many people and the ability to localize the source ofof distortion seemed to be present. who attended these demonstrations-un- sound vanishes. Or, if we shut one eye, If one reflects on the difficulties en- tilthey heard similar demonstrations our judgment of depthis gone and, countered in achieving "realistic" soundwith . Then they understood although we know better, all objects areeffects for ordinary broadcasting (mon- what was being attempted.Suddenly as on a flat surface. Compare a mon-aural),wherecrumpledCellophane"three-dimensional sound," whether re- ocular snapshot with the modern stereo -sounds like a raging fire in the northproduced binaurally over headphones or photo or, for that matter, with grand-woods and a cantaloupe dropped on thestereophonically over loudspeakers, has ma's "Trip to Egypt" via the old-fash- floor portrays the victim's head being become most popular. But this popularity ioned stereoscope. The difference is re-bashed in, he realizes that all single -has raised another question. Why don't markable. channel reproduction gives only an ap-these demonstrations work so well with Modern tape recording has achieved,proximation of the original sound-our loudspeakers ?Afterall,itismighty (a) an easy recording method, (b) the imagination fills in the deficiencies. For inconvenient to listen for any length of ability to record more than one channelinstance, a symphony over the networktime to headphones, and loudspeakers at a time on the same tape, and (c) rea-sounds like a symphony, with which wereach larger audiences less expensively. sonable expense.Itisinevitable thatare reasonably familiar. But recordings The answer is that loudspeakers do someone would think of trying to cap-of autos on cobblestones do not sound work very well when properly handled. ture sound stereophonically. Our atten-normal, for this sound is a bit unusualAnd here entered the confusion between tion has again been directed to the Belland our imagination does not help out"binaural" and "stereophonic." Labs experiments'of1933 withthe much. If we refer to Fig. 1, where we have Philadelphia Orchestra performing at Let us consider the "auto -on -cobble- two closely spaced microphones ina home and being reproduced in Washing-stone" problem briefly. Since the one re- sound field, and a two -channel system ton withstartlingrealism,factslost cording microphone, placed in the driv-connecting them to separate earphones, sight of for nearly a generation. Iner's position, had no analytical or dis-it becomes obvious that we are merely 1948, Camras2 gave a demonstration of criminatory power of its own, it recordedextending the diaphragms of our two three -channelrecordingforArmourfaithfully what it heard and, for the firstears electrically. Actually, of course, we magnetic recorderlicensees.And in time(perhaps)the autoresearchersare extending the diaphragms of the in- 1950 the author, then with another com-heard, in truth, what they didn't hear-dividual headphonereceivers,electri- pany, was asked rather bluntly at a So-a hard fact to realize. Obviously, thecally, to the physical placement of the ciety of Automotive Engineers round-solution was to restore the natural sensediaphragms ofthe microphones. The table discussion, why the recorders thatof binaural hearing by using two com-time factor may be suitably delayed by he had sold some of the members did notplete and separate sound transmissionthe insertionof a simultaneous, two - reproduce properly, with the question,channels, from microphonesto head-track recorder in the bilateral transmis- "Why does the recording of an auto go- phones, one channel to each ear. Thission system. In other words, our ears ing over a cobblestone pavement soundwas suggested, and one organization triedit with successful results.3 Oddlyseem to be where the microphones are at physically placed. We have re -oriented * Ampex Corporation, 934 Charter .Si.,enough,loudspeakerreproduction, our ears geographically and individually. Redwood City, Calif. that time, was not successful for reasonsThe theorem of proportionality exists: 1 Electrical Engineering,Jan. 1934, Vol.which will become clear as we continue. mike -to -mike spacing is equal to the ear- 53, No. 1, six articles. 3 J. Acous.Soc.Am., Nov. 1952, Vol. 24, phone -to -earphonespacing,andour 2Marvin Camras,in Proc. I.R.E.,April 1949, Vol. 37, No. 4. No. 6. brain functions normally to fuse the two separate signals arriving from each ofspeakers, we no longer receive sound our two ears. The resultant sensory per-meant for the left ear only in the left ception is astonishingly like listening in ear, and right -ear sound onlyin the the spot where the mikes were placed. right ear, but rather we receive sound Of course, we are assuming that wein both ears meant for individual ears have paid attention to the proper phasingonly. Our original mike placement simu- of the mikes, amplifiers, and headphones,lated our relative ear positions, not the and haven't interchanged sides. Disturb-loudspeaker positions. This results in a ing but interesting effects result frombrand new sensory experience which such maladjustments. And the quality ofpuzzles that superb analyticalinstru- the reproduced signal, as always, de-ment called a brain, for the sound is pends on several factors present in thetwice mixed: once in the air, because system: frequency response, distortion,each ear hears sound from each speaker, noise, flutter, wow, etc. not just from one as it should; and once But we soon discover that listening toin our brain. Since we hear left and headphones makes the sound appear toright sound in both ears simultaneously, be behind, rather than in front of us.with head diffractionlags,etc.,our This is a fact brought out by Bell Tele-brains are led to the mental conclusion phone's "Oscar" exhibitatthe1933 that we hear two speakers. This leads Chicago Worlds Fair and now per-to the inescapable conclusion that we are manently displayed at the Chicago Mu-befuddled, and this type of reproduction seum of Science and Industry. And it isbecomes a new and novel experience to also just as tiring today to be squeezedbe sure. This is certainly not the type Fig. 3.The truestereophonic system. When by a headset as when the crystal radioof reproduction for which we search, threechannelsareused,distancebecomes was in vogue. The apparent answer to namely that of reproducing the original greaterthan whenonlytwochannelsare theproblemsisobvious.Reproduce soundstereophonically(i.e."inthe employed. the signals over loudspeakers, of course. round"). But with this system there is, This will put the sound in front of us,however, one point or group of points advantages. Also, we are not too much and will be more convenient for listen-lying on theperpendicularbisectingconcerned withtherecordingstudio ing. But while the sound as we hear itvertical plane between the two speakers acoustics,since our built-inanalyzer from the two loudspeakers sounds dif-where, due to phase relationship and (brain) compensates for almost all of ferent from single channel listening, it standing wave patterns in the reproduc-the acoustic shortcomings of that room, still doesn't sound quite right. Remem- ing room, we can hear sound from the suchasexcessivereverberation,un- ber, we still have the mikes placed only left speaker predominantly in the left earwanted echo, etc. This we have learned 8 or 9 inches apart. We have suddenlyand vice versa. Here the results areto do unconsiously since earliest child- invented a new system of listening: "bi- somewhat more satisfactory. A slighthood. We use here our binaural sense stereonauralphonic" or a cross between shift of the head will make this interest-in the proper and usual manner. two different concepts-which is mean-ing phenomenon disappear, and again But now, since we wish to reproduce ingless. See Fig. 2. The placement ofboth sound sources will be heard in boththis sound over speakers, we must take the speakers is unimportant here, as no ears simultaneously. This narrow cen-the listening -room acoustics into con- arrangement will give the desired effect.tral "binaural" plane limits the numbersideration. In fact, the listening room It doesn't matter whether we set the of happy listeners to those who can standbecomes an extension of the recording speakers at an angle so that their axesclose together in tandem. room. Of course, this has always been intersect, as at (A) in Fig. 2, or so that true in the case of a one -channel system. they are square with each other. Speaker Corrective Measures but not nearly as obviously so as it now manufacturers pride themselves today The answer to this predicament is is in the case of the two- or three -chan- on the non -directionality and wide-angleimmediately obvious to anyone familiar nel stereophonic system. After all, we dispersion of their products. What hap- withacoustics.Inthecaseofthe are now trying to re-create that special pens under these conditions of close binaural setup of Fig. 1, we are not con-effect in the listening room which ex- mike spacing and wide speaker spacing? cerned with listening room acoustics,isted in the concert hall. If we stand somewhere in front of theobviously, and this occasionally has its Let us suppose for a moment that we wished to reproduce a concert, by means of loudspeakers, in the same hall as that SOUND in which it had been previouslyre- SOURCE corded. In reality we should have an in- 1.2:10.1: finitenumberofspeakersarranged across the stage and several layers high in order to reproduce a similar full wave

TWO CHANNEL front. But for the sake of economy we TRANSMISSION use only two (or three) and spread them, SYSTEM one to the left, one to the right (and preferably another in the center). If we space two of them at, say, the one-third (0 points across the stage asin Fig. 3 where a = b = c), then, by proportion- ality, the microphones used to pick up the original sound should have occupied the same positions now occupied by the ",, speakers. Sound is then radiated from thepointswhereitwasoriginally picked up. Thisisa most important owks BINAURAL factor, a factor relating to the theorem POSK ION of proportionality. The recording sys- tem has merely delayed the element of time. The speakers then deliver approxi- IA) 161 mately the same acoustic pattern into the room that was picked up at those points by the microphones. Sound from Fig.2."Bistereonauralphonic," or mixed and meaningless system. all over the stage reaches the various

78 reproduction. An overly wide spacing of the mikes will leave a "dead" spot in thepickup between the mikes(and speakers) at the center of the group. This is where the third channel, in the center, becomes helpful by really round- ing out the stereophonic system. The BellLabs experiments showedthat three channels were an optimum mini- mum number, but that two would work satisfactorily, if properly handled. The insertion of a dual AM and FM simultaneous broadcast radio link does not change any of the conditions set forth, obviously, but does limit the sys- tem to two channels. Separate-sideband AM transmission plus FM has been sug- gested for a three -channel system, but appears to be still in the future. It should be repeated that the quality of the total system will limit the natural- Fig. 4. A complete portable stereophonicrecorder, with amplifiers and power supply. ness of the reproduction. This was clearly demonstrated at the November, 1952, microphonesatvarying soundlevels tern in the room. Corner speakers doconcert ofthe University ofIllinois and times. These factors are necessarynot set up such an image pattern. Symphony Orchestra, conducted by Leo- to the proper re-creation of the speaker - In general,itis necessary to know poldStokowski,atUrbana,Illinois, reproduced pattern. where and in what manner the sound iswhere the author was invited to trans- If the microphones had been spacedto be reproduced before the proper mic-mit the concert stereophonically to an "eardistance"(i.e.about9inches rophone placement can be stated. Foroverflow crowd in a relatively large but apart). as was done in a recent demon-headphone listening, mikes should bestacoustically different auditorium on an- stration, and the loudspeakers a similarbe spaced a person's head -width apart.other part of the campus. Stereophonic distance apart to maintain proportion-A bag of sawdust between the mikes,recordings were made simultaneously. ality, rather a neat trick which was notsimulating one's head,isof unknownDuring the rehearsal two cardioid mikes done. we would have had in effect a were tried,followed by two modern one -channel system. This would accom- condenser mikes. The former, designed plish little toward our ends. But with the TAPE many years ago, have an increasingly speakers spacedwidely, we losethe 1=1=1 -O. attenuated response above 9,000 cps. The necessary proportionality and achieve condenser mikes have good high end onlyourunrealistic"bistereonaural- response. Several of the musical faculty phonic" system. Conclusion :the mikes ERASE RECORD PLAYBACK present commented on the noticeable should be spaced as far apart for this RECORD PLAYBACK difference, and the more nearly true type of recording (stereophonic) as the string and oboe tone resulting front the speakers are to be spaced. We must Fig. 6. Diagramofthe arrangement ofthe switch. No other elements of the system know beforehand what sort of reproduc- Ampex stereophonic head assembly. were changed. ing system is to be used. One controversialpointmight be Next we add the problem of placing value.For loudspeakerlisteningthe raised here. What mike pickup pattern the speakers not in the original room acousticsof both therecording andshould be used? The author prefers a where thesound was recorded,but listeningrooms must betakeninto cardioid pattern, but with a wide fre- rather in some other and usually much consideration without fail. A wide spac-quency response. This pattern seems to smaller room, perhaps a living room.ing of the mikes is indicated so that thehelp stereophonic reproduction because whose major dimension is less. perhapstheorem of proportionality will be main-the pickup thus is one-sided, while the than the distance between the recordingtained. When two channels are used, thespeaker reproduction is other -sided. This microphones. Should we, therefore, re-mike spacing and position should bewould seem to help in making the listen- duce the spacing of the mikes? Experi-chosen with respect to the group sizeing room an extension in fact of the ment says no. Experiment also showsand arrangement so that all sections ofrecording studio. that if we space the speakers in approxi- the group will appear balanced in the mately the same general lateral arrange- (Concluded at bottom of next page) ment in the smaller room as in Fig. 3, where a'. b' = c', maintaining approxi- mateproportionality,theresultsare satisfactory. In fact, the sensitive cen- tral plane of "binaural" position,re- ferred to previously, disappears and the auditor is free to move anywhere within the listening room. He will experience a Fig. 5. Stereophonic similar sensation as though he were to headassembly, em- move to various parts of the original ploying a full -track auditorium. Placing the speakers in the erase head atthe corners of the room facing diagonally left, a two -track rec- ord head, and a two - inward or placing them close to side track playback head walls appears not to work as well as at theright. spacing them a little way in from the walls and square with the room, as in Fig. 3. In so doing apparently a more normal sidewall reflection and multiple image pattern is set up in the room, thus simulating more closely the recording set-up. This helps blend the sound pat -

79 Stereophonic Reproduction

JAMES MOIR

A basic discussion of the reasons behind our ability to locate sounds simply by hear- ing them, together withpracticalrequirements ofstereophonic sound systems.

Tif ERE IS LITTLE DOUBT that the best There is merit in mere size. An or-microphone at its mouth. examples of current sound repro-chestra that makes use of all the instru- Current microphone techniques aim ducer equipment meet most of thements all the time is flat and uninterest- to hide this defect by such procedures -as knowncriteriaforahighquality ing and is rarely employed. Instead the employing a microphone boom to sup- monaural system andinconsequence listeners interest is excited and main- port the microphone just out of the pic- there does not appear to be a great dealtained by constant change in the prom- ture and over the head of the speaker- of opportunity for further improvementinence given to the various instrumental the standard film and TV practice. insubjective qualityifpresent tech-combinations. Thus the centre of the A monaural system isat a further niques are merely subject to greater re- listeners' interest moves about the stage, disadvantage in that a single microphone finement.For exampleanamplifier the remaining instruments forming ais unable to discriminate against room having a frequency characteristic flat topleasant but unobtrusive background tonoise or reverberant sound, with the re- .01 db and a distortion content belowthe focal zone on which the listenerssult thatall recording and broadcast .01 per cent is not subjectively betterimmediate interest is concentrated. studios must be acoustically treated to than an alternative design flat to 0.1 a obtain a reverberation time much below (or even 1 db) and a distortion content Reduction of Source Width that known to be optimum if a tolerable of 0.1 per cent. In spite of this state of result is to be obtained with a monaural (pseudo) perfection no competent critic A monauralreproductioniscom- reproducer system. Similarly, noise from would consider that the best possiblepletely unsatisfying in this respect, the the audience coughing and shuffling ap- monaural reproduction of anything butwhole of the 100 by 30 ft. source being pear to be enormously enhanced when a soloist could be mistaken for the realcompressed and strangledto emerge heard over a monaural system. A simple thing, and until we can deceive most offrom an 8- or 10 -in. diameter hole, with but remarkably satisfying demonstration the people for most of the time there isthe result that there is no possibility ofof the magnitude of this effect can be room for improvement in techniques. identifyingor appreciating theindi- vidual instruments or sections of theobtained by anyone with normal hearing The pleasure derived from listeningorchestra on the basis of their spatial and access to a hearing aid of the nor- to a live orchestrais compounded ofdistribution. The pleasure to be derived mal monaural type. Conversation that many factors, most which are adequatelyfrom the movement of the sound source is easily understood and appears to be dealtwith by alaboratorytypeof is irrevocably lost. without any noticeable background when monaural reproducer system, but if we A similar result is obtained from a usingtwoears,appearsagainsta are to have a perfect reproduction of themonaural reproduction of the normal marked background of reverberation and original there are many marginal factorsmovement of actors about a set or stage. other room noises requiring consider- that require further attention. An or-Movement in depth is moderately well able concentration ifitis to be under- chestra generally occupies a platformreproduced if the reverberation condi- stood when the monaural hearing aid is 60 to 100 ft. wide and 20 to 30 ft. deeptions are satisfactory but all movement used. Kock has shown that the human and this spatial distribution contributesacross the stage is reproduced as move- hearing mechanism automaticallydis- to the pleasure derived from listening.ment in depth. Thus all the action ap- criminates against noise whenitap- * 87, Catesby Road, Rugby, England. pears to take place in a tunnel with a proachesthehead fromadirection

head, twoside -by -siderecordheads It will be noted that binaural head- which record two simultaneous tracks.phone listening to orchestral music, for BINAURAL OR STEREOPHONIC? running parallel along the length of theexample, appears to give a much greater tape, and two appropriate pickup heads.sensation of a completely "new" way of (from preceding page) Tape motion is from left to right acrosslistening, and that loudspeaker stereo- the heads. Thus the tape is first erased.phonic listening (correctly done) is not then recorded upon, and then monitoredas astonishing. Headphone binaurallist- Something should be said about suit- a fraction of a second later over theening is best for many special investi- able tape recording equipment. Figureplayback system. The record heads aregative problems. But speaker reproduc- 4 shows a typical two -channel portable well shielded between the two and havetion should sound more natural than the recorder. One case contains the tape 50 (lb attenuation on cross -talk betweenheadphone method because we are used transport mechanism another contains channels. The gaps of both the recordto being ina room -acoustic environ- two separate record and simultaneous and playback structures are criticallyment. In fact, it is for this very natural- playback amplifiers. one set for each of aligned in parallel so that the time errorness that we search. the two tracks. The third contains the between the two separate tracks is less The author is indebted to his early power supplies for the amplifiers. The than .00003 sec. at 15 in./sec. tape speed,teachers in acoustics, his many custom- erase/bias oscillator, mounted in one or .00006 sec. at 73in./sec. This hasers, his associates, and several midwest amplifier, supplies bias directly to onean interesting sidelight. Tapes recordeduniversitymusicalorganizationsfor channel and through a buffer amplifier stereophonically (or binaurally) on thisbackground and experimentalset-ups to the other, so that the same bias fre- type of inline gapped head may be playedwhich have led to the conclusions set quency is used in each recording head. back on a standard full -track reproducerforth above. Some of the conclusions thuseliminatingthebeatfrequency in an entirely satisfactory one -channelreached might be somewhat controver- which would result from using two sepa- manner. This makes it possible to pro-sial. Much three-dimensional acoustics rateoscillatorsof approximately the ducepre-recordedstereophonictapesstill is. As one acoustican has said: "In same frequency. Figures 5 and 6 show which may be played on either a stereo-no branch of science is the theory so the head assembly with a full track erase phonic or a standard playback machine.simple and the measurement so difficult."

80 which differs from that taken by the ear in three major respects. has adopted the first positive peak as desired sound. This binaural discrimina- 1. Referring to Fig. 1it will be seena reference point and that in measuring tioninfavorofthe wanted sound that a sound from a source on the righttime intervals to fix the position of a amounts to as much as 10 to 15 db, and sound sourceinspace,itnotes that is entirely lost when a monaural system side will strike the right ear before the leftear, the time difference being ainterval between the first positive peak is used. maximum when the source is on onearriving at the left and right ears. This A further and somewhat unsuspected process must be repeated at fairly fre- result of this binaural discrimination is side and in line with the two ears. In thispositionthetime is quent intervals if a moving source is to a marked increaseinthe difference clarityof .00063seccorrespondingtoanearbe followed and it is suggested that the speech and an apparent decrease in in- separation of 21 cm. intervals between syllables might well termodulationdistortionwhenrepro- form the convenient gaps from which ducing music from a large source such to commence each new measurement as an orchestra. In fact it gives to the SOUND SOURCE of "time of arrival" difference. reproduction of a large orchestra the The loudness difference at the two degree of clarity characteristic of a small ears is mainly due to the presence of the orchestral combination. head between the ears. Any obstacle It is suggested that Chinn's preferred placed in a soundfielddistorts that frequency range tests using reproduced PATH LENGTH field. producing an increase of pressure program material cannot be compared DIFFERENCE \ on the side facing the oncoming sound with Olson's tests using live material as AT EARS \ wave and a decrease in pressure on the in the lattertests the audience were reverse side-a process known as dif- listening "binaurally." The difference in fraction. The pressure difference be- conclusions largely represents the dif- tween the two sides is a function of the ference between monaural and binaural Fig.1.Difference inpath length and time of ratio of obstacle diameter to wavelength reproduction. arrival when sound sourceisnotinmedian of incident sound, and for a given size It is apparent that at the present time plane. of obstacle will increase as the frequency few of the advantages of a stereophonic 2. There %%111 be an intensity differ-of the incident sound rises(i.e., wave- system can be expressed numerically ence at the two ears, this difference be-length falls). An exact calculation of the and recourse must be made to express- ing a function of the frequency of thefield distortionis a problem of great ing opinions until such time as we have sound. The intensity difference isfre-difficulty but as we are only interested some system of indicating the overall quency dependent as it is mainly the re- in diffraction around a human head, subjective appeal of a reproducer sys- sult of diffraction round the head. Weiner's measured results are satisfac- tem. A number of organizations have 3. Most everyday noises have a com-tory. These are shown in Fig. 2 and worked on the problems of stereophony plex frequency spectrum and as the dif- both in Europe and in America and have fraction losses are a function of fre- recorded their opinions on the advan- quency the frequency spectrum at the tages of stereophonic systems. two ears will also differ. Thus, tests by indicated that a stereophonic system having an These differences justify further dis- audio bandwidth (per channel) of 3750 cussion. The reason for the difference in 20 cps was considered by an audience to time of arrival at the two ears is evi- '5 have the same aesthetic appeal as a dent and requires no further explana- 10 /-\ monaural system 15,000 cps wide. J. P. tion. but the question immediately arises 5 as to which part of the sound -wave cycle Maxfield ofBell Labs has stated "I 0 would rather hear a two channel repro- is accepted by the ear as determining %COO 10000 duction flat to 6,000 cps than a single the time of arrival at that ear. On an FREQUENCY -CPS channel system flat to 15.000 cps ;itis impulsive sound having a steep wave Fig. 2. Ratio of sound pressure (in db) at ears morepleasing,morerealistic,more front it may be assumed that the arrival for an angle of 45 deg. to the source. dramatic." This is a concise indication of the wave front is recognized, but on of the writer's opinion after more than a repetitive waveform there is difficulty indicate that the pressure difference at a year's work on the subject, with the inunderstandingjusthowtheearthe two ears has risen to 8 db at 1000 opportunity of hearing the systems de- recognizes the difference between suc-cps and 17 db at 6000 cps. Pure tones veloped by three of the leading European cessive cycles with identical waveform.are notofsignificantimportancein concerns. A high -frequency wave passing fromeveryday life where the usual sounds- right to left will have several cycles pass such as speech, music, and noise-have Explanation of Stereophony therightearbeforethefirstcycle energy scattered throughout the whole Without further discussion of the vir- reaches the left ear, and the right earfrequencyspectrum.Onacomplex tuesofastereophonicsystem,the may not know just how many cyclessound the resultant pressure difference mechanism of the stereophonic effect have passed at the instant the first cycleat the two ears will obviously depend will be discussed as this is probably the reaches the left ear. This rather suggestsupon the frequency spectrumof the best approach to an understanding of the that there may be difficulty in fixing theenergy in the sound. Steinberg has cal- technical requirements of a stereophonic position of a high -frequency source hav-culatedthepressuredifferenceand system. ing a frequency such that more than halfhence the loudness difference for normal Mother Nature has provided us with to one cycle of the wave can he accom-speech and his results are shown in Fig. two ears and these enable us to sample modated in the space between the ears.3 from which it will be seen that up to the sound field at two points spaced apart Taking the velocity of sound as 33,000angles of 40 deg from the median the by the width of the head. From the dif- cm/sec and the ear spacing as 21 cm.loudness difference in db is almost di- ferences that exist at the two ears and itmight he expected that frequenciesrectly proportional to the angle turned with the henefit of long experience, the above 800 cps(half wave =21 cm) through by the head. ear - nervous system - brain combination might present difficulties in location and The third major difference noted is can estimate the position of a sound it is worth noting that this is found tothat as the diffraction effects are a func- source with remarkaLle accuracy. giv- be the case in practice. tion of frequency. with a resulting dif- ing the same three-dimensional signifi- Using a very elegant test technique,ference in the frequency characteristic cance to the acoustic environment as the Galambos has taken simultaneous photo-of a sound at the two ears. With experi- possession of two eyes givestothe graphs of the sound waveform and theence the brain may be able to use this visual environment. nerve response that results from thedifference to provide a clue to localiza- For any sound source notinthe sound. and these show that the net ve tion. median plane the sound at the left ear discharge always occurs at the first posi- For sounds originating in the median will differ from the sound at the right tive peak. This suggests that the brainplane all these differences vanish as the

81 right -ear signalscannot be allowed to contaminate each other. Atthe trans-

1c(I,.. mitter end the sound pickup consists of A EA two high -quality microphones '---..' mounted FACE FACEl 2 14tteri:.; in a space model of thehuman head to simulate S1 0 the acousticfielddistortion .4 produced by the head inpractice. After i 2 FAR amplification the signalsare conducted \ EAR -4 by two separate channels to the two earphones. The results of thisare ex- 6 tremely impressive but the \'4,.... ---4...... / necessity of DIFFERENCE wearing headphones militatesagainst its 0 20 40 60 SO 100120 140 160 100 use and it would appear unlikely to find ANGLE in DEGREES favor with the general public unless the SOUNDPROOF WALL Fig. 3. Variation in loudness asa speech source headphone cordsare dispensedwith. isrotatedin a horizontal plane around the This could be accomplishedby intro- Fig.5. A wall can be renderedsound trans- head. ducing two local (domestic)low -power parent if face A is covered withmicrophones source is then symmetrically disposedradio transmitters withminiature re- connected through individual amplifiersto loud- ceivers mounted on theheadband-a pro- speakers on face B. with respect to the two ears, a condition cedure fairly common in film that holds at all angles of elevation. It and TV cation. With present techniquestwo or is to be expected that the precision of studios for transmitting instructionsto three separate channelsare all that can the operators of mobilecameras mon- location in the vertical plane vvoula be aurally. be accommodated on tape, disc,or film, poor and in practice this is found to be so it appears advisable to check theper - the case. Discrimination between back and front is also found to be poor unless PATH LENGTH the head is free to makesome explora- DIFFERENCE AT tory movement. The slightest rotation EARS of the head provides adequate discrim- ination between front andrear,pre- sumably as a result of the brain noting the direction of the change in the time differences at the two ears. It will he seen that thereare three Fig.6. Path -length main differences in the soundsat the discrepancyusing two ears-a time of arrival difference,a closely spaced micro- loudness difference, and a frequency phones andt w o - - speaker system. characteristicdifference.Atpresent there is no conclusive proofas to which PATH LENGTH

DIFFERENCE TO of these provides the real clueto locali- MEMBER OF zation in practice. It seems highly likely AUDIENCE that all three make contribution, with time and loudness differences providing the major clues.

Practical Stereophonic Systems LISTENER Consideration can now be given to If the listener's ears cannot betrans- methods of achieving stereophonicre- ported to the studio fornizince of two- and three -channelsys- production via an electrical reproducer itispossible to tems. adoptthealternativeapproachand In a simple two -channel system, knowing that we must maintaintransport the acoustic field to the list- system, local- the time and amplitude differencespres- izationisweakest in the center just ent at the pickup points in the studio.ener. The principles involved will be where it is desirable that it shouldbe at understood -by referring to Fig. 5show- There are two approaches to this prob- ing a long hall divided into its best and the addition of thethird lem, the first that of taking two separatemicrophone and speaker bridged two samples sections by a soundprooftransverse wall across of the sound field and transferringthese the two outer channels improves theper- at A. This wall can be renderedsound formance in this respect, though two samples to the remote listener'sears transparent (unidirectionallv) by none through completely separate electrical cover- of the combinations triedare as good as ing the A face witha large number ofthe three -channel layout. systems and two headphones as shown inincrophones each connected through Fig.4.Two entirely separate systems an amplifier to a loudspeakeron the B face Trial Attempts are obviously necessary as the left- andof the wall. Any sound field approaching There the A face would be reproducedon the have been various attemptsto B face and the audience in theB section combine the advantages of the dummy- would be unaware of the dividing head microphone mounting withthe use wall. of loudspeakers, though While this procedure would hereason- so far without ably effective it is commerciallyimprac- any outstanding success. From the point tical, as a separate line andamplifier are required between each microphoneand its associated loudspeaker.Some means of reducing the number ofmicrophones and loudspeakers is required. In the vast majority ofstage plays and most films the action is largelyconcen- trated at ground level andas localization in the vertical plane is ratherpoor in any case it seems reasonable toassume that the "vertical" informationneed not he transmitted. This eliminatesthe need Fig.4. Stereophonic reproduction with head- for all the loudspeakersexcept the bot- Fig.7. When close -spacedmicrophones are phones. tom row, a very considerablesimplifi- with loudspeakers, the stereo effectislimited to the clear area.

