AUDIO ENGINEERING OCTOBER, 1948 I Linear Standard Units

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AA A 1 _ 1iÌTUÌcß " PRICES AND AUDIODISCS

Sictt,men.t On ai[h Phic2 Policy

As of September 1st, aluminum prices ccre Fortunately, our patented, precision- machine again increased. This means higher cost for the process -now used for over a decade and con- principal raw material used in the manufacture tinuously improved -gives a more consistent of AUDIODISCS. In fact, the cost of the yield of high quality discs than any method of aluminum base has always been the main item production now used. And we have tested every in the cost of production. Thus, any increase in other process in use. aluminum prices is of major importance. But beyond the cost of raw materials and So our position with respect to the present labor there is a basic factor which determines the increase in aluminum prices is this: cost of manufacturing professional recording 1. We are not increasing prices of AUDIODISCS. discs. This factor is the extent to which the particular process of manufacture enables the producer to turn out a large proportion of first 2: We shall make every effort to absorb this new quality discs. There are several methods of aluminum price raise and thus continue our production used. None of these will give anything prices at the present level. Our calculations like a 100% yield. It is, however, obvious that indicate that with some improved efficiency, as the percentage of yield increases there is a now under way, and continued large volume resulting drop in the average cost of aluminum, production, we shall he successful in this hold - lacquer and labor. the -price effort.

Audiodiscs are manufactured in the U.S.A. under exclusive license from PYRAL, S.A.R.L., Paris. Audio Devices, Inc., 444 Madison Ave., N.Y. C. EXPORT DEPT: ROCKE INTERNATIONAL, 13 EAST 40Th STREET, NEW YORK 16, N. Y. Link FM -With Sylvania Loch -Ins Covers New Jersey For Its State Police Radio System Drivers of both emergency trucks and patrol cars con maintain two -way communication Automotive equipment of the New Jersey State Police includes vehicles with fixed stations. always on the alert to deal with every emergency. Fleet is spearheaded by 180 patrol cars of the department in addition to 42 patrol cars of the State Motor Vehicle Department which is served by the State Police. These vehicles are constantly in touch with fixed FM stations located at 26 strategic points throughout the state. In addition, emergency trucks carry complete radio equipment equivalent to that of a fixed station! Link Radio Corporation, manufacturer of the communications equipment, makes extensive use of Sylvania Lock -In tubes to assure unfailing efficiency of this statewide network. Lock -In tubes stay put through vibration and jarring. They have few welded joints ...no soldered ones. Elements cannot it arp or weave ...connections are short and direct. Top Radio equipment in emergency trucks dupli- location of getter reduces losses ...separation of getter material from cotes the set-up used in fixed transmitting stations. leads cuts down leakage. See Sylvania Distributors -or write RadioTube Division,Emporium,Pa.

The superior mechanical and electrical features of Sylvania Electric's famous Lock.ln tube make it the SYLVA ideal choice for equipment on NIA the road, in the air, on the rails, marine radar, FM and television. ELEC11R1 C

MAKERS OF RADIO TUBES; CATHODE RAY TUBES; ELECTRONIC DEVICES; FLUORESCENT LAMPS, FIXTURES, WIRING DEVICES; ELECTRIC LIGHT BULBS

AUDIO ENGINEERING OCTOBER, 1948 I Linear Standard Units...

THE ULTIMATE IN QUALITY. .. UTC Linear Standard Audio Transformers represent the closest approach to the ideal component from the standpoint of uniform frequency response, low wave form distortion, high efficiency, thorough shielding and utmost dependability. UTC Linear Standard Transformers feature ... True Hum Balancing Coil maxi. Structure ... Semi -Toroidal Multiple Coil Structure .. . mum neutralisation of stray fields. minimum distributed capacity and leakage re Balanced Variable Impedance Line ... per. oc tonte. mils highest fidelity on very top of a universal

nit . . no line reflections or transverse coupling. Precision Winding . accuracy of winding Reversible Mounting ... permits above chassis .1%, perfect balance of inductance and capacity; or sub-chassis wiring. exact impedance reflection. Alloy Shields ... maximum shielding from in- High Fidelity . UTC Linear Standard Trans - ductive pickup. For e the only audio units with a guaron- Hiperm -Alloy .. a stable, high permeability ± teed euniform response of 1 DB from 20- 20,000 nickel -iron core malarial. cycles.

TYPICAL LS LOW LEVEL TRANSFORMERS Relative Max. hum- Unbal- Primary Secondary +1 db Max. pickup anted DC List Type N6, Application Impedance Impedance from Level reduction in prim'y Prlee LS -10 Low impedance mike, 50, 125. 200, 60.000 ohms In 20-20,000 +15 DB -74 DB 5 MA 525.00 pickup. or multiple line 250. 333, 500/ Isso sections 08 to grid 600 ohms LB -10X As Above As above 50,000 ohms 20-20,000 +14 DB -92 DB 5 MA 32.00 1.5.12 Low Impedance mike. 50. 125, 200, 120.000 ohms 20.20,000 +15 DB -74 DB 5 51A 28.00 pickup. or multiple line 250, 333. 500/ overall. In two LI-50X to push pull grids 000 ohms sections LB12X As above As above 80,000 ohms 20-20,000 +14 DB -92 DB 5 MA 35.00 overall. in two sections LS-28 Bridging line to single or 5.000 ohms 60.000 ohms in 15- 20.000 +20 DB -74 DB 0 MA 25.00 push pull grids two sections

LS-19 Single to 1 plate push pul 15,000 ohms - 95,000 ohms; 30- 20.000 +17 DB -50 DB 0 MA 24.00 grids Ifke 2A3. 614. 300A. 1.25:1 each side Split secondary L$x2l. Kingle plate 00 to push pull 15,000 ohms 135,000 ohms; 20-20,000 +14 DB --74 DB 0 MA 24.00 Vida. Split primary and tuns rallo secondary 3:1 overall LS -22 Push pull plates to push 30,000 ohms 80.000 ohms; 20-20,000 -h26 DB -50 DB .25 MA 31.00 LS-i2 pull grids. Spilt primary plate to plate turn ratio and secondary 1.6:1 overall low Lß30 Mixing. impedance 50, 125. 200, 50, 125. 200, 250, 20-20.000 +17 DB -74 DB 5 MA 25.00 mike. pickup. or multi- 250, 333, 500/ 333, 500 /600 ohms ple line to multiple line 600 ohms LS -30X above As As above As above 20- 20.000 +15 DB -92 DB 3 MA 32.00 LS -27 Single to plate multiple 15.000 ohms 50, 125. 200, 250. 30. 12,000 +20 DB -74 DB 8 MA 24.00 line 333. 500 /600 ohms cycles LB-50 Single plate to multiple 15,000 ohms 50, 125. 200. 250. 20- 20,000 +17 DB -74 DB 0 MA 24.00 line 333, 500/600 ohms LB -S1 Push pull low level 30.000 ohms 15111i1111 L plates 50, 125. 200. 250, 20.20.000 +20 DB -74 DB 1 MA 24.00 to multiple line plate to plate 333. 500 /600 ohms -141 LS Three sets of balanced 500/600 ohms 500,000 ohms 30- 12,000 +10 DB -74 DB 0 MA 28.00 LS 50 windings for hybrid service. cenlertapped 11111 11 .R11.111011111111REINIMIPPIIMM TYPICAL LS OUTPUT TRANSFORMERS Type Primary will match Primary Secondary I db Max. List Ne. folioed no typical tubes Impedance Impedance from Level Price '. Lß52 l'uah pull 245. 250, GVG, 42 or 8.000 uluse 500, 333, 250. 25- 20,000 15 watts $28.00 2A5 A prune 200, 125. 50. 30, 20. 15, 10, 7.5, 5. 2.5. 1.2 LS-55 LSSS Push pull 2A3's. 6Á50's, 300A's, 5.000 ohms plate 500. 333. 250. 25- 20.000 20 watts 28.00 275A's, 6A3's. 61.6's to plate and 200, 125, 50. 30, 3,000 ohms plate 20. 15, 10. 7.5, to plate 5. 2.5. 1.2 LB -57 Sane as love 5.000 ohms plate 30, 20, 15, 10, 25- 20.000 20 watts 20.00 to plate and 7.5. 5, 2.5. 1.2 raEatlEMCy 3,000 ohms plate to plate LS-511 Pusi, pull parallel 2A3's. 0A50 's, 2,500 ohms plate 500, 333. 250. 25.20.000 40 watts 50.00 300A's, 6A3's to plate and 200. 125. 50, 30, 1.500 ohms plate 20, 15. 10. 7.5, to plate 5. 2.5, 1.2 wrc for our Catalog PS -4108 LSOLI Push pull 6L6 's self bias 0,000 ohms plate 500, 333, 250. 25.20.000 30 watts 42.00 to plalo 200. 125, 50. 30. 20, 15, 10, 7.5, 5. 2.5, 1.2

150 VARICK STREET NEW YORK 13, N. Y. EXPORT DIVISION: 13 EAST 40th STREET. NEW YORK 16. N. Y.. CABLES: "ARLAB" John H. Potts, Editor Sanford R. Cowan, Publisher

C. G. McProud, Managing Editor S. L. Cahn, Adv. Director Lawrence LeKashman, Asst. Editor H. N. Relies, Adv. Mgr. 'AUDI Louisa B. Dresser, Edit. Prod. Mgr, D. Reissman, Circ. Mgr. Editorial Advisory Board Representatives EGINEERIIVG Howard A. Chinn James C. Galloway 13, John D. Colvin 816 W. 5th St., Los Angeles Catit. C. J. LeBel Dale International Publications, Ltd. 105 Bolsover St. J. P. Maxfield London W. I, England George M. Nixon Successor to RADIO Harris & Floyd Winston Wells 297 Swanston St., Melbourne I, Australia S. Young White C. Victoria,

Established 1917 CONTENTS OCTOBER, 1948 Vol. 32, No. IO

Editor's Report l' Letters 8 Magnetic Tape Recorder of Broadcast Quality -Harold Lindsay and Myron Stolaroff 13 Intermodulation and Harmonic Distortion Measurements -I. Avins 17 Measuring Amplifier Internal Resistance -Walter Richter 19 Audio Engineering Society, San Francisco Section 20 Elements of Residence Radio Systems, Part II -C. G. McProud 21 High Quality Speaker Enclosure-Herbert C. Eidson, Ir. 24 The Problem of Sound Distribution, Part III -O. L. Angevine and R. S. Anderson 28 Experimental Data on Germanium Crystal Amplifiers-S. Young White 32 3 Resistor Error in Attenuators- Herbert /. Keroes 34 DBM- Impedance Chart -Clarkstan Corp 36 New Products 38 Professional Directory 38 Record Revue -Edward Tatnall Canby 45 TECHNICANA: Notes on a 6AS7G Amplifier 50 Advertising Index 56

COVER

Sound wave emitted by loudspeaker. Photographed by Winston Wells especially for AUDIO ENGINEERING.

AUDIO ENGINEERING (title registered U. S. Pat. Off.) is published monthly at New York. N.Y., by Radio Magazines, Inc., .1. H. Potts, President; S. R. Cowan, Sec'y- Treas. Executive and Editorial Offices at 312 Madison Avenue, New York 17, N. Y. Subscription rates -United States, U. S. Possessions and Canada. 83.00 for 1 year, 85.00 for 2 years elsewhere 84.00 per year. Single copies 35r. Printed in U. S. A. All rights reserved. Entire contente copyright 1948 by Radio Magazines, Inc. Entered as Second Class Matter July 29, 1948 at the Post Office, New York, N.Y., under the Act of March 3. 1879.

AUDIO ENGINEERING OCTOBER, 1948 3 EDITOR'S REPORT r

MESSAGE FROM AUSTRALIA publication in a magazine which he knows' will reach practically all whom he has seen is one which we Ameri- The charming and gracious letter which we are cans in similar circumstances could well follow. happy to publish below was just received from Australia.

Sir: TRANSISTORS I have just returned from a world tour study- ing radio from the Broadcast Engineer's point From Geoffrey Parr, editor of our well -known Eng- of view. My trip included the United States. lish contemporary, Electronic Engineering, comes a cable I hope you can find space to publish this letter as an appreciation of the ready assistance congratulating us on the series of articles on germanium and advice that was given to me at all times crystal amplifiers now running in this magazine. S. by engineers in your country. So many of us Young from Australia and other countries visit the White is learning a great deal about the idiosyn- U.S. and I am sure at times, we must occupy crasies of these devices through his experimental work quite a lot of their valuable time. and feels that he will soon have his development work up I cannot of course, write personally to every to a point where these crystals can be put into produc- engineer who entertained me, but as I know AUDIO ENGINEERING circulates amongst practic- tion. t ally all those who have the same interests as I, this letter is almost sure to reach them. I would like to say how much I admire the ready interchange of information and ideas AUDIO ENGINEERING SOCIETY NEWS t amongst American Engineers and from them to visitors overseas. On September 14th the Audio Engineering Society AUDIO ENGINEERING is filling a need in the held a special session on disc recording at the Western broadcast field and it is just what was wanted. It avoids unnecessarily wading through many Union Auditorium in New York City. Despite the heat, articles in other magazines which are not of approximately 300 audio engineers attended and heard particular interest in the broadcast field. Keep on with the good work. a talk by R. A. Lynn of NBC on the relative merits of vertical Yours sincerely, and lateral recording, and a lecture and demonstration by V. J. Liebler of Columbia Records, Inc., on L. N. Schultz. the development of the new Microgroove records. Both Chief Engineer - 2GB, Sydney, Australia. were very well received. Before introducing the speakers for the evening, C. J. LeBel reported on the remarkable We are proud that our readers have been so courteous progress of the AES and pointed out that already em- and helpful to this visitor from abroad, and we feel that ployers are advertising for audio engineers rather than, Mr. Schultz's idea of writing a "thank you" note for as heretofore, electronic engineers with audio experience.

4 AUDIO ENGINEERING OCTOBER, 1948 RINGS VI WI ALNICO - CAST ASSEMBLIES MAGNETIC FOCUSING FOR «.1`Z t sharp clear, ¡,2/JvGW O` blur; gives eliminates first focusing focusing fluctuations; the VI magnetic a V and SETS, line voltage net a d Alnico not TELEVISION warm-up, or permanent mag are cast, even during ring-type P pictured¡cured here) advanced reception The thin size are but Arnold'se) r is the last. this use (several trouble. adjustment Arnold for It's a difficult without by first cost. and any quantity, produced save quality materials,materialúir in order to desired or other es. sintered, ringsrtngication,s in the of Alnico your these in any ggrade welcometh ma methods produce Well .N MagMagnets. °deaend on tAh °Oldppermanent can depend

THE ARNOLD ENGINEERING COMPANY Subsidiary of ALLEGHENY LUDLUM STEEL CORPORATION 147 East Ontario Street, Chicago 11, Illinois Specialists and Leaders in the Design, Engineering and Manufacture of PERMi.NEN1 ,c AGNETS

AUDIO ENGINEERING OCTOBER, 1948 Ire to

Allotutomatically totes one of 18 separate

scales into posit i as you select the range. SIMPSON MODEL 221 ROTO- RANGER Ranges HIGH -SENSITIVITY A.C. -D.C. VOLT -OHM- MILLIAMMETER 20,000 Ohms per Volt D.C., 1,000 Ohms per Here is the only multiple scale test instrument of its kind in the world. Volt A.C. Volts, It definitely reduces the possibility of errors by providing A.C.: 2.5, 10, 50, 250, 1000, 5000 a single scale Volts, D.C.: 2.5, 10, 50, 300, 1000, for each range of finest 5000 this of volt-ohm-milliammeters. As the selector Milliamperes, D.C.: 10, 100, 500 switch is moved to the range desired, an ingenious gearing mechanism Microamperes, D.C.: 100 rotates a drum, bringing into place behind the meter window the proper Amperes, D.C.: 10 Output: 10, scale for that range. Here is the equivalent of 25 separate instru- 2.5, 50, 250, 1000 Ohms: 0 -2000 (12 ohms center), 0- 200,000 (1200 ments combined in one sturdy and compact unit. (18 scale.; 7 additional ohms center), 0-20 megohms (120,000 ohms center). direct reading ranges through use of high voltage and output jacks.) The patented Roto-Ranger Size: 1234" x 101/8" x 53/8"' principle eliminates the confusion of num- Weight: 8 lbs. 9 oz. erous readings on one scale, and the multiplying factors ( mmon to Price, complete with test leads and 28 -Page ordinary multi -range testers, by providing a separate scale for each Operator's Manual j69.85 range. There are no cramped calibrations in these full sized Roto- High voltage probe (25,000 volts) Ranger scales. Each is for TV, radar, x roy and other high designed as it would be for a separate instrument. voltage tests, also available. SIMPSON ELECTRIC COMPANY Ask your Jobber, or write for 8 _ zie St., Chicago 44, III. 1 complete descriptive In Canada: Baci- Simpson, Ltd., literature London, Ontario

INSTRUMENIS MA, STAY ACCURATE

lo MA.

DO M.A. A separate scale 300 M.A. for every range- automatically! 10 AMPS

o Y.D.C-

so r.D.C- I00 vD 4 1sA.0 Z ERO OHMS t Large 5'1" meter, OHs riP vpr L with 18 separate, full length oUTPUT

6 AUDIO ENGINEERING OCTOBER, 1948 AUDIO TRANSFORMERS At ELODISTORTIONSEw IN FREQUENCY RANGES íl ide /r,i (_.ulrdeg 4.)1 orcrng comfile% new alocz llle

Full Frequency Range 30 to 1 5,000 Cycles, provides uniform response over this entire band with ± 1/2 db up to 10 watts of audio power, within ± 1 db over 10 watts. Standard RMA impedances. Hum balancing coil structures and nickel alloy shielding. Included are Input, Output, Driver, and Modulation Transformers; Modulation Reactors. Sealed in Steel construction, stud mounting, with pin -type terminals.

Public Address Range 50 to 10,000 Cycles, frequency re- ± sponse within 1/2 db up to 10 watts of power, within ± 1db over 10 watts, throughout this range. Secondary impedances match 600 and 150 -ohm lines, 16, 8 and 4 -ohm reproducing systems. Listed are Driver and Output Trans- formers. Sealed in Steel construction, flange mounting, with solder lugs or wire leads.

Communications Range 200 to 3,500 Cycles, affords response ± with variations not exceeding 1 db over the range of voice frequencies. For use with 600 or 150 -ohm lines. Input. Output, Driver and Modulation Trans- formers offered. Sealed in Steel construction, flange mounting, with wire leads or solder lugs.

l'EIIICAGO ThANSRMER DIVISION OF ESSEX WIRE CORPORATION

I L L I ADDISION STREE' C H I C A G O 8 w O S

AUDIO ENGINEERING OCTOBER, 1948 - Zeiípne A About Our June Cover Sir: I could probably win a bet that you have already been written to, or perhaps wired by, KOMO or the Austin Company about the cover on your June issue. (You lose. -Ed.) Apparently the table pictured in KOMO's control room is quite unusual, and they dial counter- clockwise instead of clockwise, like heust l'EN JACKS the rest of us. Also, the "ON AIR" sign is reversed. MPROvE What happened? David L. Farrell, Control Room, WCSS Amsterdam, N.Y.