82 of view of microphone technique the dummy -head mounting has the great ad- vantage of simplicity, two microphones in a common mount being no more diffi- cult to handle than a single microphone. The limitations appear when considera- tionis given to the time differences which result in seating positions off the room axis. Referring to,Fig.6 it will be seen that at the microphone the posi- tion of a sound source is characterized Fig. 8. Method of by a time difference proportional to the error measurement in sound location tests. path -length difference, and this holds for all positions on a line midway between the loudspeakers. At a point such as L well off the center line the original time differencea -biscompletely swamped by the time differencec -dwhich is char- acteristic of the relative positions of the listener and the loudspeakers, and has no relation to the position of the source. It might be expected that the stereo- phonic effect would not be quite so ob vious in position off the axis of the placed by a high -quality loudspeaker re-relative position of the three tracks, as room and this is found to be the case in producing speech picked up by a micro-it is probably impossible to mount three practice.Figure 7 isindicative of the phone in another studio. Three filtersrecording or scanning heads side by area in which effective stereophony is having the characteristics shown in Fig.side. Earlier it was noted that two chan- nels 6,000 cps wide, or three channels obtained. Though this appears to be a 9 were used to define the frequency range in the three tests, the results be-4,000 cps wide, gave results which were fundamental weakness ofallsystems ing shown in Table II. It will be seensubjectively assessed as being equal or using close -spaced microphones, the ad- that the majority of information onbetter than a monaural channel 15,000 vantages of simplicity in the microphone source positionis carried by the fre-cps wide, and in consequence it becomes technique justifies further work on the quencycomponents above 500cpsnecessary to consider how best to use a system. though the ear can find adequate infor-medium such as tape which can deal Figure8 indicates the technique used mation in the small amount of energywith a frequency band perhaps 20,000 in all the accuracy tests. A caller held that remains in the region above 3000to 25,000 cps wide. Nothing isto he cps or below 500 cps. With the malegained by reproducing frequencies a horizontal stave marked off in each above 15,000 cps. and a more pleasing direction from the center, and stood speakers employed in these tests, speech intelligibility was zero when the bandresult can certainly be obtained by divid- about 25 ft. from the subject under test. below 500 cps or the band above 3000ing the band into two or three separate The subject wore a light head harness cps was being used. channels each 6000 or 7000 cps wide and carrying a horizontal sighting rod which In tests we have noted that dynamicusing them for a stereophonic system. was visually lined up on the center zero localization (the localization of a mov- Phonograph records present a more of the stave at the commencement of the ing sound source) appears to be appreci-difficult problem and as they may be superseded by tapeit may never he test. The caller took up a position on a ably more accurate than the localization 25 -ft. radius from the blindfolded sub- necessary to solve that problem, but one of a stationary source. neat solution due to A. D. Blumleinof ject and read passages from a book un- The final point to be considered is theE M I is worth noting. Blumlein pro- til the test subject indicated that he was technique to be used in obtaining twoposes to record a two -channel stereo- "on target" at each position. A third or three channels between studio andphonic signal on an ordinary disc by assistant read off the error in position reproducer, probably the most difficultsimultaneously using lateral and vertical on the reader's stave, along the sighting problem of all. For %-in. magnetic tape,modulation of the single groove. Equip- two solutions are fairly straightforward,ment was built and succesfully demon- rod on the subject's head. This pro- the first being the use of two or three cedure was repeated at ten different an- strated that this was possible. E M I separate tracks on the same tape. Doublehave demonstrated 78 r p.m. recordings gular positions with five different sub- track tapes are currently available and jects and the errors averaged. For the flat to 20,000 cps and these would permit can be used for a two -channel systemtwo or three channels 6000 to 9000 cps indoor tests the same group of observers withoutdifficulty.Three tracks eachwide as suggested for tape. repeated the procedure in a small the- .05 in. wide are possible, though agree- Broadcast -band AM radio poses on ater, the listening position being 55 ft. ment will have to be reached on the (Concluded at bottom of next page) from the caller. The results of the first series of tests are set out in Table I and it will be noted that accuracies of the o - - order of 1 deg. can be achieved both in- A doors and outdoors. A comparison with 5 the errors made when attempting to align the eyes on the stave zero showed cv that the accuracy of visual alignment is 10 not more than twee that of an aural Fig. 9. Filter charac- alignment. This is surprising, as the ears teristics. (A) low- are rarely used for the purpose of posi- pass filter; (B) band- 2'5 passfilter; and (C) e tion fixing, a duty left to the eyes. high-pass filter. During the indoor tests the oppor- 20 tunity was taken to check the effect of

frequency -range restriction on theac- curacy of location. The results also serve 25 to indicate the information carried by the various regions of the frequency a 30. . ----c''- Igor spectrum. The technique remained as in 1181 PNICOIJANCVIN CYCLES KRma:mu, the previous tests but the source was re- ...-

83 A Discussion of Dividing Networks

J.P. WENTWORTH

Presenting two simple dividing networks -one of which provides a connection point for negative voltage feedback which will equalize over-all amplifier -speaker response.

IT IS GENERALLY ACKNOWLEDGEDthat aoutput transformer. which was able to handle all frequencies single loudspeaker unit can not re- An ideal multispeaker system wouldTo accomplish this aim, the network produce optimally the entire audible givetheimpressionthatthesoundshould satisfythe following criteria : frequency range. For clean reproductionemanated from a single super -speaker, 1. The network must so distribute the ofhigh-fidelitymusic,atleasttwo speakers are required, each transducing signals in a limited frequency band. To insure that each speaker receives only RICK signals within its own frequency range, it is customary to use a dividing network or networks todistributethesignal SPEAKER VOICE among the speakers. COILS Perhaps the neatest way to accomplish such signaldistribution would be to RHiSP separate the frequency bands at a low VOICE COILS power level, and to use an individual power amplifier for each speaker. How- C In forads C C T2RfcR ever, since the output stage and trans- L in henries former represent a considerable part of L 11 2R2C R in ohms L.2nt, R2C a high -quality amplifier, in terms of cost, 2Pf, power consumption, and weight,itis (A) IR/ usually more expedient to make the separation at the secondary side of the Fig. 1. (A) Parallel network, constant resistance type, giving a cut-off slope of 12 db per octave; ' 2058 E. 81st St., Cleveland 3, Ohio. (B) Half -section network giving cut-off slope of 6 db per octave.

Details of an experimental determination of the The renewed interest that has been amount of discrimination against noise that is STEREOPHONIC shown both in Europe and America in given by our binaural hearing system. (from preceding page) the problems of stereophony indicates that it will be the next major step to- 2.Harvey Fletcher, "Hearing the deter- wards obtaining a perfect reproduction. mining factor in high-fidelity transmis- even more difficultproblem,particu-Under the stress of competition from sion." Proc. I.R.E., June 1942. larly in Europe whete all stations are on An excellent paper which reviews most of the TV, stereophonic sound may appear first factors known to be of importance in obtaininig a nominal spacing of 9 kc. Single side -in sound movies and may be accompanied a high-fidelity reproduction. This paper is im- band transmission of two channelsis by the introduction of a wider screen to portant to anyone interested in the subject of possible as the data in Table II showstake advantage of the new freedom. high-fidelity. that frequencies below 500 cps are of no great importance tothe stereophonic 3.Galombos and Davies, "The response of TABLE I single auditory fibres to acoustic stimu- effect,permittingadouble-sideband lation." J. Neurophysiology, 1943, No. 6. transmission of frequencies up to this Comparison of the Accuracy of Location in Describesnervevoltagemeasurementsthat Indoor and Outdoor Environments appear to have an important bearing on the point, with single-sideband transmission mechanism of the stereophonic effect. Most in- of the left and right signals on the upper Sound Source -Original Male Speech teresting reading. and lower sidebands, as shown in Fig. Standard 4. 10. Mean Error Deviation F. M. Weiner, "Diffraction of a sound On short waves there is no technical wave by the human head." 1. Acous. Indoors at a spacing Soc. Am., Jan. 1947. difficulty. The audio bandwidth ofa of 55 feet 1.04 deg. 1.15 deg. short-wave transmitterissufficient to Outdoors at a spacing 5.J. C. Steinberg and W. B. Snow, "Au- provide two or three channels as sug- of 25 feet 1.2 2.7 ditoryperspectivephysicalfactors." gested for magnetic tape, or the carrier Elcc. Eng., Jan. 1934. may be simultaneously modulated by FM One of a group of six papers devoted to stereo - TABLE II phony. That loudness differenceisthe main and AM signals to give two channels. factor in binaurallocalizationisnot now so Accuracy of Location using Bands of widely believed as in1934, but the group of CARRIER Filtered Male Speech papers is required reading. LOWER SIDEBAR° UPPER SIDEBAND Frequency Std. CONTAINING LEFT CONTAINING RIGHT Range Avg. Error Deviation 6.J. Moir, "Stereophonic sound," Wire- EAR SIGNAL EAR SIGNAL less World, March, 1951. 53-500cps 3.8 deg. 3.55deg. 500-3000 0.9 3.8 7.J. Moir and J. A. Leslie, "The stereo- 3300-7000 0.5 3.4 phonic reproduction of speech and mu- 10 KC 5000 5000 +10 KC 53-7000 0.7 4.7 sic." J. Brit. I.R.E., 1951 Radio Con- DOUBLE SIDEBAND vention. TRANSMISSION OF LOWER AUDIO FREQUENCY REFERENCES 8. K. de Boer, "Stereophonic sound repro- 1. W. E. Kock, "Binaural localization and duction,"PhilipsTech.Rev..April, Fig. 10. Radio bandwidth usage for stereophonic masking," J.Acous. Soc. Am., Nov. 1940. transmission on AM. 1950. An account of the factors that are of importance.

84 signal among the speakers that the acous- a system having the same crossover tic power output at every frequency is the frequency and the same speaker imped- same as that radiated by the single hypo- ance. Since the capacitors used ina thetical ideal speaker. high-fidelitysystem must beofthe 2. All frequencies should be radiated high -quality, oil -filled variety, and since with the same phaserelationshipsas the values of capacitance required in a those that would exist in the case of the practical system are quite large, a re- single ideal speaker. This objective is a duction by 30 per cent in capacitance rather minor consideration in minimizing represents a very real reduction in cost. phase distortion; however, if inverse feed- Moreover, use of a smaller inductance back voltageisto be taken from the will result in a lower resistance in the speaker voicecoils,itbecomes are- inductors, with a slight resultant gain quirement of paramount importance. inefficiency, and with a concomitant 3. The speaker system should present improvement in damping. to the amplifier a constant and purely re- In circuit B, any resistance in the in-Fig. 2. Circuit arrangement for obtaining feed- sistive impedance, in order to minimize ductor winding can be written off asback voltage from voice coilsof two -speaker distortion in the output stage of the amp- system. lifier. part of the speaker resistance, and can 4. In the interest of economy of power, easily be balanced by addition of aaudio frequencies. it is desirable that the network be non - padding resistance in the other half of So far, nothing has been said about dissipative;i.e.,thatit be made up of thecircuit.However,areasonablerequirement(5)above,i.e.,that the purely reactive impedance elements. amount of resistance unbalance can bespeakers should see a low -impedance 5. In order to provide effective damp- tolerated, except when feedback is usedsource.Unfortunately,bothofthe ing, the source impedance seen by each from the voice -coil windings. In theabove circuitsare weak inthisre- speaker should be as low as possible. latter case, phase shift may become ex-spect, circuit B perhaps having a slight It is evident that conditions (1) and cessive if balance is not maintained. advantage over circuit A. Moreover, (2) will be implicitly satisfiedifre- Similarly, inductance in the speakerthe source impedance-and hence the quirements (3) and (4) are met. winding can be regarded as part of the Two networks that will fulfill require- speaker damping-will be a function of ments (1) through (4) are shown ininductor in one branch of circuit B.frequency. If speaker resonance occurs Fig. 1, where R is the impedance of the Unfortunately, however,sincethisat a frequency at which the damping speaker, and f0(the "crossover" fre- speaker is carrying the low -frequencyis ineffective, "muddy" performance is quency) is the frequency at which the components of the signal, the induct-inevitable. radiated power is equally divided be- ance in this speaker is of less impor- However, a low source impedance tween the two speakers. The input im- tance than itis in the other speaker,can be provided by applying the voltage pedance of either of these two networks which is not compensated by the cir-across the voicecoilsas an inverse is equal to R, and is independent of fre- cuitconfiguration. By applicationoffeedback signal. Feedback voltage may quency, if the following conditions are Thevenin's theorem, one may see thatbe developed by the circuit shown in met: (a) the impedances of the speakers Fig. 2. For "fiat" output, R, and R2 itispossible to regard shunt capaci-should be equal. A certain amount of tance around the high -frequency of each speaker must be a pure resist- tone control can be exercised by vary- ance. speaker as included in the series ca-ing the ratio of R, to R,, thereby vary- Circuit A of Fig. 1 provides a con- pacitor, althoughthis capacitance siderably sharper frequency division of should be negligible, even at the highest (Concluded at bottom of next page) the signal than does circuit B ; the power radiated by each speaker of circuit A falls off at a rate of 12 db per octave 1,000 in the cut-off band, whereas the slope BOO for circuit B is 6 db per octave. Which aoo 700 of these circuitsis the more desirable % depends on several factors. Use of the 600 'Is sharper slope, for instance, reduces the 500 range of frequency over which any one 400 speaker has to handle an appreciable % amountofpower. However,unless 300 % great care is exercised in locating the '...4, speakers, this same sharpness of cross- ( over may destroy the illusion that the 200 \ 0 sound is being radiated from a single + %+.9 source, since the fact that the sound is 4, being produced in two frequency bands % % is more obvious under these conditions. 100 N 90 90 Relative Advantages 70 Nik From the point of view of the hobby- 60- C - copacitonc in fnicroforods ist, circuit B offers certain advantages 50- L- inductonc in iinicrohlinris

over the more complicated circuit A. 40-R- rsislance In ohms II It is true that a saving in complexity of % the circuit is probably of negligible im- 30 CIRCUIT A CIRCUIT B portance, per se; and certainly any re- duction in size and weight of the cir- cuit will be a small percentage of the 20 volume and weight N. inherentinthe .11 speakers themselves. However, there is a material saving in the cost of the components 40,00 required-circuit B re- .000 0,000 quires only about 36 per cent of the CROSS -OVER FREQUENCYIN CYCLES PER SECOND inductance, and some 70 per cent of the capacitance required by circuit A for Fig. 3. Chart for determinir.g values for L and C in networks of Fig. 1.

85 A Transistor Phonograph Pre- amplifier for Magnetic Pickups BASIL T. BARBER Complete instructions on buildinga transistor preamplifier with miniscule power requirements, withan analysis of its performance and data on transistor circuits in general for audiouse.

SCENT AVAILABILITYOfimprovedsistor applications in audio reproduction. are required, or a tapped one. The prin- types of and their pres- Following is a brief classification of cipal problem with grounded -base cir- ent successful applicationin thethe three Easic types of transistor cir- cuitsis one of stability, especiallyif field of communications, computers and cuits with their duals in the more famil- transistors with a, the current amplifica- guided missiles, indicate that the tran-iar vacuum -tube circuit. A word of cau- tion factor, greater than 1 are employed. sistor has at last ceased to be lookedtion is warranted. Do not begin replacing Junction transistors are superior to the upon as a laboratory curiosity and isgrids with bases,plates with collec- point -contact type in this respect. rivaling in performance many functionstors, and cathodes with emitters ! A basic Grounded -Collector.InFig. 2we have that the vacuum tube has monopolizedunderstanding of transistor operation is the grounded -collector circuit, which re- for the past thirty years. essential to avoid frustration and disap- sembles the vacuum -tube cathode -fol- In the audio field, with the exceptionpointment in designing circuits around lower. The voltage gain is less than 1, of a few "transistorized" hearing aidsthem. Adequate literatureisalready but can be made as high as 0.95, a loss and AM tuners, progress has been slow, available, and although the theory of of less than 1 db.Powergains as high at least in comparison with other fields. semiconductors is not likely to be found as 20 db have been recorded. There is no This' was due mainly to the inherent easy to digest, one does not have to wade polarity reversal and external stabiliza- technical limitations of the new itemthrough Shockley's "Electrons and Holes tion with bleeders may be necessary. and its scarcity and high cost, ratherin Semiconductors" in order to design The most interestingfeatureofthe than the audio engineer's or high-fidelitysimple transistor circuits for audio ap- grounded -collector circuitisthe rela- addict's lack of pioneering spirit. Now plications. tively high input impedance which, al- that most of the original limitations of Transistorcircutrycanbe,very though it can be as high as 1 megohm, the transistor have been either elimi-broadly, put into three classes, depend- depends on the load resistor Re.. Low nated or greatly reduced, the way should ing onwhich elementisgrounded. values of Rr. will bring the input imped- be open for an increased number of tran-Functionally, each class performs differ- ance down. The output impedance can ently and its selection for a particular be made as low as 100 ohms. The tech- s Project Engineer, Sperry Gyroscopeapplication will depend to a large extent niques of self -bias can be employed as Corp., Great Neck, N. Y. on the specific requirements of the appli- successfully as in vacuum -tube circuits. cation as to gain, input and output im- Figure3 shows a common method of pedance, noise level, power output, fre- quency response, etc. Grounded -Base. Figure1 presents the first class. Electronically it is similar to a grounded -grid vacuum -tube amplifier. Input impedances of as high as 1,000 I duns and output impedances of as high as 20,000 ohms are possible. These are comparatively low ;itis necessary to usehigh -valueinputcapacitors and matching transformers if transistors are employed in cascade. Gains per stage as high as 37 db are possible, with matched Fig.1. Grounded -base circuit and its vacuum - impedances. There is no 180 -deg. phase tube equivalent. reversal as in a tube, and two batteries Fig. 2 The grounded -collector transistor circuit.

tionshipsinthecircuit may deviate both branches of the circuit. R and C seriouslyfrom those predicted on a can be trimmed with the help of a sim- NETWORKS basis ofthe nominal values. A more ple bridge, but cut -and -try seems to be satisfactory procedure would be to ad- the only feasible way to insure balance (from preceding page) just the inductance to the actualre- of the inductances, at the same time sistance and capacitance values, al:sorb- satisfying the condition, L =2R5 C.In ing the tolerances in these quantities in thisrespect,the simplicity of circuit ing therelativeoutputsofthe twoa slight shift of the crossover frequency. B proves to te a strong argument in speakers. The proper inductance may easily be favor of the latter circuit. Itis assumed that inductors will bedetermined by use of the circuit shown wound especially for the circuit, since Either of the two circuits shown in in Fig. 2; if R, and R, are equal, the Fig. 1 is readily adaptable to systems it is highly unlikely that the non-stand-correct amount of inductance will pro- of three or more speakers. For a three - ard values of inductance required willvide a constant voltage gain from a to speakersystem,a completedividing be available commercially. The fact thatb, independent of frequency. Values of network anditsassociatedspeakers L and C will resonate at the crossover Land C, normalized with respect to R, may be substituted for one of the speak- frequency can be used to determine theare plotted against crossover frequency ers indicated in Fig. 1-since the net- requiredinductance.However,thisin Fig. 3. method does not insure the relationship work with its speakers presents a con- If circuit A of Fig. 1 is used, care stant resistive impedance, the original L = RIC (orL = 2RiC.),due to toler-must be taken to maintain equal re- network will operate exactly as if work- ances in R and C, and the voltage rela-sistance, inductance, and capacitance in ing directly into a speaker.

86 at that frequency, or about 160 µf! This is a rather impressive value for a coupling capacitor. Recently available minature tantalum capacitors are ideal for this application. although their present cost reminds one of the black market days of the last war. Gain. The actual gain will depend on the degree of and, depending on circuitry, can be from 17 to more than 40 db per stage, which comes close to the capabilities of most high -gain triodes and pentodes. Fig.3. Grounded -collector circuit can be self - Noise. Noise may be the limiting fac- Fig. 5. The self -biased grounded -emitter cir- biasedinsomewhat the same manner as a tor of a transistor in audio. Remarkable cuit and the cathode -biased tube are not quite cathode -follower tube. improvements have been made during true duals. self -biasing. E is the bias obtained fromthe last few months in this respect and watt in a push -pull -parallel configura- voltage divider the noise level has been reduced from 60 tion.Indesigningpreamplifiersand Grounded Emitter. Figure 4 shows adb (at 1 kc) to below 10 db in some equalizers,however,thequestionof popular type of transistor circuit fortypes, which compares favorably with a power should present no problem since attaining good voltageamplification.vacuum tube. Junction types have lower only a few milliwatts is necessary. A Gains as high as 46 db are possible, andnoise level than the point -contact types. point of interest is a comparison of effi- the circuit exhibits a 180 -deg. phase re-The noise levelis,to some extent, a ciency between a transistor and a tube versal at its output, making possible the function of the collector voltage and a employed as preamplifiers. A transistor application of negative feedback aroundlower -power supply voltage will give requires about 30 milliwatts, while a itwithout resorting to transformers.less noise, but less gain also. Another tube would need about 5 watts,' or al- The input and output impedances areserious disadvantage of the noise present most 200 times as much power. At pres- higher than the ones of the grounded -is that its magnitude is inversely propor- ent, 80 per cent of the power used in a base class, but the problem of stabilitytional to the frequency, rising at about 3 transistor amplifier is wasted in drop- still exists, although it can be correcteddb per octave. In addition, the input ping resistorsrequired to provide a with bleeders as before. In Fig. 5, self -impedance seems to play a role in the constant -current supply from a constant - bias is again employed, eliminating oneamount of noise present and a matched voltage source and for stabilization pur- battery. load does not necessarily give the best poses. As transistors and batteries im- signal-to-noise ratio. prove, even the present small power Transistors and Audio Frequency Response. Transistor cir- drain will be considerably reduced. cuits have been designed flat to as high Hum. As thetransistorneeds no With the above brief resume in mind,as 10 mc, more than adequate even forheater supply, complete elimination of an evaluation of transistors can be madethat super -ultra -fidelity unit we have in hum, a stumbling block that has felled in terms of functions and characteristicsthe back of our minds. The average cir- many an audio enthusiast attempting to directly applicable to the audiofield. cuit will give uniform fre-build his own equipment-is a reality. This evaluation is based on the latestquency response to beyond the audio lir addition, There is no time delay for information available and is by no meansrange. The low -frequency cutoff pointwarmup after the power switch is turned complete or conclusive as, judging fromwill be determined by the value of the in- on. the present rate of improvement, only aput capacitor. Point -contact transistors Micro phonics. A transistor can with- short period of time may be required tohave better frequency response, but their stand a forced vibration from 5 cps to modify any of the main transistor char-high distortion and problematical stabil-5 kc with 100 g acceleration and shock acterstics completely. ity exclude them at present from any in excess of 20,000 g, with no ill effects From a designer's point of view theaudio applications. on its operation. Although itis hoped following may well be the most impor- Distortion. With junction transistorsthat such conditions are not likely to tant criteria on which a transistor maydistortion can be kept down to about 2 occur too often in a home audio system, be judged for its applicability in audioper cent and with negative feedback these characteristics are desirable if for work. values well below 1 per cent are possible. instance a transistor preamplifier is to Impedance.Innon-linearelements Dynamic Range. Outputs as high as be mounted directly under the motor - such as magnetic amplifiers, transistors,3 volts, enough for preamplifiers and board of a record player. crystals, and semiconductors in general,equalizers, are easily available. With a Stability. Units having current gain a, the input as well as the output imped-unit of low noise level, a dynamic rangegreater than 1 invariably present a seri- ance is low in most cases, resulting inof 60 db or more can be attained withous problem ofstability. With latest a serious problem of impedance match-excellent linearity. models of junction transistors and with ing, especially if these elements are to Power. Power requirements should, inthe use of external stabilization,this be employed in cascade. In telephonyall fairness, be considered in terms ofproblem has been almost eliminated. and general transmission work with co-the power level of the signals to be Temperature. Temperature effects are axial or 600 -ohm lines this becomes ahandled. likely to cause some trouble, especially desirable characteristic, but in audio it Present transistors can give as much in extreme climatic conditions. In nor- is likely to be a serious problem, al-as 0.1 watt maximum power output andmal surroundings temperature becomes though shielding headaches are mini-this level can be increased to about 1 no problem. As the temperaturein- mized. Impedance -matching transform- creases above normal, the transistor be- ers can be used, but their expense and comes more noisy and loses some of its their introduction of phase shift, distor- gain, but it can be improved with neg- tion, and limited bandwidth make their ative feedback. A word of caution when usefulness questionable. The input im- working withtransistors: The leads pedance of the transistor, usually less should not be subjected to prolonged than 1,000 ohms, makes necessary coup- contact with a soldering iron, as exces- ling capacitors of high value, though sive heat will result in permanent de- they can have low voltage breakdown struction of the unit. The same general ratings. precautions should be taken as when Assume that we have a transistor cir- working with germanium crystals. cuit with an input of 500 ohms. The in- put coupling capacitor should have a Life. Half-life, determined on a statis- value of not more than 50 ohms at 20 Fig. 4. The grounded emitter circuit is the dual ' Filament approximately 2watts and cps for 1 db (10 per cent) attenuation of the grounded -cathode amplifier. plate about 3 watts.

87 OVERALL GAINK 115,21db AT 1000'1, -on 42db AT 2O',

K 09. -1db _4. K -205,46db .4

22 1/2V cc

OUTPUT 10 460

A MAGNETIC C5 FOR ADDITIONAL PICKUP R8 HIGH FREQUENCY WM/ DE -EMPHASIS 8500110 47.000 (TURNOVER 400cps)

Fig. 6. The transistor preamplifier circuitry is simple and its response-including bass equaliza- Fig.8.This block diagram and the formulas tion-is excellent. The 22.5 volts may be furnished by the power amplifier bias supply,a small show that grain of the transistor preamplifier power supply, or a battery. is calculated in the same way as tube circuits. tical basis, is about 70,000 hours or close itisseenthata combination of asponse of the schematic in Fig. 6. Noise to10 years of continuous operation.grounded collector and a grounded emit-level, about - 39 db, is rather high for Since the only permanent part of the ter in a direct -coupled circuit is possible,our application. A gain of 42 db com- averagehigh-fidelitysystem is its provided that the appropriate bias levelspares very favorably with most elec- chronic susceptibilityto modifications are supplied. Direct coupling eliminatestronic tubes, taking into consideration (and improvements), the transistor is the need for a large interstage couplingthat the first transistor is actually em- likely to outlive, if not the owner him-capacitor, and also provides a relativelyployed as a cathode follower. Since the self, at least a dozen or so circuits built high input impedance. Peter Sulzer wascomplete circuit has a 180 -deg. phase around it. first to apply this.' Although this type ofreversal,frequency -selectivefeedback Reliability. With the proper powercircuitry needs two transistors, one ofcan be applied around it, modifying the supply, present units seem to exceed the which supplies no gain, the definite ad-frequency response to provide the low - vacuum tube in reliability. Uniformity vantages seem to outnumber the cost offrequency pre -emphasis necessary for in production has been a serious prob- the extra transistor, especially with theany magnetic pickup. In addition, the lem, but late units are manufactured elimination of a large -value capacitorfeedback will increase the input imped- within ±2 db uniformity in their char- and the likelihood of employing an inputance at the high frequencies, decrease acteristics, which is even better than thetransformer. the noise present, and improve the line- tube tolerances (±3 db). Figure 6 shows the final circuit se-arity and harmonic distortion. Space. A transistor is about the size lected. A CK721isemployed asa With a total gain of 42 db, the best of a coffee bean. voltage amplifier, instead of a CK722compromise may be to use half of it for Cost. Transistor cost is high at pres-giving about 40 per cent higher gain atthe middle -frequency band beyond the ent, ranging from $5 to $15, but prices about the same percentage increase inlow -frequency turnover, and the other will doubtless decrease as demand in- price. The input impedance, being abouthalf for the necessary low -frequency creases and methods of production are20,000 ohms, makes it necessary to em-preemphasis. The AES curve is em- improved.Availabilityrangesfrom ploy a large input capacitor -to get down ployed for developmental work. to 20 cps.It should have an imped- From Figs. 6 and 8 we have, stock to a few weeks delivery on mostance, at 20 cps, of not more than 1/10 types. of the input impedance of the circuit or 14= KilCi= (0.9) (205) = 18442db 2,000 ohms, which means a 4-14 mini-and therefore The Transistor Phonograph Preamplifier mum value. Taking the foregoing into considera- Figure 7 shows the frequency re- I (@/KC) =42 = 21 db12.5 - tion, it should be therefore possible to from which design a phonograph preamplifier which Sulzer, Peter, "Junction transistor cir- will compare favorably withitselec- cuit applications," Electronics, Aug. 1953, (a - 12.0)=12.5 0.07 tronic counterpart. From Figs. 3 and 5 p. 170.

Fig. 7. Frequency response of the preamplifier 1 1 Ilillil I I without its negative feedback loop. 1111111 '....N. NOISE LEVEL 2 mv,-520 - Fig. 9. With the feedback loop added the pre- .... MAXIMUMUNDISTORTEDOUTPUT 3VOLTS amplifier response comes very close to the AES DISTORTION LESSTHAN 0.5%AT 1 VOLT OUTPUT' curve, whichisindicated by the dashed line. I 11111 FOR - L. 0 N R.R,2700 II -

FOR -L. 150mh .... N.. R. 6001 OVERALL GAIN 42db Odb Ft, 20001 NOISE LEVEL 20 rnv,-39db

I I I 1 . So. MOM 1mm MOM MIS FREOUENCY IN CYCLES PEP SECOND FREQUENCY IN CYCLES PER SECOND

88 One pair of a and 13 giving an approx- type) GE cartridge, the problem of in- for rectifiers.Filteringis no problem imate solution is a=0.88 and 13-, 3/50 put impedance matching is somewhat because of the small current drain. giving R. + 121= 2700 ohms and Rs = 47,- improved and it should be possible to (4) The most obvious method will be 000, assuming that C, and Cs present cascade two junction transistors to get to use a B -battery. Burgess type 5156 negligible impedance (at 1000 cps) as the necessary gain of 57.5 db for that and the miniature hearing -aid type U15 compared to R. + R, and R. pickup have been found very satisfactory. By Since the AES curve's low-frequenc modifying the automatic shut-off mech- turnover is 400 cpi we have, 5 anism of most record players, a d.p.s.t. 1 switch can replace the presents.p.s.t. Cs - and the battery can be connected to the 2nR.f. o preamplifier through the extra pole of 1 = 8500 iAtsf. the switch. Since there is no warm-up 2x(47,000) (400) 20 100 1000 10,000 time, the unit will go on immediately The pickup inductance L. assumes FREQUENCY IN CYCLES PER SECOND when the record player is started and will be disconnected automatically after considerable importance, since the in- Fig. 10. This curve shows the result of on actual ductive reactance ofthe pickup ap- test with a Pickering cartridge and the London the last record is played, resulting in a proaches the value of the input imped- LP test record, which contains tones graded in considerable extension of the life of the ance at high frequencies. In calculating level to conform to the usual LP music equali- battery. For an average( !)record the high -frequency de -emphasis, there- zation. playing time of six hours a day, a U15 fore, the value of L. has to be taken into is expected to last about six months and consideration, and this,unfortunately, Figure 10 shows the response of the with the new mercury batteriesthis will limit the preamplifier to a specific preamplifier with a Pickering cartridge. may well he doubled. cartridge. In addition, the value of L. Model 140, and a London LP frequency will effect the linearity of the ratios a test record. Four pickups tested all gave Conclusion and 13. As a decreases with frequency, 13 substantially the same response. In lis- In this article an attempt has been increases, but their product does not re- tening tests the unit measures up very made to present some of the advantages main constant, and therefore Equation well, although we doubt that, at least at and limitations of transistors, their basic apholds present, it is going to render obsolete its circuit configurations, and theirelec- A onlyapproximately. vacuum -tube counterpart. The complete tronic equivalents and to show how these + absence of 60 -cps hum and the elimina- concepts can be appliedin designing Note that these variations of a and 13 for tion of any possible speaker mechanical audio componentsthatwillcompare a certain fixed frequency are not fixed, feedback (microphonics) gives a new favorably with their vacuum -tube coun- but depend on the value of L.. If the im- experience in the reproduction of sound, terparts. A complete preamplifier -equal- pedance of L. is small compared to the especially of the low -frequency band. izer employing a total of five transistors value of the resistance R., the gain vari- with a total drain of 4 ma at 22.5 volts ation will be less than 1 db, which may Power Supply is under development at present. Prelimi- be considered acceptable. nary runs and listening tests give satis- Figure 9 shows the closed -loop fre- Although the complete preamplifierfactory performance and an article on quency response of Fig. 6. With L. = 0 draws only 1.5 ma at 22.5 volts, this the unit will be submitted for publication and R.+ Rt= 2,700 ohms, the unit is flat advantage is somewhat nullified by the when performance tests and evaluations to beyond 100 kc, while with a simulated fact that the voltage required is nega- are completed. Pickering cartridge (R. = 600, L. = 0.15 tive and is not found in the average Although an adventure into transistors FI and RI= 2,000) the frequency response present -My audio amplifier. This bias and tantalum capacitorsislikely,at follows the AES standard curve closely. can be obtained, however, by several pi esent, to prove rather strenuous finan- In actual practice R, may be even lower, methods : cially, it is certain that future improve- depending on the manufacturing toler- ments in higher gain, lower noise level ances of the pickup. With the application (1) Employing the same bias as the and distortion, better stability and uni- of negative feedback, the noise level has power amplifier, in the event fixed bias is formity, together with lower cost, will been reduced to 2 millivolts or about used for the power tubes, since this bias prove the transistor to be an indispen- - 52 db, which compares favorably with will be in the same order of magnitudesable part of the preamplifiers and re- the average tube preamplifier. as the 22.5 volts originally used. mote controls of the future. The normal gain of 21 db is obviously (2) The center tap of the high -voltage inadequate for a low -output cartridge winding of the power supply transformer inmost audioamplifiersisusually such as GE or the L-6 Audax. For the grounded, but if the center tap is re- BIBLIOGRAPHY 10 -millivolt class we need about 35 db at turned to ground through a suitable 1 kc, and with a 400 -cps turnover we W. Shockley, "Electronics and Holes in bleeding resistor, a negative voltage is Semiconductors," D. van Nostrand, New need another 22.5 db if we expect to go developed across the resistor which can down to 30 cps, a total open -loop gain of York, 1950. be employed for power supply for the IRE Proceedings, Transistor Issue, Nov., at least 57.5 db without allowing any transistors. The value of this voltage 1952. negative feedback for the low frequen- H. C. Montgomery, "Transistor Noise in cies. Transistors giving gains of this will have to be subtracted from the total availableB -plus,however,andthe Circuit Application," Proc. I.R.E.Nov., order are not available yet. The Picker- 1952, pp. 1461-71. ing line, on the other hand, giving a method may therefore prove unsatisfac- Ryder and Kircher, "Some Circuit As- nominal output of as much as 70 milli- tory in some cases. pects on the Transistor," B.S.T.J. Vol. 28, volts, will work every satisfactorily with (3) A small power supply can be built No. 3, July, 1949, pp. 367-400. this circuit, giving an output of as much with either a line -isolation transformer R. L. Wallace and Raisbeck, "Duality as as 1 volt once the value of R, is adjusted. or two filament transformers connected a GuideinTransistorCircuitDesign." Using a low -impedance, (professional - back to back, using germanium crystals B.S.T.J., Vol. 30, April, 1951, pp. 381-417.

89 D.C. Pack for Heaters and Bias ALLAN M. FERRES

A simple and practical device which will provide hum -free heater power for low-level am- plifier stages, thus making one further step toward perfection in reproducing systems.

IN ORDER TO TAKE full advantage of the excellentfidelity now availablein home radio -phonograph installations, the hum level of the system must be re- duced to a value much lower than that required a few years ago. Moving to- ward improved realism, designers have incorporated into the systems speakers and amplifiers with better low -frequency response and, at the same time, signal sourcesandcircuitwhichrequire Fig.1. External view greater voltage gain at these low fre- ofthe author's d.c. quencies. The magnetic cartridge, which heater pack. is the accepted standard for high-fidelity record reproduction, and the increased

use of magnetic tape in the home, com- bined with "loudness" and separate bass - boost controls, all contribute to the prob- lem. A gain of 30 to 40 db at hum fre- quencies over the mid -frequency gain is often encountered in most equipment. The hum reduction problem centers around the low-level stages where the signal level may be as low as 100 micro- volts. In order to keep the hum from ex- can bring this hum down to a minimum, ceeding the acceptable maximum, usually a point is finally reached where the onlysubstituting a battery of the proper volt- considered to he 55 to 60 db below nor- method of further reduction is the useage. (This, incidentally, is a good way mal listeninglevel,careful designis of d.c. as the heater supply. (Althoughof isolating the cause of hum in equip- essential. high -frequency heater current can bement when trouble -shooting.) It is pos- Afterallpossibleprecautionsare used, it is not considered in this discus-sible that when the battery is used, the taken to reduce the hum picked up by sion due to its complications.) hum will increase. This is due to hum the input source, the residual hum is voltages from the heater having been produced by the tube operating at the Advantages of d.c. Supply out of phase with hum caused by some lowest signal level. In good design, this The use of a d.c. heater supply hasother source. It is usually due entirely to is the first tube in the circuit, that fed many advantages. As power -supply a.c.coincidence and should not be depended by the input source. The hum produced does not have to enter the shield com-upon as a method of hum reduction, as in this stage may he considered to come partment of the low-level stages, shield-the other source often produces a voltage from one or more of four sources : mag- ing requirements of grid and plate leadswhich varies in amplitude and phase. netic fields, electrostatic fields, plate -sup- are not as stringent and better high -The other hum source can now be ply ripple, and heater -supply ripple. As frequency response is easier to obtain.tracked down and corrected. the a.c. magnetic field is usually caused Cathode followers and certain types of If the d.c. heater supply used has a by the power transformer, proper spac- phase inverters and feedback circuitsripple of 1 per cent or less,it may be ing between it and the input tube will which require the cathode to be un-by-assumed to produce no more hum than eliminate this source of trouble. The passed can be used at lower signal levels. the battery. and the supply should be effect of electrostatic fields may be mini- This, at times, simplifies design prob-designed with this in mind, unless ex- mized by completely enclosing the tube lems. The type of high -frequency oscil- perience has proven that in a particular and its associated parts in an aluminum lator often used in FM receiversin circuit a greater ripple can be tolerated. or copper shield can. As a resistance - which the cathode is above ground po- coupled stage, customarily used in pre- tential is a stubborn source of modulation Methods of Obtaining d.e. amplifiers, requires plate current of the hum until d.c.is used. More leeway is In general, there are three sources of order of one milliampere or less, the permitted in the choice of the type ofd.c. which can be used. The first is to plate voltage ripple can be reduced easily tubes which can be used and the circuitconnect the heaters in series in the nega- by means of an R -C filter, and so may be will be less critical as to individual tube tive side of the plate power supply, and discounted as a "problem" in hum reduc- selection. using a shunt or bleeder resistor, as re- tion. Therefore, the remaining factor- In any given amplifier, the improve- heater-supplyripple-isthe common quired, so that the proper current will ment to be expected by changing theflow through the heaters. If a suitable cause of the residual hum. heater current from a.c. to d.c. is diffi- When a.c. is used to heat the cathode power supply isavailable, thisis the cult to predict, as too many factors aresimplest and cheapest method. The volt- of a tube, hum enters the signal circuits involved. If the amplifier under discus- by heater -cathode leakage, thea.c. mag- age drop across the heaters is sometimes sion is still in the design stage, then d.c.used as bias for the power output stage, netic field surrounding the heater, and should be used if the lowest hum level is bycapacitivecouplingbetweenthe eliminating the need of a relatively high - heater and the other tube elements and required, depending only upon the costwattage cathode resistor. circuit components. Although careful se- involved. If the amplifier has already The second method, often usedin lection of tubes and arrangement of parts been built, then the improvement can becommercial equipment, is to connect the readily measured by disconnecting the *267 W. 11th St., New York 14, N. Y. heaters in parallel and operate them from heater leads from the transformer anda low -voltage, high -current power sup -

90 the point where the audio input jacks connect to the amplifier chassis to the tube heater pin which produces the least hum. Usually thisiseither the more negative terminal of the first tube or be- tween the first and second tubes. TABLE 1 Values for R,, 14, and C. Output R2 Cl 150-v. (volts) 50-W SO -W Electro- fixed lytic

12 500 200 80µf Fig. 2. Schematic ofthe d.c. power supply. 18 500 200 80 ply. Parallel operation of the heaters isformer can be used separately, being 24 500 100 40 convenient, but suitable rectifiers andswitched to a two -pole utility receptacle 30 500 100 80 36 500 100 80 transformers are not readily availablefor that purpose. 42 250 250 40 from the usual parts distributor, and theConstruction 48 250 250 40 cost of such parts tends to be high. The mounting of the parts is not criti- 54 250 150 40 The third method, used in the supplycal, provided a normal amount of venti- 60 250 150 40 described in detail here, employs an iso-lation is furnished. The unit built by the 66 200 100 80 lation transformer with a 117 -volt sec-author was mounted on a 5 x 7 x 2 -in. n 200 100 40 ondary, a selenium -cell bridge rectifier,chassis,the parts being arranged as adjustable series resistor for voltage ad- After the ground connection to the shown in Fig.1. The cardboard cov-heaters has been made, then the fixed justment and filtering, and two 150 -voltered filter capacitors,R, and the fuse are electrolyticcapacitors.Allpartsare bias is adjusted to the desired value by mounted underneath the chassis. On themeans of the 50,000 -ohm pot. standard items, readily available. Thefront can be seen the bias -adjusting pot part values are chosen so as to furnish The advantages of using d.c. on the and the a.c. on -off switch. On the backheaters of low-level stages can be fully 150 ma at any voltage from 12 to 72, asof the chassis are the line cord, the 4 - may be required, with a ripple of 1 per appreciated when it becomes possible to terminal output socket, the two -pole re-turn the gain up to maximum-with no cent or less. Although there is no objec-ceptacle, and the secondary switch forsignal input-without any output from tion to operating any of the tubes in anthe transformer. The four rectifiers are amplifier on d.c., it is usually not nec- the speaker, except a possible increase mounted by passing a threaded rod 4 in.in tube hiss. When such a condition is essary for hum reduction in stages wherelong through the center holes and sup- the signal voltage averages one volt or reached, the system may be said to com- porting itat the ends with two xpare with professional installations, as- so. In practice, the phono, tape and mi- TA -in. anglebrackets.Smallrubber crophone pre -amplifier and the tubes in suming that its other characteristics are grommetsprotecttheleadspassingequally ideal. the equalizing pre -amplifier are d.c.-op- through the chassis. The values of R,, PARTS LIST erated to advantage. It may also be de-R, and C, are selected from Table 1. sirable to supply d.c. to the heater of the Cr, C, 40 of, 150 -volt, elect. Figure 2 is the complete schematic for C. See text and Table 1 high -frequencyoscillatorofthe FMthe unit. tuner, if modulation hum is encountered. F, 1 -amp. fuse For increased tube life-and in someP, 2 -pole female receptacle As almost all6.3 -volt tubes currentlycases for lower tube hiss-it is recom-P. 4 -hole socket used in oscillators and low-level audio mended that 12 volts be applied to 12.6 - R., R. 50 -watt wirewound ; see text and stages have 12.6 -volt equivalents, no dif- volt tubes and 6 volts to 6.3 -volt tubes. Table 1 ficulty will be experienced with tube se- A fine adjustment of the heater voltageR. 50,000 -ohm potentiometer lection. The tube heaters should be con-is made by varying the position of the R, 47,000 ohms, 1 -watt nected in series with the first, or lowest - tap of R2. It should be noted that there S. SPST toggle switch level tube, connected to the negative ter- S. DPDT toggle switch isno directconnection betweenthe SR,..., .150 -ma, 130 -volt selenium rec- minal of the power supply. heater supply and the chassis, which is tifiers Another feature of this supply is itsgrounded. This is done so as to permit T, Isolatontransformer,117/117 ability to furnish up to 90 volts or so ofa wider choice in the selection of a volts (Merit P-3096 or equiv- fixed bias for the more efficient operation heater grounding point in the amplifier. alent) of Class A or A, output tubes. The biasThe proper point to ground the heater Fuse clip is adequately filtered and adjustable fromstring is determined by turning on the 5 x 7 x2chassis zero to the maximum value. For experi-amplifier and then running a lead from Knob for R, mental or test work, the isolation trans - Line cord and plug A Corner Horn For The Small Listening Room WAYNE B. DENNY Large -aperture horns, desirable for adequate bass response, can be made to appear much smaller than they really are by effective use of a corner.

MOST DESIGNERSAND USERS of often require 20 or more cubic feet oftens finds that the system-particularly higher -quality reproducing equip- space for efficient radiation of low -fre- the speaker enclosure-"takes over" the ment agree that adequate bass quency sounds. This requirement poses room to the point where it dominates response requires rather large speaker a problem to those who must confine the entire listening area visually as well enclosures. Horn speakers in particular their listening to apartment -sized living as aurally. This may not bother the rooms. Too often, the person who in-high-fidelity addict but itislikely to * Grinnell College, Grinnell, Iowa. stalls a better than average audio sys- engender a high noise level in the form

91 visible and it is this feature which makes the unit appear much smaller physically than its actual acoustic size. The entire volume beneath the top panel is of the order of 20 cubic feet ! Yet it looks no larger than a moderately large corner table. Part of this deception is caused by the ratio of width to height. The height is just under 33 inches-about right to hold a table -model television receiver where it is most easily viewed. This is important in those rooms which provide but one unobstructed corner suitable for viewing and listening. Major dimensions are shown inFig. 2. As shown, the horn approaches ex- ponential shape and provides for a the- oretical low -frequency cutoff at about 60 cps. The mouth area-just over 1000 square inches-is large enough to sub- due serious reflections and this aperture is augmented by walls and floor. The structureisvery solidly con- Fig.1. Thefinishedhornseems verysmall physicallyin comparison toitseffective size. structed of 3/4 -inch plywood. All joints are secured by wood screws and glue of vocal complaints from thedistaff except the top which is, of course, re - Fig. 3.I hisviewoftheenclosureturned around fromits side. moveable. Weatherstripping material is normal position, was taken Is it possible to build a speaker horn from above and behind. It is useful in showing used to ensure a tight seal between the many details, though somewhat deceptive from whichisadequate in size but which top and the walls of the room. Internally, a photographic standpoint. To avoid misunder- does not completely dominate restricted 1 x 2 -inchreinforcing strips are used standing, remember that thesidewalls are living quarters? After some preliminary along all corner joints. These strips are rectangular, the top parallel to the floor. study it appeared possible to solve this not shown in the diagram ofFig.2 but At prices prevalent in the Midwest the problem satisfactorily and the unit to their use is illustrated inFig.3. be described is offered as one solution. entire structure can be built of fir ply- Construction is relatively simple and wood for about $25. The use of birch No new principles are involved but need notdeterthe average amateur effective use is made of some well known or similar outside ply will double the carpenter or cabinetmaker. The only cost. ideas. difficulty-a minor one-is caused by Inspection ofFigs.1 and 2 will serve the fact that several panels meet at Performance Characteristics toillustrate the fundamental features angles other than 90 degrees. A little of the horn. The driver is back -loaded A number of speaker enclosures have care is necessary to insure a solid and appeared in the literature during the at low frequencies in the conventional tight -fitting joint. All panels can be ob- manner. The horn expands toward the past two years which employed the tained from a single sheet (4 x 8) of General Electric S1201 -D speaker as rear-that is, toward the corner of the plywood as shown inFig.4. It is sug- room-and then expands toward the drivers. A few of these descriptions have gested that all outside panels be cut as included impedance curves. For purposes front at either side of the structure. The shown so that the grain of the wood is large portion of the horn is formed by of comparison, a similar driver was in- uniform in direction. The smaller panels stalled in the corner horn described here the top panel, side panels,floor, and are cut from the residue and are pref- room walls. Room walls and floor are and impedance curves were prepared erably cut oversize and planed tofit. with the aid of the circuit ofFig.5. The results are shown inFig.6. Com- parison of the solid -line graph with the corresponding curves for other enclo- sures shows that the present unit has unusually small fluctuations of imped- ance with respect to frequency. Although no claim is made that such curves are completely indicative of speaker per- formance, the inference is that increased airloading makes theresponseless variable with frequency. Analysis of the data with and and without fiberglas absorbing material in the space behind the cone indicates that impedance fluc- tuations are smaller with the fiberglas installed. Percentage -wise, the fluctua- tions are small in either case within the requency range from 30 to 400 cps. A rule of thumb procedure for checking the damping of a loudspeaker consists of starting and stopping a small electric current through the voice coil. After the circuit isclosedthe speaker is subject to heavy electrical damping in addition tomechanical and acoustic damping. After the circuit is broken an open cir- cuit exists and the damping is exclu- sively mechanical and acoustic. The dif- ference in sound. if any, provides some Fig. 2. These three views show the general makeup and major dimensions of the enclosure. Dimension details can be changed to suit other speakers. (Concluded at bottom of next page)

92 A Tuned -Pipe Enclosure for Bass Enhancement

RAYMOND H. BATES

A simple and easily -built enclosure will give excellent sound quality from any good 12- inch loudspeaker, with low -frequency reproduction equivalent to many larger cabinets.

A(CORDING TO LITERATURE presented Ihave developed a fairly simple yet ticalsize.This configuration permits inthe September 1952 issue of effectivecornerenclosurefor12 -in. direct radiation of the middle and high AUDIO ENGINEERING-I refer spe- speakers, shown in Fig. 1. frequencies while the lows are radiated cifically to the article written by Mr. The configuration was generated by directly and are also augmented by the John E. Karlson entitled "A New Ap- consideringfirstalongtriangular back radiation through the short ex- proach in Loudspeaker Enclosures"-a shaped pipe 6 ft. long with the speaker ponential slot. That was borne out by long closed -end pipe, having a long notch mounted at one end and the other end impedance measurements, and was veri- at the open end, will give practically having an exponentially tapered slot, as fied by extended comparative listening continuous radiation over the frequency shown in Fig. 2. Since this is obviously tests. Note in Fig. 4 that the free air range required for good audio reproduc- a back -loading device, and we are inter- resonance of 65 cps for the Electro- tion. The pipe length must, of course, ested primarily in enhancing the low - Voice SP12B is broken up and smoothed be considerably larger than the width frequency response, the slot or notch at out with the curve essentially flat down and depth in order to achieve the proper the open end can be made reasonably to 30 cps. The non -resonant character effects. With these fundamental charac- short, considerably less than the 2/3 in- of this enclosure has been demonstrated teristics 'of the closed -end pipe in mind, dicated for broad coverage. Further, if by extended listening tests using such we now fold the 6 -ft. pipe, as shown in records as the Cook organ records as * 67 College Place, Hampton, Va. Fig. 3, we get a package that is of prac- well as the Capitol test record. There is

hibits a slight resonance and thisis i3 shown by the shape of the impedance 12 CORNER HORN curves of Fig. 6. There are two humps 0 (front preceding page) in the impedance curve where the im- 01, a 0 pedance rises about 30 per cent above 10 the nominal voice -coil impedance. The indication of the coupling between the ao two humps are nearly symmetrical about 0 cone and the air. Using a single dry -cell the free -cone resonant frequency. This 0,01 as a current source, this test was applied suggests that the resonance frequencies 7 repeatedly to the corner horn enclosure 20 50 00 200 400 of horn and driver are the same, a FREWENCy IN CYCLES PER SECOND and no audible difference between makedesirable condition. There was no de- and break was found. 'It was inferred,liberate attempt to bring about this de- therefore, that the damping was ade- Fig.6.The solidlineshows the impedance sirable state of affairs, the curves just cQrve of the GE speaker in the enclosure, with quate and that transient response wouldcame out that way. Fiberglas completelyfilling'the space behind be satisfactory. The latter conclusion If the hornisused inconjunction the driver. The dot; show irregularities which was substantiated by listening tests. Onewith drivers for low -frequency repro- occur when the fiberglas'is absent. The dashed simple test is to listen to a good record-duction only, it may be expected that the line shows impedance peak of the driver with- ing of percussion instruments. Bartok'sresonant frequency of the driver may be out anenclosure. Sonata for Two and Percussionsomewhat lower than for the one shown. ( Victor)issatisfactory for this pur- this. reason some of the internal dimen- If so, a slight readjustment of the in-sions have been omitted. For example, pose. ternal panels may he desirableItis for No finite horn, no matter how care- slight changes in construction will per- fully designed and constructed, is com- mit the use of a 15 -inch driver. Under pletely free from resonance phenomena. such circumstances the throat area is The horn which has been aescribed ex - easily increased, with a consequent de- crease in horn length. Fortunately, the design of the enclosureissuch that T changes of this type are easily made SIDE 7 )p after construction is nearly complete.' 43 Apparently t 9. notallGeneralElectric SIDE 1 Model S1201 -D cones have the same reso- ,,. FRONT nant frequency.Ifa driver of different Fig.5. Circuitusedforimpedance measure- resonant frequency is used it should not be 1 i ments. At each frequency the decade resistance expected that the impedance curves will be 32 4. 14- 32---..1 isadjustedforsimilar voltmeter readingsin identical to the ones shown in Fig. 6. How- both switch positions. The resistance isthen ever, if the fluctuations in impedance with Fig. 4. How to cot allthe main panels from equal to the absolute value of voice -coilim- frequency are not severe, good results may a single4 8 -footsheetofplywood. pedance. be anticipated.

93 The basic enclosure is now ready for finalfinishing.If you like the simple modern appearance that mine has, as shown in Fig. 8, you may now screw a foot piece to the bottom, cut the same size as the bottom piece, and then glue a 1A -in. sheet of foam rubber thereon for acoustic insulation from the floor. The enclosure should now be set in whatever corner you have chosen for it (prefer- ably one on the longer axis of the listen- ing room) so that the sides of the en- closure do not touch the moulding along the base of the corner walls of the room so you can measure the top finishing Fig.1. The author's piece. This piece should be sufficiently cabinet in finished large to fit snugly against the two corner form.Light appear - once of front is due walls and still jut out over the front grill to the use of natu- of the enclosure approximately 1 inch ral -colored monk's for good appearances. In my case this clothforthegrille. turned out to give dimensions of 20 x 20 28 in. Mount this piece to the top by inserting wood screws through from the

ikr [ 1 1-1 I - I % --INFREEAIR 20 r \ -INCABINET--- / I - i 10 ----\ ao r../' .... -..

0 100 1000 FREOUENCY IN CYCLES PER SECOND no audible hoomy or boxy sound. the top and bottom pieces should be as- For those who are interested in con-sembled, again using nails and wood structingthisenclosure the diagramglue, for a neat joint. Now the two Fig. 4.Measured impedancecurveof E -V SP -128 loudspeakerinthe author's cabinet. shown in Fig. 5 will serve as a guide.lower left pieces of Fig. 6 should be Note reduction of natural resonant peak and The usual precautions of using woodassembled using nails and wood glue. extension of low -frequency range. screws and glueing all joints for an air-and then this sub -assembly should be tight seal apply. The slot dimensions areglued and screwed to the two sides al- indicated on the drawing. inside of the enclosure. Nuw glue a thin ready assembled. See Fig. 5 for properstrip of foam rubber along the edges Figure 6 indicates the various pieceslocation of this sub -assembly. The front needed to assemble the enclosure. In as-panel should next be fastened as indi-that contact the wall in order to obviate sembly the two sides should be nailedcated in Fig. 5, using wood screws andany vibrations being set up at the points together first after wood glue has beenglue generously to insure an air tightof contact. Next the Kimsul pad can be applied to the contacting surfaces. Nextsealatallcontacting surfaces.Thisfastened to the top with glue and a few whole package should now be allowed to set for whatever time is indicated onfurniture tacks. You are now ready to the can of wood glue you may have used.mount the speaker. A 15 x 15 in. speaker I used Casco and let the enclosure setmounting board should be cut from q - overnight, and should resemble Fig. 7.in. plywood stock, with a 10% -in. circu- lar hole cut in the center for mounting 0 the 12 -in. loudspeaker. Speaker lead-in ELONGATED CLOSED - END PIPE, WITH OPEN wires should he connected through two END NOTCHED holes conveniently placed in one

V side of the enclosure near the top. The to speaker board may now be mounted

A 4 1 ever the 13 -in. hole in the front panel 1100 FOLDING using eight wood screws equally spaced ."24 6-PIPE about the periphery of the mounting f 'Y 46,, RESULTS board as shown in Fig. 9. Finally the INCOM- PACT EN- front grill frame, 38Y4 x 23 in., should CLOSURE be fabricated from u -in. square pine stock. This frame should be covered with lumite in your choice of color. The completed front cover may now be fas- tened to the enclosure front by means offour cabinet doorcatches.These Fig.2. Design of cabinet was developed from should be mounted on the sides, two near Fig.3.Appearance of cabinet afterfolding this long closed -end pipe with notched front. from the original "pipe" of Fig.2. the top and two near the bottom. This

94 314 PLYW000 THROUGHOUT SCALE I' ie EXCEPT AS INDICATED. SCREW, DIMENSIONS NAIL, AND GLUE ALL JOINTS IN INCHES FOR AIR -TIGHT SEAL EXPONENTIAL

SLOTDIMENSIONS

W. .46 Fig.5.(right). Sec- 2 .52 tional drawingsof 3 .64 the simplified corner T 4 80 enclosure. Fig. 6 5 1.00 (below).Detailsfor 6 1.24 7 1.52 cuttingplywood 1.88 pieces for the cab- 9 2.22 inet.15x15in. 10 2.64 speaker mounting 11 3.12 panel. 12 3.62 13 4.18 14 4.76 21 15 5 42 16 6.10 17 6.82 18 7.60 19 8.42 20 9.30 21 10.34 1,4-11 1/4

2 PIECES, FOR SCALE: 1' 18' TOP AND BOTTOM DIMENSIONS IN INCHES expedient allows for ready access to the 1 FRONT I SIDE 1 SIDE speakerincase you want to change

21 14 t/4 speakers etc. In my case, the top was 19 ff-IS 10 1/2 -11 finished by sanding and then applying clear varnish to bring out the natural 81/2 grain of the mahogany. The over-all

161/2 effect is very modern. The simplicity of 45 1,121 41 this basic enclosure permits of flexibility 19-.I in final exterior finish so that if you do 1/2'. PLYWOOD not like the modern approach you may apply some other type finish.