Nickel -plated heavy Sir: gauge steel frame What a place KOMO must be with turntables which revolve counter -clockwise (note tht left- handed pickup), telephone dials that turn from right to left, and an "ON AIR" sign which reads backwards ... . E. F. Phillips

. Apparently we can't get away with anything insofar as control room pictures are concerned. We reversed this purposely; otherwise our "Ammo ENGINEERING" slug of Nickel Silver would have covered interesting portions springs the picture. But we didn't realize that our eagle -eyed readers would pick up quite so Silver contacts -tale Nickel -plated many tell points. Ed. brass sleeve High Fidelity Sir: I have followed with a great deal of interest the many articles on high fidelity. Like the rest I have my own views on the subject. Perhaps they may be of interest to your other readers. 'flo % 9JLtlf' 9121 I specialize in audio experimenting together with its associated fields. As far as ALL ADC JACK ALL THESE BENEFITS I'm concerned, high fidelity means this to PANELS NOW CONTAIN me - a highly flexible audio amplifier THESE NEW ADC JACKS and the best reproducing system possible. To clarify that statement, here is my own personal set -up. G. E. pickup through my The new streamlined design of ADC own preamplifier employing 300. 500.800 cycle crossovers, bass boost and gain control Strength Jacks uses heavier gauge metal for greater to feed Meissner Tuner. This enables me {er to match radio side of tuner to phono to . tarea .I strength. The frame is die -formed and main audio amplifier. Main amplifier uses Rigidity!igid ty 6B4's (I agree with the triode hounds!), & I press- welded for utmost rigidity and variable feedback from voice coil covering dimensional accuracy. three stages, treble boost or droop and of Corrosion! course, main gain. Less Contacts! Silver -alloy contacts and genuine All this, of course, doesn't take into consideration the bass and treble boost or Better Nickel Silver springs guarantee corro- droop of the tuner which is used for the general I balancing of phono or radio. My Life sion resistance, even under the most own ear dictates an approximate ortho curve, so preamp and main amplifier are set for humid conditions. the average best listening. The tuner controls take care of the varying individual All the way through, the new ADC is a characteristics of the program material. better jack! If you require top quality From this I hope you can begin to per- See your jobber ceive my point of view. The amplifiers and precision -made components, ADC should indeed be high fidelity to cleanly or write today reproduce any frequency throughout the au- for ADC Catalog Jacks merit your first consideration. dio spectrum and it should be wide range with flexibility. My own runs 35 to better than 20,000 cycles and I can make it flat or build as I desire. Above all, it's cleaner 14ti;dGQ DEVELOPMENT CO than most in its reproduction throughout. f 1'Padfo DueCsva tle '7taeat use a Lansing 12" floating on felt and rubber. [Continued on page Ill

8 AUDIO ENGINEERING OCTOBER, 1948 for A -M BROADCAST MEASUREMENTS

TYPE 1931 -A MODULATION MONITOR

TYPE 1932 -A DISTORTION & NOISE METER

General Radio Company has supplied pre- FEATURES FEATURES cise measuring equipment for broadcast The TYPE 1931 -A Modulation Monitor The TYPE 1932 -A Distortion & Noise stations for over twenty years. G -R instru- allows the following measurements to be Meter is continuously adjustable over the ments are designed for accuracy, ease of made continuously: Percentage Modula- audio -frequency range and can be set to any operation and long, trouble -free life. G -R tion on either positive or negative peaks; frequency quickly, since it has only one broadcast instruments are quality instru- Program -level monitoring; Measurement main tuning control plus a small trimmer. ments designed specifically for broadcast of shift of carrier when modulation is With it measurements can be made on a -f station use. applied; Transmitter audio- frequency distortion in radio transmitters, line ampli- These two meters are standard equip- response. fiers, speech amplifiers, speech input equipment in most a -m broadcast stations; they Requires r -f input of only 0.5 watt; car- ment to lines; noise and hum levels of a -f are essential "musts" where transmitters rier frequency range 0.5 to 60 Mc; terminals amplifiers, wire lines to the transmitter, and associated station equipments are to be for remote indicator; distortion less than remote pick-up lines and other station operated continuously at peak efficiency. 0.1°,x; 600 -ohm audio output circuit for equipment are made with ease. The Modulation Monitor indicates con- audible monitoring; modulation percentage Full -scale deflections on the large meter tinuously the percentage amplitude modu- range 0 to 110%; flashing over -modulation read distortions of 0.3`ó, lac, 3 %, 10% or lation of broadcast and other radiotele- lamp operates over 0 to 100, on negative 30% ; range for carrier noise measurements phone transmitters. The Distortion & peaks; overall accuracy at 400 cycles is 2% extends to 80 db below 100% modulation, Noise Meter measures the a -f distortion in of full scale at 0% and 100% and 4% at any or 80 db below an a -f signal of zero dbm transmitters or audio equipment such as other modulation percentage; a -f frequency level. The a -f range is 50 to 15,000 cycles, line and speech amplifiers. It finds many response of meter indication is constant fundamentals, for distortion measurements uses in communications laboratories and in within 1.0 db between 30 and 15,000 cycles and 30 to 45,000 cycles for noise and hum. production testing of radio receivers. when used with the Type 1932 -A Distortion Typs 1932-A Distortion & Noise Motor .. & Noise Meter. $575.00 Typo 1931 -A Modulation Monitor $395.00

Cambridge 39, GENERALRADIO COMPANY Massachusetts 90 West St., New York 6 920 S. Michigan Ave., Chicago S 950 N. Highland Ave., Los Angeles 38

AUDIO ENGINEERING OCTOBER, 1948 9 aCtua This is o.oN.FMNp S RECORDING 7 performance

When you can secure the quality of results shown Db. - in these graphs, can you afford to use any record- 0 ó p0 a8 ó8 ._. -U N IAtil r-- .- Q ing equipment that does not approach such a standard? .5 On the basis of definite performance, the Maurer 16 -mm Recording Optical System - the -5 only postwar designed equipment of its kind on FREQUENCY RESPONSE CHARACTERISTIC the market - is unrivaled in its field regardless OF GALVANOMETER USED IN MAURER of price. MODEL 10 RECORDER II And the Maurer 16-mm System has already been selected by two of the four major radio networks for their television requirements. Ob. 6 6 i ó ó 6 6 ó +5 in ó 8Q O8 v Ñ M In 4. -0---

Anticipating a demand for finer quality of record- D ing, including the higher frequencies, the Maurer -5 Y.- Recording Optical System was designed to pro- -10 FREQUENCY RESPONSE OF PLAYBACK duce an extremely fine line image that makes ON MAURER FILM PHONOGRAPH APPROPRIATE possible the recording of frequencies beyond -15 WITH EQUALIZATION IN RECORDING 10,000 cycles, with very low distortion. The galvanometer of the postwar Maurer Model 10

System is tuned to 12,000 cycles. This is the model *For those who may have been educated to believe that such a result is not possible with 16 -mm film, we shall be glad to demon- that has been sold to the trade for two years. strate that it not only is possible, but practical commercially NOW.

_rñáúrgr- "mill/ Professional Motion Picture J. A. MAURER, INC. Cameras and Recording Equipment for the Production of 37 -01 31st Street, Long Island City 1, N. Y. Industrial, Educational and Training Films

IO AUDIO ENGINEERING OCTOBER, 1948 Want page 8J Thus to me, high fidelity means reproducing any material available, limited or ffrr, TC-3 which to my own ear sounds as its most natural reproduction. The material which isn't more or less ffrr usually doesn't get played! I doubt if there are many ears which hear flat, but there are no doubt many trained ears to detect unnaturalness and distortion. Let your musical sense determine the curve and your engineering its ultimate reproduction result. Let's keep AUDIO ENGINEERING "audio "! Acoustically yours, J. P. Cook, 2609 Buena Vista, Bakersfield, Calif. The solution of filter network problems, has been greatly simplified through the use of toroidal coils wound on molybdenum permalloy Federal Promotes Stone TC-2 cores. Design engineers have learned The election of Rear Admiral Ellery to depend W. Stone (USNR) as president and Gen- upon them since discover- eral William H. Harrison as chairman of ing that only these toroids possess all the board of both the Federal Telephone the necessary qualities of a good and Radio Corporation and its world -wide high "Q" coil. madttfacturing and sales subsidiary, the International Standard Electric Corporation, Of the 30 different items now was announced today by Colonel Sosthenes being manufactured, Behn, chairman and chief executive officer the most of the International Telephone and Tele- available types now being sup- graph Corporation, parent concern of the plied ore two companies. Fred T. Caldwell, former- TYPE ly president of Federal Telephone and Ra- RANGE dio and International Standard Electric TC -1 Any Ind, up to 10 HYS was elected vice chairman of the boards of TC -2 Any Ind. up to 30 HYS both corporations. These appointments be. TC -3 Any Ind. up to 750 MHYS came effective September 1. 300 250 TC-3 200 Q 150 100 50

100 200 500 1M 2M 5M 10M 20M 50M 100M

FREQUENCY- CYCLES

Admiral Stone, since 1931, has served in various executive capacities with the I. T. FIL:,T' & T. System, most recently holding the post of executive vice president of Federal Tele- phone and Radio, domestic manufacturing We are producing toroidal coil filters which con. subsidiary of I. T. & T. General Harrison, sistently demonstrate the value of toroidal coils. vice president in charge of Operations and These filters cannot be matched in stability, accu- Engineering of the American Telephone racy and sharpness by filters mode with the usual and Telegraph Company, recently was laminated type of coil. elected president of I. T. & T., also to be- Orders for samples or production quantities are come effective on September 1. During the equally respected. All inquiries will war he served as a Major General, acting be promptly as Director of Procurement, Army Service handled. Forces, Washington, D. C. In addition to various campaign medals for service during World Wars I and II, Admiral Stone also holds both the United States Navy and the United States Army BIrvucell a ea. Distinguished Service Medals. He is a Knight Commander of the British Empire, DESIGNERS AND MANUFACTURERS a Knight of the Grand Cross of St. Mau- OF ELECTRONIC PRODUCTS rice and St. Lazarus (Italy), a Grand Of- ficer of the Crown of Italy,. and a Knight 45 WARBURTON AVE. YONKERS 2, N.Y. of the Grand Cross of San Marino.

AUDIO ENGINEERING OCTOBER, 1948 Pick the POWER you need take the performance for granted 30 WATTS

20 WATTS

8 WATTS 728B Ideal for use wherever extra high power is needed. Frequen- 756A cy response 60 to 10,000 cycles. Coverage angle 50 °. 12 11/32" for the Ample power capacity in diameter, 3 25/32" deep. 3 majority of applications - cubic feet of enclosure space. great Frequen- plus peak performance. to 10,000 cycles. cy range 65 in angle 60 °. we Coverage Enclosure diameter, 3%" deep. space 21 cubic feet.

755A quality Combines outstanding space THESE THREE direct radiators will meet practically cost and range with low Frequencysma all your requirements -in broadcast stations, wired requirements- Coverage j0 to 13,000 in age music, program distribution and sound systems, high- diameter, feet 70°. 2 cubic quality Pick angle equires only home radios and record players. the ones deep. Requires you need on the basis power requirements. You'll enclosure space. of of find superior quality of reproduction in all of them. Get the full story of this Western Electric line from your local Graybar Broadcast Representative -or write Graybar Electric Company, 420 Lexington Avenue, New York 17, N. Y.

DISTRIBUTORS: IN THE U. S. A. - Graybar Electric Company. IN CANADA AND NEWFOUNDLAND - Western Electric Ltd. Northern Electric Company, - QUALITY COUNTS - 12 AUDIO ENGINEERING OCTOBER, 1948 MAGNETIC TAPE RECORDER

Or QRaIiLy

HAROLD LINDSAY* And MYRON STOLAROFF*

Design details of a new high -fidelity instrument for exacting applications.

- of editing, and re-use of the MAJOR EMPHASIS in design- obtained only under carefully simplicity have been amply covered in ing the Ampex magnetic -tape controlled laboratory conditions. medium, recorder has been on the pro- Overload Charactorlstk previous literature. duction of unsurpassed recordings Another advantage of tape record- Recorder Details of with reliability and continuity of per- ing results from the gentle overload The electronic system consists the power formance to satisfy the most rigid pro- characteristic. Unlike other methods four plug -in chassis units: for controlling fessional standards. Production model of recording, tape distortion increases supply, a relay chassis recording amplifier, 200A, shown in Fig. 1, has the follow- gradually as the signal level is in- all operations, the The in- ing capabilities: creased to the point of saturation. Be- and the playback amplifier. and the a. Full coverage of the audible spectrum cause of this, severe transient over- put of the recording amplifier amplifier are ( ±1 db from 30 to 15,000 cps). loads do not cause breakup and there- output of the playback b. Low distortion. From input to out- fore do not spoil the program. This 150 or 600 ohms. The whole chassis put terminals the system shows 4 per assembly is resiliently mounted. cent intermodulation distortion at peak feature allows considerable latitude in meter reading -with harmonic distor- setting up for recording and elimi- The power supply consists of a 360 - tion not exceeding S per cent 10 db nates the necessity for rigid monitor- volt plate -voltage source for the erase, above peak meter reading. bias, and playback output tubes, and range. The over-all ing of the input signal. c. Great dynamic a 300 -volt supply for the re- unweighted noise level of the system Other outstanding features of mag- regulated (measured flat from 30 to 15,000 cps) netic -tape recording -such as perma- cording and playback amplifiers, the is 60 db below full modulation, or 5 nency of record without deterioration, oscillator, and erase and bias screens. per cent harmonic distortion.

. The significance of the above statements is that reproduction on this purposes, A system is, for all practical substantially perfect. The use of separate recording and playback heads and amplifiers allows essentially instantaneous monitoring and makes possible extremely critical comparisons between recordings and original pro - grams -comparisons which cannot be readily made with other systems. Such a comparison -where the output of the compared with the recorder is directly Fig. I. Author Lind- material being recorded -has been given the term "A -B Test." The in- say takes some meas- coming signal is fed into the recorder. The monitoring amplifier is then urements on the Am- bridged alternately across the recorder pex tape recorder. input and output terminals. When live programs are recorded - using the best possible input equipment -and when monitoring with the highest -quality amplifier and speaker systems, critical listeners have not been able to determine which is the original program and which the reproduction. Comparable results with other methods of recording can generally be 'Engineers, Ampex Electric Corp., San Carlos, Calif.

AUDIO ENGINEERING OCTOBER, 1948 In addition, there is a d -c filament u ' 6AC7 higher measured dynamic range. ply for the playback amplifier, a .Ji; Therefore, the dip setting was aban- volt a -c output to provide -through a TO RECORDING doned. selenium rectifier -24 volts d -c for all EAD Playback Noise relays and selenoids, and filament supply for all tubes. he main problem on playback is to obtain best signal -to The essential problem in the record- -noise ratio. The maximum signal ing amplifier is to convert the signal obtainable from a given design of playback voltage into proportional current in bead is limited by capacitive loading the head. This is accomplished by the of the head at high frequencies. Therefore, circuit shown in Fig 2. A 6AC7 was the maximum number chosen for the output tube because of turns is placed on the head for which of its high mutual conductance. The loading 300 v O.0 does not occur within the desired fre- current output of this tube is further quency range. Because of the tremen- increased by a transformer. The sec- dous amplification required for the low ondary current is fed back through Fig. 2. Circuit of the output stage of frequencies, great care has been taken the cathode of the output tube. This the recording amplifier. Current feed- to eliminate hum pickup. The play- assures that the current through the back arrangement shown here contrib- back head is contained in two recording head is proportional to the utes to proportionality of head current. mu -metal shield cans with a copper signal voltage and is free of distortion. can in between, and the entire head Ample undistorted recording current is by proper design of heads and excellent placed in a cast -iron head housing. provided for any of the commonly - current waveforms. Matching caps cover the front of the available tapes under any desirable The actual setting of the bias is not playback head when the head gate is mode of operation. critical as the same undistorted output closed into the playing position. Direct The design of erase is and bias cir- obtained (4 per cent intermodula- current is used on tube filaments cuits is extremely critical tion distortion) to from the over a wide variation eliminate hum from this source. standpoint of achieving the lowest pos- in bias current once a definite minimum Wide -range frequency response is sible tape noise. With a properly - value is exceeded. Over -all sensitivity not a problem at 30 inches per sec., as designed system, the is reduced residual tape with higher bias, as it takes 15,000-cps response is readily obtained noise caused by erase and bias greater recording cur- current to supply at lower speeds. However, consider- rents will be below that of the same the play- output, but since the record- able equalization is necessary to back amplifier. It has been found ing that amplifier has ample capacity this achieve flat response because the volt- the quietest erasure is can be made by of no consequence. The bias is nor- age induced in the playback head placing the is entire roll of tape in a mally set slightly above that value giv- proportional to frequency and because powerful gradually- decreasing 60 -cycle ing maximum output for constant re- of demagnetization effects in field. the tape The erase and bias currents in cording input and thus is conveniently at shorter wavelengths. For maximum the recorder should not measurably set by ear. At lower tape speeds the signal -to -noise ratio, the recording cur- increase the noise level of the tape bias setting becomes more critical, as rent must be equalized to provide above that obtained by this type of high values of bias reduce the high - equal probability of overload at all fre- tank erasure. This result is achieved frequency response. quencies. The exact nature of this There is one bias setting which, at equalization requires careful study as first, seems very attractive. It usually the energy distribution of program ma- occurs at a value of bias just below terial may vary widely with different the current giving maximum response, types of pick -upe. This problem has and results in a dip in the over -all now been studied for a considerable intermodulation distortion. The set- time and it has been found that the ting is quite critical, but, for the low- frequency end can be boosted 5 db same limiting distortion, 8 -db higher at 50 cps with a 50-microsecond pre - output can be obtained. Because of emphasis on the high end without this apparent increase in dynamic overload under a wide variety of pro- range, the bias was always adjusted to gram material. The playback amplifier this value during the early stages of is equalized to provide flat over-all this development. However, it has response. since been discovered that this setting Taps Typos results in a poor overload characteristic. When A -B tests were made it Several varieties of tape are now was found that, at the point where available in this country. In general, differences between program and re- the lower -force tapes have proved most production could be detected, the out- satisfactory for broadcast applications put level was higher with the higher because they are easier to erase and bias current in spite of the fact that more quiet, and have lower modulation the critical setting resulted in 8-db noise. Tapes in this category are made by Audio Devices, Inc., and Min- nesota Mining & Mfg. Co. (Type RR). Fig. 3. View of units suspended from Type RR is outstanding because of its top plate shows rewind assembly, left uniform coating which results in low rear, capstan drive, center, and takeup modulation noise and extremely uni- assembly, right rear. Brakes can be form magnetic properties and output. seen below both turntable drives. In the latter property, this tape sur-