16 2

i MAHOGANY FOAM RUBBER TOP STRIP

MONKS

FOAM RUBBER SHEET7

Fig. 8 (above). Sketch of finished appearance. Fig. 9. Semi -finished cabinet with loudspeaker in place on its mounting panel. Note brackets Fig. 7 (left). Cabinet in stage of construction on edges offrontpanelformountingthe before mounting loudspeaker panel. trimming frame.

95 Evolution of "The Horn"

EDWARD V. KETCHAM, JR

The conception, design, and construction of a speaker enclosure consisting ofa closet -con- tained low -frequency horn with a midfrequency horn and two high -frequency sound producers.

THE HORN"isa home -builtloud- recommended mouth area. Working di-the garage workshop using a Cook test speaker system.Itislarge insize mensions were figured from (d) by as- record and 10 -watt amplifier. and magnificent in performance. Itsuming widths as large as closet space The first try ran at moderate volume. was conceived as a result of reading allowed and dividing these into the listed Waiting for the test record to run its about Albert Kahn's huge home audiovalues, the quotients denoting height orcourse provided anxious moments as installation publicized as a "Residence depth dimensions. The drawing of Fig. 2treble tones weakly wandered forth. One EntertainmentCenter"by Electro- details the results of these calculations.thought was paramount. "When will it Voice, Inc. Mr. Kahn is president of this Lest your interest sag at this point, take hold? Will it take hold ?" At about organization. The "Center" features a the end product is a low -frequency horn 500 cps the test tone became solidly bass section comprised of a large horn from which pours full, clean, unforced effective and then progressed lower and driven by two 18 -in. woofers, and occu-low -frequency sounds-a tuba player at lower.Positive response was audible pies one entire end wall of a sizable an interview trying for his lowest note,down to 30 cps and apparent response room. Mr. Cook's organ records, a bass viol,below thathowever, the pickup used More specifically, "The Horn," truly drums-without resonant peaks or "be-and this layman's ears preclude accu- an amateur's project, was first consid- hind -the -door"muffledeffects.First ratejudgmentinthisrock -bottom ered as a half-size edition of Mr. Kahn's indications ofits performance becameregion. Theoretically "The Horn" func- installation, turned on end and backedevident upon completion of the basictions to 25 cps and the speaker resonates into a rectangular hall closet. As com-structure and the installation of the 18 - at 27 cps. pleted (Fig. 1) it became a combination in. woofer in its tightly sealed cavity. A A repeat test at high volume produced ofKlipsch speaker cavity and horn test was made on the concrete floor of sufficientunadulteratedlow -frequency throat principles, feeding through an ex- ponential horn, plus design features and driver components of the Electro-Voice Patrician. The entire assembly was de- signed and built tofit existing closet space and exhaust into one end of the living room listening area. Technicians may find faults in its layout, but it per- L. F. HORN forms superbly. MOUTH General procedure was as follows. De- tails of the "Residence Entertainment Center," the Patrician, and some basic rules of horn design were obtained. Paul 600cps CUTOFF Klipsch's papers on folded horn prin- MULTI CELLULAR ciples and the Jensen monograph on HORN horns were studied. The basic idea of a low -frequency horr with its mouth di- 200-600cps 1111111/ vided in two by a compartment contain- HORN TWEETER ing associated apparatus was drawn to 1500cps CUTOFF rough scale. It became apparent that it would be advisable to face the low -fre- quency woofer speaker away from the listening area, and folditsassociated horn around two right-angle turns to bring the latter's mouth out in the de- sireddirection.Thisdesignfeature would provide the longest path and most gradual taper within the limits of avail- able space. Concentrating on this low -frequency horn, an approximate path length was established. After several false starts a L. F. HORN list of cross-sectional areas was com- MOUTH piled. The list itemized the recommended area of (a) the mask opening in front of the 18 -in. woofer. (b) a pre -throat chamber, (c) an eight -inch -long throat, and (d) calculated areas at six-inch in- tervals along the length of the horn. Fig- ures from (d) established an exponen- tial curveor expansionratetothe Fig.1. The finished enclosureis mounted ina closet with the divided horn exhausting into the listening area. Between the two sectionsisthe TV receiver. The upper sectionwillbe * 50 South Bay Ave., Brightwaters, N. Y. covered with agrillsimilarto that over the lower section.

96 sound power to be a potential cause of Having successfully provided for low -channels source material signals to the physi....al damage in a finished room. Vi- frequency range, the next stage was theproper drivers. brations created a sense of being sur-building of a 200- to 600 -cps horn for a With the assembly of all components roundedbysomethinguntouchable. 12 -in. woofer, as prescribed by the Patri-completed, further tests were in order. Small objects moved of their own ac-cian details. This was patterned directlyAfter preliminaries with the test record, cord.Others vibrated audibly.Small from the furnished data except for twothe writer proceeded to launch units of wonder that a chance visitor arrivingvariations. Available width being lessthe New York Central Railroad upon a upon the scene, replete with the odor ofthan specified, the height was increasednew route via Cook's "Sounds of Our drying paint, accused the writer of ne- to maintain prescribed mouth area. The Times" by way of an open garage door. farious activities relating to the produc- second change consisted of arranging the The open mouthed admiration of a four- tion of illegal beverages ! horizontal and vertical axes off center so year -old neighbor was wondrous to be- as to aim the unit toward the exact cen- hold. Construction A word now from the voice of experi- The photos of Figs. 3 and 4 illustrateter of the listening area at ear height. ence! No doubt you know of the fellow construction details. Upper and lower Then theupperfrequencies.Pur- who built a boat in his basement and mouth sections,throat and back arechased Partician components were ar-was unable to get it out. Similarly let it built separately, then assembled. The ranged to point at the listening area cen- be understood that a big audio horn terpoint. From 600 to 3500 cps a speaker cavity came last.Joints are once built.. .has to be installed. In glued throughout. After testing at loud horn -type driver unit operates throughplanning, one must consider not only volume strong bracing was added to thea multicellular horn with 600 -cps cutoff. available area for the installation but back of the speaker cavity to reduce theTwo tweeters operate from 3500 cps up. also difficultiesin reaching that area. possibility of developing extraneous vi-One, a horn -type driver operates through "The Horn," while restricted in width brations. It is believed this can be bettera multicellular horn with 1500 -cps cut-to that of a hall closet, is ceiling high accomplished by building the cavity ir- off. The other is an 8 -in. cone speakerand cannot pass through doorways in an regular in shape while maintaining its modifiedforextremehigh -frequency upright position. A five-foot closet depth cubic content. operation. A four-way dividing network and corresponding front -to -back dimen- sion, however, make transportation pos- sible face down. This does not entirely conclude the problem as the over-all k 55 diagonal measurement from bottom front to top back momentarily is all important I when raising the assembly to an upright position. In the case of "The Horn" one I inch to spare proved sufficient. HOAN 7' If Finishing was handled in a novel man- ner, as the photos reveal. The wall was first torn away exposing the closet in- I stallation area. The unit was backed into position. Wedges were driven between the unit and surrounding walls. Lag I bolts through the sides into adjacent studsmade everything"solidasa

I I house." With "The Horn" finally at its I destination,panellingwas installed I t I f.-I. across the entire end of the room on top cs 0, of existing plaster. The panel seams I were spaced so that the wall opening at I- -- - A the closet measured exactly four boards wide. as can be seen on Fig. 5. An up- le:-49 11- '.------. I - and-down sliding panel of like material I /4 and similar width was constructed on concealed slides so that it can be raised

. 1...... , out of the way through the ceiling. A counterweight simplifies this maneuver. In passing, the writer admits partial de- stage as the panel was 4 22 1 featatthis divided and hinged across it center mak- , , ,A r ., ing a visible seam. The onlyalternate k._ was to "raise the roof." The next project will be to provide push button control of

1 this panel with an electric motor mecha- /.10A1 ,Ir Plo.n. nism. %It A sliding drawer or cradle permits .e--22; I% the TV chassis to be serviced; a TV front panel and a plastic grill cloth pro- I vide finished appearance. Source mate- rial comes from amplifier, FM tuner, 1 and record player in a fireside wood N closet at the far end of the room.

Other Speakers vk- /0 /0 /0 /0 Inexperimenting with "The Horn" FRONT .51-crio,v A- A by changing relative volume levels of the differentdrivers,the writer has f- formed an opinion that may be of gen- eral interest. It is that most approaches Fig.2.This drawing gives allthe dimisions ofthe author's versionof "The Horn." Some alterations may have to be made by other constructors tofit existing spaces. to extended low -frequency reproduction

97 60C cps CUTOFF MULTICE LL LIL A R HORN

WOOFER 200-600 c ps CAVITY' HORN

THROAT OF L. F. HORN

Fig. 3 (above). Side view of "The Horn" shows some idea of the construc- tion.Fig.4(left).This three-quarter view shows placementa' the 200 -600 -cps ond lower -frequency horns.

inthehomeindirectlycausepoor mg the wide -range, even response that speaker response in what may be termedis the outstanding characteristic of this upper bass. To substantiate, the bass - multiple speaker system. boost curve of home amplifiers peaks be- Building and installing "The Horn" tween 50 and 100 cps. Enclosure designs was an adventure. Test record tryouts are available that are remarkably effi-and lengthy listening sessions have re- cient in this range, and when used withvealed wide -range efficiency most re- a low crossover network and level con- warding for the effort expended in its trols usually make amplifier bass boostcreation. It utilizes equipment found in undesirable. Under such circumstancesthe most elaborate factory assembled the direct -radiator cone speaker em- systems and several enthusiastic critics ployed for midrange reproduction lacks consider that its performance ranks with bass boost that it can use to advantage. the best professional demonstrations, yet Said boost not being present, the over-through home construction many dollars all response curve of the speaker system were saved, and the system was built in. droops in the upper bass range. ThisThe writer formerly used a high-priced droop may lead some to think their low - coaxial speaker mounted at the location frequency units perform better than isnow occupied by "The Horn." The actually the case, due to the relative dif- closet served as an infinite baffle and the ference in output volume between the same source material equipment was in low and midrange speakers. use. There is little comparison. Direct - The remedy is to employ a separateradiator speakers are not in the same upper -bass speaker. Its efficiency must league with several properly -horn -loaded compare favorablywith the extremedrivers. low -frequency unit. Such a device is the Yes,it was a satisfying adventure. 200 -600 -cps horn with 12 -inch driver Look around, friend! Have you a work- Fi;. 5. This photo shows how the entireinstcl- described above as a component of "Theshop? A strategic closet ? Ever knocked la -ion is covered by the vertically slidingpanels Horn." It plays a large part in produc- down a wall ? You have? Well ? when it is not in use.

98 A Corner -Mounting Infinite Baffle

M. V. KIEBERT, JR.

A presentation of some of the problems involved in loudspeaker enclosures for high -quality re- production, and the author's solution with a suitable design to house an LC -1A speaker mechanism.

pAST EXPERIENCE has all too often veering businessdictates,a brick or elsandhencecomparabledynamic made us painfully aware that whatstone enclosure thus being ruled out. ranges. Consideration was given to the was often called a "loudspeaker" In casting about for a suitable device effect of monaural listening as compared was just exactly that and not the "repro- itisimmediately apparent that there to binaural listening and to the aural dis- ducer" that it should have been. were but two basic design paths to fol-crimination intrinsically present when In order to attempt to satisfy a verylow; the horn -loaded unit, and the di- sight is used to supplement sound. critical listener who happens to be a rect -radiator system. With monaural listening it appeared rather exacting engineer by profession The problem of power output wasthat an acoustic level of one to two and an out -of -hours musician by hobby,next considered. Measurements of theacoustic watts could do a satisfactory but who knows what he likes when itacoustic power levels of the various in-job, but that an additional 10 to 20 db comes to musical reproduction, a surveystruments were reviewed; a sound -levelof dynamic range would be desirable. was made of the over-all reproduction meter was used to survey typical levelsThe measured dynamic ranges of direct problem. Seven years of test, explora-encountered at choice locations at sev- (live concert of the Philadelphia Sym- tion,and development followed.Not eralPhiladelphiaPhilharmonicand phony)performancesweresurpris- only was it necessary that it must notLewisohn Stadium concerts. ingly low (presumably because of the offend the eye, it was also required that Noise -level surveys at these concerts averaging due to reverberation in the the assembly should be capable of beingwere also made and compared with typi- hall). Twenty totwenty-five db ap- moved from place to place as the engi-cal home listening levels in order to as- peared to be a maximum. sess more accuratelyascalefactor This study provided initial clues as to which would provide equal desired -sig- therequired optimum designtrends. * Applied Research Inc., Chicago, Ill. nal -plus -background to background ley- Early experience with theatre horns and their 20 to 25 per cent conversion effi- ciency, that an amplifier with a clean 4.5 watts of electrical output would do a good job covering 600 to 1500 people. Typical direct radiator sys- tems have conversion efficiencies of 3 to 16 per cent so the power amplifier must have between six and thirty electrical watts output to do the same job in a satisfactory manner. From our point of view the low -effi- ciency,power -demanding,direct -radi- ator systemisnot good design. The lower efficiency units generally are that way either because of inadequate mag-

netic circuits, or as a resultof dia- phragm break-up-both being cases to Fig.1. A simple yet be avoided if IM is to be kept to a low neat appearance valve. The higher more efficient direct- characterizes the au- radiator units such as the Altec 604C thor'sspeaker en- or the RCA LC -1A permit a good de- closure which pro- sign compromise by only requiring a videsfor a 15 -in. nominal six- to twelve -watt power am- single -unit speaker plifier which isrelatively easy of at- mechanism. tainmentwithlow -voltage,low-cost power supplies and rather nominally rated and operated power output tubes.

Frequency Range The next item for consideration was frequency range and the sound distribu- tion patterns desired for average instal- lations. Many engineers and hobbyists rather indiscriminately add high -frequency and low -frequency units into a system and frequently do achieve some measure of frequency balance insofar as level as a

99 functionoffrequencyisconcerned. However- from thewriter'spoint of view this procedure would be analogous to having an artist paint a background in one picture, the theme or object in another, and the high lights in a third. Then by hanging all three pictures side by side we should get the integrated Whole. Leonardo Da Vinci didn't do it -so we're not trying to do it either. Considerable experience in the design of directional antennas (which may be likened to acoustic radiating systems) emphasized the distorted spatial distri- bution patterns that always occur when spaceseparationexistsbetween two sources of radiation even when these are in phase. Add some phase variation and the spatial pattern becomes all the more non -uniform. From the foregoing it appeared that horns and multiple -unit systems were I"X ALL MEMBERS EXCEPT not too desirable and in fact critical CENTER POST listening tests by musicians and non - musicians always seemed to favor the Fig. 2. Constructional details for the corner- mounted infinite baffle. single -source direct -radiator system when this was given A -B comparison Forourrequirements a corner inforcing reflection paths which might tests with multiple -unit horn -type sys- speaker held several desirable advan-otherwise degrade the response curve. tems. During these tests the observerstages. The corner design improves the The unit is mounted near the top of the did not know which system was in op- radiationcharacteristics,asiswell cabinet in order to provide the optimum eration. The almost universal responseknown; the triangular design permitted spatialdistributionwiththesound was that one system seemed "smoother" veryrigidconstruction;thedesign source as close to ear level as possible. than the other. easilypermitsasymmetricalspeaker The interiorislined with ozite ce- One axiom in enclosure design is this:mounting and consequent avoidance ofmented on with rubber cement. Addi- if when you touch the enclosure you tional absorption was utilized by drap- spurious peaks, and the system pre-ing strips of ozite across the internal can feel the low notes, it's not good! sented a minimum decoration problem G.A.BriggsofWharfedale got cross -bracing membersasshownin around this difficulty by using a brickinsofar as integration into a living space Figs. 3 and 4. enclosure. John V. L. Hogan minimizedwas concerned. The performanceofthisspeaker this difficulty in an early (around 1936) With a low -frequency cone resonance mechanism in this particular cabinet is WQXR receiver by heavy cross -bracingof between 35 and 40 cps, and a desire considered adequately rewarding for the of the enclosure. The greater portabil-to minimize the rate of roll -off belowwork involved in designing and con- ity of the latter arrangement appealedthis resonance aswellasto ensurestructing the enclosure, anditisfelt to us and according this design proce- smooth low -frequencycurve immedi-that the complete system is as good as dure was followed. ately aboveit,a volume of approxi-the writer can make it-until the urge mately 15 cu.ft. was decided on, andto redesign shall again arise to start a completely enclosed structure was con- the entire cycle over again. sidered essential. Fig.1is front view of the enclosure.

Construction In order to fit snugly against the wall, three legs 4A -in. high were used in or- der to raise the cabinet above the aver- age baseboard. This is not too desirable because of the small resonance peak oc- casioned by the cavity formed between the enclosure and the floor. For this reason the legs are removable and we ultimately visualize installation of the enclosure against the ceiling. The enclosure is made of 7 -in. ply- wood with construction details as shown in Fig. 2. Assembly is with wood screws and casein glue with particular care taken to insure a rigid and air -tight as- sembly. All peripheral and cross brace stiffen- ers were made of 1 x 2 -in. fir. The top - to -bottom member was 2 x 2 -in.fir tied intothe three setsofinteriorcross braces by cemented -in blocks aided by wood screws. U The speaker mechanism chosen-the Fig.3.Internal view with one back panel re- moved to show cross bracing and unsymmetrical RCA LC -1A, more commonly known as Fig. 4. Scrap strips of Ozite lining are draped mounting of the speaker. Also note start of the Olson speaker-is mounted asym- overinternal bracing to further dampen the Ozite lining in lower part of cabinet. metrically in order to break up all re- cabinet.

100 Coupled Loudspeakers

CHARLES W. HARRISON, JR.

A discussion of the principles of multicone speaker operation and a description of a composite corner -mounting assembly composed of four acoustic baffles of trapezoidal cross section.

THE EFFECTIVENESS of a sound source efficiencyisof no realpractical im-of 35 cps a speaker of gigantic dimen- depends in an important way on theportance in the design of a high -quality sionsisobtained.Even thetheater phase relationship between the nor-loudspeaker for home use. woofers which utilize exponential horns mal velocity of the radiating surface and The question might now be asked as approximately four feet long on axis are the force reaction (or sound pressure)to why one would want to construct a nothing more thandirectionalbaffles of the medium on the surface. The prin-loudspeaker array giving a performance below 60 cps. This fact can be verified cipal value of a horn is that it will per-similar to that afforded by an expo-readily by reviewing the theory of the mit sound to be generated by the vibra-nential or catenoidal horn. The answerexponential horn found in any text book tion of a small diaphragm, but radiated is merely that the physical size of theon acoustics. The only solution, if one from an aperture large enough to keepcomparable horn must be many timesinsists on using a low -frequency horn, pressure in phase with particle velocitythat of the coupled loudspeaker system. appears to be that of building one in the downtorelativelylowfrequencies. Unfortunately mouth reflections in horns yard out of brick or concrete. Such a Hornsofexponentialorcatenoidalgive rise to air column resonances, and shape are conformable to practical ap-unless the mouth is comparable in di-speaker should never be built out of plication. mension to the longest wave -length toplywood, for serious spurious responses Itis customary, in calculating thebe reproduced, tremendous variations inare sure to be obtained from the "ring- throat impedance of a horn offinite sound transmission must be tolerated. A ing" of the horn walls. Yet in spite of length (assuming it to be baffled so thatsecond design criterion for finite horns the large size of a properly 'baffled horn, radiation is confined to 2x steradians),is that the cut-off frequency of the cor- its use in the theater is justified because to replace the mouth of the horn by a responding "infinite" horn should be atefficiency and power handling ability massless diaphragm of suitable shapeleast an octave below the lowest fre- for frequencies above about 50 cps are working in an infinite baffle. This dia-quency in the desired transmission range important factors. The multiple loud- phragm has no effect on the operationif reasonably smooth response is to bespeaker using well -damped drivers un- of the horn, but permits one (for mathe-obtained. When these requirements arequestionably affords the best method of matical purposes) to terminate the horntranslated into a horn capable of "prop- getting good bass response in the home in an impedance equal to the radiationerly" reproducing notes in the vicinity where space is restricted. impedance of a piston operated in an infinite baffle. From the basic horn equa- tion, in terms of the velocity potential, one obtains an expression for the pres- sure and particle velocity at anypoint within the horn. These expressions, to- gether with the piston functions, per- mit determinationofthethroatim- pedance. The important thing isthat the external sound field set up by the horn can be duplicated by a vibrating pistonofcomparable radiating area. The basic problem is that of obtaining an aperture large enough to keepthe sound pressure and air -particle velocity in time phase down to low frequencies. For practical purposes the large piston radiator (a direct -radiator loudspeaker Fig. 1. Photograph mechanism of suitable design) may be ofacousticbaffles replaced by a system of properly phased employedincoupled tightly -coupled smaller drivers. How- loudspeaker tests. ever, it is to be remembered that in the useful frequency range of a horn, the input resistance at the throat is high and this affords additional damping over that availableinthe diaphragm sus- pension system of the horn driver. This is important at low frequencies if non- linear effects are to be minimized that are generated when large diaphragm excursions take place. For comparable performance themultiple loudspeaker mustemploymorehighlydamped drivers than are used with a horn. Such drivers areinherentlyinefficient, but * Commander. U. S. Navy. 611 Massachu- setts Ave., Norfolk 8, l'iryinia.

101 The writerconstructedamultiplebook in the field of acoustics.' One findsthetically)the two diaphragmsthat loudspeakerconsisting of identicalthe reaction force on one elemental area it is "permissible" to use into quadrants. wedge shaped acoustic baffles of suchof the piston due to the motion ofEight sector -shaped diaphragms are ob- angular dimensions that four of themanother elemental area. By a processtained. Is it to be supposed that an in- fitsnugly into a 90 -deg.corner. Asof summation (integration) one findstolerable phasing problem has now been shown in Fig.1, each front panel isthe force on a given elementalarea encountered? The writer thinks not. To just wide enough to accommodateone due to the motion ofallother ele-throw more light on this kind of fal- direct -radiator loudspeaker mechanismments. Inclusion of allof these ele-lacious reasoning consider the following mounted at its center. This insurestight mental areas on which the force hasnumerical illustration: Suppose thata acousticcouplingbetweenadjacentbeen computed by a second integrationphase -conscious observer (if onecan be drivers when the speaker enclosuresare (taking due account of the number offound) is located some distance froma assembled in the corner ofa room for piston radiator mounted in an infinite operation. Notice that the radiating times a given element is included in thebaffle, and that the line from hisear to ele- calculation) gives the total forceact- ments of the speaker resemble themouth the center of the diaphragm never makes of a sectoral horn. In the ing on the diaphragm. This forceis an angle less than 30 deg. with respect following sec-equal and opposite to the forceexerted tions the reasons behind thisdesign are to the baffle plane. Further assume that set forth. on the fluid medium by the piston. The latter force, divided by the the observer does not wish complete cone veloc-destructive interference to obtain atany ity, gives the mechanical radiationim- pedance of the speaker. As might frequency in the range from 60 to 10,000 Radiation Resistance of Coupled be cps. Surely the "phase problem" is at its Loudspeakers expected, the radiation resistance(inworst at null points in the angular dis- mechanical ohms) turns outto be pro-persion pattern. Reflections from all ob- Itis well ktioii that theradiation re-portional to the square of the effectivejects are ignored, and it is assumed that diaphragm area at low frequencies. sistance (in mechanical ohms)of an iso- the velocity of sound is 344 metersper lated sound source whosedimensions The analysis has been outlinedbe- second. Using these data one finds that are small compared to the wavelengthcause it brings out two essential facts if the "phasing problem" is to be avoided of the radiated sound isproportional topertinent to this discussion: -even when using a single piston radia- the square of the effective radiating (a) The diaphragm of tor,the diameter of the loudspeaker area. Furthermore, the value ofthis re- any loudspeaker must not exceed 1.9 inches. It is obvious sistance is independent of may be considered to be madeup the physical ofa number ofsuitablyshaped that one may ignore the radiationpo- shape of the soundsource. Applying this smaller diaphragms. tentialities of this loudspeaker at 60cps. principle to the multipleloudspeaker (b) The interactionor coupling between If the general problem of phase is really under discussion,one concludes that these smaller diaphragms hasbeen of significance (except as discussed later when the multiple loudspeakerisiso- properly taken into account inthe in this paper) one might very wellcon- lated from all surroundings(no longer analysis. clude that operated in a corner) thatthe radiation resistance of the four-speaker array at Statements(a) and(b), together (a) Multiple microphone pickup ofany low frequencies is about16 times thewith the theory of thepiston radiator programisto be avoidedifthe radiation resistance ofone of the driversoutlined here. permitone to draw cor- outputsofthese microphones are comprising the rectly the conclusion that the to be electronically mixed and trans- array in the absence of radiation mitted over a single channel. acoustic coupling. It isclear, therefore,resistance of a tightly coupledmultiple that the radiation speaker system installed inan infinite (b) Effortsto obtain a diffuse sound resistance of eachbaffle is proportional to the field,i.e., a sound field of random driver when operatedin multiple is in- square of the phase, in a studio or auditorium by creased by a factor of4 over that ob-effective diaphragmarea at low fre- the use of splays and asymmetrically tainable from thesame driver operated quencies. Naively expressed,a person placed patches of absorbent mate- as an isolated speaker. It is might say that the radiationresistance rial, should be abandoned. to be empha- of a multiple loudspeaker is sized that the physicalorientation of greater than the drivers ina baffle which is smallthat of one of its driversmounted in The writer has attempted to point out compared to the wavelength an enclosure because the driversaidin this section that a multicone loud- of the radi-each other in compressing speaker is capable of excellent bass re- ated sound is not importantinsofar as the air in the radiation resistanceof the array isthe vicinity of thecones, causing eachsponse (by virtue of the fact that at low concerned. This factpermits locationdriver to "think" that it isworking intofrequencies the mechanical radiation re- of the drivers ina suitable spatial rela-a more dense fluid medium. Amore sistanceis proportional to the square tionship to insurea elaborate and quantitativeanalysis ofof the effective diaphragm area), and desirable sound coupled loudspeakers, based that an array of speakers presents pre- pressure pattern. If the multipleloud- on the prin- speaker is now located ciple of symmetrical phasecomponents,cisely the same problem with regard to in the corner ofhas been published.2 phase as any direct -radiator loudspeaker three large mutuallyperpendicular rigid planes, the radiation It now seems pertinentto discussof small diameter, or any other vibrat- is confined toa ing body. No horn is immune from the solid angle of n/2steradians, and thisbriefly another aspect ofstatement (a) "horn" loading further that the diaphragm ofany loudspeaker"phase problem" for the mouth of the enhances the horn may be closed by a massless dia- radiation resistance ofthe array. Butmay be considered to be madeup of a such a situation is number of suitably shapedsmaller dia-phragm with no deleterious effect on physically unrealiza- phragms. the performance of the horn. Before ble in an enclosedspace (excluding an In a recentarticlesitis anechoic chamber) strongly implied that two low-frequencyconcluding this topic the writer would and theideaof like to suggest a problem of considera- sound transmission ina solid angle ofspeakers are highlysatisfactory,but n/2 steradians four such speakersare taboo becauseble theoretical interest concerning the must give way to a rigor- coupled loudspeaker system under dis- ous analysis of soundtransmission phe- "they present a problem ofphasing." nomena in rooms. The writer is inclinedto divide (hypo - cussion. Suppose one were interested Possibly a more elegant in calculating the radiation impedance way of dem- on the mechanical side of one of the onstrating qualitativelythat the radia- tion resistance ofa multiple loudspeaker 1 L. E. Kinsler and A. R. Frey, "Funda-drivers forming the four -element array, is proportional mentals of Acoustics", John Wiley &Sons, assuming the array to be operated in to the square of the Inc., New York, 1950, effective diaphragmarea at low fre- pp. 187-195. No the corner of three perfectly rigid mu- quencies is to comment audio engineer should be without thisbook. tually perpendicular semi -infinite planes briefly on the 2 S. J. Klapman: "Interaction impedance under free field conditions. One equiva- theory of the circularpiston radiator of a system of circular pistons," I. mounted in an infinite baffle. Acous. lent mathematical model consists of two This prob- Soc. Ain., Vol. 11, Jan. 1940,pp. 289-295. suitably oriented symmetrically disposed lem is, discussed ina straightforward 3 W. C. Shrader, "Audio in the home." manner in a recently published excellent speakers driving an infinite wedge. The AUDIO ENGINEERING, July 1952,page 30. sides of the wedge are linearly tapered,

102 and if extended would meet at an anglethrough (c), one can evaluate unknown Under steady-state conditions, a point of 22.5 degrees. The height is twice thecoefficients. A little mathematical manip-source of sound gives rise to a stand- height of an enclosure. The top and ulation then enables one to arrive at aning -wave pattern having nodes and anti - bottom (which are missing) have theexpression for the sound presstire atnodes at fixed locations within a room. cross-section of an isosceles trapezoid.any point in the room. The next step isTwo or more such sources separated a The basic problem is to find an expres-to move the sound source analytically sion for the acoustic pressure in theinto a corner. It will be found that thesmall distance and operated simultane- vicinity of a driver satisfying the wavepressure at any point in the room hasously tend to fillin nulls in the sound equation and all boundary conditions.been maximized. However, and this ispressure pattern. The four -driver loud- This is an advanced problem in mathe-the point of this entire discussion, aspeaker array may be considered a point matical physics. max. max. value of sound pressure isor simple source of sound only at low obtained when the sound source is lo-frequencies. When the frequency is in- catedin acorner, and the sound pres-creased somewhat the point source must Operation of a Loudspeaker in the Corner be replaced by the appropriate distrib- of a Rectangular Room sure is computed at the same point. The force of the fluid medium on theuted sound source. The large aperture Probably the simplest boundary -valuediaphragm of the point source is theof this speaker makes it rather unlikely problem in applied science is the deter-vector sum of the pressures atthis that objectionable pressure minima will mination of the acoustic pressure distri-point due to all possible modes multi-exist anywhere within the room. bution in a rectangular enclosure havingplied by the surface area of the vibrator. six perfectly rigid sides under free os-As mentioned before, the negative of cillatory conditions. The wave equationthis force, divided by the cone velocity, Sound Pressure Pattern in terms of the velocity potentialis gives the radiation impedance in me- separated in cartesian coordinates. Thechanical ohms. The real part of this Although the intensity of sound in a impedance is room depends greatly on the properties acoustic pressure is equal to the density the radiation resistance.of the enclosed space, most of the ex- of air multiplied by the partial deriva-The larger the acoustic pressure react- tive of the velocity potential with respect perts agree that while the ear tolerates to time. The air particle velocity is equal a certain degree of non -uniformity, a to the negative gradient of the velocity loudspeaker having a smooth response potential. The boundary conditions are under free field conditions will generally that the normal component of particle be more acceptable under all listening velocity at all bounding surfaces of the conditions.5Consider the loudspeaker room must vanish. One immediately ob- array shown inFig. 1,to be oriented tains an expression which shows that in the corner formed by the intersection the acoustic pressure is always a maxi- of three mutually perpendicular per- mum in the corners of the room re- fectly rigid semi -infinite planes. Assume gardless of the room dimensions and also that the adjacent walls of the wedge the order of the mode of free oscillation. shaped enclosures are perfectlyrigid However, itis to be noticed that this (and thus non -absorptive). The equiva- analysis says nothing about how the lent mathematical model of this system normal modes of oscillation are estab- is a perfectly symmetrical circular array lished, and in such a room there would composed of 16 wedges not displaced in be no decay of sound, the vibrations length. Each wedge is twice as high as continuing forever.It turns out that a wedge comprising the physical loud- when the walls of an enclosure are sound speakersystem.Twodriversare absorptive, a perturbation of the simple mounted in each wedge, one-fourth the theory just advanced enables one to way down from each end in the front compute the pressure distribution in the panels. The mathematical model dis- room for an individual mode during penses entirely with the three semi -in- the transient decay of the sound field finite planes. It is easy to see intuitively (no sound energy supplied). The pres- thatat low frequenciesthe acoustic sure distribution for each mode isstill pressure pattern in the azimuth plane a maximum in the corners of the en- Fig. 2. Scaled drawing of multiple loudspeaker. is essentially uniform since there is an closure. angle of only 22.5 deg. between adja- Now whena"cavityresonator"ing on the cone the larger is this re- cent speaker axes. To maintain this uni- bounded by sound -absorptive wallsis sistance. It is apparent, therefore, thatform pattern throughout the frequency driven by a simple source of sound, onefor maximum cone loading under steady range of interest requires the use of proceeds as follows to determine the state conditions the speaker should bedrivers having a beam width of more sound pressure distribution in the room:' designed for corner operation.For prac-than 45 deg. at the highest frequency (a) The form of the wave equation usedtical purposes cone loading is increasedin the range to be reproduced. The pat- must permit the injection of soundonly for sustained low frequency notes.tern of the array in the azimuth plane into the fluid medium by the simple For transient signalsit would appearcan be computed easily by graphical source. that the use of a corner for speakermethods. One simply adds up vectorially, (b) The flux of air from the source can i.e.,in proper phase relationship, the be represented by a source func-location would be no more beneficialsound pressurescontributed by each tion, and this function can in turnthan locating the speaker anywhere else be expanded in characteristic func-in the room. driverin the angular direction of a tions at the source location (a point There is one important advantage ofgiven point. This process is repeated at anywhere in the room). locating a speaker in the corner of a (c) The steady state sound pressure atroom in addition to the increase in cone 5 H. F. Hopkins and C. R. Keith, "New any point in the room can be simi- loading obtained on sustained low fre- loudspeaker system"I.Soc. Mot. Pict. larly expanded in series. quency tones. Since all of the normal Eng., 51, pp. 385-398, Oct. 1948. By using certain information obtainedmodes of a room have pressure maxima The azimuth planeisdefinedasa from the problem of the transient decayin the corners, a speaker so located will plane midway between and parallel to the of sound in an absorptive room, and (a) two planes that individually contain the excite all possible modes. These modescenters of 16 speakers. This plane is at are few and far between (with respectright angles to the axis of the circular 4 A good reference is P. M. Morse and to frequency) at very low frequencies, array.Theverticalplaneincludesthe R. H. Bolt "Sound waves in rooms",Re- and it is considered important to excitearray axis and the centers of four drivers view of Modern Physics,Vol. 16, No. 2 allof them inthe interest of "uni--two in one wedge, and two in the dia- April 1944, pp. 69-150. formity" of transmission. metrically opposite wedge.

103 other points until the array pattern has parallel sides of the enclosures cannotwith sound absorptive material would been constructed. Obviouslyone must vibrate because there are no force dif- be furnished the primary be more effective than the correspond- pressure dis- ferentials acting on them. The fronting baffle of rectangular shape in damp- tribution patterns of the drivers taken panels are just wide enough to under free field conditions. ac- ing out standing waves within theen- It is not at all commodate the drivers. These can be closure. Unfortunately, normal modes difficult to compute adequately braced. The back panelis exist for trapezoidal shaped bafflesa' the pattern in the vertical plane, again only several inches in width. Ifitis they do for rectangular, spherical, and knowing the primary patterns of the properly securedtothe non -parallelother -shaped enclosures. The solution drivers used. More beaming takes place sides its vibration can be ignored. Theto this problem is to fill the wedge with in the vertical plane. The writerdid top and bottom of each enclosure is not use two drivers in each wedge (one small rectangular parallelepiped shaped small in area. The bottom certainly will blocks of fiberglas using strongper- above another) because the soundpres- not vibrate because of the load on it, sure pattern in the vertical plane would forated curtain materialas a retaining and the top can be "held down" bya wall to prevent the fiberglas frompress- become too narrow even at relatively heavy tweeter. low frequencies, and the perfect ing on the driver diaphragm. Thenor- sym- The sides of a rectangular enclosuremal modes will now be dampedout by metry of the array would be destroyed. for several drivers will necessarily vi- as will be explained. the viscous drag of air particles oscil- brate because the array lacks symmetry. lating in the small pores of the fiberglas The multiple loudspeaker described This can be minimized only by construc- and other absorbing material inthisarticle can be used withor present. tion of an enclosure of extreme rigidity. In addition the effective volume ofthe withoutatweeter.Iffour12 -inch Because of the symmetry of the wedge drivers, such as the Western Electric enclosure will be increased by theuse enclosures one can be certain thatif of fiberglas because itcan be demon- type 728B or 754A are used,an ex- identical drivers are installedin the cellent loudspeaker is obtained without strated that a sound wave is propagated four baffles that what one diaphragmmore slowly infiberglas than inair. the use of a high -frequency unit. For does will be necessarily duplicated byFor a given frequency the wavelength theinitialteststhe writer employed the others at allfrequencies. It doesof sound radiated from the back sideof four WE 728B drivers in thewedges, not follow that when four or more driv-the diaphragm into the fiberglas filled and a WE 594A loudspeaking telephone ers(possibly of different sizes)areenclosure is shortened; accordingly the with 31A horn for treble. Thecross- installed in a common rectangular over frequency is 800 cps. One impor- en- inside dimensions of the baffle, interms closure that all cones will go in andof the wavelength, are increased.The tant asset possessed by this dual loud- out together at all frequencies. Theme- absorption of sound in a fiberglas filled speaker isthatthesoundpressure chanicalconstantsofthe diaphragm pattern in the azimuth plane for both enclosure is approximatelyan isother- suspension system, the mass of the dia- mal rather than adiabaticprocess. At the tweeter and woofer join smoothly phragm and drivingvoicecoil. at cross -over. Persons employing, for the low frequencies the effective volumeof stiffness of the enclosures, and theco- the enclosureisabout1.4 times the example, folded horns for bass liketo efficient of acoustic coupling betweenactual volume. At higher frequencies forget about the problems arising from drivers may be of such value as tocauseessentially free field conditions obtain phase differences nearthe crossover one diaphragm to move in irrespectiveon the back side of the diaphragm be- frequency, abrupt discontinuities in the of the fact that the polarity of the volt- cause no acoustic waves are reflected. distribution pattern atcrossover, and age appliedtothedriver terminals the phase difficulties associated with the Thus the enclosure may he regardedas would normally cause the diaphragmone of infinite size. The impedanceseen sharp bends even atfairly low fre- to move out. This is not a new concept.by the driver, looking into the enclo- quencies.Noneofthesedifficulties In antenna array design itis not un-sure, is the characteristic resistance of plagues the speaker describedinthis common to measure a negative resist-air. It is worth noting that at low article. ance at the terminals of one of the fre- Another feature of the coupled loud- quencies very little power is radiated dipolescomprisingthearray.Tofrom the backside of thecone into the speaker isits excellent mid -rangere- achievestability,i.e.,minimizedia- sponse,i.e., enclosure where it must be absorbed. response in the range of phragmslip,allmultiple -loudspeaker This is because the baffleis small in frequencies from say 300 to 800cps. enclosures should be compartmentalized Apparently a folded horn works well terms of the wavelength, and theacous- and thedrivers employed should be tic pressure is very nearly in timephase only in the region from 30 to 300cps. highly damped. The fact that highly This necessitates crossingover at about quadrature with respect to the particle damped identical drivers are used in thevelocity. Thus at low frequenciesan 300 cps. But thisisundesirable be- writer's multiple loudspeaker and that cause it has been found an advantage. enclosure may berepresentedasa none of them is operated in a common lumped compliance (purely reactiveele- from the listening point of view,to enclosure insures that to within a fewment) in the equivalent acoustical cir- have any effects due to out -of -phase degrees all diaphragms do go in and outcuit; a fact that is well known. conditions between the low- and high - together. Observe that the use of wedge Construction of Multiple Loudspeakers frequency speakers come above there- shaped enclosures is not optional. The gion of maximum energy transmission. symmetry afforded by this construction Baffles having the cross section ofan Evidentlytheuseofathree-way requires the acoustic forces acting on isosceles triangle will serve equallyas speaker system, employinga folded horn each diaphragm to be the same. These well as those having the cross section of for bass, does not obviate this difficulty. forces are not equal, for example, evenan isosceles trapezoid. The angles of when four drivers are operated ina each baffle art fixed by the requirement On the Choice of Acoustic BaffleShapes for Multiple Loudspeakers compartmentalizedrectangularenclo- that four baffles must ht side by side sure. This fact may be verified im-in a corner. This fixes the angle be- Acoustic baffles havingisosceles mediately by applying the principle of ween adjacent speaker axes at 22.5 trapezoidal cross section have several acoustic images. It is a serious matterdeg. If three enclosures are employed highly desirable attributes. As pointed if any out -of -phase condition exists be-this angle is 30 deg. which is probably out in the last section, if four identical excessive from the point of view of ob- tween diaphragms, because the chieftaining a good azimuth pattern at rela- baffles are used, the angle between ad- purpose of a multiple loudspeakeris jacent speaker axes is only 22.5 deg., thereby lost; namely, that of achievingtively high frequencies. If this three- and if only one driver is installed in speaker array is used as a woofer only, a good low -frequency performance. Thethe 30 -deg. angle between speaker each baffle excellent angular dispersion phase relationship of the vibrating dia- axes of sound isassured. The choiceof phragms can be checked by stroboscopicis probably not too objectionable. wedge-shaped bafflesisimportant for Driver specifications should be fol- means, or by measuring the sound pres- lowed with regard to the required vol- other reasotr. For one thing thisarray surepatternofthespeakerinan is symmetrical. If the four wedgesare ume ofeachenclosure.Where the anechoic chamber. speaker system is used without a tweeter, assembled in a corner formed by the At first glance it mightappear that floor and two rigidwalls,the non- the writer has found satisfaction with a baffle of trapezoidal cross section lined Western Electric Type 728B or 754A

InA drivers. A smaller version of this multi- The series or series -parallel connec-is desirable to construct frames to fit ple speaker could be made up using tion is recommended. It is worth men-over the front panels as suggested by Western Electric Type 755A 8 -inchtioning thatiffour identical driversthe drawing. A brass wire screen meas- drivers. Such a speaker would be about having a nominal input impedance of 8uring about 13 x 36 in. should be stapled optimum for home use. Because of the ohms are connected in series the nomi-to the front panel to prevent accidental 755's extended high -frequency response, nal input impedance of the array is in-damage to the speaker cone, and then no tweeters would be needed. Where the creased from 32 ohms to a somewhata suitable grill cloth tacked in place. multiple speaker is used as a woofer higher value by virtue of acoustic cou-If the frames have been properly re- only, experience has shown the Bozak pling between drivers. This effectis cessed they willfitsnugly over the B-199 to be a sound choice due to its notverypronouncedwhenhighly edges of the grill cloth and wire. The lowresonantfrequencyandcorrect damped inefficient drivers are employedframes may be secured to the front damping. in a multiple loudspeaker, but is to bepanels using small oval headed brass The enclosures must be completely kept in mind when matching a highlyscrews and brass cup washers that have linedwithKimsulorothersound efficient horn loaded driver to an ampli-been blackened by chemical treatment. absorptive material. With certain driv- fier.Readers are advised to conductObserve that different screws are used ers it is an advantage to fill the baffles listening tests as an aid in deciding theto secure the front panel to the enclo- completely with blocks of fiberglas1 x best driver connections and the propersure and the frame to the front panel. 2 x 4 inches in dimensions. Type PF-314 amplifierloadtaps.Ifincorrectly If wedges of trapezoidal cross section fiberglas is recommended. The blocks of damped drivers are used, hangover ef-are constructed an air column will exist fiberglaswilladheretotheKimsul, fects are likelyto be observed. Thisbetween the speaker and corner of the thus preventing packing. Alternatively, difficulty may be minimized or elimi-room. For enclosures 4 ft. high, reso- to prevent packing the enclosure may nated entirely by making a large feltnance of this air column will occur at be compartmentalized using clothre- washer for each driver with center holeabout 70 cps, and again at about 211 taining walls. This treatment smooths of sufficient size to slip over the magnetcps, etc. It should be filled with sound out the speaker response, i.e., increases frame. The periphery of the washer isabsorptivematerial.In makingthis the speaker damping at low frequencies, secured to the front panel with carpetelementary calculation no end correc- and increases the effective volume of tacks. The air set in motion by the back tion for the pipe was applied. the baffle. It is recommended that fiber- side of the diaphragm must pass through glas treatment be kept well clear of the the pores of the felt. Driver damping Acknowledgment diaphragm of the driver. The enclo-becomes, therefore, a function of the sures should be essentially air tight andporosity and thickness of the felt used. The writer is indebted to Mr. H. F. noportsarepermitted. The design The baffles for the writer's multipleHopkins of the Bell Telephone Labora- should be such that the front panel isloudspeaker were made out of h -in.tories for making available certain in- just wide enough to accommodate the plywood. The outsidedimensionsof formation concerning multiple -speaker driver mounted in the center. This in-each wedge are as follows : experiments carried out at the Murray sures close coupling of the drivers at Height -48 in. Hill Laboratory. low frequencies. The baffles will have Width of front panel -14.M in. The significant comments on the use to be approximately 4 ft. high in order Width of back panel -5-5/16 in. of fiberglas in an acoustic baffle to im- to obtain the requisite volume. When Slant depth -24% in. prove speaker performance are due to they are setside by side and then These figures do not include the di-ProfessorJordanJ.Baruch ofthe jammed into a corner, excellent baffle mensions of the front panel frame. Massachusetts Institute of Technology. effect is obtained. The polarity of each A scaled drawing of the multiple loud- Messrs. Theodore John Schultz, John driver should be checked, using a small speaker is shown in Fig. 2. Bouyoucos and A. A. Janszen of the dry cell, beforeitisinstalledinits The front panel of each wedge is re-AcousticsLaboratory, Harvard Uni- baffle.Naturally, the speakers are to movable,theotherfivesidesbeing versity, reviewed the manuscript and be connectedelectricallysothatall permanently secured.Intheinterest made useful suggestions for improve- diaphragms operate in the same phase. of obtaining a finished appearanceit ment. The Concrete Monster

JAMES FERGUSON*

It could only happen in California-an exponential horn built into the sideof a house! It's a neat job and must sound fine. We envy his originality-aswell as his apparent disregard for domestic relations.

ONE OF THE MOST INIPORIANT links in and smaller excursions of the voice coil zompromise on quality. the audiochain,and oftenthe keep it in the linear portion of its travel. After considering this knotty problem weakest,isthespeaker system, The articles say that the mid -range for some time and building several en- especially the speaker enclosure or baffle. and high frequencies offer no space prob- closures which promised "big bass" per- Radio and audio magazines devote fre- lems for horns, but when it comes to the formance in moderate size spaces, I still quent articles to them. Many preface the low bass the horn's mammoth size (awas not satisfied. So I decided to build test by saying that the exponential horn length of some 16 feet and mouth diam- an exponential horn outside the house, is the best and most efficient method ofeter of 10 feet) necessary to reproduce and merely let it poke its mouth into the transferring acoustical energy from theproperly organ pedal notes and other room. And for the benefit of the many cone or driver to the free air in a room. bass down to 30 cps or lower simply rule perfectionists who read this magazine, Its high efficiency reduces distortion by it out. The little woman would not toler- the following is a brief outline of the not requiring so high a setting of theate such a monster in the living room, system I have constructed. volume control for a given sound level, even if the hi-fi bug would. So the arti- I used three speakers, all exponential - clesoffer you somethinglessbulky horn -loaded, and two dividing networks * 11900 Westminster Place, Los Angeles which sounds less well at the bass end. crossing over at 200 and 3,500 cps. The ?4, Calif. When you compromise on size, you often "listening -room" is a combination living -

105 and dining -room (Fig. 1), 31 feet long The Horn and 18 feet wide, narrowing to 12 feet length, including the mouth formed by in the dining section. Looking from the Now for the big horn and air chamber the walls and ceiling, is over 16 feet). living -section into the dining -section, the (which is not built of wood, but of steel The first 3 feet, forming the throat,was cabinet containing the mid -range horn and concrete) illustrated in Figs. 4 and 5. made of 22 -gauge galvanized steel in and four -in -a -row high -frequency horns, The speaker, a 15 -inch Stephens 103LX two sections. It was coated on the out- Fig. 2,is located in the right corner "woofer" with low -resonancecone,is side with asbestos -filled pitch or asphalt, under a window. mounted undergroundina 55 -gallon and wrapped with a solid layer of binder This cabinet is shaped like a cube 2 steel drum-the type that has aremova- twine. Then it was given another coat feet square and 27 inches high, with the ble head-and is sealed witha gasket of pitch and wound spirally witha 6 inch projecting corner sliced off diagonally so and ring clamp. The speaker is mounted strip on a44 -in. of water -proof crepe paperas that its face is 21 inches wide. The lower plywood ring whichis shown in Fig. 4. This provides excellent 20 inches is occupied by the mid-range fastened to the underside of the metal horn damping of spurious vibrations in the and back -of -coneairchamber drumhead. In addition, a second wood metal walls. The metal portion of the which surrounds it. It has a throat area ring of smaller diameterwas used to fill of 50 square inches which doubles every in the space between the mounting board horn telescopes into the next, concrete, 6 inches of horn length, being 400 square and the head, due to the curvature of the part for removal of the speaker. inches at the mouth. It is driven by a 12- head. The concrete horn proper was made inch Wharfedale speaker which hasa A 10 -inch circular holewas cut in the as follows, built up as shown in Fig. 6. porous cloth cone suspension. The four - metal head, and Y2 inchwas turned up Quarter -inch round iron bars were bent cell high -frequency horn(each horn for a flange to receive the first sectionof to form the corners, then square frames with 4 -inch -square mouth and 10 -inchThe horn. That made the opening 11 made of the same material were placed length) is located in the upper part ofinches in diameter and the hole in the along the length of the frames at about the cabinet with room for planterson wooden rings was tapered from this size 2 -foot intervals and tied together with each side. It is powered by a Stephens to 13 inches next to the speaker. The air- wire where they intersected. Not shown P-15 high -frequency driver. tight cavity of the drumserves to balance are some extra rods which were added in The sound from the big bass hornthe impedance of the back side of the the broad parts of the frame; these rods enters the room in the upper left cornercone to the horn -loaded front side.It were spaced about 12 inches apart in each of Fig. 1 through an opening 4 feet widealso prevents the sound of the back-wave direction. Also not shown are some diag- and 5 feet high, Fig. 3, with its top at the from being radiated to the neighbors. onal bracings of a -inch rods. The frame ceiling. The angle of ceiling and walls Incidentally, the sound from the entire was next covered with heavy -gauge gal- forms the last 5 or 6 feet of the horn'shorn outside the house is hardly audible vanized metal lath, which was secured to length, and its effective mouth diameter from a few feet distance, even when it is the frame with "hairpins" of 18-gauge at this point is approximately 10 feet.delivering considerable volume in the galvanized wire, spaced about 10 inches The large opening has in noway de- house. apart arid twisted. tracted from the room's appearance inas- much as the grill cloth covers not only The drum was partiallyfilled with large stones to reduce its volume toap- the opening, but the rest of the wall proximately 4Y2 cubic feet. The stones above a dado and over to thecorner break up the space into several small window. The opening fills thespace to cavities of varying sizes and shapes, the left of the mirror shown in the pic- producing a distributed resonance, rather ture. An uninformed visitor would never than one large peak, as would be the suspect the presence of the horn's mouth case with one large cylindrical cavity. from inside the room. The horn starts from the drumhead at The distance of a few feet that sepa- ground level and emerges into theopen- rates the bass and mid -range treble horn ing in the wall, making abouta 95 -degree gives a spatial effect to orchestras which turn, and having an actual mean length is very pleasing, if not third -dimensional. of about 11feet (though its effective

Fig. 2. Closcup of the mid- and high -frequency speaker section in the corner cabinet.

Fig. 3. Mouth of the low -frequency hornopens

into the listening area through a grille -cloth- covered port approximately 48 by 60 in.

e

Fig.1. The author's dining room, which is part of the livingroom. Midrange and treble speakers arc in the small cabinet beneath the window at the right, while the hornopens into the wall at upper left.

106 Fig. 4 (left). The author with his externally located exponential horn. Wonder what the neighbors thinkitis.Fig. 5 (right). Cross section of horn structure with relation to the dining room wall. "X" indicates the 2 < 2 x1/4angle irons embedded in concrete blocks to support the structure. Next, the opening was made in thewater -proofing and hardening compound,makes a transition from round at the wall by removing the stucco from thesuch as Anti -hydro, Sealcrete, etc.) was small end to square (with rounded cor- outside and lath and plaster from the in-trowelled onto the metal lath. After this ners) at the beginning of the concrete side, exposing two studs passing throughhad set for 48 hours, more coats werepart. After the concrete had thoroughly the opening. One was removed for thetrowelled on the inside and outside until dried out, the inside was coated with a length of the opening for easy accessa thickness of 2 inches or more had been sealer and then with gloss lacquer to during construction, and the other "air-built up. Each coat was scratched or make the surface more reflective. roughened so the next would adhere The opening into the room was cov- stream" shaped and left in place. firmly, and the last outside coat wasered with closely woven white nylon (I must confess here that I was reluc-"floated" with a sponge -rubberfloat. cloth about 1 inch back of the grill cloth tant to tell my wife of my intention toThe inside was trowelled smooth. A (or at the stud line), and which was used cut a barn -door size hole in her dining -metal ring was used at the small end ofto prevent a "dark look" at the opening. room wall, so I maneuvered her into anthe frame, which acted as a screed' forIt doesn't seem to make any difference in over -night visit to her daughter in aplastering, and to make a neat ending forthe sound. An open -mesh drapery mate- nearby city. When she returned home.the concrete portion of the horn. rial was used for the grill cloth. the deed was done and it was too late The horn has an expansion rate which Idrive the speaker system with a for her to object.It seems that manydoubles in cross-sectional area every 27home -built ultra -linear amplifier, using wives don't share their husbands' enthu-inches of horn length, beginning with thea Fisher audio control, with phono, tape siasm for hi-fi.) 11 -inch circular opening in the drum-recorder, and an AM -FM tuner as sound The frame, or cage, was supported inhead. The sheet -metal section, Fig. 7,sources. TV sound comes from a 630 - place and a coat of cement mortar (com- type television set. posed of one part plastic cement and 1 Screed: A strip used to gauge the thick- And how does it sound? Well, in pre- three parts sand-to which was added a nessof the plastering. vious articles on speakers and enclosures, the superlatives have been pretty well used up, but I've been listening quite FINAL DIMENSIONS OF INSIDE OF HORN WILL BE 1' OR e LESS THAN SHOWN a while and this is the best I have heard DUE TO THICKNESS OF PLASTER yet on the low end. The highs are good. too.

LOCK SEAM

9" SOLDER 4e" 22 GA GALVANIZED STEEL 1/4' ROUND IRON RODS WIRED OR WELDED AT INTERSECTIONS 45 3/4 SHEET SQUARES SPACED26. APART 44 14" ON MAIN AXIS tiNtW S LOWER SECTION 2 r INSIDE OF SHEET METAL RING MUST FIT LARGE END OF METAL SECTION TO2.BELOW TOP Cass 1/2 O 3- 1/4"X 1""L" STRAPS FOR HOLDING 46 GASHEET METAL RING THROAT TIGHTLY TO CURVES FOR UPPER AND DRUMHEAD LOWER SECTION FORMED LOWER END OF CONCRETE HORN MORE BARS ARE ADDED BETWEEN BY 60" RADIUS SHOULD BE SUPPORTED BY TWC 1/4" X 2" X 2" THE FRAMEWORK SHOWN, AND THE ANGLE IRONS IMBEDDED IN FOOTINGS AS WHOLE COVERED WITH 32 GALVANIZED JOINT AT THE ANGLE OF THROAT AND SHOWN AT X IN FIGURE 5 EXPANDED METAL LATH. IT REQUIRES DRUMHEAD SHOULD BE SEALED WATERTIGHT ABOUT SIXSHEETS WITH ASPHALT MASTIC

Fig. 6. Basic framework of 1/4 -in. rods used as reinforcement for the horn. Framework is covered Fig.7.Patternsfor the construction of the with metal lath before cementing. telescoping throat end of the exponential horn.

107 Real Theater Sound inaSmall Package

THOMAS R HUGHES

Part 1. The author, after years of analysis and desire, has reshuffled old facts and ideas to make a fresh approach.