14 AUDIO E NGINEERING OCTOBER, 1948 passes samples obtained from Germany where this method of recording saw its early development. Higher coercive -force tapes, such as Minnesota Type A or Type B, have 10-db higher output for the same distortion but also have greater modulation noise and are difficult to maintain quiet. A great deal of research is being conducted to improve tape characteristics further and promising results have already been obtained. They indicate that the dynamic range of the medium will continually improve. Tape Speed Use of the 30 -in. speed warrants Fig. 4. Capstan drive includes resilient coupling, above motor, and crank - some justification. Very excellent re- actuated idler which presses tape against driving shaft, above top plate. sults have been obtained at half this Assembly is adjustable for tape alignment. speed. In fact, it is possible to obtain flat response to 15,000 cps with the e. Uniformity of frequency re- permits, in broadcast operations, the same intermodulation distortion and sponse. At extremely short wave- use of machines in close synchronism unweighted noise level as quoted for lengths the response of tape in pairs, to afford complete program the 30 -in. speed. To obtain 15,000 -cps varies considerably between rolls protection. response requires 15 -db additional gain and even within a single roll. That the handling of the necessarily at 10,000 cycles in the playback ampli- At 30 inches, the shortest wave- substantial quantities of tape required fier, which brings up the hiss level and length used is still long enough for high -quality performance might be harmonic distortion accordingly. The not to exhibit these inconsis- sufficiently convenient to satisfy practi- measured noise level is not increased tencies. This is proved by the cal requirements, very high rewind and because the noise in the system consists fact that all available tapes, fast -forward modes of operation have primarily of low- frequency components though varying greatly in mag- been provided. These speeds have been and at 30 inches per sec. the hiss level netic properties, give exactly the made as great as possible without ex- is more than 15 db below the compon- same frequency response with no ceeding the maximum safe operating ents which produce the noise reading. change in equalization at 30 tension for the tape during accelera- While it is still impossible to distin- inches per sec. tion and without reaching tempera- guish recorded material from the orig- d. Independence of bias setting. As tures when passing over tape guides inal in A -B tests at 15 inches per sec., explained above, the bias set- that would cause thermoplastic defor- the higher hiss can be detected on the ting is not critical and at 30 mation of the tape. reproduction during quiet passages inches high values of bias do not Total rewind time for 36 minutes of when monitoring at original orchestra affect the high -frequency re- program material is approximately 1 level. sponse. At shorter wave- minute, 45 sec. This same rate of Because of the higher noise level lengths the bias tends to erase speed is available as the fast- forward and harmonic distortions, it is not high frequencies and it is neces- mode of operation and is particularly practical to make re- recordings at the sary to set the bias more criti- useful where it is required to pick up lower speed as these deficiencies are cally. selections within a program. During doubled in the copy. Therefore, for e. Greater stability of motion. The these high -speed functions the tape the highest -quality recordings which 30 -inch speed contributes greatly will, at times in its transport from one can be re- recorded without detectable to the achievement of a steady reel to another, acquire velocities of deterioration, 30 inches per sec. is in- tape motion, resulting in unde- about 760 inches per sec. Stopping dicated as the best choice. tectable wow and flutter content the tape, reels, turnables, etc., when Further advanta:;es of the higher even in the most susceptible traveling at these speeds requires very speed are as follows: program material. effective and consistent brakes. They a. Extremely low hiss level. Al- f. Head alignment less critical. In are capable of making a stop from though the measured noise level order for recordings made on maximum reeling speed in 5 sec., or is the saine as at 15 inches per one machine to play back on an- from normal playing speed in 0.2 sec. sec., because of the ear charac- other without losing high - The drive system consists of a re- teristic at low levels, the noise frequency response, all the gaps movable supply and take -up reel, each background actually heard on of recording and playback heads mounted on a turntable carried on the programs is predominantly tube must be very critically aligned vertically- extending shafts of the re- and tape hiss. Therefore, at 30 perpendicular to the tape. The wind and takeup drive assemblies; a inches the "listening" dynamic higher the speed the less critical capstan -drive assembly with its tape - range is considerably greater. is this adjustment. At 30 inches, locking idler; a reel idler; and compen- b. Ease of editing. At 30 inches, there is little danger of heads be- sating tension arms. it is possible to remove syllables coming misaligned as they wear. On the German Magnetophon, tape from words or clicks and pops Mechanical Features was reeled onto plain hubs without from a recording without dis- A synchronous drive with very posi- side flanges and was thus removed turbing the actual program. This tive tape coupling characteristics gives from the equipment as a solidly - would be difficult or impossible playback time reproducible to 0.5 sec. packed self- supporting roll. In the de- at lower speeds. per half hour of program time. This sign of the Ampex machine it was

AUDIO ENGINEERING OCTOBER, 1948 15 necessary to take into account the fact portion of the spindle projecting with the differential action, provides a that the reels were to hold over 50 per through the reel hub and are pin - braking system allowing rapid stops cent more tape than Magnetophon keyed to the hub for manual reel- without tape being broken or snarled reels and would consequently be much ing of tape in editing and threading. from the throwing of slack loops. The less safe to handle as self -supporting Drives and Brakes spring-energized brakes offer consider- rolls. Accordingly, tape is handled on The rewind, or supply -reel assembly, able advantage here over the use of reels consisting of a plastic hub (4-in. located to the left rear of the top plate dynamic braking. Reels are not too free diem. by 3/8 -in. thick) mounted on of the recorder, is illustrated in Fig. to turn but offer some resistance to a single 14-in. diem. (0.050 -in. thick) 3. It consists of a specially- designed the removal of tape. This is particu- aluminum flange providing, when larly important when - vertically- mounted ballbearing two - the machines are wound to within 3/8 in. of full diam- phase capacitor -type induction motor used in remote operation where, at eter, storage space for 5400 ft. of 0.002 - with double shaft extensions and rest, the tape will not lose tension and in. thick recording tape-36 min. of flanged end bells. Electrical design of stall the machine through operation of continuous program time. the motor is 'such that the maximum the tape -runout switch associated with In operation on the recorder, these torque is developed at or near zero the takeup tension arm. reels are placed on turntables carrying speed to effect a uniform tape tension The function of the capstan -drive a central extended centering- spindle throughout the reel. The upper flange assembly shown in Fig. 4 is to main- and three equally- spaced drive pins. serves as a means of attachment to the tain constancy of tape speed during re- The centering spindle is somewhat supporting structure while the lower cording and playback operations. This longer than the drive pins to facilitate flange carries the brake assembly. is effected by the use of a synchronous centering and positioning of the reel. Brakes are spring -applied and sole- motor driving, through a mechanical The hub carries three equally- spaced noid- released external- band -and-drum filter system, a precision -ground cap- 5 /8 -in. diem. holes, each centered with type, the drum being carried on the stan shaft. This runs in precision respect to a corresponding flange- drive- lower motor shaft extension. The brake bronze sleeve bearings, grooved and pin (3/16) hole. A radially -placed design has been worked out to effect graphited and provided with a gen- tape -measure slot 332 -in. wide con- a differential braking ratio of approxi- erous lubricating system. A close- nects each of the 5 /8 -in. diam. hub mately 2:1, using the self- energizing coupled non -slip drive from capstan to tape is obtained by clamping the tape holes with the hub periphery. effect of band wrap. Rewind and take - between a rubber -tired ball- In threading, the tape is held be- up assemblies (which are substantially bearing idler and tween thumb and index finger to form identical otherwise) are arranged sq the capstan surface. The a short narrow lapp, the short end of their brakes are self -energizing in op- idler is carried on the end of a bell - crank arm which is next to the hub, and inserted posite directions of rotation and in and is solenoid operated. through a hub slot and over a drive such order that regardless of which Including synchronous motor, cap- pin to form a hitch. This threading direction the tape is traveling, the stan shaft and housing, capstan idler hitch is rapidly done and is self- freeing reel supplying the tape will always re- with bearings and linkages, solenoid, on runout. ceive the stronger braking action. motor capacitor, and terminal strip, the complete assembly While this type of reel does not re- Obviously the brakes must operate in is mounted on a common base plate. This quire a hold -down device. editing unison and this is assured by the sole- is suspended on the under side of the knobs are provided. They slip on the noid control, which when combined main top plate with spring -loaded spherical - cone joints arranged for critical adjustment of tape tracking. Fig. 5. Closeup of chassis units shows plug -in arrangement with leadscrew On either side of the centrally - and crank for insertion and removal. Entire base plate is shock and located head housing are the tension vibration- mounted. arms. The one on the left is associated with the reel idler while the right -hand arm is combined with a cutoff switch. The spring -loaded tension arms serve to momentarily equalize sudden changes in tension during starting or stopping and the lesser fluctuations resulting from the passage of a splice. They also preserve tension on the tape while the equipment is at rest. Maintenance Ease of servicing and maintenance :is been provided in the arrangement the functionally -styled front- access- > pe console. Further, the general de- uzn facilitates simple removal of units ,,r replacement by spares. All chassis units are withdrawn from their plug- in connections and replaced by the combined action of a special leadscrew and crank. The complete head housing is also of plug -in design -held in place by a pair of crank- operated screws. Intermodulation and Harmonic Distortion Measurements

J. AVINS*

order intermodula- Ir has long been recognized that of convenience and convention as to alyzer, the second 800 cps cannot be non -linear elements may produce whether harmonic distortion or inter- tion component at Measurement of the considerable distortion which can- modulation distortion is used as a readily measured. compon- not be correctly evaluatd by harmonic measure of the system non -linearity. second order intermodulation where distortion measurements. This distor- For reasons which are described be- ent is particularly necessary A tion takes the form of cross -modulation low, intermodulation-distortion meas- the non-linearity is symmetrical. method is shown or intermodulation of the component urements are now being used more block diagram of this f frequencies in the signal, and as a re- extensively than heretofore to supple- in Fig. 2 sult new sum and difference frequencies ment harmonic distortion measure- Amplification Independent of are produced which impair the quality ments and in some instances to sup- Frequency of reproduction. plant them. The simplest type of non-linear Both harmonic distortion and inter- At the present time there are two system is one in which the non- linear- modulation distortion are caused by principal methods used to measure in- ity is the same at all frequencies, i.e., the same non -linear elements. When termodulation distortion. These may the transfer characteristic is inde- a system has a characteristic which be referred to conveniently as the pendent of frequency. For this con- does not vary with frequency, there- "modulation method" and the "wave - dition Warren and Hewlett1 have de- fore, the degree of non -linearity or the analyzer method ". monstrated analytically and experi- distortion can be measured by either In the modulation method, a low mentally that the system distortion the harmonic -distortion or the inter- audio frequency fi of the order of 100 may be expressed in terms of either modulation-distortion rdethod with cps and a high frequency fz of the harmonic distortion or intermodula- substantially identical results. In the order of 4,000 cps are applied simul- tion distortion. Depending upon the more usual case when the amplifica- taneously to the system. The amount type of non -linearity- whether it is tion varies with frequency, the har- of intermodulation distortion is then representative of a single -ended ampli- monic method may not provide a observed by noting the extent to which fier characteristic, a symmetrical push - correct measure of distortion. Here the high -frequency signal has its out- pull amplifier characteristic, etc., the the intermodulation method is the put amplitude modulated by the low - distortion as measured by harmonic more flexible means for measuring frequency signal. As shown in the distortion will be approximately one- the distortion present. Its very flexi- block diagram, Fig. 1, this measure- third the distortion as indicated by bility, however, leads to difficulty in ment can be made by using a band - intermodulation distortion. Thus, interpreting the results obtained, since pass filter to separate the high -fre- there is a good correlation between these results are a function of the quency wave and its sidebands in the the degree of non -linearity as expressed choice of frequencies, their relative am- output, rectifying them, and observing by either harmonic or intermodulation plitudes, etc., and to date no satisfac- the amount of low- frequency modula- distortion, and it is a matter of con- tory standards exist which are gen- tion in the rectified output. The per- venience and convention as to which erally applicable. cent intermodulation distortion is usu- measure is used. Harmonic and Intermodulation ally referred to the upper frequency Effect of Frequency Characteristic Distortion f2 which commonly has one -fourth the When the non -linearity of the sys- If a single frequency is applied to amplitude of the lower frequency fi. tem is a function of frequency, there a non -linear system, the output con- In the wave analyzer method of is no longer a correlation between the tains distortion components at the har- measuring intermodulation distortion, distortion as measured by harmonic monic frequencies. If, on the other measurements are made at a number distortion and as measured by inter- hand, two signals of different fre- of points throughout the audio-fre- modulation distortion. The principal quency are applied simultaneously, quency range. The amplitude of fi reason for the lack of correlation is then the output will contain new fre- and f2 may be equal and the frequen- that if we attempt to measure the quency components which are due to cies may be varied in such a way that non -linearity at a particular frequency intermodulation of the two frequen- the difference frequency f2-fl is held in terms of the harmonics which are cies. The intermodulation components constant. The relative amplitude of produced, the harmonics may be at- produced have frequencies equal to the the intermodulation component at the tenuated (or amplified) with respect to sum and differences of integral mult- difference frequency f2-fi is then con- the fundamental as a result of the fre- iples of the original frequencies. Since sidered a measure of the intermodu- quency characteristic, and therefore non -linearity produces harmonic dis- lation distortion. Where a wave an- the harmonic distortion terms in the tortion as well as intermodulation alyzer is not available, the difference 1W. J. Warren and W. R. Hewlett, "An to be a matter be equal to 400 distortion, it appears frequency may chosen analysis of distortion methods by the In- cps to permit the use of standard Vol. *RCA License Laboratories, 711 5th Ave., termodulation method ", Proc. I.R.E., New York City. filters. However, without a wave an- 36, pp 457 -466, April 1948.

AUDIO ENGINEERING OCTOBER, 1948 17 output do not represent accurately the two middle frequencies is usually flat, ments inadequate. For example, in distortion or non -linearity at the fun- no correction is required. circuits using tone controls, noise sup- damental frequency. Effect of Pro -emphasis pression circuits, filters, or crossover In general, the intermodulation dis- Measurement of the over -all distor- networks, non -linearity may be present tortion method is more flexible and tion in frequency -modulation receivers and the conditions may be such that can be used to provide a measure of is indicative of the problems which when testing by harmonic distortion the non -linearity even when a frequen- occur where the transfer characteristic methods the harmonics fall outside the cy characteristic is present. Thus the is a function of frequency. If, for passband. Under these conditions, it non -linearity may be explored by ap- example, distortion takes place before is clearly difficult to correlate harmonic plying two appropriately chosen input de- emphasis (for example, in the fre- distortion measurements with inter - signals one or both of which are in the quency discriminator), the intermodu- modulation distortion measurements vicinity of the frequency at which the lation components produced by high - on the basis of the approximately 3 to 1 non -linearity is being measured. Clear- frequency terms will be effectively em- ratio which obtains when the transfer ly, the figures obtained by thus measur- phasized with respect to the terms characteristic is independent of fre- ing the distortion cannot be interpreted causing the distortion. Thus, if two ghency. readily on an absolute basis because of frequencies at 8,00 and 8,400 cps are The special case where an amplifier their dependence on several parameters, applied, intermodulation components is divided into two or more frequency principally the frequencies used and at 400 and 800 cps are produced. These channels requires comment. For ex- thir relative amplitudes. are not de- emphasized, but the 8,000 ample, consider an amplifier having a Limitation of Harmonic Distortion and 8,400 cps components are. It fol- "bass channel" extending to 1,000 cps The following example will show a lows that the intermodulation distor- and a "treble channel" handling fre- typical case in which harmonic distortion will be 12 db higher than if the quencies above 1,000 cps. It will be tion measurement fails to evaluate same distortion occurred following de- assumed that non -linearity is present properly the non- linearity present. emphasis. in both channels. If the modulation Consider a radio receiver in which the In the above example, the modula- method is used to check intermodula- second- detector design is such that tion method using a low frequency and tion distortion, with applied frequen- high distortion is produced at high a high frequency .to modulate the sig- cies at, say, 100 and 4,000 cps, it is modulation percentage at frequencies nal generator would not yield results clear that no intermodulation will be above 2,500 cps. The cutoff frequency which are as direct as those obtained observed. In actual operation, how- of the audio amplifier will be assumed by the wave -analyzer method. In par- ever, intermodulation of the frequen- to be 4,000 cps. Under these condi- ticular, it cannot be assumed here that cies within each channel will be pre- tions, the second and higher harmonics the high -frequency component is mod- sent, and hence, the modulation method are outside the passband and therefore ulated by the distortion present at low gives a misleading result. harmonic distortion measurements, frequencies. On the other hand, the wave ana- which must be made with the funda- Low -Frequency Distortion lyzer method will give a useful indi- mental above 2,500 cps, will not reveal Consider an example of low- frequen- cation so long as the two exploring fre- the distortion in the system. Never- cy distortion such as may be produced quencies are chosen properly, so that theless, in operation, distortion is pre- by an output transformer at high le- these frequencies and the resulting in- sent as revealed by the presence of vels. For this case it is clear that termodulation components lie in the intermodulation terms in the output, either harmonic distortion measure- channel under test. extraneous frequencies which-unlike ments made with the fundamental in Conclusion the harmonic distortion components- the non -linear range or intermodula- The extent to which a particular in- lie within the receiver passband. tion distortion measurements with at termodulation distortion measurement If an intermodulation least one of the frequencies in the non- l distortion test will reveal the existence of distortion 1 is made in this receiver using the wave linear range will serve as a measure is determined by the frequencies used, analyzer method, the distortion will of the distortion. The choice as to their relative amplitudes, the number show up readily if two frequencies of which method is to be used here will or order of the intermodulation dis- equal amplitudes at 3,000 and 3;400 depend upon the equipment available tortion components which are meas- cps are used to modulate the signal and the current practice. ured, and the variation in the trans- generator. The distortion then is in- Sources of Error fer characteristic with frequency. dicated in terms of the first order in- In general, there will be many in- In general, the measuring of inter - termodulation component at 400 cps stances where the amplification varies modulation distortion by exploring the and the second order component at with frequency in such a way as to spectrum with two signals which are 800 cps. Since the response at these make harmonic distortion measure- [Continued on page 55]

Fig. I (left). Setup for modulation method of measuring intermodulation distortion. and Fig. 2 (right), same for analyzer method.

LOW - FREQUENCY OSCILLATOR f, ( 100 Cydes) OSCILLATOR AMPLIFIER f, UNDER TEST HIGH -FREQUENCY WAVE OSCILLATOR AMPLIFIER ANALYZER f, (4000 Cycles ) UNDER TEST (f, -f.). x(1, -10 OSCILLATOR LOW- PASS HIGH -PASS DEMOD- INTERMOD. FILTER FILTER -- DISTORTION fr ULATOR METER (i5óó Col,.) (C50Ó Cycles)

18 AUDIO ENGINEERING OCTOBER, 1948 Simple Method of Determining Internal Resistance

WALTHER RICHTER*

How to find the internal resistance of an audio amplifier or similar apparatus.