To THOSE WHO have followed the Our objective is to reproduce (as ac- A Fresh Approach articles on loudspeakers and enclos- curately as is physically possible) fine ures in radio magazines it must seem music, of all types, in the average size The solution of all our problems starts that the subject has long been exhausted. home. The problems which have to be with the proper generation of the lower There are as many different suggested solved for most of us are : the high cost frequencies and here is where we part ways of enclosing speakers and as many of available systems ;their large bulk company with the designers and authori- extravagant claims for the set-ups as and difficulty of fitting into the decora-ties. We are not so interested in a flat re- there are pages in the dictionary but we tive scheme; the problem of having tosponse down to 40 or 50 cps because know that in many cases our ears and operate recommended systemsattoo musical scores rarely call for funda- memory are just deceiving us. high a volume level in order to achieve mentals so low, nor do orchestras or The ears, after a good night's rest, the effect called "presence"; the problem ears provide a flat response. What we are too ready to tolerate anything in of obtaining life -like definition of instru-demand is the clear generation of the the musical sound that is stimulating ments and effects when the musical score full harmonic structure of the cello, with and different. But the very divergenceexplores a polyphony of superimposed its rugged vigor, or that of the bassoon, of design and choice in speaker systems parts and melodies ; and lastly the prob-with its alluring intensity, and the jungle is proof that no satisfactory standard lem of extra amplifier capacity required vibrance of the marimba bass notes. has been achieved. for the available high -quality systems. Someone has said that most speaker The speaker system we have developed Engineers conquered the problems ofdesigners listen to music through their solves,for thefirsttime,allof theadequately reproducing fine music in slide rules. However, the effects we are speaker problems connected with high- the theater many years ago, but thereinterested in can not be distinguished quality music reproduction in the aver- has been a lack of realistic approach toby instruments in a laboratory. Further- age size living room. This has been ac-the needs of the normal home listener. more, they are difficult to achieve in the complished by a departure from the rutHaving played in both symphony andhome with normal speakers. So we will designershave beeninforseveral dance orchestras in younger years and years-that of trying to adapt theater -being an engineer by profession, thegive you our specifications for simple type speakers to living room use. writer has been struggling with these solution. In order that you will not assumeproblems for many of his older years. In spite of the fact that many folded- thisisjust another speaker enclosureHavingdozensof music -lovingac- horn corner -speaker enclosures are be- that happened to charm the ears of itsquaintances who would be in the market ing builtfor home use with15 -in. master, we ask you to bear with us infor the proper article, if it were avail-woofers, the sales points for a large a thorough discussion of the principlesable, we determined to develop a stockwoofer are only applicable when itis involved, before we finally give you theitem that could be moved into any roomused in a box or baffle enclosure. And details of construction and accomplish- without disrupting it. Since the prob- it should never be used for symphonies ment. lems are so interwoven, we will not take in an average size living room. 3721 Hillcrest Drive, Los Angeles 16, up their solution in any regular sequence Where it is used in a bass -reflex cabi- California but will refer to them at relevant points. net or any other box resonator, the prime objective is to have the natural lower cone resonance fall below the cutoff point of the program material. This can only be accomplished with a large cone of careful design. But, with horn loading of our woofer, we are not muchcon- cerned withresonantpointsbecause

a horn levels them out to a great extent and still maintains acoustical coupling below these points.

Fig. 1. The com- On the other hand, there aremany pleted three -horn factors against the use of the large corner reproducer in woofer in a home installation. Themost a typical living -room important one is that, when it is worked setting. at its intended amplitude,it produces outraged cries from family and neigh- bors. You don't get the robust timbre

of bass instruments or singers froma large speaker that is just coasting along at comfortable room level. Anotherfactoragainstthelarge woofer is its cost. If you are determined to hear and feel the impact of that bass drum beat and you open up the input to

108 your big cone, you will have to have a fromalldirections.And, when the heavy field magnet for proper attack speaker systemisplaced in a room and decay at full amplitude. That's when corner, there is no use in talking about the cost starts mounting but that isn't 120 -deg. spread of a multihorn tweeter, the main factor in the cost of a really because the walls only allow 90 -deg. fine woofer of theater proportions. Un- spread. in less many detailsareconsidered Advantages of Horn Loading proper shaping, strengthening, and sus- pending of the large cone, you will not The greatest advantage of a horn get a clean attack and decay-even with over all forms of boxes and resonators the heavy field magnet. is the much smoother response curve These are problems you have to ac- with its freedom from sharp peaks and cept in producing adequate volume for a dips as different frequencies are reached. theater or auditorium. but why should The only difficulty of reproduction or the home owner have to be saddled with transmission by horns is in the natural them? By the use of a small, light, but nonlinearity of the air column when the greater amplitudes of the low frequencies stiff cone, we have avoided all these dif- are impressed upon it. ficulties to a great degree. The nonlinearity of air coupling ap- Therefore, we will start with a woofer plies equally to any form of box or which has a small hard cone-not over resonator. At the same time thereis an 8 -in. nominal size with a 6 -in. effec- Fig.2. View of the structure described by the trouble in boxes and resonators from tive working diameter.Phenolic im- authorinearlystageofconstruction.The both the mechanical resonance points of pregnated paper works very well for the back wave from the cone passes to the front speakers and the acoustical resonance material of the cone. The angle its sur- ofthecabinet, back around a partitionto points of enclosed air, as well as that face forms with the axis is smaller than therear,thence out throughthesidepas- of standing and reflected waves. Stand- in the normal speaker because we are sages showninFig.1,asindicatedby the ing waves are set up by the meeting of not concerned with direction of sound arrows. equalwavesreflectedfromopposite projection. In other words, it is a steep sides of an enclosure, such as a bass- cone. Harnessing the New Creature reflex cabinet. This is contrary to all the literature The loading of thisdriverisac- Standing waves are only produced on the subject, but we want it to workcomplished with two horns. The outerwhen the wave length is shorter than as a piston right up to the 1000 -cpsmounting ring of the speaker framethe internal dimensions of the enclosure crossover, and only a small cone willbolts to the sheet metal horn, picturedand when the sides of the enclosure are do that. When a larger cone is used, it parallel. Thus,itiseasy to produce ceases to work as a piston at muchalong with the speaker. This horn hasstanding waves in a living room with lower frequencies, and trouble is likely a square throat and anex- two bare walls facing each other and to be encountered with spurious vibra- ponentialflarethatdoublesinarea the speaker axis parallel and centrally tions in the walls of the cone, producingevery 1.).iin. of axial length and ends spaced with these walls. Standing waves subharmonics and the familiar buzzingin a mouth about 13 in. square. From produce overemphasis or de -emphasis of on sustained low notes. the formulas for exponential flare and certainharmonicssothataclear To drive the cone, we use a voicesize of mouth opening,itis seen thatrounded voice or tone structure is not coil with a relatively large diameter for thisgives a low -frequencycutoffat heard. an 8 -in. speaker-not less than lA in. around 450 cps. Reflected waves at high frequencies and preferably 1% in. or larger. This Since the woofer is designed to work (the wavelength shorter than thein- places the thrust more nearly in line ternal dimensions of the box)cause with the outer suspension as a more -or -less rigid piston through- ring and, outitsrange,its response falls off distortion within any enclosure because with the steeper sides of the cone, pre- they are out of phase with the direct vents generation of subharmonics. The rapidly for signals at much above 1000 - cps, but the dividing networkis radiated frequencies. Thus the box must coil is round copper wire, as we are de- be lined with loose hair felt or other more concerned at low frequencies withsigned for 1000 -cps crossover. Thus the acousticalmaterial which willeffect- low resistance to signal currents andworking range from this center horn is ively proper heat dissipation than we are approximately 450 to 1000 cps. trap such unwanted highfre- with mass. The back of the woofer is loaded withquencies within the box. The larger the Another important enclosure the more out of phase these featureofour a folded horn which depends on thepaths become and the more important speaker is its heavy magnetic field. Thewalls and floor at the corner of a room first woofer we used consisted of the theirtrapping becomes.Thisises- field pot from an old RCA 104 loud-for extension ofits mouth and flare.pecially true in a labyrinth passage for speaker, witha new cone made ofDesigners know from long experiencea single wide -range speaker. phenolic -impregnated paper. This wasthat frequencies above 450 cps will not But we are not concerned with either an electromagnetic speaker which re-follow the tortuous path of a foldedstanding orreflectedwavesinour quired around 25 watts offieldex- horn,sothis horn handles thefre- system because we don't generate any citation. quencies from 450 cps down tothe shortwavelengthswithinourlarge Both the spider and the outer sur- lower extremity of its response. horns. The shortest wavelength thatcan rounding suspension ring must be freely be generated at the crossover region Speaker expense is held to a minimum is complianttoavoiddistortionfrom by the absolute simplicity and foolproof slightly under one foot, and could cause none ofthese troublesinthe mechanical nonlinearity. However, the design of the woofer.Itsounds too outer surround must be sufficiently tough small passages of the folded horn. It is to endure the rigorous punishment itsimple and easy to construct butit the combination of low crossover fre- receives, since the stiff cone does notreally works. And, for use in a smallquency, horns, and corner position in absorb any of the flexing as an ordinary house, anygoodlow-pricedhorn the room that removes these problems. felted paper cone would.A ring of tweeter can be matched to it. Before you can appreciate the happy imitationleathersimilartoplastic We are not concerned with direction set of circumstances which makes this materials used in upholstering makes an of sound projection in a normal livingdesign so ideal for music reproduction excellent surround. room for it bounces off the walls at us theprocessofsoundtransmission

109 through horns at different frequencies diffract around the mouth of the horn, with the annular movement of bass notes. must be understood. outside air is going to try to rush inward But there is little comparison here-as Sound wavesarelongitudinalin along the axis and cause both reflected a plan view will show-for the angle of motion and tend to disperse in all di- waves toward the speaker and distorting spread in a 90 -deg. corner of a room rections in the form of a spherical wave. eddy currents with the outcoming sound. is so great that their mixing is hardly If originating between two buildings, of consequence. In fact, this is where for example, the wave readily bends or Natural Distortion from Instruments the nonlinearity of the surrounding air diffracts around corners and disperses works to our advantage. Remember that in all directions. This is true mainly Next time you are near some brass instruments, if you listen carefully you the low notes are trying to expand side- for the longer wavelengths, however, wise to form a sphere and have no as they tend to cause actual compres- will notice that the lowest notes from the bass horns have a crackling sound. Like tendency to move inward to mix with sion and dilation of the air with enough a series of sharp pops-similar to thenotes from the center horn, while the energytoovercomethefrictionof bulk of our cabinet in the center pre- changing direction. crackling noise from exhaust pipes of large truck engines as they accelerate.vents the eddy movements in the same The formula for surface area of a way the mute does in a trombone or sphere is 4ars while that for the portion To a lesser degree, you can distinguish of a spherical wave subtended by the the same crackling in vigorous low notes other brass horn. walls andflooraround our speaker of the baritone horn and followup In the room with our speaker system location is arz/2. Thus the sound gen- through the alto horn, trombone, andone has the feeling of sittingin the erated in the corner is eight times as low notes of the trumpet. You don't get orchestra with the instruments playing effective as it would be if generated out- completely away from the crackling ef-all around him. Within a few feet of it doors on the top of a tower. By similar fect till you listen to the smaller "toy" you are, in effect, sitting in the mouth of comparison it is apparent that the same trumpet used in much of Handel's music. the horn. If one wants to hear a sym- sound source would be many times more Examine a mute for use in one ofphony with the feeling of being in the effective ifits wave propagation were these instruments and you will see thattwentieth row of a concert hall, he can confined to the almost parallel sides of itis designed to close out the outside place the speaker in a room next to the a long horn. air from the center but permit the an- living room and let the notes blend be- The function of a horn, then, is to nular passage of the fundamental notes fore coming through a door into the start the sound wave out as a minute along the walls of the horn. Thus it living room. sector of a spherical wave and gradually suppresses the upper harmonics of the allow the sector to increase in surface fundamental notes and kills the eddy area tillitisfinally turned loose in currents set up in the mouth by outside Part 2. the atmosphere. The lower the note the air. stronger the walls have to be to con- These brass horns can not take ad- The first wo,)ier had such a large tain it and the more readily it spreads vantage of the walls of the room to ex-field coil that the cabinet had to have and diffracts around the edge of the horn tend them, so they have exaggeratedconsiderable depth from the front, or mouth when released. By using the flares or "bells" at their mouths, to soften face, back into the corner of the room. corner of the room as an extension of the sudden release of annular wave mo- This meant that the face was wider the horn, the wave is never allowed tion as it reaches the end of their re-also; as you can see, the base of an to diffract around the edge and, conse- straining walls and to, achieve better equilateral triangle increases with the quently, the volume of those low notes acoustical coupling. Thus musicians livealtitude. The second assembly was built is conserved for the listener to enjoy, every day with this harsh distortion ofaround a woofer with a permanent mag- in balance with the high notes. the brasses, and the expression "brassy" net and was scaled down somewhat in Itisnot so simple asitsounds, has an understood significance. The en- dimensions because the smaller magnet though. could be fittedinto the corner more Tests have proventhatthe thrallingrustic vibrance of a "brass snugly. greatest movement of the air particles choir" would be lost without this effect. The woofer may be built up by re- takes place along the walls of a large All of these features of the horn work moving the frame from the permanent horn when low notes are being trans- to advantage in this system. The funda- magnet structure of an old 12- or 15 -in. mitted.Thisisknown as"annular mental ranges of the brass horns fall be- speaker and replacing it with a sturdy movement" and is comparable to "skin low 400 cps and therefore are in the 8 -in.frame. However, a high -quality effect" when high -frequency currents range of the folded horn. The annular field -coil magnet can be used, if avail- travelanelectricalconductor.This effect is what makes it possible for usable, provided it has adequate wattage effectdiminishes as notes of higher to bundle the whole works up in a small so as to provide a large magnetic flux- frequency are transmitted. If we start cabinet and bring the bass notes out ofsomewhere around 10,000 lines per sq. low notes down the sides ofa large cm. Itis necessary that the structure narrow slits along the wall. selected have a deep voice -coil slot, and horn and have higher notes takinga Itis these very bass notes that will more direct course down the axis of that the pole pieces are long axially. the horn, it readily swing around the folded passages The voice -coil diameter should be at is. easy to see that there is of our horn while the higher notesare least 1% in., and 1% in. or more would going to be considerable turmoil. snuffed out by loss of energy in acousti-be even better. The solution to this problem isto cal resistance. Thus we have an acousti- Suitablespeakerframes-including hold the large horns to a relatively cal filtering and less worry about inter- the magnet structure-can usually be narrow range of transmitted frequencies. frequency distortion. At the same time, obtained for a relatively low cast from This is what we have done by having we have the effect of a large horn moutha shop which specializesin reconing two horns to handle the frequencies without the distortion from eddy move- speakers. While many of the modern below 1000 cps. The tweeter, foruse ments, because the center of it is taken high -flux -density speakers would pro- at a higher crossover, can usea simple up by the main body of our cabinet. vide an ideal magnet for this purpose, horn. with considerable saving in price. it would be futile to suggest disassem- But, not only do you have distortion You may say that since. we are es-bling one of them for the special type between a wide range of frequencies sentially using the corner of the roomof woofer we desire. It was stated pre- beyond our cabinet as the mouth of our viously that such speakers were rela- in a large horn-you have distortion horn, we have theoretically reproduced between single low notes and the tively expensive, and that this design out- the same features we said caused inter- was economical. side atmosphere. Naturally, ifyou send frequency distortionin a large horn. Having secured asuitable magnet a burst of annular ripples down the side For we bring the higher frequencies of structure,it might also be possible to walls and then allow themto suddenly our center horn out into the same mouthobtain from the same source a frame

110 axis across the center. Then plot points on it starting with the 4% -in. throat, so that you will have a side elevation of the horn standing on its mouth, as in Fig. 3. Measure off three divisions of lA in. each along the axis and strike off a chord at each division, perpendicu- lar to the axis. The area of the throat opening is 20.25 sq. in. By doubling this area and extracting the square root, it is found that the span along the side at the first chord will be 6-1/3 in. Plot half of this distance along the chord to each side of the axis. At the next chord Fig.3 Detail of one sideofthe metal horn the dimension will be 9 in. and at the covering the ange from 450 to 1000cps. mouth it will be 144 in. Fig.5.Detail of the mounting of the speaker Next, trace a smooth curve through on the flange and horn. or"basket" for an s -in. speaker. Re- the four points plotted on each side of tworows of holes at the corners member that this basket should be as for starting a saw blade in the cuts. deep as possible-the one used on some theaxis.Thisisbest done with a of the earliest dynamic speakers is the "French Curve;" you may not find a Clamp the circular edge in a vise so curve thatfallsexactly onallfour that a 4% -in. side of the square is flush type to obtain. Assuming that you suc- points but retrace the different sectorswith the edges of the vise jaws. If the ceed in obtaining a permanent magnet to get it as close as possible. Then usevise does not have jaws wide enough, unit, be sure to cover the voice -coil a flexible rule to measure the length ofthe width can be extended by two small opening immediately with Scotch tape- pieces of angle iron. Bend the triangu- either the cellophane type or that known this curve to determine how long to cut thesheet metalside from throat to larpiece, between two diagonal saw as masking tape-to preclude the slot cuts, inward to a 90 -deg. angle against being filled up with metal particles. Once mouth. To make these sides, get some 22 orthe vise jaw. Then trim off the triangle these particles become lodgedin the to leave a V2 -in. voice -coil slot, it is almost impossible to 24 ga. furniture steel. 20 ga. is a little lip as shown. Bend remove them-although one of the most stiff for shaping but would give greaterover the next triangle, being careful to effective ways to "try" to get them out rigidity of the horn walls. On the steel,choose the side that will let the tin snips is to fold a piece of this same tape over draw the axis again and then lay offclear thefirstlipjust cut. Continue the end of a thin strip of metal, which the distance along the axis that was aroundforallfourtriangularseg- measured around the curve. Divide this ments so that a 4% x 4% -in. mouth can then be used to "wipe" the slot. opening with /-in. lips results. A large percentage of the filings can be distance into three equal divisions for removed in this manner, although some the chords and lay off the former dis- The lip extensions, at the throat line are almost certain to remain. tances across the chords for the points of the horn sides, should be cut to match After you have mounted the smaller of the curves. the portion of the flange they mate with, frame on the magnet the rest is up to The curves will be the same for all around the mouth. These lip pieces may four sides of the horn, but two of thebe bent over at approximately 90 deg. the speaker reconing expert. He must sides must have a /-in. strip added and the extensions on the bottoms of two design a steep cone of stiff parchment, sides may be bent over in the same di- such as phenolic impregnated paper, and along the edge as shown in Fig. 3. At the mouth of these two sides leave anrection, to an angle of approximately a voice coil to handle the wattage re- 45 deg. Then the two sides without ser- quired for a small woofer. To match the extension of an additional 1in. to bend average horn tweeter and utilize smaller over. At the throat of all four sides,rated edges may be clamped against the leave an extended lip as shown. Cut outflange, facing each other, with the /-in. capacitors in the dividing network, the the two pairs of sides as drawn, andlip of the throat extending inside the voice -coil impedance should be around then snip out little wedge-shaped pieces horn and their upper lip pieces against 16 ohms at 400 cps. This isn't critical, from the /-in. extended edge left along the flange on the side away from the however; a value of 8 ohms isstill the curves of two of the sides. speaker gasket, as in Fig. 5. usable. The flange shown in Fig. 4 must be While these pieces are clamped along The most criticalitemisthe sus- pension of this cone. It will have a much made from furniture steel of at least 20 the edges, drill small holes and rivet greater ga. or heavier. Cut out a circular piecesmall tinner's rivets or nail stubs, so excursionthananynormal the same diameter as the outside edge that they fall within the circle you drew 8 -in. speaker, so both the center spider of the 8 in. speaker frame. Scribe ato clear the speaker gasket. After rivet- and outer surrounding ring should be circle on it which will just clear the ing the two sides on, they can be care- of some -plasticized cloth fabric or soft inner edge of the speaker gasket where fully shaped over a large pipe or other leather rather than the usual molded mat it bolts on to this flange. Then markcylindrical surface, for the horn flare of paper. These suspensions must be off the 4% x 4;4 -in. square in the center curvature. Next, form the horn flare in free in movement (not stiff) but able of this circle and saw across the di-the two remaining sides and then bend to stand the punishment of the greater agonals with a hacksaw, as shown. Drillover the saw -teeth edges to the proper flexing. The effective working diameter angle with pliers. of the cone proper-its outside diameter These saw -teeth are to reach along -will probably be around 6 in. the outside of the other two sides and The Intermediate Horn allow the soldering iron point to apply heat effectively and to flow the solder Bolted to the mounting ring of the between mating edges. After you are 8 -in. woofer is the direct -radiating in- sure every thing fits properly, these re- termediatehorn.Throughthishorn maining two sides may be riveted to the passes the major portion of the funda- flange at the speaker end. Some me- mental notes and the drum and cymbal chanics like to tin the mating surfaces crashes, etc. For the low notes of the before assembling. Use a little"cut" outside horn, the shape of the walls is muriatic acid on the edges and solder not so important but forthisinter- with half-and-half solder and a large mediate horn we must carry the flare soldering iron. Solder an inch or two out in a smooth exponential curve. at a time while clamping the edges in To meet allrequirements, a throat snug position with "C" clamps or other opening of 4% in. square works out Fig. 4.Detail of the speaker mounting ring to suitable means. best. Take a sheet of paper and draw an be attached to the horn ofFig.3. After all corners are well soldered,

111 this was our first assembly. In later ST, models, the fin -like separator pieces pro- ject at right angles from the center of each side of the sheet metal horn, to act as struts between partitions, and they are slotted across one side to let the 3/16 -in.truss rods pass through them. These pieces must be shaped to fairly well match the surfaces theyare to push against, and to obtain a snug fit so that partitions will not besprung Fig. 6. Completed horn, shown with the early outofpositionastrussrodsare tightened. electrodynamicdriverunitusedinthefirst Fig.9. Method of mounting horns to front of model. A truss rod passes from the side of the sheet metal horn to the outside of cabinet structure. place the throat opening over aflat the cabinet in each of the four directions, extension of the vise or an anvil and at the center of the sound passages. After Where the metal horn fits through use a hammer lightly to shape the inner partitions for folded horn soundpass- the front panel and where the partitions /-in. lip back against the inner surface ages have been cut and shaped tofit were fit to the wooden collar around of the horn for soldering. For riveting together correctly, thesefin -like sepa- the speaker, a thin layer of plastic (non- in tight places or for this shapingpro- rators must be lined up with the direc- hardening) putty was applied between cedure, a stiff bar or angle iron can tion of sound and tacked in place with surfaces. This was to seal from passage be clamped to extend out from a strong small brads. As the horn partitionsare of air and to prevent any spurious vi- vise. This lip should be soldered to the assembled, a 3/16 -in. rod (such as steel brations at joints. horn wall to kill any chance for spurious welding rod) can be threaded through the group to help line up the separators No.8 wood screws,ofdifferent vibration between the horn and flange lengths, were used in our first cabinet. sections. Figure 6 shows the finishedap- while they are tacked in place. Lengths were chosen so that 1 in. or . pearance. Finally each truss rod may be cut to When you are through soldering there length and threaded on each end. Screw more of thread was seated in the holding must be no visible air leaks up the cor- a nut on one end with a thread or somember and they were spaced 1% in. ners of the horn or where the flange issticking through; clamp the rod in the apart in short runs and 2% or 3 in. attached. The speaker mounting ringvise with the nut resting on the jaws; along the main sides. A small holewas must have a continuous gasket to mate and rivet the end with a ballpeen ham- drilled to guide each screw and prevent with this horn flange, for completing mer. This end is used at the outside ofsplitting of edges of panels. Then the this air -tight effect. Before the speaker the cabinet with a large washer to keep hole through the held piece was enlarged is bolted to the horn, solder a piece of the nut from seating into the wood with for the shank and countersunk. A bit of light hardware cloth across the throat the vibrations. The other nut is screwedfloor wax was used to ease eachscrew of the horn flange, leaving no free edges on the end which sticks into the sheethome in its long run. to vibrate. metal horn, with a good lock washer to Design of the Main Case keep it from working loose. Allow plenty Arranging Inner Passages of thread on this end for tightening. The stircAway to cut and assemble In installing an 8 -in. speaker frame In computing the sound passagearea the sections of the wood cabinet without on the larger magnet an additional 16-past the speaker mounting flange, the mistakes is to make a plan drawing ofga. metal flange can he included in thecross sectional area between partitions, its assembly to scale, similar to Fig. assembly, as shown in Fig. 7, to format A in Fig. 7, is figured and the cir- 7 or 8. This is especially true if thea tie or hub for the partitions of thecular area of the speaker flange de- plywood is of different thicknesses. In sound chamber. This member seals offducted. It will be seen that the sound the cabinets we constructed we used /- the passage of sound to the rear andwaves have to turn a 90 -deg. angle at in. plywood throughout, but the front gives a more direct support for thethe space marked A before departing of the cabinet, sides, top, and bottom heavy magnet. With our first wooferfrom this chamber back of the mounting we used a wooden collar, fabricated by could be made of thicker plywood if it glueing two layers of Win. plywoodring. Thus frequencies of the intermedi- is on hand. All pieces must be at leasttogether, as shown in Fig. 8. One layerate range are filtered out by acoustical /-in. thick, however. was given s -in. slots to let a 3/16 -in.resistance before they can cause much Figure 9 shows the metal horn at-tie rod pass through on each side of thedistortion in the folded horn passages. tached to the front by wood screws but speaker magnet. Continuing alongtheinnersound

GUI MIN[ RUE MERE DODO TD SLIA ""( IN SHAPED NAHSEPARATORS

TIEDOLTS tale4.67.41 SMUT WE'RE SEND

SECTION V -V Sr CT OW It -s SECTION V-, SECTION

Fig. 7 (left). Horizontal and vertical sections through the center of the speakerunit for the second model, using a PM dynamic drivor. Fig. 8 (right). Sections through first model, using the driver showninFig. 6.