AN AMPLIFIER can be consid- it should be reduced to a minimum. amplifier gives results inferior to that

re- Bred as a generator, or source of During the time that the loudspeaker of a triode amplifier; the plate electric power, the output voltage cone describes this undesired oscilla- sistance of a pentode tube is very high, (or current) of which is at any given tory movement, a voltage is generated which causes the voice coil to `look instant proportional to the value of in the voice coil Because the latter back" into practically an open circuit. the signal voltage applied to the input moves in a strong magnetic field, the A conventional triode circuit generally terminals of the amplifier. In the voltage will produce a current which, is designed so that the load resistance early days of radio and amplifier de- according to Lenz's law, will be in such is equal to twice the plate resistance sign, it was generally believed that a direction as to oppose the movement. of the output tube; consequently, in two amplifiers capable of furnishing This current, therefore, represents a such a stage the load is `looking back" the same amount of audio power over force tending to damp out the un- into a resistance approximately one- the same frequency range, when ex- wanted movement. The amount of half of the load resistance itself. This cited from an audio frequency gen- current which will flow due to this means that an 8 -ohm speaker, operated erator, would produce identical re- voltage generated in the moving voice from an amplifier with triodes in the sults so far as reproduction of music coil will depend on the resistance of output stage, and connected to the B- was concerned. But listening tests the circuit in which it is flowing. It ohm tap of the output transformer, showed that although two amplifiers is evident that the internal resistance will `look back" into a resistance of may have identical frequency response of the source is part of the total re- approximately 4 ohms. Compare this curves and identical power output, with the condition prevailing for a they may still produce different tonal pentode, for which the manufacturer results, one of them giving muddy re- recommends, for instance, a load reproduction while the other one gives sistance of 5,000 ohms while the plate crisp and clear results. The most im- resistance is approximately 50,000 portant reason for this phenomenon i, ohms. In such a case, the 8 -ohm the internal resistance of the ampli speaker would be "looking back" into fier. a resistance of approximately 80 ohms, Hangover instead of 4 ohms! When a generator furnishes electric When negative feedback came to be recognized as a powerful tool of im- power to a purely resistive load, the Fig. I. Equivalent circuit of an performance, an ana- internal resistance of the generator - amplifier. proving amplifier apart from the possible influence on lysis of it showed that one of its ad- its efficiency- is of no importance. sistance, and we can therefore expect vantages was the apparent reduction When, for instance, the generator sud- that an amplifier with a low internal of internal resistance of an amplifier denly ceases to produce a voltage, the resistance will give us better damping employing it. By employing negative current in the resistive load will drop than an amplifier with high internal feedback, it is quite possible to reduce suddenly to zero. But when such a resistance. (For oscillograms showing the internal resistance of an amplifier source of electric power is operating this effect, see: Olson "Elements Of using pentodes to a value near or even a loudspeaker, the situation is consid- Acoustical Engineering ", Second Edi- below that shown by triodes. (The erably more complicated. Upon sud- tion, pages 162 -163.) author definitely does not wish to en- den cessation of the current through, The loudspeaker is connected to the ter the presently raging controversy or the voltage across, the voice coil amplifier through the usual output between pentodes versus triodes in the of the loudspeaker while the cone is transformer. The primary of this output stage of an amplifier; he does in motion, the cone will not instantly transformer is connected to the plate not wish to state that an amplifier come to rest, but will make a few os- circuits of the output tubes. The with, pentodes in the output stage, and cillations which evidently will not damping current will therefore be de- employing sufficient feedback to re- represent a reproduction of the input termined by the magnitude of the duce its apparent internal resistance signal to the terminals of the ampli- plate resistance of the output tubes re- to a value near that of a triode -oper- fier. This effect is known as the flected into the voice coil circuit in ated stage, will produce identical tonal "hangover". Because it is undesirable, the ratio of the square of the turns - results. He wishes only to state that ratio of the output transformer. This without such feedback, the pentode 'Electronics Section Allis -Chalmers A Jg. Co., not have a chance Box 512, Milwaukee, Wis. is the reason why a straight pentode amplifier will even

AUDIO ENGINEERING OCTOBER, 1948 19 to give results comparable to that of (2) R(i2-il) = Rii1R2i2 = et -e2 According to this equation, we connect a triode- Aperated amplifier.) In few which results in a resistive load Rs, preferably equal cases áoà the manufacturer See fit to (3) R = el e2 to or a little larger than the rated give the value of the internal impe- i2 - it e2 /R2 - el/RI value, to the amplifier and observe the The dance of an amplifier, or the amount internal resistance of the gene- voltage existing across it, and then rator can therefore be of feedback employed. Even if the obtained by reduce the load in the ratio p. The simply feedback were given, few of us are dividing the change of voltage voltage across the reduced load will occurring when resistance sufficiently acquainted with the réhi- the of the also be lower, and the internal resist- load is changed tions between it and the reduction of by the change in cur- ance R will then be given by equation rent place output impedance so that we could taking due to this change 5. in load. For the determination of R, readily calculate the apparent output Levei it is therefore only necessary to apply impedance. It is, however, an easy The measurements are a constant signal of suitable frequency preferably matter to determine the actual out- taken at reduced in to the input terminals of the amplifier output, order to put impedance of a given amplifier by be sure no overloading and to observe the two output volt- that takes place. means of a simple measurement, the As an example, let it be required to ages es and e2 resulting when the load basis of which is developed in the find the internal resistance of an am- resistance is made equal to Rs and R2. following: plifier supposed to furnish 10 watts of The result expressed in equation (3) Referring to Fig. 1, a generator with power to a speaker with a 20-ohm voice can be brought into a somewhat dif- an internal resistance R furnishes coil. According to the formula P = ferent form, as follows: Let us as- power to a load with a resistance Rt.. E2 /R, a voltage of 14.1 volts across sume that we make the load resistance Let E be the internally produced volt- 20 ohms would produce rated output. at first and then reduce it to a age of the generator, which in the case Rs, We replace the speaker with a 20-ohm value. R2 R1, where p is a number voltage applied to its grid. We now = p resistance, and adjust the signal ap- less than unity. Due to the reduction of a vacuum tube would be Inc the plied to the amplifier, so that the volt- resistance from R1 to p R1, let the load RL assume two values of of load age appearing across the 20-ohm load the load -voltage el will be smaller than resistance, namely R1 and R2. Let will be well below 14.1 volts, for in- the voltage es. Let this also be ex- the current through, and the voltage stance, 5 volts. We now must reduce factor" in across the load, when the load has the pressed by a "reduction the load resistance to a new value; other words, let e2 q et. If R2 and value R1, be it and es respectively, = this can be accomplished most con- e2 are introduced into equation (3), whereas current and voltage in the veniently by simply placing an addi- we obtain case of RL = R2 be ie and e2. Since tional resistance in parallel to the 20 in both cases the current multiplied (4) , lr-q) ohms. If we make this parallel resist- by the total resistance in the circuit, _., /s, f Dp, ance also equal to 20 ohms, the new i.e. (R+RL) must equal E, we have load resistance R2 e, (t -q) I -q represented by the (1) E it (R i2 -1-R2) R, % = -4-R1) = (R %( h_, two 20 -ohm resistors in parallel, will From this follows R, [Continued on page 53]

audio Enciinzaing S/,rief San 3hanciAco SECtiOn

Nearly 100 members of the newly formed San Francisco Section, Audio Engineering Society, attended the August meeting when they made a comparison test of twenty types of commercial loudspeakers. i ) Side view of AM tuner.

[IQ»LQnJ L RESIDENCE RADIO SYSTEMS

C. G. McPROUD* PART II

Continuing the description of units suitable for a high -quality Residence Radio installation.

AN ideal AM tuner for a Residence choices. A still further application involved, and it is always ready for Radio System may naturally take for the fixed tuner is for recording instant service simply by switching one of many forms, hut the qual- studios which need high -quality re- its output to the recording or audio ity enthusiast is most certainly likely ceivers for off -the-air recordings. A amplifier. The principal disadvan- to insist on a t -r -f tuner to ensure com- separate tuner may be set up for each tages are the cost and the space re- plete listening satisfaction. Wide station frequently recorded, with the quired, but the design shown reduces range t -r -f tuners are complex instru- assurance that the audio signal i= the cost considerably, and the units ments, and while it is not difficult to optimum for each. are small enough that a six -station as- design such a tuner, the execution of A single station tuner has at least sembly can be mounted behind a sin- the design may be somewhat more in- two advantages over a conventional gle 54-inch rack panel with ease. For volved. To be sure, several such tun- receiver. It may be tuned carefully recording studios which must record ers are available commercially, either and accurately, with the band -width several programs simultaneously, the as complete units or in kit form. set to the best value for the conditions strip -tuner arrangement is actually However, for the system covered by less expensive, since the conventional com- this series, a different arrangement arrangement would require a was desired. plete receiver for each recording chan- The reasons for the development of nel, which actually duplicates the fa- the fixed- station tuner units to be de- cilities. For example, ten strip tuners scribed are several. Primarily, the will provide service from ten stations particular installation planned by the to any number of recording machines writer required that four AM stations at greatly reduced cost over an equal be accommodated, and that they be number of conventional receivers selected by push buttons which actu- which would be necessary to record ated relays. Additionally, fixed sta- the ten separate stations simultane- tion tuners permit a large residence ously. installation to be made with provision The quality requirements, together for feeding an audio signal from each with those of simplicity, dictate the desired station throughout a home, use of a t -r -f circuit for the single - with a selector switch and an individ- Fig. I. Two methods of obtaining station tuner, and an arrangement usual amplifier and speaker at each lis- band -pass coupling by the use of two ing two r -f stages and an infinite -im- tening point. This allows a number tuned circuits and an impedance corn - pedance detector serves adequately for of listeners to choose their own pro- mon fo both. (A). High- impedance the purpose. Because no more than gram fare without disturbing others' capacitance coupling; (B). Low- imped- one station is to be received at any time, no audio stage is included, and *Managing Editor, AUDIO ENGINEERING. ance capacitance coupling

AUDIO ENGINEERING OCTOBER, 1948 21 series resonant circuit is used for tuning the input, and the peaks of the over -coupled circuits are flattened out appreciably. The tuning, circuits are seen to consist of a peaked series resonant circuit ahead of the first tube, a band -pass circuit between the two r -f stages, and another band -pass circuit between the second r -f stage and the detector. Station Swkehing The method used to select the de- Fig. 2 (left). Curve resulting from over -coupled circuits of Fig. I. The usable sired channel is quite simple, involv- band width is represented by the limits ', 2 -sf. The valley between the peaks ing only the switching of the plate can be filled by using a single -tuned circuit in conjunction with the double - supply. This is made possible by us- tuned circuits. Fig. 3 (right). Method of coupling the antenna to each of the ing an infinite -impedance detector for tuner strips. This enables tuning the antenna circuit for each station and each channel, and making a part of provides adequate signal input to the first stage. the cathode -ground resistance of each common to all the switching from station to station detector channels, as For the circuit of (B) in Fig. 1, K shown in Fig. 4. Then, when is done by switching the plate supply the is approximately equal to C. /C., so plate voltage is switched, the from unit to unit, as will be described. detector C. becomes considerably larger, and in the active channel passes This method eliminates complicated current, thus easier to adjust easily. This with switching arrangements, and functions the common portion of the ca- type of coupling is known as low -im- thode- ground resistor to perfectly. For good quality, the in- serving de- pedance capacitance coupling, and is velop the signal voltage. This voltage terstage coupling is band -pass, using employed in the tuner strips. is then fed to the audio low- impedance capacitance coupling. amplifier. The circuits of Fig. 1 represent two methods of band -pass coupling, using capacitance for the common imped- Fig. 4. Switching ance. The tuned circuits are both ad- between channels justed to the frequency of the desired is accomplished station, and the coupling capacitance by simple switch- C. adjusted for optimum -or the ing of plate sup- maximum usable -band width. Ac- ply. Since Rio is tually, with this form of coupling, a common to all change in the value of Cr. causes the channels, the a -f two resonant peaks to move apart, but output is devel- not equally from the previous single oped across it for peak. However, they may be set pro- the one channel perly during alignment, and no which is provided trouble arises from this effect. with plate supply. The response ourve of either circuit of Fig. 1 is shown in Fig. 2, with of being the separation between the peaks. The usable band width can be shown In order to reduce the effect of the Since the value chosen for the com- to equal V2 Af, and of is approxi- valley between the two peaks, a single mon section is 18,000 ohms, while the mately equal to Kfo, where K is the co- tuned circuit is employed for the an- remaining section is 82,000 ohms, the efficient of coupling. For the circuit of tenna coupling, as shown in Fig. 3. shunting effect of the remaining chan- (A) in Fig. 1, K is roughly equal to The coils used have adjustable cores, nels is negligible. Holding relays of C./Cs when the two inductances have and the tuning capacitances are fixed. the type described in Part I are used equal values of Q. Since a 20 -kc band Since a number of tuners are to re- to switch the plate supply in the four - width at 1,000 kc gives a Af of 20/V2, main connected to the antenna at all channel unit being used in the com- or 14.14 kc, K becomes 14.14/1,000 or times, the coupling to the antenna is plete system. .014, and C. is a relatively small value such as to load the circuit only at the The audio output level is fixed with of capacitance when practical values of frequency of operation, rather than respect to the detector, and since no L and C. are used. throughout the entire band. Thus a ave is used, the gain is adjusted by

Bottom view of AM tuner.

AUDIO ENGINEERING OCTOBER, 1948 given in the text. Table I, and Fig. 6. Fig. 5. Over -all schematic for the AM tuner strip. Values for C. and Cs' are chassis since only one varying the bias on the r -f stages. tuning capacitances provide a high LC conventional time, and there is Each r -f stage has an isolating mini- ratio, acid the gain is fairly constant operates at a given of interaction between mum bias resistor, with the common for all the bands. The approximate no chance section being selected for suitable sig- values for C. for a 20 -kc band width them. nal to the detector. are shown on the curves of Fig. 0, and Adjustment alignment almost de- Construction for convenience in adjusting, it is Satisfactory use of a frequency modu- Convenient mounting of a number suggested that C. be composed of a mands the an oscilloscope if of strip tuners demands that they be fixed mica capacitor of suitable value lated oscillator and µµf of band width and reasonably small, with a 23/4x10 in. paralleled with a 320 -to -1,000 proper adjustment mica padder. This will permit a rea- correct flat- topping of the response sonable adjustment of band width. curve is to be obtained. The second TABLE I The coil shields are 1 -3 in. square coupled pair of coils should be aligned Turns of wire to be removed for to the grid of various ranges by 2 -34 in. high, and are sufficiently first, feeding the signal Freq. C. Turns large to allow for good performance the second r -f stage. After the de- Range µµf Removed from the coils. The construction is sired response is obtained, shift the 550 -800 50 0 straightforward, and after laying out oscillator to the antenna terminal and 800-1,000 50 40 two r -f 960 -1,175 50 70 the chassis, should offer no difficulties. align the coils between the 1,075 -1,300 50 90 There is no reason why several such stages. For convenience, a potentio- 1,300 -1,550 25 80 tuners should not be built on a single (Continued on page 48] 1,500 -1,700 25 100 5 chassis being about the minimum for 1111111 the necessary parts. Five coil shields I I with space for := are required, together .0045 owl mi. mi. ism three tubes and a filter capacitor for I. ow. plate supply. The neces- t-----I---Ism-NNE INN Elm win Eno ' the detector ----IME MM sary bypass capacitors are in two in- Fig. 6. .o04 -----II----BlBl84581--I/- EMI verted cans, each containing three Curve 0.1 -mf sections. showing ó ------ó - The complete schematic is shown in values of -I---- - 5. For the r -f coils, OTC' per- M .0035 ----r----EMI MIN NMI IOW B-BlO Fig. C. for I 1 NB------meability -tuned coils type LSM are - 20 -kc band --- =M=11tl_ E --MIN 11MI------several _ made in / used. These coils are width with 0 --MIM MI MI IM IF inductance values, the 1-mc coils be- I .003 -om.---- the coils IMMI - =====rirmrigi ing used for this application. Since recom- z B) , the inductance variation is not large, . mended, MI ME ------/NMI must be removed from the coils MI ENV `2 turns and the d.0025 Go - to make them suitable for the ranges 8181I------MIPS------IMI= MO PidE NMI NM values ii WAS MM OM - number MIM IMO in the broadcast band. The given for z fS /MI ---MIM are shown in I. ---- of turns to be removed J A=------S- .002 NMI Table I for use with a 50-µ4 capaci- II------MUM --IMO o 1 o -- tor for Co from 550 to 1,300 kc, and NMI 25 -µµf capacitors for Co for frequen------_ .0015 1,300 ko. The relative small -__--Bl------SIMI L3 1.4 1.5 1.6 L cies above .55 b .7 .6 .9 1.0 1.1 12 1Cambridge Thermionic Corp., 445 Concord FREQUENCY - MEGACYCLES Ave., Cambridge 39, Mass. AUDIO ENGINEERING OCTOBER, 1948 23 High Quality Speaker Enclosure

HERBERT G. EIDSON, JR.*

Constructional data for an excellent speaker cabinet. Author with complete cabinet.

THE following speaker cabinet was by too small an air chamber, no re- cations, from a local lumber company. conceived after several commercial radiation of the sound by the bailie or The sections follow: types and one home -made one were speaker cabinet, or speaker diaphragm, One Front 30x23 inches found lacking in naturalness of speech and no false bass emphasis created by One Back 291Ax22 inches reproduction and low -frequency res- resonant sound chambers. One ponse of musical reproductions after One company states, in relation to Bottom 22x16 inches several months of listening tests. In one of their main line 12" speakerat One Top 23x12% inches the writer's opinion, a cabinet which "Enclosure required: three cubic feet." Two Sides 2954x(16% -1234) ins. encloses an excellent speaker should The cabinet was designed with this Associated Material Needed: serve one main purpose: To allow the figure in mind, for it is felt there is 1 Speaker baffle (made of inch Celo- speaker to operate as though it were little to be gained by making the en- tex, 19x27 inches) ' mounted in an infinite baffle. In this closure larger and a bulky appearance 2 Wood Strips 27x1 3/4x3/4 inches. way, the full true quality of the speak- would then result. 2 Wood Strips 19x1 3/4x3/4 inches. er may be realized, with no restrictive The outer enclosure is made of one- 6 Welding rods (brass) 36x3/16 inches. 1 Grill cloth (silver and black) 19x25 dampening of the rear side of the cone half inch pine plywood and all sections inches. were purchased already cut to 2 Back molding 'Chid Engineer, WIS, WIS -FM, specifi- strips 163/4 inches. Columbia, 2 Back S. C. 1Western Electric 728 -B molding strips 223 inches. Kimsul sound -proofing material. (Enough to cover 13 square feet.) 2 Base boards 11 3/4x3 3/4x3/4 inches. 2 Base boards 19x3 3/4x3/4 inches. 15' Wood stripping one inch square. 6 Sheets #1 flint paper. 6 Sheets # 180 -C carborundum waterproof paper. 1 Box fine steel wool. 2 Two inch paint brushes. 1 Pint turpentine. 1* Small can wood putty. Is % pint can paste wood filler, trans- parent. 1* Pint enamel paint, medium gray. 1. % pint enamel paint, black. 1' % pint satin finish varnish. 1' % pint clear varnish. Sherwin- Williams products.

Above: Front, side and back dimensional drawings of speaker cabinet. Right: Detail drawings of outer parís of cabinet.

24 AUDIO ENGINEERING OCTOBER, 1948 The cost of construction of this Allow one full day for drying. Now speaker cabinet is much lese than a sand lightly with fine grade paper, go good commercial cabinet and it is felt over surface with moist cloth, let dry that performance is much better. The and apply second coat of gray enamel. local lumber company's bill for lumber When this is dry, work well with fine and cutting cabinet pieces to size was steel wool, wipe with moist, clean cloth, $16.00. If the plywood were purchased let dry and apply coat of satin finish in one sheet and cut with a hand saw, varnish with new, clean brush. This the price would be about $8.00. The last coat will tone down the lustre pro- rest of the material listed cost approxi- duced by the enamel and give the cab- mately $10.00. So it is possible that inet a finished look. Above: Cabinets with front and rear the cost could be held to $18.00 or less. During the painting and drying portions removed. Enclosures for 8 Construction cycles of the cabinet, the base should and I 2-inch speaker units are shown. All edges of the different sections be treated with wood filler as the cab- For best results, speakers should be are thoroughly sanded down with #1 Met has been and given two coats of- especially designed for operation in sandpaper. The front and top pieces black enamel, lightly sanding after the .4,1 this type of cabinet. are first nailed together, using finish- first coat. The enamel should be thin- ing nails 144" long. Care should be ned with turpentine in the same man- exercised in fitting the two to form a ner as the gray enamel. smooth joint. The two sides are nailed The four back molding strips should on next, then the bottom. One-inch receive the same treatment as the cab- square reinforcing stripe are nailed in inet. It will be noted in the picture the inside top and bottom sections that their ends are cut at 45° to in- (across back end) for back to fasten sure neat and tight fitting. to (see photograph of interior of speak- If a medium -dark golden oak finish er enclosure). Also the same type is desired rather than the gray enamel stripe are nailed in the inside of the finish, purchase the following: cabinet at junction of sections for 1 Pint Golden Oak Stain further rigidity. (Sherwin- 11'iItiams ) 1 Pint clear varnish The four boards that form the base (Sherwin-Williams) are nailed together and then centered After the cabinet has been sanded with the bottom of the cabinet and (with all nail holes and cracks filled nailed into place. with wood putty), brush wood filler Using a six -penny nail as a punch, carefully on duly the exposed edges of drive all finishing nail heads below the plywood and wipe as previously de- surface of the wood about inch. riá scribed. After twenty -four hours, brush Above: Interior of speaker enclosure, Mix about three tablespoons of wood on golden -oak stain, being careful that showing I2 -inch Western Electric loud- putty and fill in all holes left by the stain does not run. After twelve hour. speaker mounted in place. above operation. Fill in all cracks that go over lightly with fine steel wool, may have been left by improper fitting wipe with moist cloth, allow to dry of sections. After twenty -four hours, and apply second coat of stain. When to allow for drying, sand cabinet thor- dry, go over again with steel wool and oughly with #1 sandpaper and finish moist cloth, let dry and apply two coats with fine sandpaper. of clear varnish, using steel wool and Preparing For Painting cloth after each. The final coat is Stir wood filler so as to mix well that of satin finish varnish, applied and add turpentine if necessary to ob- with new, clean two -inch brush. The tain a consistancy of thick cream. base and back molding is given the Using two -inch brush, apply wood fil- same treatment as the cabinet. ler over all outside surfaces, brushing Final Details across grain at all times. After five Although the rods which protect the minutes, to allow for absorption, rub front grill need be only 213 inches with large clean cloth across grain, long, the standard length welding rod thus removing all excess wood filler. is 36 inches, so each brass rod will have Beware of wood fillers that require no to be cut to it's proper length. The rubbing! They will not give the de- rods are made clean and bright by R. inting the enclos- sired results. Allow filler twenty -four Above: speaker rubbing with fine steel wool, then in the text. hours to dry. Now, using fine steel ure. Details are given dipped into clear varnish so that oxi- Back and new wool, go over the outside lightly, leav- Below: molding strips dation will not dull this lustre. Dip- Western Electric 755 -A 8 -inch speaker, ing a smooth, flat surface. ping was done in the following man- designed especially for enclosure of Painting and Finishing ner: this type. Stir new can of grey enamel well. A bumedout, 40 -watt fluorescent tube was Thin with 1/5 volume turpentine. This cut in half by applying nearly one turn will allow paint to flow more freely. of #20 Nichrome wire around its half- Apply one coat, using long even strokes way point. Six volts with high current was applied to the iron wire, and after with the grain of the wood. When being red -hot for about thirty seconds, this cabinet is covered, stroke the edges point of the tube was dipped in cold water, over, to be certain no paint is running. (Continued on page 52]

AUDIO ENGINEERING OCTOBER, 1948 25 RCA 15 -watt

DE LUXE AMPLIFIER MI -12295

You've never seen a value like this lar low output magnetic pickups. Phase distinctively styled RCA 15 -watt All - inversion and inverse feedback used to Purpose Amplifier. You can't beat it for obtain low distortion and uniform fre- high quality performance, compactness, quency response for various load condi- beauty, and simplicity of operation tions. Unique balanced tandem tone under all sorts of operating conditions. control circuit. Choke filtered power supply for minimum hum and uniform Electronically engineered to give top regulation. Large functionally designed performance. Frequency response (30 control knobs. Accessible output termi- 15 watt push -pull Output- to 15,000 cycles). Two high impedance nal board and replaceable cartridge ' Noise -free electronic mixing - microphone input channels 116 db. gain type fuse conveniently located in rear. Switch for high -low power (RMA), each with individual volume Set of locking -type 3 pin Cannon plugs output requirements - Under- control. Two hi- impedance phono chan- and receptacles for each microphone writers- approved. nels, one for hi -level and one for low - input. Also available with 2 low imped- level pickups. Can be used with popu- ance inputs (250 ohms).

RCA 50 -watt De Luxe Hi- Impedance Amplifier MI -12293 Additional Unparalleled Complete with RCA Tubes Now only 197'° Amplifier Values RCA 30 -watt De Luxe Hi- Impedance Amplifier MI -12296 Complete with RCA Tubes Now only '115 °° SOUND PRODUCTS RADIO CORPORATION of AMERICA ENG /NEER /NG PRODUCTS DEPARTMENT, CAMDEN, N.J.

In Canada: RCA VICTOR Company Limited, Montreal

26 AUDIO ENGINEERING OCTOBER, 1948 ore business for you because It s RCA. More profit because of these new values

RCA SPEAKER MI -12422 12 -inch high efficiency, Alnico V, Cone Type Mechanism. Special cone provides a selected listening characteristic -a result of coons ove of listeners'past e RCA MOLDED SPEAKER BAFFLES reactions over the past 10 A Molded Plasticized Fibre Baffle with at- years. 10-watt capacity. 15 gray hammeroid finish and con- ohm voice coil. tractive trasting grille cloth. 12" x 15" x 51 ", sloping front, strong, moisture- resistant, tying. Available Also available with and fors ",óy2 " and 8 " mecha - 4 7 J matched 10- wattmulti- nisms as MI -6378, MI -6379. tap line transformer as and MI -6380, respectively. M1. 12421. Similar baffles for 5 ", 61/2", 8 ", 10" and 12" mechanisms also available its molded Bakelite and wood. All prices shown are suggested list prices cuñiert to nnrnsnl Aanla. dicresntt

NEW BANTAM VELOCITY RCA VARACOUSTIC MICROPHONE MICROPHONE MI-12080-B MI- 6204 -C Smaller than a pack cigarettes. First Three microphones in one. Here you have of 1948 the popular cardioid microphone (uni- used at all directional) plus standard velocity micro- political conven- tions. Broadcast phone (hi- directional) plus standard quality. Built -in pressure microphone (non-directional) all Make the RCA in one general purpose unit. Variable voice -music switch. 15 feet of cable. characteristic obtained by ingenious slider Sound Products Distributor mechanism on rear of housing. Quality in your area your and performance found heretofore only in SOUND HEADQUARTERS. more expensive polydirectional Broadcast Microphones. High sensitivity, shock ßn750 mounted, high impedance. 30 feet of cable. THIS COUPON FOR FREE CATALOG

;ell me m mu au

RCA SOUND PRODUCTS (Dept 7 -J) Radio Corporation of America Camden, N. J. BUY THE BEST Please send me free copy of RCA's new Catalog on complete line of matched FOR LESS sound products and price list.

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AUDIO ENGINEERING OCTOBER, 1948 27 The Problem of Sound Distribution

O. L. ANGEVINE, JR.,* and R. S. ANDERSON **

PART III Valuable data on planning sound installations.

IN the two previous articles, general reverberation usually found in class- power of 10 watts to produce a level of data were presented for planning a rooms would be more objectionable at 105 db (symphonic orchestra) in a liv- sound system. This article will give higher levels. ing room 12 feet by 20 feet by 8 feet- examples showing the use of such data A similar example is the hotel room this level may be reduced by adjacent in specific problems, and those pre- where the usual power is 0.1 watt, cor- apartment dwellers pounding on the sented illustrate some of the common responding to a sound -pressure level wall. types that occur. References will be of 86 db in a room of 1600 cubic feet In the examples given, a single cone - made to figures in the other two (see Fig. 1). Here again it is neces- type loudspeaker is normally used and articles. sary to keep the levels low enough so as is located where convenient. Usually A logical sequence in planning a not to disturb neighboring rooms. A it is mounted at the front of the sound system without retracing steps is: like situation occurs in a hospital room. room, high enough from the floor eo The home radio receiver, that high frequencies are not obstruct- 1. Determine what the system is sup- another posed to do. example of a sound system in a small ed by furniture or people. 2. Select the speakers. room, ranges from approximately 1/2 While the reverberation time of a 3. Compute the electrical requirements. watt for the small table models which small room may be above optimum, it 4. Determine the location of the speakers. for is not generally so bad that shaping 5. Decide what, if any, frequency shap- are used reproducing speech and ing is needed. music at low levels. Figure 1 gives a of the system frequency response is Minor adjustments in speaker place- required. Likewise, rooms of this type ment and level can be made after the are quiet and so do not require varia- installation has been completed. Some tions of frequency response to over- noise. provision for reserve power should be ride made to take care of these adjustments, Largo Rooms and doubling the power provides a 3 db Music distribution in a large room margin for this purpose. will be illustrated by examples in an industrial plant which presents a wide Sinai Rooms range of noise and acoustical condi- Music is distributed to large num- tions. The same approach can be used bers of small rooms in such buildings for sound systems in stores, restau- as schools, hotels and hospitals. The rants, and for many applications of average classroom is about 24 feet wide wired music. by 30 feet long by 12 feet high and will Assume an office 50 by 100 by 12 ft. seat 36 to 40 pupils. The volume of this A B Volume = 60,000 cu. ft. room is 8640 cubic feet. As was pointed Average noise level = 70 db. out before, a peak sound -pressure level Entire office to be covered with "music - of + 80t db is required for adequate while- you -work." Reverberation: Somewhat higher than coverage of speech, and 95 + dbt for optimum, as the floor, walls and reproducing a small orchestra. If 2 %- ceilings are hard and the furniture efficient cone -type loudspeakers are is made of wood. used, Fig. 1 shows that these levels A number of cone -type loudspeakers would require .1 watt and 3 watts, re- will be distributed throughout the spectively, of electrical power capacity. room to produce a uniform sound level. Since the sound level must be kept low Since the music is background music enough so as not to disturb adjoining only, the level will be chosen as 10 db rooms, the commonly used figure for above the noise. While in actual use classrooms is 1/2 watt, which produces t the people listening to the program a level of 87 db. Although this level is may desire a somewhat lower level, the 8 db below the optimum level required 10 -db figure given provides enough for even a small orchestra, it is quite margin for any case. Music selected satisfactory, especially since the high C for this type of service generally has a restricted dynamic range without Chief Sound Equipment Engineer, Strom - Fig. II. Four possible speaker place- either extremely low passages which berg- Carlson Co. ments in an office 50 x 190 feet. A & cannot be heard, or extremely loud Acoustical Engineer, Stromberg-Carlson C-cone speakers in two -way baffles. passages which will require more Co., Rochester, N. Y. B & D speakers in wall mounting power. tError in Part I of this article makes these -cone levels read minus instead of plus. baffles. From Fig. 1, an 80 db level in a

28 AUDIO ENGINEERING OCTOBER, 1948 to these room of 60,000 cubic feet requires an be provided in the aisles, so that stran- There are two approaches of speaker selected electrical power of 0.35 watts. This gers who have not been conditioned rooms, and the type upon which is used. One power may be divided between two to the high noise level can be paged. is dependent is use of a large number speakers as shown in Fig. 11, A and B. Although the over -all system response approach the operating at low solution providing more even dis- of cone -type speakers A the listeners, use of four speakers levels and located near tribution is the architecture al- D the same figure. permissible where the shown in C and of mountings for the open -back housings are used, very lows permanent If This method is so similar nearly the same amount of sound will speakers. problem that it will not radiated in both directions, so they to the previous be o out in the example. The may be considered two -way baffles, as be carried second approach is to use a single clus- illustrated in A and C of Fig. 11. horn speakers, or horn and cone Although the small open -back housing ter of speakers combined if music is to be has a low- frequency response usually reproduced. considered adequate for this applica- The example chosen is a sports arena tion, an extended low- frequency re- o 50 feet. open-back 300 by 200 by sponse requires a larger cu. ft. f B Volume = 3,000,000 baffle than the equivalent closed -back A Average Noise Level = 80 to 90 db-if housing.17 Fig. 12. Two possible horn locations in the crowd is enthusiastic, the noise may be considerably higher. will a factory building 50 x 100 feet. A- level The speaker location determine with speech horn -type Entire arena to be covered the amount of wire required for con- straight reentrant speaker. and incidental music. nection, and some layouts are more 8-360 radial reentrant horn -type To override the noise, the program level economical of wire than others. In speaker. must be 10 db higher or 100 db. From Fig. 11, C will use less wire than D. should rise 3 to 6 db per octave for Fig. 3, 75 watts is required for the The only frequency shaping required good speech articulation, speakers of horn speakers and from Fig. 2, 250 for this type of installation is compen- the type chosen generally have an watts for the cone speakers. Horns will sation of high- frequency line losses if adequate rising characteristic. be chosen that have a low frequency wired music is used. Large Rooms with Bad Acoustics cut-off at 200 cycles so that the horns The same solution will suffice for In large rooms with hard walls and may be used alone for the reproduction speech. The 80-db level used, is the high ceilings such as railroad stations, of speech. When the cone speakers recommended level for speech, and the gymnasiums, and sports arenas, the re- are used in addition to the horns, a frequency response and speakers satis- verberation time is extremely long and level 3 db higher than that stated in factory for music are certainly ade- distinct echo is usually found. In the problem will exist in 'that portion quate for speech. rooms of this sort acoustic treatment of the frequency range where both For an example of music distribu- can be very helpful, and if installed types of speakers are radiating. To tion with a high background noise, at the time of construction of the reproduce symphonic music at the assume the same size room full of tex- building the cost is not necessarily ex- recommended level of 105 db, addi- tile looms producing a noise level of pensive. In addition to simplifying tional power must be used, but this 104 db. This is an example of the case the sound system installation, acoustic is inadvisable in a room so excessively discussed earlier where a program level treatment will reduce the noise to a reverberant and is not usually at- l 10 db higher than the noise level ap- more comfortable level. tempted. If it must be done, acoustio proaches the threshold of pain. Since In such rooms, the usual use of the treatment of the room is imperative. the noise is continuous and the peo- sound system is for paging and an- Figure 13 shows a cluster of cone ple exposed to it have become condi- nouncing, with an occasional need for and horn speakers to provide proper tioned to it, it is not necessary or de- music. The people paged are more distribution of sound in a sports sirable to provide a level of more than likely to be passing through the room arena. The horn speakers are 40° by 102 db. This example has been taken than regular occupants. Frequently 80° cellular horns and have been aimed from an actual installation. word articulation is very important - to provide an overlap of the radiation Horn speaker having a cut -off of in a railroad station the garbling of a patterns in the direction of greatest 250 cycles will be selected for this ap- few words can mean a long wait for "throw." Emphasis has been placed plication as the high noise level pre- the next train. on beaming the highs to all parts of cludes any advantage that may obtain if lower frequencies are radiated. Note, however, that the low-frequency output TABLE I Total of the amplifiers should be reduced so Square of Multiplied Power that the speakers will not be driven be- Path by No. of Power per (watts) (wafts) low their cut -off frequency. Figure 3 Length (feet) Speakers Speaker 36 shows that an electrical power of 6 2 End Speakers 1202 = 14400 28800 14400 x 75 = 18 watts is required to produce a pro- 61700 32 gram level of 102 db. 4 Side Speakers 802 = 6400 25600 6400 x 75 = 8 Because the program is being repro- 61700 duced at a level somewhat below the 8 4 Floor Speakers 402 = 1600 6400 1600 x 75 = 2 noise, at least four speakers will be 61700 necessary to keep the sound distribu- 900 900 x 75 = 1 1 tion uniform, arranged as shown in 1 Center Floor Speaker 302 = 900 61700 Fig. 12, A or B. Again this approach Sum of Squares of all well with speech, 77 applies equally ex- Path f .ngth= = 61700 cept that a slightly higher level should

AUDIO ENGINEERING OCTOBER, 1948 29 the arena to keep the articulation high each speaker is proportional to the cient flexibility be provided so that this at every seat in the auditorium. As square of the length of the sound path. adjustment can be made after the the high frequencies play a major Thus the proportion of the total horn system is installed. part in maintaining good articulation, speaker power required for each Since the cone speakers are non - any seats that are slighted will be poor speaker in this example is computed directional at the low frequencies they seats for a listener even though the loud- in Table I. are to reproduce, it is necessary only ness level may be adequate. When the The square of the path length of a that they be aimed outward from the articulation is poor due to lack of high particular speaker divided by the sum cluster at the center of the arena. frequencies, the natural reaction is to of the squares of the path lengths of Figure 14 shows the portion of the desire more level, although raising the all of the speakers is the proportion frequency response contributed by the level may only increase the reverbera- of the total power that should be al- cone speakers. This figure also shows tion and further decrease the articu- lotted to that speaker. Therefore, this that the low- frequency response must lation. What is really needed in this proportion multiplied by the total be reduced to match the apparent in- case is an increase in the high fre- power will give the power required for crease in loudness due to greater re- quency response at those seats. that speaker. verberation at low frequencies. Al- Even distribution of the sound to The computed values may need some though this correction has made the all parts of the arena will require that adjustment after the installation has system response flat over the fre- various amounts of power be fed to the been completed. Likewise the balance quency range, it has not changed the horn speakers, depending upon the adjustment of cone speakers to horn shape of the reverberation curve, and length of "throw" that each speaker speakers must finally be made by ear, as the room is excessively reverberant has to make. The power required for therefore it is necessary that suffi- at the mid frequencies, it will be much worse at the low frequencies. If the low frequency response is not still further attenuated, this reverberation Fig. 13. Sports arena: Horn speaker array using 40° vertical, 80° horizontal will manifest itself as long rolling of directional pattern used to obtain coverage cellular projectors. A- Overlap echoes. The range of the bass control throw. B & C directed downward may be along axis of geatest -Speaker must be adequate to allow adjustment a in floor with switched off to permit use of microphone the center of the for the changes in reverberation time minimum feedback. Overlap in B is accidental and not serious. as the audience size varies. Auditoriums- Indoor In auditoriums the usual application of sound equipment is voice and music reinforcement, and buildings that have been designed for the presentation of speeches, stage plays, and concerts \`t,¡:. \ usually have fairly good acoustics. The ...\ \\ example which follows is an auditorium which has a volume of 500,000 cu. ft. and is irregularly shaped in accordance with the best design prin- ciples for good acoustics and visi- bility. It is desired to reinforce voice and solo instruments of a symphony orchestra and to maintain the impression A- PLAN that all sound is coming from the stage. If this objective is attained, the audience will be unaware of the reinforcing system. Too often the sound system is required to produce an artificial effect in order to satisfy the performers and management that the system is working. For reinforcement of solo instruments, a sound pressure level of 95 db is recommended. Since this is not back- OVERLAP ground music but is the reason for the B - SECTION THROUGH WIDTH audience being present, and since the program is originating in the presence of the audience, a high-quality sound system is necessary to reproduce the full range of the reinforced sound. There- fore, theater -type two -speaker systems will be employed. As these speakers have an efficiency of approximately 5%, from Fig. 2, an electrical power of 20 watts will produce a sound pressure level of 95 db. If voice reinforcement