112 passages, the cross sectional area at the wall at D, Figs. 7 and 8. Any mold- The voltage is so low that insulation is each change of direction is figured for ing which extends intothis opening not a critical item in these coils. doubling every 12 in.in axial length. from the wall would curtail the annular Surplus capacitors can generally be Since there is no continuous 12 -in. run movement of the lowest notes along this used by paralleling different sizes to in any part ofit, get the right capacitance. These can be this can only be a outer wall. the oil -filled paper type with working rough approximation, by proportions. Like most experimenters, we have voltages at 25 or 50 volts, if obtainable. Remember to allow for the same degree never gotten around to finishing our If the higher voltage types have to be of expansion as the sound bends around cabinet because we are too busy on used they will be much larger in size the end of partitions becauseit may other projects. Louvers should not be and higherincost. There are some travel four or five inches axially in these used over the tweeter and any cloth used electrolytics for sale in "bathtub" cases, bends. Leaving the sound chamber at A, overitsopening should be of fairly so beware of them. The regular oil - the low notes pass toward the front ofloose weave and hard,wiry thread, filled paper types have no polarity mark- the cabine.: and are bent back at B to preferably plastic. ing of terminals but the electrolytics pass rearward along the top and bottom Both the tweeter and the center horn either have a positive and negative mark panels to C. must be mounted flush with the front or the terminals have different colored Many corner speaker cabinetsde- surfaceofthepanel (disregarding insulators. And, of course, the electroly- pend on the walls of the room to form louverswhich extend beyond). The tic is much smaller. the final folded passage, but this design reason for thisis, of course, to avoid We used the series -filter system for uses complete side panels with the truss any discontinuities in the sound path our dividing network, since authorities rods terminating in them. Pieces of /- where the horn joins the open air. seem to agree that itis the most de- in. hair felt were glued along the out- Sounds like a lot of work, doesn't it? sirable. It requires coils and capacitors side of these panels to prevent theirIt has taken yearsof struggling toof different sizes for the two legs or making vibrating contact with walls of achieve this ideal and the real listening circuits. The coils may be bolted to the the room. Rubber feet were installed on satisfactionfor a true music fanatic. sides ofthe speaker cabinet-in the the bottom. Thus, thereispractically Let's put it this way : A sextet of saxo- uppercompartmentallottedtothe no sound transmission through the room phones of different ranges can imitate tweeter. Place them several inches apart walls at normal operating levels. the brass choir, but the comparison in on adjoining sides, so that their axes are Curved inserts of sheet metal were texture is about the same as comparing at right angles, and use brass or alu- bolted to the side walls and center sep- an ordinary12- or15 -in.resonator - minum bolts. arator to help the sound around the last boxed speaker with our system-except bend at C and prevent possible distor- that the saxophones are a little muddier Preliminary Listening Tests tion in this large chamber which sur- than the box speaker. Connect the two coils and two capaci- rounds the magnet. This sheet metal tors together in proper relation with may be any gauge from 28 to 22, for joints soldered in a temporary manner easy shaping. Use lock washers on all Part 3. for testing. Then place the tweeter about bolts, which are 8-32 machine screws 8 or 10 feet away from the main cabi- in most locations. As previously stated, the tweeter may net for the first hour or two of listening. Before tightening the truss rod nuts he any good make of horn tweeter, with By this means you can tell what you are in the metal horn, on the final assembly, a horn that cuts off well below 1000 cps. getting from the different components. twist a turn of small wire under each Some manufacturers have versions that are inexpensive ($17 to $35) and suita-It is much easier to judge, if you have nut and allow the two ends to reach ble for our purpose. As stated earlier,an audio oscillator, but a frequency test out the front like hairpins. Then take we are not so concerned with the anglerecord can be played on your record an old gauze or lace curtain and wad it of spread of sound from the tweeter,player to check the response at cross- up and pack it loosely in the horn to where it is to be used in the corner ofover between the two speakers. Using fill it almost to the mouth. Place a piece an output meter across the speaker ter- a living room. The rated impedance isminals or a microphone placed in front of light hardware cloth over this curtain most desirable at 12 or 15 ohms but itof the speakers is of little avail because and secure in place with these hair pins. may be 8 ohms. In any case, it is simpler of the vast difference in the character If the hardware cloth is not light and ifithas the same impedance as the woofer used. of the two speakers and differing spread pliable, its ends may receive vibrations of sound dispersion. from touching the metal horn. Within the woofer's operating range, If this is your first experience of try- its voice coil impedance does not range ing out two speakers and a dividing The purpose of this curtain wad is much above itsd.c.resistance value. twofold. Naturally, the direct radiated while the impedance ofthe tweeternetwork, you may he highly confused at sound is much more effective than the varies over a larger range, in relation first. Things do not happen like you ex- lower notes which have to traverse the to the frequencies itis generating. So pected and you wonder if you are a com- folded horn. So this material acts as a plete flop. But just be patient and make we can design the components of the no decisions until you have grown ac- mute to cut down the intensity of the dividing network for the impedance of customed to listening to them for sev- emerging notes and provide a distortion - the woofer and forget about the tweeter. eral hours. If you are getting response free coupling with the outside air. Thus as long as itis not over 100 per centfrom each speaker you know that there a fair balance can be obtained between from that of the woofer. and we can is nothing open -circuited and it is going the two horns and further experiment- match impedances by using add tional resistors, if desired. to take time to appraise the balance. ing may be resorted to in order to suit Many good articles in the literature If this is your first experience of lis- the needs of the particular speaker and have covered the details of designing teningcriticallyto two horn -loaded horn assembly. andconstructingdividingnetworks. speakers, you not only have the sur- Don't be too concerned about the These articles give charts for estimating prises of where the sound is going to looks of the sheet metal horn or other the number of turns of wire to wind on hit you from but your ear will have to features around that part of the cabinet each coil and the dimensions of the coil get used to richer notes packed with because it will have to be covered with spools, etc. So we will not take up space harmonics you were missing before. So, some cloth or other finishing. Picture with such details here. even the playing of your most familiar molding was used to form open louvers For winding coils, remnants of No. records can be disconcerting. Thus we 16,17, or 18 magnet wire can be ob- repeat: your first decisions are going over the horn on our first cabinet. There tained from motor winding shops andto waver back and forth and you had are many ways to dress up this cabinet can be spliced together with a thin cop- better not make any. to fit in with the surrounding environ- per sleeve, sweated over butted ends. In the end, the thing you will have ment. Do not run any wide molding up Thin "spaghetti" can be slid over the todecide(maybe withthehelpof the edge of the horn opening against sleeve, after soldering,for insulation. others) is whether you have a satisfac-

113 tory balance in volume levels between the pad is out of the circuit. But the pared or either treble or bass response the two drivers. We mentioned, in an pad must be used to attenuate the re- earlierissue, can be sorted out for comparison with that a University 4408 sponse through the dividing network. that of another speaker. If you are try- tweeter so closely balances the output when comparing our horn -loaded speak- of our horn -loaded woofer that they ing to explain to someone that a celloor are ers with ordinary box -mounted cones. Ezio Pinza excite a broad band of har- just connected directly to each leg of the To check the balance between woofer dividing network. If one of your drivers monics, you can cut out ether driver and tweeter (after the connections of and show that there is still a consider- overbalances the otheritcan be fed Fig. 10 have been made and an attenuat- from a potentiometer connected able complement of harmonics issuing across ingpotentiometerforadjustingthe from the other. On the other hand, you its leg of the network, as described in tweeter -woofer balance has beenin- can show that a lyric soprano has no articles on dividing networks. This may serted) a normal cone speaker, around be a 10 -watt wirewound resistor with lower harmonics while a mezzo-soprano a 10 or 12 inch diameter, is plugged in. has to have woofer response for rounded sliding clamp contact for takeoff adjust- Then adjust the pad while operating notes. It is uncanny what can be done ment. Its resistance may be from 100 to switch 1 hack and forth until the volume 200 per cent of the voice coil impedance with it. is the same in either position. Now play Les Paul started the novelty of re- rating of that driver in ohms.It's a severalorchestraorchoralrecords good idea to measure the d.c. resistance cording several sound tracks on one tape through the new system and at points with different modes of playing the same of voice coil and resistor with an ohm- where you doubt your judgment switch meter, as an approximate check. steel . With his record of "Little to the other speaker and see if there is Rock Getaway" you can completely any great difference. Means of Comparing Speakers erase the highest or the lowest sound We want to emphasize that the bal- tracks with switches 3 or 4, because You are going to find that itis no ancing is to be done on the passages there are no stray harmonics of these easymattertodecidewhether you around the crossover region. Do not try tracks on the other side of the crossover. should be satisfied with the balance or to judge the system on solo flute or We can take certain records of Lily not. You play an album of records and violin music, for example. The com- Pons and erase Pons and leave the or- then you change it one way. Later on parison speaker will be of little use two chestrastillplaying along fairlyef- you play some other records and decide octaves away from the crossover. fectively. itstillisn't right. But remember that When we are trying to point out to in recording you have differing charac- Cossfidenca is Appraisal untrained ears how the efficient response teristics in microphones and their plac- Listening atallthe different horn of the little woofer will etch the sharp ing, not to mention difference in record- mouths of the new speaker system will intensity into cello or English horn pas- ing equipment and operators. show you that all kinds of thin squeeky sages, their attention is likely to be dis- Since your main concern is with the noises come from the tweeter and choked tracted by the sharp excursions of some region that the crossover falls in, it is squawks come from the lower horn in flute or violin. So we can cut off the a big help to compare your balance with some alternate passages. These things tweeter and focus their attention on the the output from any single cone speaker happen when the dividing network cuts feature we are trying to demonstrate. of fair quality. This is just one of the the broad band of harmonics of a note Then there is the fellow who thinks reasons you are going to need the con- (in the crossover range)into shreds his pet speaker is just as good as yours trol hookup we use. Another good use and you were listening to the narrowest and he has infinite faith in his hearing for this hookup is to prove to the family of the shreds. Several feet back, this is and memory. Tell him to bring his own and friends that you aren't going slowly not apparent, butif you really want speaker and his pet records over and you off your nut, when you try to explain the can really give him a comparison. When purpose of this new system and what is to hearit you can flip either switch 3 or switch 4 and sampleitreadily. he hears his own records he cannot different about it. claim any fakery in the line up, if his When a real music lover comes to Then, if you thinkit shouldn't sound thatway,pluginthecomparison speaker is shown up. visit you and listens to a good over-all speaker. Then when you flip the switch, system for a while, he will soon make Hold That Distortion comments or ask questions about the the sound will come out of thecom- means of obtaining this realism. As the parison speaker and ytiu can see how it All of the foregoing discussions of the sounds there. If you have to use the T reader doubtless knows by now,itis pad to attenuate the tweeter or woofer virtues of this speaker systemare predi- fruitlesstostartanexplanationof response for comparison, it will be neces- cated on the assumption that high -qual- speaker systems.But itis a simple mat- sary to flip switch 2 back and forth with ity music signal is being fed into the ter to demonstrate all aspects by flipping the other switch. By this means you in- dividing network.Thereistheold the switches in our control hookup. sert the T pad with its attenuation, for argument between techincal experts- All thatis needed to complete the the horn -loaded speaker, and drop it out whether one should improve the speaker hookup of these controls and the di- for a straight through run to the com- system or the other sound equipment viding network (as shown in Fig. 10) parison speaker, withitspoorer effi- first. We believe that the speaker is the are four DPDT toggle switches, a phone ciency. end to start on because you can doa plug jack, and an L or T pad. The pad This arrangement provides a versatile lot of see -sawing around in the feed- may be 12 or 16 ohms, assuming the sampling hookup. The quality or re- impedance of the drivers is in that re- sponse of over-all systems can be corn - (Concluded at bottom of next page) gion. All of the equipment shown in Fig. 10 is installed in the upper com- partment with the tweeter and we have switch handles protruding into one of the side openings of the bass horn, next to the wall, where curious hands are not likely to discover them. The jack also opens into this passage as does the han- dle of the T pad. Switches 1 and 4 are used most so they were placed on the ends of the line of switches, so their handles are easily found by feel. Switch 2 is thrown to the position which inserts the pad only when another speakeristo be Fig.10. Schematic of the dividing networkto show the four switches used in making com- plugged into the jack. Most of the time parisons between the over-all system and otherspeakers, andfor checking performance of the individual components.

114 to B,,ace speakv, eatd.,./ Vibration Reduction in Loudspeaker Enclosures G. B. HOUCK

Presenting the reasons for bracing a bass -reflex loudspeaker cabinet, and showing how to do it with the assurance of improved performance when the job is completed.

BEN JAMINFRANKLIN once observed that "Empty barrels make the loud- TABLE 1 est noise." In this case he was re- PhysicalProperties of CommonCcnstruction Materials ferring obliquely to the common phe- Mass Thick. nomenon of uninformed vociferation, not aterial lbs./ ft.' In. ibS./ ft.' Ms/Mi V./ V, describing the performance of a loud- Dry packed sand 105 8.75 114 10.7 speaker enclosure. G. A. Briggs, in the Brick 125 3 31.2 406 20.2 second chapter of "Sound Reproduc- Concrete 150 3 37.5 488 22.1 tion," was commenting on the latter Plaster 1 8.0 104 10.2 when he wrote: "The indications are White Pine 26 7/8 1.9 24.7 4.9 White Oak 46 7/8 3.36 43.8 6.6 that the effect of cabinet resonance has 2-7/8 wood panels with1" sand between 12.55 163.5 been underestimated in the past." He 12.8 * General Precision Laboratory, 63 Bed -- observed that the tone -quality of repro-unit volume of air having a mass /di, ford Road, Pleasantville, N. Y. duced sound was greatly improved whenand an instantaneous maximum velocity the loudspeaker cabinet was constructedV, strikes a unit volume of panel having of materials having a high density. In a paper presented before the IRE PGA', FrankMcIntoshpointedoutthat "boomy" sounds are caused by acoustic radiation due to decaying vibration of the panels in a poorly braced cabinet. Briggs offers one solutiontothis UNIT BEAM problem-make the panels massive and 7 they won't vibrate. The principle in volved is that of relative momenta. Con- sider the effect of a moving mass of air striking a panel. Referring to Fig. 1, if a Fig.1.Kinetic energy effect of Unit Volume IRE ProfessionalGroup onAudio, of air M,, impinging on Unit Area of panel, M,. No 10. September, 1952. Fig. 2. Typical panel as used in the discussion.

sivepickups is from vertical vibrations at moderate prices.But, here again, and is mostly eliminated by change to a moderntubes, circuits, and output trans- THEATRE SOUND modern magnetic or variable reluctance formers will probably make a better (from preceding page) pickup. The reason being that they pro- showing in reproduction of the exquisite duce practically no response from verti- detail in later recordings, as displayed cal vibrations. The other fifty per cent by thenewspeaker system. ing equipment without being fully con- still remains from lateral distortion of In the writer's opinion, simplicity is scious of results unless you have a sensi- stylus movement caused by a heavy arm the watchword-from the diamond sty- tive speaker system. with stiff swivel joints, working against lus right through allstagesto the di- It is about like the old tale of the man the whip of eccentric or warped records. viding network.Simplest tubes with who found a horseshoe and bought a After the man got a horse he had to self -biasing, no interstage transformers, horse to putiton. Our new speaker get a stable and a riding habit. So a simplestpower supply withoversize system will mercilessly display the de- preamp stage ahead of the amplifier components, and no special circuits in fectsinrecords and equipment. The will he necessary, unless it already has the regular line up. first thing you have to do to soothe it one. Then we start in on equalization- It is hoped that this description may is get rid of record surface noise with because the speaker system shows that have been sufficiently full of detail that a magnetic pickup and a new changer- up. There are almost as many ideas for anyinterestedhobbyist who wants one of the better modern changers, with attacking this problem there are ideas speakerperformancewhich,inthe only a few grams needle force and a for speaker enclosures but we have not writer's opinion, is well above the av- ball -hearing arm swivel, soitisfree experimented with these to the same ex- erage, will be able to duplicate the en- from the different forms of distortion tent. Success in the matter is not diffi- closure type with a minimum of effort. contributed by drag of arm on groove cult. however. The pleasure given by a speaker of this walls. The amplifier is the oldest member type is usually wellNvorththe trouble If youhavenot done this already, you of the team, having started its career taken to build one-a fact which is at- have noideahow different the records before the loudspeaker. So there are tested by a number of the writer's ac- can sound. Fifty per cent of the sur-no secrets about amplifiers and plenty quaintances who are using the same face noise and distortion from inexpen- of satisfactory performers can be had type of speaker housing.

115 le- 26

---f SUB -PANEL .5. i 2 X BRACE

12 SUB -PANEL I co 53 BRACE 2 Y4 BRACE --1 -1-- 1

- 21 25 U5 B4 PANELE i !-- 12t 2-45 in 253 SUB -PANEL BRACE IA) B) C

Fig.3. Typical bass -reflex enclosure, with dimensions asusedinthe sampleproblem.All stiffening beams are mounted so that their maximum cross-section dimensionis perpendicular to the plane of the panel. a mass M., initially at rest, both masses if one attempts to relate various design will have a resulting velocity Vt. This parameters to a resulting acoustic out- Fig.4.Examples ofcornerpostconstruction. relationship may be written : put. It is entirely beyond the scope of -M.01= (M,+ MOW (1) this discussion to examine these theoreti- ence to appropriatetablesreveals that cal considerations in minute detail. Fur-the modulus of elasticity of commonly Note that for the optimum condition,thermore, it can be shown that a muchused lumber varies from about 1.00 x V. approaching zero (panel does notsimpler method of analysis provides the 106 to 1.6 x 106. move) it is necessary to have the ratioessential information necessary to make The moment of inertia of these beams of M./Mt as large as possible. Since Mt, very substantial improvements in cabi-equals bd'/12, where b is the width of the mass of air is fixed at roughly 1/13 net construction. the beam and d isits thickness. (See lb./cu. ft., M, remains the only variable. For all practical purposes it is reason- Table II) It is interesting to note here By varying M., it is possible to obtain ably sufficient to consider a panelas that doubling the thickness of a panel the ratios shown in Table I for several made up of an infinite number of small (and hence the thickness of a unit -width different materials. It appears obvious beams arranged side by side as shown in beam) reduces the deflection by a factor from a glance at this table that a small Fig. 2. Notice that in this type of analy-of eight; tripling the thickness reduces improvement in V. (hence a reductionsis, the beams are represented as ex-deflection by a factor of 27; and so on. in vibration) may be had only at thetending across the shorter dimensionBefore demonstrating a typical solution expense of a large increase in weight. of the panel. Assuming the edges of theto a cabinet design problem, it will be For example, a panel made of concrete panel are supported, it is logical to sup-found helpful to introduce one further would tend to vibrate (other things such pose that the beam exhibiting the mostbeam equation into the discussion. In at the modulus of elasticity being equal) severe deflection when subject to a load, the case where a panel contains an open- as much as one made of will be one near the center of the panel ing (speaker mounting hole or reflex wood, but would weigh 20 times as such as beam A. Now, without attempt- port), the beams which have one termi- much. Thus a typical cabinet ing to determine an exact coefficient for nation at an opening are classified as weighing50poundsconstructedof the stiffness factor, it can be shown that cantilevers. The deflection for this type wood, would weigh 1000 made of con- the maximum deflection of beam A is of beam is written as : crete-probably too much for the aver-dependent on a few easily determined age living room floor to support. Even variables. Actually, since the beams are (4) ifthis were permissible, such an en- integral parts of a homogeneous plate, 8 E1 closure would be virtually immovable the deflection will be somewhat less than For convenience, the terms in the and would present the baffling (no punthat of a single unattached beam. equations which remain more -or -less intended) problem mentioned in a recent The equation for the deflection of aconstant in any one design, are W and editorial.2 rigidlysupported,uniformlyloaded E, and they may be lumped into one con- Fortunately for the cabinet designer beam may be written as stant, K. The equators are then re- there is another solution to the problem W V written as: of reducing panel vibration. Instead of Z - (2) relying on weight alone to accomplish 384 E I Simple beam-no end supports 5K LI the desired results, he can make the in which Z equals the maximum deflec- Z - panels stiff, and join them rigidly to- 384 1 (5) gether. tion, W equals the load on the beam (maximum instantaneous value), and I Stiffness of Panels equals the moment ofinertia of the iir-12 1/B' DIA (VARIES WITH cross-section of the beam. If this equa- CASKET SPEAKER/ Several factors determine the stiffness /4. DEEP tion is compared with that for a non- ..',.)-..-....'..a" ' BREAK TO FIT of a panel. The chief factor of course is I. 771 SPEAKERSPEAKERR the geometry of the panel. In most cases rigidly supported beam, in which case AS REQUIRED itis very difficult to analyze the be- 5 W L' L10 //4' DIA. IVASIIICKMII havior of a vibrating plate, especially Z- (3) 384 E I ALTERNATE METHOD 2 EDITOR'S REPORT, AUDIO ENGINEERING, it will be observed that the deflection is March, 1953. five times the magnitude of the former. Of course in actual practice these ex- /-GLUE WITH PRESSURE. USE SCREWS IF PRACTICAL TABLE 2 tremes are almost never encountered; Moment of Inertia about the Center of Gravity no panel however loosely secured would for Common Lumber Sizes(max. Value) exhibit a vibration five times as severe Nominal Dressed or Moment of as one firmly attached to an immovable Size Finished S;ze Inertia support. Nevertheless, this simple com- In. In. In.' parison emphasizes the need for rigid I X 1 7/8 X 7/8 .049 support of the panels. 1 x 2 7/8 >c1 5/8 .31 The maximum deflection of such a 1 x 3 Vsx 2% 1.32 beam may also be reduced by decreasing 2 x 3 15/8x 25/8 2.45 its length as far as is practicable. On the 1 x 4 '/8 x 3.44 2 x 4 x 6.45 other hand, little benefit is derived from attempting to vary the value of E: refer- Fig.5.Loudspeaker mounting details.

116 Simple beam-fixed end supports upon request,supply suitable gaskets SAMPLE PROBLEM K L3 for this type of mounting. Consider the bass reflex cabinet shown (6) Z=384 I at (A) in Figure 3. Assume that itis Conclusion constructedof 1 -in. plywood (7A -in. Cantilever dressed), and that all corners are joined K L3 Z - It has been the purpose of this dis- in such a manner that the panels are rigidly 81 (7) cussion to analyze the problem of spuri- supported. ous acoustic output caused by excessive The top panel will exhibit the smallest Figure 3 and the sample problem maximum deflectionsbecauseitisthe show the suggested method of analysis panel vibrationsin a loudspeaker en- smallest panel. The deflection of unit beam as applied to a typical cabinet. Cabinets closure, and to suggest an approach 1 is given by equation (6). of different shapes will present stiffness which will result in a definite reduction in the effect and a vast improvement in K problems somewhat different from the Z1)1112 384 I sampleillustrated.Inthiscase,the the over-all performance. As might be analysis is used to determine the size,expected, the vibrations dealt with are In this case I, the moment of inertia, from shape, and number of braces requiredthose which occur at low frequencies. Table II is .049 and The various equations do not take reso- K 13' to improve the performance of a cabinet ZMILL = 117 K originally constructed without full con-nance into account but explain the phe- 384x .049= sideration of the factors just discussed. nomenon below resonance where dy- Since unit beams chosen from the bottom Naturally the need for extensive bracing namic and static deflections are nearly or sides would have identical dimensions, is reduced if the enclosure is properly the same. If one remembers that the the maximum deflectionof these panels designed from the start. natural resonant frequency of a solid would be the same as the top panel. varies inversely withitsweight and The maximum deflectionof the front Construction directly with its stiffness, it is obvious panelisdetermined by considering unit to conclude that constructing panels as beam 2. In this case There is more to the problem of de- K 26' rigidly as possible helps to raise the Zmax= =936 K signing a rigid speaker cabinet than resonant frequency to a region where 384 x .049 mere choice of adequate panel thickness damping is more easily accomplished and arrangement of bracing, however. The deflection of the back panel is similar. by the use of padding and sound absorb- Because of its shape, the area of the front As was observed from the discussion of ing materials. Thisiseasily demon- panel below the port (6 x 26 in.) exhibits the beam equations, above, the methodstrated with the test set up illustrated a deflection which is characteristic of unit of supporting the panels is highly im- in Fig. 6. Connected in this manner, the beam 3, which is a cantilever. The deflec- portant. Since most panels are made of oscilloscope indicates the power factor tion is given by plywood, it is necessary to insure that all of the load. At resonance, the power K K 6' Zmax = = 551 K joints are constructed to achieve thefactor is unity and a straight line ap- 8I 8 x .049= maximum possiblerigidity. A simple pears on the scope. Power factors at but effective method for accomplishing In determining the bracing required to non -resonant frequencies show up as reduce front and back panel vibration to a this is to use corner posts to which theloops of various widths. If the oscillator value consistent with the top panel, con- panels are securely anchored. A few is adjusted until a panel resonance is de- sider the load as the area defined by panel typical joints of this type are illustrated tected, it may be observed that the ap- "P" in (B). Its width is roughly 10 inches. in Fig. 4.It will he found that such plication of hand pressure to the vibrat- Since a unit beam has a width of1 inch, methods also contribute to the over-all ing panel will cause the straight line to and the deflection is 936/117 or roughly appearance of the finished structure. open up into a loop, and by returning the8 times as severe as for unit beam 1, the At first glance, the prospective builder requiredstiffnessof the brace beamis oscillator it will be found that the reso- 8 x 10 or 80 times that of unit beam 2. From may be somewhat dismayed by the sug- nant frequency occurs at a higher fre- Table II it may be seen that the best brace gestion that special tools are needed to would be a 1 x 4. The stiffness factor equals prepare the joints for assembly. This 3.49/.049, or 71, which is close enough to type of work is best done using a joiner the desired value. Actually it may be more or router, although the patient crafts- desirable to use a 2 x 3 as shown at (B) man may use a plane with very good with a stiffness factor of 245/.049, or 50, in results. some instances where a brace 1 x 4 would In many instances the whole adversely affect the acoustics of the cabi- problem can be greatly simplified by net. Since such effects are generally not having a local mill cut each piece to serious near the back of the cabinet, it is size and prepare the joining surfaces recommended that 2 x 4's be used. Here the from drawings furnished by the de- stiffness factor is 6.45/.049, or 132. If three signer. The task, then, is merely one of braces are used as shown at (C), each will assembling the finished parts. support approximately one fourth of the During this assembly operation itis total load. This makes the stiffness factor best to screw and glue all joints together equal to 8 x 33K-4- or 66 K. If the above cal- so that the finished cabinet will be tight culations are applied successively to the and solid. The front panel of the en- braced panel shown at (C), two deflections closure can also be permanently an- Fig. 6. Test circuit used for checking imped- are obtained. chored if the loudspeaker is mounted as ance andresonantfrequencyofspeaker in Since both sub -panels and braces vibrate shown in Fig. 5. It is fairly well recog- enclosure. in unison, the final deflection is obtained by nized that speaker performance is greatly adding the two and the sum will be found improved by mounting the speaker in to equal 117 K. Because of the short length of unit beam this manner. As illustrated, an alternate 3, sub -panel Q (A of Fig. 3) requires less method using two superimposed panels quency. During this test, of course, one bracing than sub -panel P. Just to be on the obviates the necessity for special rout- must not pick an oscillator frequency safe side however, a 2 x 3 should be in- ing. In either case captive nuts attached whichcorrespondswiththenatural stalled as shown. to the back of the panel receive the acoustical resonance of the cabinet. In The reader will note that all the braces speaker mounting bolts from the front. this case applying pressure to the panels used add up to a total volume of IA cu. ft. Many loudspeaker manufacturers will, will have little effect. which is about 1/12 of the cabinet volume.

117 Feedback and Loudspeaker Damping

JOHN A. MULVEY'

The author proposes a solution to the problem of obtaining a feedback signal which is equivalent to the movement -generated e.m.f. of a speaker voice coil.

THE RELATIVELY RECENT articles on circuit can nearly constitute a short-e.m.f. itself causes no current to flow. positive current feedback used to im- circuit for the coil, permitting muchIt only acts to influence the voice -coil prove loudspeaker damping have, tocurrent to flow with just a small move-current which is moving the voice -coil, this reader, been most engrossing. In ment -generated coil e.m.f. The more cur-by subtracting from the voltage causing spite of studies made of the articles and rent which flows as a result of this e.m.f. it. This is true all the time except dur- arguments on this subject he has, untilthe better will be the dynamic braking ing "hangover" periods. During these quite recently,felt much in the darkof the movement, that is, the quickerperiods the physical forces of inertia or when it came to feeling able to claimwill the movement cease. But even with much of an opinion of his own on the suspension compliance cause the move- relative merits of such a system.Ita short-circuited coil moving in a mag-ment -generated e.m.f. to exceed the driv- seemed that such a subject should benetic field the cuurrent through itis ing voltage and so cause its own reverse resolvedintosimplercontradictions definitely limited. It is limited by thecurrent to flow. Notice here that this is than any presented so far in order toresistance of the coil itself. A methoda "reverse current," one of opposite make one feel he has made a wise choiceto overcome this limitation and furtherpolarity to that originally causing the of one opinion or another. enhance the dynamic braking action ismotion. This reverse current then will This need has led to this attempt tomost desirable. Such a method, to beact on its own and damp the motion reduce the subject into simpler terms successful,would havetopermita and so limit the hangover effect. Since by digging into the fundamental aspectsgreater current flow than a short-cir- positive feedback-whether voltage feed- of the subject. The author feels that thecuited arrangement would allow. At first back or current feedback-tends to in- analysis whichishere presented canany such method might seem impossible for it would seem that nothing could al-crease the output signal, at this time if show some things not apparent in fore-low more current than a short circuit.positive current feedback were intro- going discussions, and finally may offer ducedit amends to those who think they disagree, But on second thought it should be real- would increase the current as well as to make some proposals thatized that a second source of voltage inlimiting the hangover effect so as to the engineers more actively engagedseries with the voice -coil could be pro- squelch it quicker. may find promising. vided to increase the current above what From this the main point may be ap- According to Lenz's law, the voltagesimply a short-circuited voice -coil wouldparent but it is simply that positive cur- induced in a coil due toits passageallow. This suggests a circuit employing rent feedback does not oppose voice -coil through a magnetic field is always of aas a second source of voltage one greater movement until the movement -generated polarity such as to produce a currentbut proportional to the movement -gen-e.m.f. exceeds the driving voltage and which would oppose the motion. In othererated e.m.f. and in phase with it havingproduces a reverse current. At this time words, an e.m.f. due to coil motion inalso a lower source impedance, as inthere is a phase reversal which, in effect, a field tends to stop the motion. ThisFig. 1. So far we have spoken only ofchanges positive feedback to negative presumes, however, a closed circuit at movement -generatedvoltages.Inthe feedback. If this point is not made clear the coil ends, for this e.m.f. can only actcase of a loudspeaker, where it is voice -there is yet much room for controversy. if it can act to cause or influence cur-coil current which causes the motion in It should be noted that the comparison rent through the coil. When such a coil thefirstplace,movement -generated was said to be only an effect similar to is a voice -coil there is always such ae.m.f. is still present and acts to oppose inverse feedback. The e.m.f. generated closedcircuitexisting. The pathis the voice -coil current. In this way it in-by a coil moving in a magnetic field is around through the windings of the out-fluences the actual current and always inverse feedback. Any voltage which put transformer secondary. This closedacts to stop the motion the same as ifaids that voltage is, with regards to it, some physical force was causing thea positive feedback voltage, but with 3080 S. W. Spring Garden Road, Port- motion. However, generally under these regards to the total effect it is a negative land 19, Ore. circumstances the movement -generated feedback voltage. There is a good com- parison in the rather familiar circuit of an amplifier using a positive feedback loop connected to the cathode end of an unbypassed cathode resistor. The feed- back voltage is definitely positive with respect to the signal voltage appearing across the cathode resistor, but just as definitely negative with respect to the over-all effect. Ea MOVEMENT -GENERATED VOLTAGE But the story doesn't end here. To E, SERIES GENERATOR VOLTAGE, WITH OUTPUT some, the advantage of improved damp- GREATER BUT PROPORTIONAL TO ARO IN ing during hangover periods will be PHASE WITH Ei thought to be too highly paid for by the inherent disadvantages existing the rest of the time, since the rest of the time Fig.I.(Al Theoretical circuit, and(B)practical circuitof feedback obtained from current the in -phase feedback is regenerative. in a voice -coil. (Concluded at bottom of next page)