C- SECTION THROUGH LENGTH only is the purpose, Fig. 2 shows that

30 AUDIO ENGINEERING OCTOBER, 1948 Fig. 14 (left). System response in highly reverberant room. A- Apparent frequency response using a flat sound system. Enhancement of bass is due to increasing reverberation time at low frequencies. B- Contribution of horn speakers. C- Contribution of cone speakers attenuated to compensate for effects of reverberation. Fig. 15 (right). Elimination of acoustic shadow in indoor auditorium. A- Location of speaker at A throw, shadow at S. B- Location of speaker at B eliminates shadow.

the same speakers would need less than echoes in the seats that receive sound badly needed. Any sign of feedback en- one watt to produce a sound pressure from both the main and auxiliary courages the performer in his conviction level of 80 db. On the other hand, if speakers if there is a large difference that the only way to use a microphone remote pickups or recordings of a full in the length of the sound path from the is to "cuddle" it, and his confidential orchestra are to be reproduced, 200 two sources. Care in placement of the whispers to the microphone sound no- watts of electrical power will be neces- speakers and choice of level will mini- thing at all like his natural voice. If sary for a level of 105 db. mize the effect. the user avoids this error, he is likely to While the exact location of the speak- The case of voice and music rein- put the speakers so far from the micro- comes se- ers is not important and is dependent forcement can be handled similarly to phone that the sound from illu- on the architecture, they must be kept the above example whether the hall is veral different directions and the of the single source is lost. Of near the stage so that the sound coming a smaller auditorium, a church, or a sion installations from them merges with the sound com- night club. Of course the smaller audi- course, many of these poor ing from the stage rather than appear- toriums may not warrant the expense are the result of a hasty job which is to be temporary. ing to come from separate sources. The of theater -type speakers, and those only speakers must be kept far enough from chosen must be the best compromise be- Outdoor Sound Systems the microphones to allow an adequate tween cost and the quality required. Single -horn speakers may be em- l amplifier gain before acoustic feed -back There may be more of an acoustic prob- ployed in many outdoor sound systems occurs. Obviously, concealing the lem in these halls, and a further reduc- such as ship hailing systems, systems speakers will further heighten the illu- tion of the bass response may be needed for directing fire fighting, and mobile sion that all sound comes from the to keep the reverberation from being too systems mounted on police cars. To de- stage. annoying. termine what can be done with such a system, assume a conventional re-en- As in the previous example of the As a temporary installation in small horn having an 80° coverage an- sports arena, the bass response must be halls, a portable sound system can be trant 150-cycle With 25 watts reduced, but because the acoustics of an entirely adequate method of rein- gle and cut-off. of power available, Fig. 5 shows a 95- the auditorium are better, much less forcing solo instruments and voice, al- level on the axis of bass attenuation need be used. though a permanent system will do a db sound pressure at 80 feet and 80 db at 440 A not uncommon problem in audi- somewhat better job. While the small the speaker feet of 30° coverage. To toriums is "acoustic shadows" Since baffle of the typical portable speaker for speakers with the 80° the higher audio frequencies essential to does not permit radiation of the very obtain the same levels is neces- good voice articulation and naturalness low frequencies, this deficiency is off- speaker mentioned above, it in music travel in straight lines simi- set by the facts that the primary need sary to multiply the power required by larly to light, there may be "shadows" in such halls is high -frequency rein- the square of the ratio of the respective if an obstruction such as a balcony gets forcement and that the bass response coverage angles : 25 (80/30)2=177 in the "beam" of the speakers. Thus, may need reduction anyway. Unfor- watts. In this case, however, the power it may be necessary to restore the high tunately, most portable systems are hor- is given, so to determine the levels pro- frequencies to seats under the balconies. ribly misused because of a complete duced by an 80° speaker, divide the The best and least expensive approach misunderstanding of their application given power by the squared ratio: to this problem is to relocate the speak- by the people using them. The speakers 25/(80/30)2 =3.5 watts. Using -this a ers so that the balcony will not cast a are often located too close to the micro- value for power, read from Fig. 5 shadow-see Fig. 15. If this cannot be phones because of the short speaker sound pressure level of 95 db at 28 feet done satisfactorily, several directional. cords supplied, and acoustic feedback and 80 db at 160 feet. For speakers high-frequency speakers may be placed results. The operator stops the feed- with different horizontal and vertical to beam the sound into the shadowed back by turning down the treble control coverages angles, multiply the power area. This solution may introduce and thus loses the high frequencies so first by the (unsquared) ratio, using [Continued on page 47] AUDIO ENGINEERING OCTOBER, 1948 31 Experimental Data on Germanium Crystal AmpliFiers

S. YOUNG WHITE*

Further data on the characteristics of amplifying crystals.

THE writer has made an inten- the total output of the generator or former for convenience of having sive effort to investigate the fun- the drop across the grid circuit. The higher readings. damental characteristics of crystal ratio of these voltages and the value Testing procedure is in several amplifiers, with the obvious objective of the setting of R2 give us the input steps. Set the grid to plus 0.5 volt. of finding those points of similarity impedance. It also allows us to put (A side remark might be in order -we with vacuum tube theory that would any signal voltage on the grid that the never find the optimum gain or over- allow maximum utilization of this vtvm can measure, so we can measure load condition at 1 volt, so far in our well -developed art. It must be con- the impedance at a variety of input work-it always lies between 0.4 and fessed that many more dissimilarities signal values. The meter M is a 10- 0.6 volts, and we always seem to end were found than had been expected, so ma type. up at 0.5.) It is not necessary to ad- that the writer at the moment is in a The plate circuit utilizes an over- just closer than 0.1 volt to the opti- somewhat confused state of mind. sized output transformer. This is a mum value. We have a supplementary which doubtless will be reflected in push -pull to 500 -ohm line, 10 -watt unit we plug in from time to time to this article. After thirty years of unit, and has good coupling character- check grid performance from plus 22 daily work with electronic tubes it is istics. Calibrate it for stepdown ratio, to minus 22 volts, but it is seldom difficult to reverse many concepts, and which will run from 6 to one to 2 to used. Also we do not use the volt- in addition, many characteristics are one on the various taps. By varying meter shown, as we calibrate the ten - shown by at least certain crystal spots the secondary load R4 from about ohm pot in terms of volts and this which are very new, and have no par- 1,000 ohms down to 100 or so, we can is close enough control in most case% allel in amplifier work. load our plate from about 36,000 ohms A treated crystal will draw about 4 After several attempts at a univer- to 400 ohms, a sufficient range for ma; an untreated one about one ma. sal set -up for gain and impedance treated crystals. We then increase the plate voltage. measurements, the circuit of Fig. 1 The plate milliammeter has a 5 At 40 volts, an untreated crystal will was developed and seems to meet the ma range and a one ampere shut, for draw perhaps 3 to 7 ma; a treated requirements rather well. From the flashover conditions. Resistance Ra can one, 1 to 2 ma. Swing the grid down grid we have a low resistance induct- be cut in to limit the surge on flash- to zero volts and note effect on plate ance of about 2 henries and 30 ohms. over, if desired, and may have a value current. Then reverse grid and plate - This is to provide a d -c path for the of 10,000 ohms less. Condenser that is, take the catwhisker just used grid current, or C and if most measure- is to by -pass this resistor, and may be for a grid and clip it on the plate ments are made at 1 kc. the imped- 2 if or more. supply, and also take the former plate ance is sufficient to be a bridging contact and use it for a grid. Again The plate potential is shown as a value. note control. tapped battery. A tap every 5 or 6 A convenient source of variable Rodprodty bias on the grid is a ten ohm potentio- volts is sufficient to run curves. If you use a normal power supply with a An effect of great importance meter across a No. 6 dry cell, which has potentiometer across it, be sure the been noted in all work to allows zero to 1 1/2 volts to be ap- our date, plied, and it is desirable to have the system is clear of grounds, as it is and we need a new term to describe used with polarity reversed it. We two d -c resistance of the inductance L low and the have contacts, each of one normal ma area so that no allowance need be made for practice of grounding the neg- and separated about 2 mils. may the d-c voltage drop in it, as from ative may cause difficulty. If such a They have identical contact pres- is by -pass cases time to time we shall put up to 15 source used, it adequately. sures. In many their forward currents at one volt will be very close ma range and a one ampere shunt, for For preliminary investigation we - The audio generator (a Hewlett use 1 kc, and no attempt is made to say 4 ma and 4 1/4 ma respectively. Packard) has a 500 -ohm output and make anything uniform over the Their back currents may be, at 40 volts, 2 2 1/4 all about 30 ohms d -c resistance. It will spectrum. Our work indicated the and ma each. For provide over 30 volts output. In series, transistor itself is flat over the band practical purposes, they are identical, one will control the second, we have R2 which can be a 2500-ohm of 20 to 15,000 kc, so after taking a yet but wire -wound pot, or a decade box of 10 good look at this characteristic we the second one will not control the first. They lack reciprocity. This is to 1000 -ohm steps. A vacuum tube concentrated on 1 kc. voltmeter is arranged to switch across not absolute -one may control the Testing other say 1 ma, but when reversed the The output vtvm can be clipped on control is less than 25% and usually Consulting Engineer, 52.12 Van Horn St., Elmhurst, L.1., N.Y. the high impedance side of the trans- about 10 %. Sometimes the control is

32 AUDIO ENGINEERING OCTOBER, 1948 zero in terms of observable variation pedance is in hundreds of ohms, this before we can do this, while the wires of a 3 inch meter, and yet reversal is a power loss. we have been using have been rounded gives normal control. to 0.2 mil radius. Stabilizing by Tapping and We must emphasize we are using Some slight effort has gone into plat- standard IN34 units and have no Overshooting ing spots on the crystal to do away One nice thing about all these crys- other source, so our remarks apply with the rather indefinite catwhisker tals is withstand a very large only to them. that they contact. The small size of the spots plate wattage, not only without dam- This lack of reciprocity will be a makes for considerable difficulty here, difficulty in production, as we must age, but usually with both stability and we have not developed a technique gain and increase in amplification. have a unit we can plug in the circuit It suitable for it. has become standard practice in our and have the grid on the grid side, Germanium has many isotopes, and tests to overshoot to 500 ma or so sev- and vice -versa. So if we find a nice several radioactive ones. There seems eral times and be slow in reducing the active spot that is reversed, we must to be much room for atomic activity in voltage-taking a second or so to do so. abandon it and find a new one of it. We shall try common forms of At the same time, we tap the mounted proper control direction. radiation and report on the effect. crystal rather hard several times. A At the present time we have no idea Negative Gild Control good point will always survive this how rapid the heating is, or how rapid treatment, so it is recommended as A point little discussed in the the rise in plate current is when we standard practice. theory of the transistor is the possi- overshoot. We are acquiring a two negative. In bility of running the grid Lack of Hooting Effect in the Crystal team oseilloseo1x to study this. several instances, we have found spots One phenemonon that we do not like that exhibit decided control with nega- Characteristics of Treated Crystals to mention, as it has very peculiar in- tive grid, so the theory must bear effect noted is the current ferences we have not had time to check, First that this in mind. We have only fouled at 1 volt rises from an average of is the relative lack of heating up when plus these with the shoulder gap unit, 3 the to the crystal is passing say 600 ma at 40 ma for untreated crystal where the face (of N type crystal) is above 10 ma, another reason for run- used as the plate, and the side (un- volts or more. This is a current density amperes per square ning at 0.5 volts. polished) is used as the grid. The of about half million tip, and rapid The plate current is about half, be- phenemenon occurs when the edge of inch at the catwhisker incandescence should occur. It does ing two ma instead of 4 to 7 am aver- the ::rystal is irregular. The gap not. On a cubical basis most of the age for the untreated. spacing used is .003 inches. There is large interaction of the two currents, the grid current increasing from perhaps 3 ma at plus 1 volt, to Fig. 1. Diagram AUDIO 6 ma when plate current is drawn. GEM. Negative bias on the grid reduces both of set -up for grid and plate currents, and at -10 volts the grid current is zero and the testing amplify- plate 3 ma. The grid current then ing crystals. 41.0 i1111tA reverses, the curve being linear up to 11111111 1111 -15 volts or so. With this negative grid, much higher plate voltages can be used without flashover. drop is probably at the reduced cross The plate impedance comes down section of the tungsten, and also a very from about 50,000 ohms to an average of Untreated Crystals Characteristics thin layer on the crystal face. Many of 9,000 ohms, which we shall call These are not of too much interest, assemblies have only a cubical volume 10,000 ohms for convenience. as treated crystals are much more equal to a tenth watt resistor, and if The plate current control by the promising. We have hundreds of you put 20 watts in such a tiny resistor, grid follows a different law. The per- will briefly sum- curves of these, and heating is immediate, and the results centage change in plate current is a marize them. spectacular. We have no such rapid constant, broadly speaking. At 20 volts The plate impedance runs 80,000 to temperature rise, and the unit is almost plate, for instance, the current may be 40,000 is, in unaf- ohms, and general, impossible to burn out. 300 microamps. 0.5 positive volts on fected by grid bias or plate current, the grid will increase this to 370. At a of plate voltage only. It is noted that if we treat such a being function 60 volts, plate, the current may be This is a radical departure from va- spot to this enormous power, it returns 3 ma, and the grid at 0.5 volts will in- of course. to its original condition when cooled- cuum tube performance, crease this to 3.7 ma. In vacuum the effect that is, it is still operative as a straight The d-c control curve of tube terms, the first gm is 140, the sec- is at low plate cur- amplifier, and draws the same current of the grid this: ond, 1,400. rents and voltages the control on an as before the abusive treatment. So we have not melted the tip, in both cases active spot is negligible; at 0.3 ma evidently The plate impedance or area substantially. is unaffected by plate current the control may be 5 %, increased its is the same, and again for instance. At 4 ma plate current, The G.E. welded crystal passes only the grid bias. This is most peculiar, control may increase the plate current 200 ma through the point, melts it with but we have checked quite thoroughly. for to 51/4 ma, a change of about 25 %. this current, and forms a large tear- The gain is about the same both The gain goes up roughly in propor- drop shaped end on the tungsten, which conditions. This is indeed puzzling. tion, and the plate impedance comes is firmly embedded below the surface The grid or input impedance is down. The gain with a 50,000 load may of the crystal. It is interesting to note very largely affected by the plate. Grid be the disappointing one of say ten we must form a very sharp point on impedance by the series resistance in voltage, and since the input im- the wire of only a few millionths radius [Continued on page 51)

AUDIO ENGINEERING OCTOBER, 1948 33 Resistor Error in Attenuators

HERBERT I. KEROES*

How to determine resistance tolerance limits for a required accuracy of attenuation.

and variable attenuators are By taking the natural logarithm of may be identified with that in equa- manufactured to a resistor ele- both sides we obtain; tion (lb), and by substituting N for elf, FIXEDment tolerance. This tolerance (3)0. this becomes; bears no easily discernible relation to LOa[ +ZN +1] +2N -Loa[ -1J (6) a(4/4) the resulting attenuation error, and ae N -1 This may be differentiated as fol- N +t Ir/Zr users of attenuators are usually at a lows; For small variations from the nom- (4) inal attenuation de may be replaced de by the attenuation errorae 2 1 +2T2. Then; {r 1 1 (7) 1 +1 1+2 12x1 1 +2r/I* /2n -1 Upon simplif9ing we obtain; The quantities m and n are the re- l31 a (2/z.) spective percentage accuracies of re- V 2.+2. 2./z. sistors Z. and Z.. The quantity under the radical sign ¡Continued on page 541 Fig. I. The L Attenuator Circuit. disadvantage when it becomes neces- Fig. 2. Chart for determining error in Pi and Tee type attenuators. to specify the resistance tolerance sary 1 ZN ZN of attenuator elements. A simple re- .9 lation is herewith developed which has _- .8 _- .7 been placed in nomograph form to fa- .2 cilitate rapid calculations. -.6 - Attenuation networks of the II, Tee .5 e TOL. ZM and ladder types are composed of two M= I+ .3 - or more L network sections connected -.4 IOC in tandem. The L section is shown 7 TOL. ZN -.3 e I+ .4 - diagrammatically in Fig. 1, the series 100 branch shown as Z. and the shunt .5- branch as Z.,. The loss expression' is -.2 given by the square root of the ratio M 7- between the impedances viewed from Ñ .8 - one terminal to ground when the other +A -A .9 - terminal is first shorted to ground and 1 - 1.2 .8 then opened. The loss of the L section .I is one -half that of a full II Tee section .09 composed of two such sections. Let e 1.1 .9 J be the loss of the L section expressed J8 2- w m -.06 1.06 .94 CO in nepers. v Then U -.05 w w - 1.04 .96 ° (to) ronn -.04 3- 2 Zo, Z.+Z. ° ° 1.02 .98 .X-.-1* - .03 4- á .v. +.-M. m 5 - (lb) 1.01 _ .99 .=1 T. 6 - Z.+ Z. 02 7 - This may be solved for e9 which is 1.005 .995 8 the ratio between the input and out- 9 put voltages for the complete Tee or 10 - II network. (2) .01 + +1 7.009 20 _ .008 30 - _- .007 co - ' "Electric Circuits and Wave Filters," A. T. -.006 Starr, p. 174. Pitman and Sons, N. Y. C. .005 The Daven Co., 191 Central Ave., Newark, - N. J.

34 AUDIO ENGINEERING OCTOBER, 1948 STANDARDS ENGINEERED TO THE HIGHEST FM and AM BROADCAST

NEW

Ifigh fidelity Dynamic N

8Rd Microphones

1 Model 650 , (Output -46 d6¡

4/7:::;1

\ FEATURES LIKE THESE Model 645 ' WIN TOP RATING (Output -50 db) 6 Statloa aad Itereocvek £agiaee't4f

Flat out to 15 kc! Extremely high output! Impedance selector! Dual- type shock -mount! Remarkably rugged! Individually calibrated! Developed in cooperation with station and network engineers, the new "650" and "645" meet exacting requirements of modern high fidelity FM and AM broadcast service. Proved in studio and remote use. Polar pattern is non -directional at low frequencies, becoming directional at high frequencies. Recessed switch gives instant selection of 50 or 250 ohms impedance. Exclu- sive Acoustalloy diaphragm withstands toughest use. Many other important features assure the ultimate in broadcast quality. Satin chromium finish. Fully guaranteed. Model 650. Output level -46 db. list $ 150.00 Model 645. Output level -50 db. List $100.00 Broadcast Engineers: Pat the "650"or "645" to the test in your station. Know the thrill of using the newest and finest. Write for full details.

ELECTRO- VOICE, INC. BUCHANAN, MICHIGAN Export: 13 East 40th St., New York 16, U. S. A. Cables: Arlab

NO FINER CHOICE THA gLeretracc

AUDIO ENGINEERING OCTOBER, 1948 35 DBM- IMPEDANCE CHART F do. 7n . FD +211 00 .N 11111.1A ._/IIII. 111111.... II...... - ií.\`CLII/I.I....'. ...1111111. 111MI...o /11% :..... =MII\MIL.MMIPIOMIIMMO\`WdrIUM-IMI%/%\iV.:III%II%\\I O.M.WINIIKOMOMINIe1O5-i.\.I.. J r:07`IIum11II oiMSWWMan V11 !p.1 .211111M11M1111MO\/mm WZ\Mg NI\ I O:/01 h. .i alMM11I ÌMMI\ I_MI F, +1; 74111111iMIEWSi5i1i!/11110ï111i.. I ..1111//,/..I.II.i.tI,/IIII/,1:..--. -t,. 'IIi .,. I i'OiI,'il '0,1I L +ius 10 mW 1.1 %I/.II:I_1I /11:I .. I...6 11 \.\`.` W/IIMIIEOrI...9+.... i\11I\i`7~1/-=1I\.111L.i\11110111\P:AnffIMIN-AMII. /IIIII ...... \.O1% 11110I. %101i% wrI mi .... - mI smo mournme%ei`aiiaramII I1/.\1\\IM110` .WI 2311 \IIIIMM\\I !doAMMIIIIIIIllnal1m% lmea MI\I/ïí1i1E_\/ï1/%11 .MIIllIi\I/ïI51iIC!I1111 I :/ïM IMM//111I ' t .1i111` \ I/.:1.11,:31,i111I t \ 11111111FAIIIIII1011111111L411111111 .a._,I.... -/411------WINS'---- _....\` -----,------I.... .` /fOr211.PMIIQ.0L.0i\11.51MOIUM'AMP11I---II-II- ` ---II-II1101 -..----\` 41:SMII IIM%IIMI. OIMIM eno1 wnm/\xmo.5.\\i`5 nomm\\w% WMIIII11Pb11/AN=INOW/GliIC10%/M11WAMIMMOI I I /i1 20511M169 /=\ p:I.iPI L 102WMPAIMIIMMI13 I 0/01 KV. MMIIII:ME\ I/ï1:11 KMMI111:MINILII/ï SIM :N,i¡¡1 1/1IIII-i IiN1C11i:ii,,,iq .,10, INW.\'.1--I W.S. I.il..IN.. Wi\M\.I-1.II..L-IIIIIL..- _/1:111IW.... \.L.-I_-M.SIIi\..L. UMEINIMM\.I'.1I III.... 111I%10ii`Ía omor/w%M'I011wWAIIIMIWOiiird 4117741111M%IS%1111 WAMMEN\ II '1I M74 EMI/ a\11 w,mon` :1111VW IIIram PIMI\ I :/i!1 L 1111P%CI I \// /ï 01101/M11 -Is - _I/ï1':1:ICNIIII: i \\/.ï1'11!III:1 -1://.11,i-111rIt'` I i ` .i'll \GI11' ' ' FD \ \ . I 'it O` .01 m . ....W.S..%-MIM.....W...i1:iI%iiiÌ 11111IIW WI WM/ Ì..010 ON...-II11II60...... IMIINglMMC.I.. MIR, A.ISNOMIrmIMIEhl IOW .JSMOUNI i1:i rI 1=P 41111Ii \.5_-CINIOMMIO.101ieL AI II aa11/mm\ib1 Ìwas11mmm\\I/1 IM/N15 O:/iF1 L .2 MM/%lCI MIMïri - %=\%'ii'1 111\ Illil 111iÍI %ïÍ li t' .11111mI/.r.!/.11,1-gIIII%r.!.1Ipr.I't y.r

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.1 uM P20511.ol -u- M..,+111MrIlimIll011=i11i ...v.1111i Ì%'i11 MI.....1IIMIMMOSIMMONC.. 1i1:i1 LvIIIW Ì....W111110 WNW LMIWIIMIIMOW....OVAIIMEMNIAC.OW II%7I%\ii`:!III%\I\%\I-Ii\.IS.IiSI-Ii\.5.\Iiv 5 731II Nas11 /ii`iG1 Gw%I N'AMI1 I`E1 MOME1MINEWI\ !:/ï!1 N.. 121111./11PIIMII\ =\ / '/ï.: I11:1 \\%'i 1; III:1 %ïiÍ I,i -1IiIIII/..!1.11.\1,iIlII/,:1:11"t'` /' -r"'t' ` I I 4E411111ligeogill ¡ 42 -..-- .01 uW ._1Lb. AI 7W AEiI I dD. c S C S i0 _, Cc C C Aac x o 70 ho Ó I.I

An 00.0/ If Recorders came

with Mileage Meters...

Presto 6N would be /illES 4HE.4D

Yes, day after day and year after year over 3,000 Presto 6N recorders are hard at work h broadcasting stations, recording studios, educational institutions and government agencies throughout the world.

6N recorders purchased ten years ago are performing as well today as when they were new. This outstanding record of the 6N recorder continuous in action is proof again that Presto design is built for hard, duty and Presto materials are the finest obtainable.

By actual So when you're looking for a new recorder, remember: test the best recorder for the most people is Old Faithful, the Presto 6N.

P. O. Box 500, Hackensack, N. J. RECORDING CORPORATION, Paramus, New Jersey' Mailing Address:

In Canada: WALTER P. DOWNS, Ltd., Dominion Sq. Bldg., Montreal and discs World's largest manufacturer of instantaneous sound recording equipment AUDIO ENGINEERING OCTOBER, 1948 37 NEW PRODUCTS

BRIDGE TYPE A -F METER Model H -81 Bass Reflex Sector Cabinet E -V BROADCAST MIKES Discontinued fits anywhere -in 90- degree corners, during the war to permit flat on Two completely increased walls, or at intersection of wall and high fidelity, high production of more urgently ceil- output dynamic Broadcast needed instruments, the ing. They may be mounted singly, in pairs, Microphones General Radio announced by Electro-Voice, Inc. Company's bridge -type frequency meter is or in clusters of four around a post, to at- again available, in answer to requests from tain wide-angle distribution of sound. This many laboratories. enclosure is perfectly adapted to nearly all interior sound installations because of its small size and low cost. It may be used with any 8-inch speaker. This cabinet is formed of wood composition around a frame of hard wood. Finish is of brown opaque lacquer with chrome trim. It can be covered with color to match the locale of the installation. Height, 22- 1/2"; width, 17 3/4"; depth, 8 -% ".

I he11 features of the bridge -type instrument are its wide range (20 to 20,000 cycles) and its high accuracy (0.5%). Precise frequency settings can be made, because of the null method used, and the null detector can be either head telephones Developed in cooperation with network or a vacuum -tube voltmeter. The inherent and station engineers, the new Models 650 and 645 are designed frequency response characteristics of the H-8I to meet the exacting Wien bridge circuit also permit the instru- requirements of modern high fidelity FM ment to be used as an adjustable- frequency and AM broadcast service. filter. Consistently accurate wide range high For further data, write the General Ra- fidelity response out to 15 kc, extremely dio Co., 275 Massachusetts Ave., Cam- high output level, and rugged shock- resis- bridge 39, Mass. tant construction make them suitable for either studio or remote pick -up. Manu- L-P VARIABLE RELUCTANCE CARTRIDGE factured to close tolerances, under labora- A new variable reluctance cartridge, tory quality control, every unit is individ- designed especially for the new long -play- ually calibrated. ing records, has been announced by the Model J -61 is a Peri- dynamic enclosure The E -V Models 650 and 645 Broadcast designed Receiver Division of General Electric's to house 6 -inch speakers. This microphones are equipped with a newly de- Electronics Department at Electronics Park, cabinet is finished in simulated brown veloped shock mount. It incorporates dual Syracuse, N. Y. leather with grained effect and chrome trim. Lord shear -type mountings which elimin- The new cartridge, which features a low Height, 16.3/4 "; width, 12 -3/4"; depth, ate undesirable vibrations transmitted from mass stylus assembly and high compliance 6.1/4 ". Both of these cabinets are fur- the stand, and reduce side sway of the mi- for more faithful tracking, is one -third nished with brackets and screws for mount- crophone without reducing the efficiency of smaller than previous models, according to ing on wall or post. the isolation unit. The microphone head R. S. Fenton, sales manager for the divi- may be tilted through an angle of 45 de- sion's component parts section. grees, yet it retains its center of gravity The new, improved shape of the cart- with respect to the shock mount and stand. ridge makes it more universally adaptable Impedance Selector Switch recessed in to various tone arms. It also affords greater the front of the Microphone stud permits clearance for record changers. instant selection of either 50 or 250 ohms The stylus of the new cartridge is a sap- impedance, balanced to ground. This phire, measuring one mil in diameter as switch is easily actuated with the tip of a required by the new microgroove record- J-6I pencil or screw driver. Being recessed, it ings. carmot be moved accidentally. Built WALL- MOUNTING in Cannon XL-3 connector permits ENCLOSURES tilting of the microphone head without Teo new wall mounting enclosures, one strain on cable or connector. The quick - for 8 -inch speakers and the other for 6- disconnect cable connector permits separ- inch speakers, have just been announced ate handling of microphone and cable for to the trade by Jensen Manufacturing remotes. Company, Chicago, Ill. (Continued on page 401

PROFESSIONAL DIRECTORY

C. J. LEBEL AUDIO CONSULTANT Custom -Built Equipment Winston Wells 370 RIVERSIDE DRIVE NEW YORK 25. N. Y. U. S. Recording Co. Designer and Consultant © Acoustical and Electronic Research v V 1121 Vermont Ave., Washington 5, D. C. PNOL C' DESIGN INSTRUMENTATION 307 East 44th St. MU 4 -3487 PSYCHO ACOUSTICS SOUND RECORDING District 1640 New York 17, N. Y. EtECTROMEDICAL SUBMINIATURE AUDIO

38 AUDIO ENGINEERING OCTOBER, 1948 POYOUvises an pottARs

'.` -.. .

Two years ago Soundcraft introduced the `Broadcaster', a premium quality instantaneous recording disc. The "super" blank record costs more to make, is worth more to the broadcast engineer. but a small portion of the cost of a recorded program or record release, the Representing one is yet the heart of any recording project. No platter can be more expensive than blank disc appre- that loses a program or originates doubtful pressings. Soundcraft knew that key engineers ciated these facts. Soundcraft's judgment in offering the high -reliability `Broadcasters' (master- selected discs in been vindicated by standard sizes) and the wax -like `Maestros' (oversize discs for processing) has widespread acceptance on the part of makers of the best off- the -line recordings, transcriptions, and phonograph records. For real economy don't cut your disc budget, cut the best disc! Clip the coupon today for the story of how Soundcraft discs are made, the list of Soundcraft distributors, and your sample `Broadcaster'.

For real economy don't cut your disc budget, To: cut the best disc! REEVES 10 S E. 52nd 1 AF ' St., N. CORP. Please Y. 22, N. sample send Y. `Broadcaster' totstribntot list, and free

fiPanY,...... REEVES CORP. Address. 10 EAST 52nd STREET NEW YORKt 22, N. Y. Export Address ' V RF.EVIIVTF.., N. Y.

c , / c'] c f / e .Jf:e GUroadcafer Jlte trlayback uifion Jhe

AUDIO ENGINEERING OCTOBER, 1948 39 scanning photo -electrically a synchronously Will operate with any standard oscillo- NEW PRODUCTS rotating disc, modulated photographically scope. from a precision pattern. The accuracy of FREQUENCY Aron page 33j the discs assures a positive signal which RANCE -40 cps to 10 kc with 60 cycle AC. Polar pattern is non -directional limits intermodulation distortion, frequency, at low fre- and other discriminations. quencies, becoming directional at high fre- MARKER PULSES at I, 3, 5, 7, 9 and 10 kc. quencies. The sweep frequency signal generated is used for instantaneous SWEEP FREQUENCY governed by`20 synchro- For further information, write evaluation in produc- for Broad- tion testing, as tool nizing pulses per second. cast Bulletin to Electro- Voice, Inc., a in development labora- Bucha- tories, as a simplified teaching nan, Mich. method in KURMAN colleges and universities, for over-all and RELAY component checking of SWEEP FREQUENCY GENERATOR radio station and A new relay, with a split armature, motion pictures sound equipment, as well called the "24", The Clarkstan Corporation, is being manufactured by Los Angeles as magnetic recorders, and for testing and Kurman Electric Co., Inc., 34, Calif., announces the Clarkstan 35-18 37th St., Sweep servicing all types of aircraft and broad- Long Island City 1, N. Y. This New "24" Frequency Generator, an electronic device cast receivers and transmitters. Relay which was developed for automatic con- shows the behavior of audio and The specifications of the Sweep trols, other alternating Fre- keying, antenna changeover, burglar electrical apparatus with quency Generator, Model No. 125, are: alarms, and closed circuit respect to frequency and associated applications. phen- OUTPUT volts, open circuit; 50 milli - omena. -7 The following It is designed to operate in the au- IMPEDANCE- Internal impedance 200 ohms. characteristics are incor- dio range porated in this in conjunction with an oscillo- POWER CONSUMPTION -25 watts, 115v, 50 relay: scope. The complex signal is produced by and 60 cycles. Rated Sensitivity: .014 watts d. c. .3 V. A. a. c. . Adjusts to operate at .005 Watts, and from .01 to 155 Volts d.c. or a.c. The "24" contains the following general features:

. Contact flexibility Ingenious New . Magnetic circuit . U- shaped magnetic circuit of high permeability nickel alloy, especially heat Technical treated Methods . Steep curve provides dependable constant operation at low To Help You current values ARMATURE: balanced, solid, or split with Increase Efficiency insulated sections. Bakelite or mycalex bases are obtainable. MICROGROOVE CARTRIDGE Engineers of The Astatic Corporation, Conneaut, Ohio, worked directly with Colum- bia technical experts in designing their new FL-33 Pickup and LP -33 Crystal Re- placement Cartridge, for use with the new,

Light Projector Increases Thread Grinding Production Production of thread grinding machines can now be increased through the use of a light projecting device called the Thread Pack -up Projector. The thread profile appears long playing Microgroove discs produced in a viewing screen, magnified 20 times, thereby per- by Columbia Records, Inc., the mining accurate visual pioneer adjustments. pickup and microphone manufacturers have In operation the Thread Pick -up Protector is placed alongside announced. the thread grinding machine. A Dalzen Thread Grinder, Model No. 1, is shown above. While the machine is grind- As a result, they are being manufactured ing the thread, the operator, using the Light Pick -up to meet precisely the specifications set forth Projector, adjusts a "dog" on the next piece to be ground. Thread Pick -up Projector by the record manufacturer. When the "dog" and piece are then placed in the thread grinder the thread profile is automatically in location and An outstanding feature of the Astatic ready for grinding immediately. equipment is that the LP -33 Crystal Cart- Even the most Inexperienced personnel can "pick up the ridge, which has a permanent sapphire thread" using this instrument after only a few minutes needle with .001-inch tip radius for Micro- demonstration. Grinding is also done more accurately groove recordings, is readily interchange- and the viewer permits measurements of reliefs, notches, able with a companion etc. to .0005 inch. cartridge LP -78 for playing conventional 78 rpm records. The Efficiency of production can also he increased through the cartridges may be changed in seconds, by use of chewing gum. The act of chewing helps relievc anyone, because they fit firmly into position nervous tension and seems to make the work go easier in and faster. For these reasons, Wrigley's Spearmint Chew- the pickup on the same slip -in principle ing Gum is being made available more and more by by which cap and body of some modern plant owners everywhere. fountain pens are joined together. By Complete details may be obtained from merely slipping in either cartridge, you Acme Scientific Company have the proper pickup for 78 or 33-1/3 1457 West Randolph, Chicago ', Illinois rpm records and at the same needle pres- AC-75 sure of five grams.

40 AUDIO ENGINEERING OCTOBER, 1948 A novel design at the pickup's base is represented as eliminating tone arm resonances and providing assurance of perfect SPEAKING OF MICROGROOVE tracking. Output is approximately .5 volt; frequency range, 30 to 10,000 c. p. s. The FL Filter, an accessory for the pick- RECORDINGS up that assures best performane with high- quality speakers, is also being pro- David Hall Says . . duced by Astatic. GOODELL AMPLIFIERS the The )linnesota Electronics Corporation. from 204 Oppenheim Building, St. Paul, Minn. gained 100 the resultsCARTRIDGE were has added three new amplifiers to its lin, at of Goodell Dynamic Noise Suppressor Am- D -1405 we were plifiers. DICKERING and Model NSA -2 is a six -tube Dynamic Noise satisfact stortion percent of thti en- Suppressor with separate range and sup- we had once relieved which pression controls -two complete high -fre- troubles quency reactance tube filter sections with tracking one low -frequency reactance tube. Response is maintained under dynamic operating countered..." conditions to beyond 12.000 cycles per second with effective reduction of objectionable noise amounting to more than 25 decibels!

AVID HALL is director of repertoire of Mercury Classics and also author of "The Record Book," the most complete D compendium on recordings. He's an outstanding author - ity-he knows recordings. You can take the experience of David Hall as fact and apply it to all Microgroove recordings. You'll find that the Pickering D-1 40S Microgroove Cartridge eliminates distortion and track - ing troubles. It plays back exactly what has been finely en- graved in the microscopic grooves, insures a good signal -to -noise ratio and has the widest fre- This amplifier has all the facilities of quency range of commercial the larger model and is designed with ample reserve power for most home installations. reproducers. Separate inputs for radio and phono with front panel switch. Tube socket connec- D -1405 SPECIF!CATIONS tion provides power and input facilities for Stylus Radius 0.001' magnetic pickup pre -amplifier. Tracking Pressure 5 grams Model NSA -I is two chassis construction Whole and 18-watt output. This includes the pre- Stylus Material amplifier using 2 -6AT6 tubes as standard Diamonds, not chips equipment. This model includes dual G. E. Mounting Keystone Clip indicator eve tube for visual observation for Interchangeability of gate action. Dynamic Noise Suppressor, with Standard Pickering tone control sections and pre -amplifier are built on one 15 "x3 "x7" chassis. Power Cartridges supply and power amplifier on separate Finish Satin-Chron, chassis 17 "x3 "x7 ". Two -chassis construction minimizes hunt and simplifies cabinet installation. All power- supply filter capa- DUE TO INCREASED PRODUCTION AND REVISED citors oil -filled, 600 volt paper types. Push - pull 807 output tubes conservatively rated MANUFACTURING TECHNIQUES PICKERING CARTRIDGES at 18 watts. FOR STANDARD RECORDINGS ARE NOW AVAILABLE A demand for the Dynamic Noise Sup- AT NEW LOW PRICES pressor in an amplifier using triode output tubes has led to the development of a spe- For the highest standard of excellence in record reproduction, the cial two -chassis model similar to the above cleanest and smoothest response, perfect tracking, minimum record in every respect except for the use of a wear, widest frequency and the lowest distortion, specify Pickering push -pull driver stage consisting of 1 -6SN7

driving 4.68'4 output tubes in push -pull par- Cartridges . . . they're available allel, conservatively rated at 18 watts. through leading dealers and radio For further data, please write the manufacturer. parts jobbers.... model S -120M has a .0027" Sapphire stylus and made) RECORDER In the past ten years there have been D -120M has a .0025' Diamond stylus. no improved designs in recorders. The Robinson Recording Laboratories, 2022 Sanson St.. Philadelphia. Pa., now intro- Oceanside, New York.