118 Damping of Loudspeaker Cabinet Panels

M. RETTINGER A discussion of the need for damping of enclosurepanels and suggestions for realizing the required result. Considerableimprovement in speaker perfor- mance can be obtained with relatively little expenditure ofmoney or energy. THE LOUDSPEAKER IN A CABINET often Thus stiffening tends more to change the door panels. Such materials act in two produces undesirable vibratory mo- character of the radiation than to elimi- ways: (1) to convert vibratory energy tion of the cabinet panels. It is well nate it. into heat by mechanical resistance; and known that any surface excited into flex- Another undesirable effect of panel vi- (2) to lower the vibration amplitude by ural vibration has families of resonant bration is poor transient response of the providing a higher mechanical imped- frequencies, so that the radiated spec- radiated sound. This is produced by the ance. trum is not the same as that produced by long buildup and decay periods of the ex- the primary signal forces. How to avoid cited panels, compared to the short pe- It should be noted that damping is the effect which reinforces certain fre- riods associated with a well constructedmost effective at the resonant frequen- quencies and thus provides selective ra- loudspeaker. The qualitative effect is one cies, since the larger bending amplitudes diation is an old problem, and the usual of poor vocal and instrumental definition, alternatelystretch and compress the recommendation for cure is to brace the of blurriness of tone and lack of crisp-damping material, which thus dissipates enclosure walls. However, increasing the ness and clarity. Such panels are spokenthe vibrational energy in its mechanical stiffness merely raises the resonant fre- of as having a high mechanical Q orresistance. Adding mass to the panel quencies. True, the amplitudes of the ratio of mechanical reactance to mechan-causes it to vibrate with a smaller ampli- higher resonant frequencies can be more ical resistance. easily damped ;yet the total radiation tude for the same applied exciting force. Increasing the damping of a mechani-If the panel were mass -controlled,its may be more pronounced on account of cal vibratile surface reduces its radiated the larger number of vibrating surfaces. sound output. A common example ofvelocity would be F/M where F is the damping is seen in the application of anvibromotive force and M is the mass of RCA Victor Division, 1560 North Vine the panel. Street, Hollywood, California. undercoat, such as mastic or glassfiber- board. to automobile hoods. fenders. and

LOUDSPEAKER DAMPING (from preceding page)

The basic problem is one of separating the movement -generated voltage from Fig. 2. Typical circuit the driving voltage so that only and at employed a move- ment - generated all times will the feedback signal resem- feedback voltage ble the actual cone movement, in -phase The useofresistor current feedback succeeds in doing this R isoptional, since part of the time only. True it serves thefeedbackcould best during thosecriticalperiods of be appliedin any of hangover, but hangover is only part of a number of ways. realism. It follows usually in the wake L, vacE Coll of transients which themselves would L2 FEEDBACK COIL be much improved if motional feedback T, SPECIAL NEUTRALIZING TRANSFORMER Es, NEUTRALIZING VOLTAGE THIS VOLTAGE ISEQUAL TO could be put to work on a full-time job. AND 1110.OUT OF PHASE WITH THAT COMPONENT or E2 WHICHIS DUE TO THE INDUCTIVE COUPLING OF Source of Feedback Signal L, AND L2 It is not difficult to produce separate andindependentmovement -generated and this is likely the simplest solution: Although the author has believed for voltages. Every microphone does it. A use of neutralizing voltage. Feedback sonic time now that a feedback signal separate winding on the voice -coil for- windings wound over a voice -coil, or due to voice -coil movement would be mer suggests itself but thisis only anear one end would be the same as a considerably better to use than any sig- partial answer foritwill have some mutual inductance with the voice -coil transformer ; current through the voice- nal existing within an amplifier itself, that will mean voltage will be induced in coil would induce a voltage in them. If thispositive feedback discussion has it other than that due to motion. The another simple transformer of special helped to stimulate and crystallize think- apparent solution is to shield the wind- design were used inseries with the ing on the subject. It can be seen that ings from each other magnetically. This voice -coil to allow the same voice -coil theoretically the resulting improvements seems a difficult problem since the pick- current to produce a secondary voltage of using an independent feedback sig- up windings should be as close as possi- equal to that induced in the feedback nal as described would not be limited to ble to the voice -coil from the standpoint windings it could be used to cancel the improved damping.Itwould actto of minimizing time lag between driving voltage so induced. This transformer make the voice -coil and cone follow voltage and pick-up voltage. Such a time could have a low -impedance primary more exactly whatever voltage wave - lag is due, of course, to sound traveling since the secondary can consist of many at a rather slow speed even through a turns to produce the right voltage. By shape was applied, and at all times. solid. If it were not kept at a minimum this device then,it should be possible It is felt that much credit belongs to serious phase shifts would occur at the and feasible to use motional feedback Warner Clements who seems to have higher frequencies and would limit the with all the advantages that the prin-stirred up this thing in the first place introduction and effectiveness of feed- ciple implies. Figure 2 shows a sug- and to Ulric Childs who gave him some back. Another approach suggests itself gested circuit. good arguments.

119 is to use the damper as a filler of the (compound) panel. If additional (sound- absorptive) material is required on the inside of the cabinet, this constitutes an additional requirement. An exact evaluation of the damping quality of a material is not simple, how- ever. What is usually done in the labora- tory is to suspend the panel from the

ceiling at one or two points by fine wires Fig. 1. Damped ware train resulting from stim- or cables. A ball of wood, rubber, or cork NO DAMPING MATERIAL v ulustopanel, showing two wave maxima a -a regulation baseball has been found WITH DAMPING MATERIAL given number of cycles apart at whichmeas- very satisfactory for this purpose-also urements are made and on which computations hung from the ceiling, is permitted to fall of logarithmic decrement are based. from a predetermined height to strike the panel. to which a vibration pickup has Damping Panels been fastened. Itis important that this There are a number of ways by which pickup be very light ; otherwise it will Fig. 2. The effect of resonance on a panel is damping can be added to a loudspeaker itself act as a damper. The output of the much decreased when damping material is used cabinet panel. One means consists in pickup is displayed on an oscillograph, as a filler. building the panel up in layers of wood, so that the successive reduced amplitudes its output was first transmitted to an in- fastened together at numerous points by of the transient wave -train can be mea-tegrating network to obtain a measure of screws. Bending of the panels causes the sured. The quantity of interest is thethe change in panel deflection amplitude. sandwiched layers to rub against each ratio of successive amplitudes spaceda Measurements on various types of other, thus creating frictional damping. full cycle apart. Some books prefer to woods show that the logarithmic decre- Another means is to use laminated panels consider the amplitudes when spaceda ment of %-in. panels is as follows : half -cycle apart: but a study ofsucces- composed of alternate layers of plywood LOGARITHMIC and felt, cork, rubber, or lead. sive amplitudes on the same side of the WOOD time axis appears more common. DECREMENT A convenient if perhaps not very rig- Pine orous way to determine the frictional co- What is known as the logarithmic .02 decrement-the vital quality in all these Beech .03 efficient or damping constant of sucha Plywood .04 compound structure is to takea sample tests-is given by of the material between thumb and fore- A. The superiority of the plywood as a finger of the left hand and rap it with d= log material with high internal damping is the knuckles of the right hand or strike readily evident from these figures, which where.4, is the first and A. the secondpertain to untreated surfaces. When it with a stick and observe the duration measured amplitude.Inpractice,the a of the resultant sound. A vibratile plate evaluation of d does not usually comprisemastic compound is applied to them and so excited behaves in much the same way taking the logarithm of the ratio of twopermitted to harden, the values can be that it would if excited by soundwaves, still further increased. and the duration of the noise produced successive amplitudes (since they may The above tests require considerable is a fair measure of its damping be but slightly different to allow theirlaboratory facilities, and hence cannot be proper- exact determination), but consistsof ties. If it radiates a dull thud the damp- determining the ratio of two amplitudesreadily carried out by the average music ing is high, while a long ring would in- which are a number N cycles apart. Inlover, home owner, or hi-fi enthusiast. A dicate low damping. Some experience. that case (see Fig. 1) number of less complicated tests have of course, is necessary to correlate the been proposed to show up the resonant 1 A, sound given off with the frictionalre- d= - log vibrations of a speaker cabinet panel. sistance,particularlyifmeasurements eAN AN Thus, ripples in a glass of water set on are to be made on a wide variety of test where A, is the first andANthe nth am- the enclosure can be generated, and fine panels. plitude. sand, salt, or sugar crystals sprinkled on In the writer's experience, two types It is, of course, also possible to excite the panel can be made to form patterns, of cabinet panels have proven especially the panel acoustically with a loudspeaker so-called Kundt's figures. However, rel- effective as a laminated structure to give atively large panel amplitudes are re- placed near it, and to record the output quired to achieve noticeable results with damping of a high order. One consists of from a vibration pickup fastened to the two 2 -in. -thick plywood panels with these tests. A rather simple procedure a panel as the frequency is varied. This consists in placing two pencils on the -in.-thick neoprene layer sandwiched might be termed a steady-state transmis- panel, and a light tin can filled with lead between them. The other consists oftwo sion measurement (with and without the shot on top of the pencils. When the such plywood panels witha sandwiched damping material applied to the panel). filler of corrugated, waffled, speaker is driven with an oscillator, the or honey- Or the vibration pickup may be fastened lead shot will rattle in the tin pan very combed plastic of the type frequently directly to a wall of the cabinet with the audibly when the undamped panel is set used foraircraftpanels.Using the speaker excited sinusoidally by an oscil- into vibration at the resonant frequen- damping material as a filler is farmore lator. Figure 2 shows a measurement cies. After properly damping the panel, effective than applying it toone side of made in this manner, first without and and properly energizing the speaker the panel only, because as a filler itcan then with the damping material applied again with the same current used previ- act in shear, while applied to the exterior to the interior of the enclosure. Since the of a panel its action is less restrained. vibration pickup was inertia -operated. (Concluded at bottom of next page) If the damping material not usedas a filleris sound absorbent, certain other Fig. 3. This chart 0 AMPLITUDE ININCHES desirable results occur, of course. These shows the accelera- 100 .01 ooe c000e have frequently been discussed incon- tion required at vari- z nection with the interior acoustic treat- ous frequencies to ment of loudspeaker cabinets. Briefly, produce given cone such materials reduce the standing-wave excursion amplitudes. fr-t effects in the enclosure and make foran Accelerationsore W10 very large and the 0 acoustically larger volume. Somecom- movements "hit" cab- promise may have to be effected in this inet panels very hard, respect in practice, because the economic so that panel reson- 4 aspect of a cabinet is also important. But ances produce maxi- 1 20 100 from a purely idealistic point of view. mum effects. 1000 OPOO the most effective way to dampa panel F FREQUENCY IN CYCLES PER SECOND

120 Speaker Treatment for Improved Bass

CAMERON BARRITT

A simple method of lowering the effective low -frequency limit of inexpen- sive speakers, but not one that is recommended for high -quality woofers. ADESIRABLE FREQUENCY RESPONSE characteristic for an audio repro- ducing system requries adequate lows as well as highs. Amplifiers are available which can handle the necessary range, but the loudspeaker is the bottle- neck in the drive toward more perfect audio reproduction. The problem of ren- dering sufficient highs can be met by various means such as the employment of "tweeters"-some of which can go close to the upper limit of human audibility- Fig. 2. Methodof butthelow -frequencyspeakersor slitting cone, as de- "woofers" usually fall far short of the scribed by the author, lower limit. Sound is propagated at fre- and location of shel- quencies as low as 16 cps, and 20 cps is lac sealing ring. usually considered as being audible. LP records are capable of going down to 30 cps, by way of further example, but not many speakers go this low. The lower limit of a properly baffled speaker is determined by the mechanical resonant frequency of the vibrating pis- ton or cone. A certain amount of propa- gation occurs below this frequency but response falls off rapidly beyond. The resonant frequency of a mechanical vi- brating system is determined by the mass and the compliance-the inverse of stiff-required. On the other hand, the com- SUSPENSION COMPLIANCE ness. These two are the electro-mechani-pliance of the speaker depends on the CONE MASS of ACOUSTIC AIR LOAD cal analogs of inductance and capaci-"give" in the cone suspension or mount- tance.The mechanicalresonantfre- ingand no unde sirable consequences quency formula is fr.//2xVmC. The willresult from increasingitsvalue. ACOUSTIC CAPACITANCE mass of a speaker cone is determined forThe total compliance or capacitance of OF CABINET the most part by constructional require-the speaker system also depends on the ments for rigidity, and is not readilyacoustic capacitance of the air volume in 0 altered. Furthermore, if one were to in-the baffleifitis the totally enclosed crease the mass very much in order totype. The equivalent electrical circuit' of Fig.1. Equivalent electrical circuit of a loud- lower the resonant frequency, phase re-a loudspeaker mounted in an enclosed speaker mountedinabaffle. sponse would suffer and poor transientbaffle is shown in Fig. 1. Notice that the response might result. For best repro-two analogical capacitances are in series. * 1200 Susquehanna Ave.,11'.Pittston duction of transients a low m/C ratio isIf the baffle is made as large as possible, Po. it will he so much larger a capacitance than the cone compliance that the limit- ing value will be in the suspension. The remainingreactance - theinductance DAMPING OF CABINET PANELS representing the quadrature component (from preceding page) of the speaker's air load-is usually small enough to be ignored (except in the case ously, no rattling of the lead shot should One means of reducing the transmissionof the R -J speaker enclosure where it is occur. of these forces into the speaker panels is exaltedto make possibleasmaller Figure 3 shows the acceleration forto employ a damping material such as a speaker enclosure). Consequently that various cone amplitudes. It is seen, for rubberorcorkgasket betweentheleaves just the cone suspension com- instance, that at 100 cps, a cone vibrat- speaker and the cabinet. If the speakerpliance on which to work in improving ing with an amplitude of 0.1 in. under-is not very heavy, the gasket may bethe bass range of a speaker. goes an acceleration .of 100 G's. Since acemented to both the speaker and the Since the baffle is of no concern, one modern dive bomber can rarely exceedcabinet, and the speaker thereby morecan use the "free -space resonant fre- 10 G's without shattering, the accelera- effectively isolated from the cabinet.' quency" of the speaker as the working tion for loudspeaker diaphragms is very RCA makes a speaker cone with a rub-criterion.Althoughthemassofa large indeed. ber damping ring. This ring should not bespeakercone cannotbecontrolled For the same reason, there are greatused for cementing the speaker to the cab- forces acting also on a speaker enclosureinet, because the ring is intended for an- 1 Harry F. Olson, "Elements of Acousti- which is closely coupled to the speaker. other purpose. cal Engineering." McGraw-Hill.

121 enough to varythisfrequency very be obtained from large chemical supply this operation is finished and the sealer much, the cone suspension can be treated houses.2 Dibutyl Phthalate appears to beis dry, the plasticizer may be applied to make it more compliant and this a good plasticizer in not drying out over with a cotton swab. Distributing itin would lower the resonant frequency the a period of time ;but it does have theeven amounts around the periphery of same as would increasing the capacitor marked disadvantage of "creeping," that the suspension is desirable, but there is in an electrical "tank" circuit. There is, spreading through the cone by capil- no worry about spreading it laterally as are two methods of doing this, and by lary action. This would be disastrous if the Phthalate spreads itself. If it should utilization of both it is possible to lower allowed as the whole cone instead of just begin to work past the barrier after age- the frequency response limit 30 per cent the suspension would thus be made pli-ing. another swath of sealer may be or more. A relatively inexpensive 8 -inch able and piston action would suffer, re- applied to stop it. speaker treated by this system can be sulting in low efficiency and weakening made equal in bass -range capability to of high frequencies. To prevent this con- a good quality 12 -inch speaker. dition the edge is first made impervious Results Obtained to capillary travel by sealing the pores Methods of Increasing Compliance with an application of shellac, varnish, or By utilizing one or both of these two The first method is called "slotting" by cement. This means the addition of a methods of suspension treatment, the re- the makers of Permotiux loudspeakers. small amount of mass to the cone, and a sults shown in the chart may he obtained. As shown in Fig. 2, the speaker is made further lowering of the resonance point The improvement in low -frequency ca- less stiff by radial incisions in the sus- is sometimes noticeable but rather negli- pabilityisevident. We have thus far pension. The improvement derived from gible. A small soft brush is used to paint stressed only the advantages in bass, but this treatment depends on the merit of the cone in a swath about 44 -inch wideno really deleterious effects to the higher the suspension design beforehand. The (forlarge speakers)justinsidethe ranges have been noted. It would seem suspension area of some good moderncurvature of the first corrugation-on that the slitting operation could conceiv- speakers is extremely thin, as can be bothsides of the cone. Clear shellac wasably cause edge effects resulting in ir- seen when itis held up to a light, and used in most of the treatments attempted. regularitiesinthemiddle frequency the small amount of improvement pos- and with a fair amount of success, but it range. One of the most pronounced peaks tends to break down under reaction fromof a speaker's middle frequency response sible is effected with a relatively small the Phthalate if the cone is very well number of slits. In older speakers withfelted. Recent but rather inadequate ex-is due to the resonance of the edge.3 The stiff suspensions it appears that more slits perience with so-called "radio" cementmechanical alteration of the edge might are better. A wise approach is to makewould seem to recommend it because itaggravate the peak somewhat on the less a small number of slits at first-aboutdoes not appear to be affected by theexpensive speakers. A small change in sixteenfor example-thenifthe im-plasticizer. It is important that a goodfrequency of some of the peaks in a provement is not what was anticipated. job be done in blocking the creep of theloudspeaker's curve has at times been to try more between the firstset.A pair Pthtalate, because if it once gets startedobserved, but the extent of the varia- of drafting dividers is used to space the through.itisdifficult to stopit. Onetions has not been quantitatively ascer- slits evenly. They are set by trialso should apply several costs of the sealer that when "walked" around the edge ofif inspection deems it advisable. When 8Corrington and Kidd-"Measurements the first corrugation they return to the on loudspeaker cones,"Proc. I.R.E.Sept. 2Eimer & Amend, New York City, for1951. p.1021. same spot with integral spacing. Then example. they are walked around again allowing (Concluded at bottom of next page) the points to prick the cone and thus mark the correct location for each inci- TABLE I sion. A sharp razor blade is used to slit carefully as close along a radial as pos- Results of Speaker Treatment sible, from the middle of the first cor- Speaker Resonant Frequency Treatment Before After rugation (which is the cone edge) across Treatment Treatment the rim to the area where it is cemented 3" Cinaudagraph P2AI 230 205 Eight slits to the "basket." The rim should not be 190 Sixteen slits torn and the cuts should go all the way 4" x 6" RCA 446S2 195 155 Sixteen slits to the bottom of the following corruga- 147 Thirty-two slits tions. If the suspension at the apex of 140 Same, plus cement sealer the cone. the so-called "spider," is also 130 Same, plus three applications of plasti- of the corrugated type. slitting here will cizer, ageing after each 5*x 7" RCA 257S1 110 80 Thirty-two slits afford an additionallowering ofthe 75 Same, plus shellac sealer resonant frequency. Often this is difficult 58 Same. plus five applications of plasti- or impossible to do, however, and might cizer, ageing after each affect the high -frequency response. 6" x 9" RCA 269S1 100 80 Thirty-six slits The second means of improvement is 72 Same, plus sealer and plasticizer called "plasticizing." Plasticizing is ac- 70 Same, plus ageing complished by applying a suitable chemi- 63 Same, plus second application of shellac cal agent to the suspension to make the sealer and ageing 55 Same. plus four slits in spider and more fibers more pliable and yielding. The ageing plasticizing described herein was accom- 8" R&A ( British) 880P 108 90 Twenty-five slits plished with Dibutyl Phthalate which can 80 Fifty slits 72 Same, plus plasticizer 62 Same, phis second application 58 Same, plus two more applicationsof plasticizer. and slitting spider 10" PM 10-12 75 63 Thirty-six slits 48 Same, plus three applications of plasti- cizer and ageing 12" GE S-12011) 73* 70 Sixteen slits 68 Same, plus plasticizer 50 Same, plus fourslitsinspider, three more applications of plasticizer, and ageing.(Capillarytravelisvery Fig.3. Circuit arrangement used to determine marked in this cone.) resonantfrequencyofloudspeaker. Some speakers ofthis type come with a resonant frequency as lowas 63 cps.

122 Improving Loudspeaker Performance

DAVID B. WEEMS

How to improve low -frequencyresponse from inexpensive loudspeakers with the expenditure of less thana dollar and an hour or so of your time.

Among the most important charac- where fr = resonantfrequencyofthe several answers. The first is that at nor- teristics of the ideal speaker are wide- speaker in cps, mal volume levels such movement will range frequency response and low dis- Mo = mass of cone, voice coil, and probably not occur. Secondly, if it does tortion. The fundamental cone resonance air load in grams, occur the use of multiple woofers will of a speaker is closely associated with Cm. = compliance of the suspen- eliminate it and at the same time provide both its bass range and its distortion sion system in centimeters one of the most practical means available content at low frequencies, because the per dyne. to the home constructor of accomplish- output falls off rapidly with an increase Assuming a constant air load, exami- ing truly superior low -frequency repro- in intermodulation effects and frequency nation of this formula shows that as duction. Finally, regardless of the theo- doubling below resonance. It is a well either the mass or the compliance of a retical possibility of excessive voice -coil known fact that the proper baffling of a cone is increased, the resonant frequency movement, the net result of higher com- loudspeaker in such mountings as bass- is lowered. While increasing the mass of pliance for cheap speakers has been bet- reflex and horn -loaded enclosures lowers the cone lowers resonance, it also limits ter sound. In speaker design we cannot the resonant frequency, and an amplifier high -frequency response and results ina always predict, like Old Man Mose, what that presents a low impedance to thedeteriorationoftransientability.In- will provide subjective enjoyment. With speaker will minimize the effects of reso- creasing compliance thus seems to be the this premise firmly in mind and with nance. We are concerned, however, with best approach toward lower fundamental courage in our hearts, we shall now pro- the qualities of the speaker itself and how cone resonance. High compliance also ceed to the task of speaker improvement. we can improve them. aids in obtaining wider range and lower The fundamental resonant frequency distortion. of a loudspeaker may be determined Procedure mathematically from the formula: It may be argued that higher compli- The materials to be assembled are an ance in cheap units may produce greater - 1 intermodulation distortion due tothe 2n V MrCins movement of the voice coil beyond the * 11327 Missouri Ave., Los Angeles 25, area of maximum flux density. There are Calif.

Fig. 1. Materials needed for treating speakers_ chamois,solvent,cement,tweezers,scissors, razor blade, scalpel, and in some cases clothes - Fig. 3. Inserting shims to hold the voice coil in sprinkler head. Fig.2. Removing the dust cover with tweezers. position.

'scope input used to calibrate the audio can also be detected audibly by noticing oscillator, an operation which should be when the cone tends to chatter, which is performed at frequent intervals, particu- caused by itsgreater displacement at SPEAKER TREATMENT larly with a beat -frequency oscillator resonance. The 'scope method, which (frompreceding pig, When working with low frequencies and depends on the rise in speaker impedance such small increments, accuracy of cali- at its resonant frequency, is reliable and bration is important. By setting the oscil- can also be used for a speaker mounted tained because of the apparent inconse- lator to 60 cps and varying the calibra- in abaffle thus serving to indicate the quence. The plasticizer, however would tion adjustment until a stationary one- reduction in bass range that the enclosure have no pernicious effect as long as it is to-one Lissajous figure is obtained might cause. kept within its prescribed area. If one de, ellipse, or inclined line, depending As a before -and -after test of this treat- were treating a speaker intended only on phase), the oscillator can be set read- ment, LP records with low organ notes* as a woofer, there is nothing to lose in ily without much trouble. To find the are very useful. Often the improved the complete treatment and much to resonant frequency of the speaker turn speaker will bring out notes in the lower gain. If both methods are applied, the the 60 -cps input gain to zero and hold register which were not even noticed be- slitting operation must be done first be- the speaker up in the air. Vary the fre- fore treatment cause slitting a plasticized rim is quite quency of the oscillator until the maxi- difficult. mum deflection is noted on the 'scope. 4 Columbia ML 4120: Saint Saens, Sym- In checking progress the circuit shown The frequency indicated by the dial is phony No. 3; Columbia ML 4329: Poulenc, in Fig. 3 has advantages. The 60 -cps the resonant frequency. In most cases it Concerto in G Minor are good examples.

123 inexpensive 8 -to -12 -inch speaker, plus narrow strips to suspend the cone. The (Fig. 1) a small piece of the softest, gasket should be trimmed of original thinnest, and most pliable chamois avail- cone remnants and reglued into position. able, radio solvent (optional), household The dust cover may or may not bere- glue, tweezers, small scalpel (optional), used, depending on the ultimate purpose barber's scissors or other pointed scis- of the speaker as described later. In sors, a new razor blade, and an aluminum every instance the glue should be used clothes sprinkler head. The chamois may sparingly, especiallS, on the cone. When require some shopping around because the chamois is glued to the frame, the of the enormous variation of thickness glue should be spread near the outer rim and pliability, but usually the small, most of the frame only; thiswill allow a inexpensive pieces found at the five and longer chamois suspension and preserve dime will include a few that have the its pliability. desired characteristics. The aluminum When the job is completed, we have a sprinkler will only be required for cer- cone that is suspended at four points by tain very inexpensive units as will be the spider, and at four points on soft described. chamois at the outer rim. It would be The operation should be performed on difficult to conceive of a more compliant a clean bench or table. First, tear away suspension. An ordinaryreplacement the dust cover with the tweezers (Fig. speaker, purchased for about five dollars 2), taking care not to disturb the voice- in a radio supply store was thus treated coil leads. Next, place shims of film nega- and then compared to a 15 -inch speaker tive or paper around the center pole to costing more than 10 times as much. The maintain the voice -coil form in position, cheaper 12 -inch unit appeared to be dis- Fig.4.Slitting the outer edge of the cone to as in Fig. 3. Usually four narrow shims tinctly cleaner. although the overall re- removeitfrom mechanical engagement with located at equal intervals around the sponse was admittedly somewhat rough the frame. pole are sufficient. The solvent may then and shrill due to inherent characteristics be used to saturate the juncture of the and the removal of cone resonance asa outer rim of the cone, the frame, and the contributor of bass. gasket. While the solvent is acting on This phenomenon of shrillness may be the glue, the rim of the cone may be cut removed by two general methods, the away at the middle of the inside cor- use of the speaker as a woofer only, or rugation (Fig. 4) leaving a "lip" past frequency correction of the speaker it- the smooth part of the cone. The entire self. The latter may be achieved bya outer corrugations and gasket may now mechanical means that the writer has be lifted out. If solvent was not avail- found surprisinglyeffective and that able, this part can be loosened with a seems to smooth the high range. It is to knife after it is cut loose from the cone. be recommended only for inexpensive The spider should be cut away with speakers of the replacement class (and either the scalpel or pointed scissors, then only on 10 and 12 -in. units), but leaving four strips about to y, inch in these are the units that most need fur- width at equal intervals (see Fig. 5). ther alteration. Now attach four strips of Scotch tape to The method consists of gluing a small the outer rim of the cone, evenly spaced, perforated aluminum dome over the cen- and then fasten them to the frame with- ter of the cone in place of the dust cover. out applying tension to the cone. If de- This dome may be obtained froman sired, the shims may be removed and the aluminum clothes -sprinkler head. The speaker tested for trueness of centering. sprinklersareobtainablefrommost If it will respond to loud musical pas- variety stores for about a nickel. The rim Fig. 6. Four strips of the soft chamois are glued sages that include low frequencies with- that clamps the dome in place should be in place to hold the tone centered. outrattling,the assumption may he iremoved by inserting a sharp knife blade made that the centering was adequately under it and liftingit away from the done, and the shims may be replaced as dome. A small amount of household glue before. should then be placed around the rim of The Scotch tape should be replaced, the dome, and after positioning,a small one piece at a time, by pieces of chamois weight on the dome will hold it in place about 2 inches wide (see Fig. 6). The until the glue has set. The speaker should chamois pieces should first be glued to now he ready to mount and try out. the lip left when cutting away the rim of the cone, then to the frame. The chamois should be tightened just enough to re- move wrinkles but not stretched. After this operation the speaker may be tested again. Now the four large gaps around the cone rim may be filled by cutting large pieces of chamois tofittheM. These large sections should be installed very loosely, leaving a deep wrinkle between the cone rim and the frame as shown in the end -product photo of Fig. 7. Their purpose is to eliminate the exchange of Fig. 7. Spaces between the four original chamois air between the rear and front of the Fig. 5. The spider is cut away except for four cone, not to support and impede it ;so strips are filled with loosely hung chamois to sectors which hold itin position while allowing help sealfront off from rear air withoutre- they are left loose, allowing only the four greater compliance. stricting cone movement.

124 1

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