AUDIO ENGINEERING OCTOBER, 1948 41 duce an entirely new lathe -type recorder screw pattern ". Five pitches are available for professional work in radio stations and with either inside or outside start. These recording studios. are 88, 96, 112, 120 and 128 lines per inch. The use of a perfected belt drive and Quick feed or spiral is obtained by a large dynamically balanced components the hand wheel. Automatic spiral and auto- "wow" factor has been reduced to an un- matic cutter pop -up is available on special believable .01% or one part in 10,000 at order. Provision is made for eccentrics and both speeds. Individual micrometer ad- safety groove cutting with the advantage justments on both 78 and 33 1/3 speeds that eccentrics are cut at the same speed make exact stroboscopic speeds at all times. and with the same stylus used on the re- Electronic balancing of motor armature; a cording. twenty -pound spring mounted sub -base; An accurate diameter scale, built -in mer- with a patented "right angle" belt drive, cury switch and convenient lift levers are Oilite bearings with reduce "rumble" to-65 db below record. designed for fast operation. The cutter car- tapered shafts ing level. throughout provide adjustment for years riage accomodates any type of cutter. A of wear and eliminate The highly precise ground thread feed unique suction tube is mounted under the all side play in the screw completely eliminates the usual "feed carriage. turntable. The maintainance required is at a minimum. The entire machine is built of hand finished aluminum castings and chrome plated. The cabinet of two -tone Duco, provides rack panel space of 154 inches, with ample storage space below. Rubber wheel casters provide adequate floor isolation. "Professional Junior" A feed -back Recording Amplifier, 60 watts, with high and low equalizers is also offered. COLLAPSIBLE For further data, please write the manufacturer. DOLLY for TV NEW RIBBON MICROPHONE The new Studio Ribbon Microphone CAMERA WORK 1., Ampérite has been designed to meet the Now being used by RCA, most exacting reproduction requirements of NBC, CBS & DuMont Broadcasting Stations, Recording Studios, and Public Address. The frequency range Video Stations. covered is 40 to 14,000 cps; output -56 db; harmonic distortion less than 1 %; discrim-

This Dolly made by specialists in custom - built TV and motion picture equipment.

Sturdily supports TV camera on tripod, also cameraman. It assem- bles quickly without tools. Collapses into carrying case 1' sq. by 6' ination with angle-60 to 10,000 cycles, less long. Ideal for Studio than 1 /10th that of a diaphragm micro- á Location TV work. phone. Unit is shock- mounted in rubber. The case is unusually rugged and will withstand a great deal of mechanical abuse. Rentals, Sales & Not affected in any way by altitude, moisture, or temperature. Available in 200 Repairs of all types ohms output, 50 ohms on special order. Also available in hi- impedance. 16mm & 35mm cameras, When used for public address, the feed and editing equipment. back is unusually low due to the fact that the frequency response of the microphone s is free from peaks. For further information, please write: Representatives for Hou st on D e v e l o p i n g Amperite C.., Inc., 561 Broadway, New Machines; Auricon SS camera & recording equip- York 12, N. Y. ment; Bardwell & McAlister lighting equipment; MAGNETAPE RECORDER Moviola and Micro Engineering editing equipment. LABORATORY Model SP850, a ruggedly constructed, FRANK C. ZUCKER Complete staff of trained technicians high -fidelity magnetic tape recorder, specifically designed for broadcast, recoding studio, CAmERR EQUIPrEnTC. available for consultation. and all semi- professional applications. is the 1600 BROADWAY IIEW YORK CITY latest addition to the Magnetape Recorder

42 AUDIO ENGINEERING OCTOBER, 1948

NNW line, manufactured by the Amplifier Corp. of America, New York City. This completely self -contained unit consists of an efficient tape -pulling mechanism, a recording amplifier with supersonic bias and erase oscillator, a playback amplifier, monitor amplifier and speaker, compactly housed in an enclosed relay rack cabinet, measuring 20%" high x 15" deep x 21" wide. Additional built -in equipment includes a vacuum tube type VU meter with an adjust - able sensitivity range of 18 db for indication of proper recording level on all types of magnetic tape. The meter is also used for measurements of bias and erase voltage amplitudes. An automatic program timer is also included for unattended recording of any desired program or sequence of programs. Among the outstanding facilities offered are selection of three tape speeds for any desired degree of frequency response. At 4" per second the frequency range is 60 to 5000 cycles, I± 2 dl) with a full hour's pla OFMAI of standard 1250 foot roll of magnetic tape; at 71/2" per second the frequency range is /,,,,,.w 50 to 9500 cycles, with one -half hour's play; at 15" per second the frequency range is 40 MODEL to 12500 cycles, with 15 minutes of play. II// tt737A" ///// h:ar.e h ////1 //// 4.41 , THE SUPER-

CARDIOID CRYSTAL MICROPHONE

Here indeed is the crystal microphone of all time -a high-output, unidirectional microphone Five input channels accommodate three that ranks far, far above ordinary microphones, one phono pickup or telephone crystal microphones. Why? Be- line, and a radio tuner. Individual adjust- cause it is a SUPER- CARDI- ments are provided for the supersonic fre- OID microphone. What does quency bias and erase voltages. The super- this mean? It means that the sonic bias frequency range is adjustable "Monoplex" is twice as midi- ' ` - o from 30 to 80 kc. Total recording and play- rectional as the Cardioid. It has wide angle pickup across the front back distortion at 400 cycles is under 3 %. of the microphone but it reduces sound pickup from the rear by with lese than The playback amplifier, 15 db -over a broad range of frequencies, and reduces pickup of 1% distortion at rated power output of 15 random sound by 73 %! The "Monoplex" employs the same type watts, is equipped with individual high and network used in the highest -cost Shure low frequency controls. Treble equalization of acoustic phase -shifting Broadcast microphones. New "Metal -Seal" crystal. The case is ranges from + 13 db. to -10 db. at 10,000 sound or up- cycles. Bass equalization ranges from +10 pivoted at the rear and can be pointed toward desired excellent db. to -10 db. at 100 cycles. Twelve dif- wards for horizontal plane pickup. The "Monoplex" is ferent output terminals, ranging from 2 to for thigh- quality public -address, communications, recording and 500 ohms, are included. similar applications. It will operate under adverse conditions of For additional information regarding this background noise and reverberation where a conventional micro- laboratory magnetic tape recorder, includ- phone would be practically useless. Make the most of the "Mono - ing a complete set of technical specifications, plex " -it is destined for a performance record unique in crystal write for free catalog # 4904 to Magne- microphone history! phone Division, Amplifier Corp. of America, Yaa./aetured under Sh.re pafwY iw.d awl 'mediae. 398-4 Broadway. New York 13. N. Y. r,.n.n.ed .nde. patents of ar..r. o.a.ropa..a. C.. REMOTE CONTROL AMPLIFIER Model "797A" CODE: RUMON List Iris*: $35.50 A new remote -control model of the Brook high quality amplifier, designed especially for custom -built radio -phonographs, has recently been introduced by Brook Elec- SHURE BROTHERS, INC. tronics, Inc., 34 DeHart Place, Elizabeth 2, New Jersey. Microphones & Acoustic Devices (Continued on page 44) 225 W. HURON ST., CHICAGO 10, ILL. CABLE ADDRESS: SHUREMICRO

AUDIO ENGINEERING OCTOBER, 1948 43 Designated Model 10C3, the new ampli- Nau< vast can /facie fier is built primarily for use in the home by music lovers and serious high-quality- *DYNAMIC NOISE SUPPRESSION audio enthusiasts; however, its audio char- with Your Present Radio -phonograph or Amplifier acteristics are such that it is equally well- 3 suited for application where highest engin- These simple steps add eering standards prevail. In designing the realism to your music new model, attractiveness and flexibility reproduction. were given equal consideration with electrical performance. 1. Plug in the "Little Won- der" *Dynamic Noise Suppressor between your pick -up and amplifier. 2. Plug in the socket adapter to the power - tube socket. The remote control, for setting the 3. Insert the matched low - exact degree of suppression you find needle -talk pick -up in most pleasing, can be mounted wher- your pick -up arm. ever you wish . at the instrument That's all that is necessary or even in another part of the room. to The 3 -tube suppressor unit can be reduce background noise with placed anywhere in the cabinet. negligible loss of depth and The "Little Wonder" realizes the The unit follows the Brook -established brilliance . . . you giving a full capabilities of your present practice of using low -mu triodes through- gratifying sense of "presence" equipment; can be used, with suitable out. Frequency response is flat within 0.2 in your music reproduction. pick -up, on the new, long -playing db from 20 to 20,000 cycles. LOW PRICE $82.50 list records, too. For full specifications, Full technical specifications and descrip- Includes tubes, matched pick -up, remote write Dept. A. Or, even better, hear tive material will be supplied free by the control, cables, fittings, adapters, instructions. a demonstration at your distributor's. manufacturer. *Licensed under U.S. and foreign patents pending and issued. MIDGET ATTENUATORS The Daven Company, of 191 Central Avenue, Newark 4, N. J., announces the addition of a new line of attenuators featuring decreased dimensions. These controls of reduced size were designed for use in high quality equipment where mounting space is limited and precision construction required. The same mechanical and electrical construction offered in the larger Daven attenuators are supplied in the new type units. The manufacturer will gladly provide further information upon request. big MICROPHONE BOOM STAND mates The new microphone boom stand pictured er below. specially designed by RCA, permits lanutactur instantaneous one -hand movement of a studio on microphone over an arc up to 13 feet in NUTS!NU radius and as high as 21 feet above the floor. The stand is adjustable in height from SASAVINGS six to nine feet, and the boom when closed has a radius of only five feet. Consisting of

"WE HAVE LOPPED a big slice from our annual outlay for nuts," reports large appliance manufacturer ... "By using Westfield Nuts and concentrating on fewer types and sizes. Production was speeded on assembly lines." Westfield Nuts are uniform and accurate. Standard or Custom Made, they're milled to exact specifications . . . Send drawings and data for recommendations and prices. two telescoping four-foot sections. the boom Or have a Westfield engineer study your full requirements ( RCA Type KS-4A t has a controlable and suggest arc improvements and savings that really count. of approximately 180 degrees, and is counter- Address Department,, 02 balanced so that it may be freely elevated and rotated to any point within its range. The sturdy stand is of all -steel construction with satin chrome finish, except for the low gravity cast iron base, which is finished in dark umber gray.

44 AUDIO ENGINEERING OCTOBER, 1948 RECORD REVUE EDWARD TATNALL CANBY PzoleQV.íoKaQ (Because of unforeseen difficulties, we are obliged to postpone until next month Mr. Canby's interesting discussion o/ the characteristics o/ the various noise suppressors now RECORDING on the market. However, his usual monthly TAPE review o/ recordings is presented below.) eatÆFM QUALITY

Richard Strauss, Salomé: Dance of the a«d EASY PORTABILITY Seven Veils. Moussorgsky, Khovantchina: Dance of the Persian Slaves. Royal Philharmonic Orchestra, Beecham. RCA Victor 12 -0344 (I) 12 -0239 (I) Two singles from Sir Thomas Beecham's orchestra, both with a pseudo -oriental touch, both opulently, magnificently played, both full of fine orchestral color, atmosphere, neither of much real importance musically. Technically, there is the usual mystery-both of these have gorgeous acoustics, fine big liveness, impeccable balance of various parts of the orchestra. But the Moussorgsky regis- ters with me as having almost no higher range over 5000 or so; the Strauss seems to have useful components in the 8000 range. Theo- Th e b a sic Mognecoraer retically the two should be a pair. Is it the recorder mechonism (PT6.A) music that is different? Was there in fact a difference in the recording -even if merely a wet day and a dry day? ... In any case, The New 3 Element neither of these can compare in range and in cleanness, especially in loud passages, GHECORDER with the average ffrr and similar recordings, but excellent acoustics, fine micro - phoning easily make up for the lack. MA The Magnecorder meets the highest broadcast standards, November. (The Seasons, Tchaikowsky, June; and it costs you less. You buy and combine only the op. 37A) units need: Iturbi, pianist. you Magnecorder (PT6 -A) - Basic recorder mechanism. RCA Victor 12 -0242 (I ) (PT6 -P) Recording and repro- Two short and melodious piano pieces Portable Mixer -Amplifier - from a series of monthly tid -bits composed ducing portable field amplifier. Can be used as high - by Tchaikowsky about the time of the 4th quality remote amplifier. Mixes three low -level micro- Symphony. Both make attractive short- phones. piano listening. "June" will be period (PT6 -R) Recording and reproduc- familiar to most listeners as a salon -music Rack Mount Amplifier - stand -by of the better sort-a really lovely ing amplifier for studio rack mounting. With PT6 -A melody, wisely not over -exploited (at least makes complete studio recorder -reproducer. 41 in this, its original form) by the composer. Iturbi is good for this stuff. Piano is over- percussive; not much on the bass side. Ted Pt:vrt.a$Ee! 7t' d 7lexcdle! Weight - PT6 -A, 23 pounds; PT6 -P, 29 pounds. Liszt, Sonetto del Petrarca, no. 104. Wow and Flutter - .2% William Kapell, piano. Frequency Response - 40 to 15,000 cycles; + or - 2 db. RCA Victor 12 -0342 (I ) Tape Speed - 15 inches or 71/2 inches per second (Inter- Another interesting short piano work, ro- changeable). whipped finger work, mantic, full of cream Synchronous 1/50 HP. but not as crudely flashy as some of Liszt's Motor - rather outdated pianistic fireworks, and this Single Control has a good leal of seriousness to it that Rewind - 45 seconds. makes it an attractive piece of its kind. Reels - Standard 7 -inch 8MM film reels. Kapell might be accused of insufficient never throws tape. Instantly interchange- depth in the playing; personally, I prefer Current failure it; Liszt, these days, so easily gets too able from portable to rack mounting. gushy! The piano recording here is excel- Write today for detailed specifications lent, the best Victor has produced recently. I'd say. Percussion notably absent, plenty of warmth and closeness, good brilliance too.

Bizet, Les Psehsurs de Perles: A Cliffs Voix. Halevy, La Juive: Rachel, quand du Seigneur! Richard Tucker, Columbia Opera Orch.. INCORPORATED Pelletier. aree(1414d Columbia 72577 -D ( 1 ) 360 N. Michigan Ave. Dept. A Chicago 1 A conventional French opera single, with the advantage of very fine recording. Mr. Tucker knows how to give, but he should

AUDIO ENGINEERING OCTOBER, 1948 45 mind his French accent. There is much singing about "La noowee" here that will make French students blanch with horror. (The French probably wouldn't mind a bit.) The Bizet is a nice sob -song, the best The NEW Brook of the two by far. Debussy, (1887)_ La Demoiselle Elue (The Blessed Damozel). 10 WATT Bidu Saviìo, Rosalind Nadell, Women's Chorus of Univ. of Penn. Philadtlphis Triode Orch. Ormandy. All- Columbia MM 761 (3) A beautiful performance of an early Amplifier work of Debussy, highly impressionistic, Modal 12/13-Two-unit remote- control amplifier. Doc - atmospheric, reminiscent of "The Afternoon orator -styled control cobinel for living -room use. Also of a Faun" in style. Bidu Sayio, well known available as Modal 12A2 for table or rock operation. for her more joyous performances of opera, HERE it is at last -an amplifier of intermodulation and harmonic dis- is surprisingly fine here in the extremely incomparable performance - tortion reduced to the vanishing French music. The chorus has been trained built up to the highest standards of point at any power up to maximum. well and blends perfectly into the prevail. Brook engineering -in the moderate Frequency response is flat within ingly nostalgic atmosphere. The music is price field. 0.2 DB from 20 to 20,000 cycles. s bit diffuse and perhaps too long- drawn- Within the range of its power rat- Now for the first time, the distor- out, but few of us will mind that, in view ing the new Model 12A3 is equal in tion -free all- triode performance of the purity and sincerity of style it shows. all respects to the world- renowned which the Brook Amplifier alone A difficult piece to record -long silences, Brook 30 -watt amplifiers. provides is available at a new low stretches of very low level, little to cover cost. Orders will as The use of low-mu triodes in all be filled rapidly up when the 78 rpm " rhythm" so annoying as production permits. stages, together with Brook-designed when you can hear it. Everything here is transformers available in no other Write TODAY for copy of de- exposed to the worst that the phonograph amplifier, permits the cleanest am- tailed Distortion Analysis and Tech- medium can do. Considering this, the al- plification ever achieved ... with nical Bulletin AK -10! bum is excellently done from the technical Dealer Inquiries Invited- Standard DIcccccts Apply viewpoint, though perhaps it might have been better to bring up the level in the low spots, in spite of Debussy. (Incidentally, The BROOK High Quality Amplifier this is an excellent recording on which to experiment with various forms of noise sup- - Designed by LINCOLN WALSH pressors, since the removal of hiss and especially of pounding 78 rpm effects is vital 6RL.IOK BROOK ELECTRONICS, Inc., to the music.) 34 DeHart Place, Elizabeth 2, N. J. Songs of the Auvergne. Madeleine Grey, orch. conducted by M. Elie Cohen. Columbia MM 758 (3) This is a reissue, brought about directly INTERIJLATION and HARMONIC ANALYSIS by a surprising wave of public demand fos- tered by our publication -in -law (via this column!), the Saturday Review of Record- Faster Simpler ings. Interesting to find that such a campaign by individual record buyers actually influences a big company. These are most More Complete interesting French folk songs, sung capably tt ith th, by Madeleine Grey in the best French man- ner, with a very elaborate and coloristic PANORAMIC SONIC ANALYZER orchestral accompaniment. Personally I don't like the latter; I feel that folk music Backed by over 15 years of Panoramic know -how and ingenious forward looking engineering, the of any sort, (unless flatly borrowed as Panoramic Sonic Analyzer now introduces the thematic material for larger works) is much many time -saving advantages of the Panoramic better left to tell its own musical story- technique to the field of audio measurement. The which it can very well do. But if you don't Panoramic Sonic Analyzer Model AP -1 is a scanning instrument which automatically measures object to elaborate settings, you will find the frequency and amplitude of audio components these songs worthwhile and unique. The between 40 and 20,000 cps . . . thus distortion recording is clear, but limited in range and analysis with the AP -1 is somewhat distorted. Nothing to bother a Faster... a continuous overall view of the audio spectrum is displayed in one second music lover. Some signs of worn masters. ... the effects of changes in design parameters or audio voltages can be instantaneously Diamond, Music for "Romeo and Juliet ". For Use In evaluated Little Orchestra Society, Thomas K. Scher - Audio Research Laboratories Simpler ... tedious roint by point tuning is man. Recording Studios eliminated . the instrument is highly stable Columbia MM 751 (3) Broadcast Stations and requires minimum balancing a self contained intermod- Sound Film Studios More complete .. "Romeo and Juliet" is re- interpreted on ulation analyzer section enables measurement latest ideas, every Universities than the stage, according to of individual envelop components rather few years in music the same seems to Production Lines just total cross modulation distortion . . . -and Uses distortion as low as 0.1'S is measureable . be inevitable. But though the 1880 "Romeo" Other tuning by and miming weak components is Noise study would no doubt seem preposterous on our eliminated . . . wide range voltage readings in music the style in High frequency vibration -50µv -á0"v present -day stage, analysis Romeos and Juliets has long been set by Investigate the advantages of using the AP -1 Tchaikowsky and it's not easy for many a Acoustic investigation for your audio problems. WRITE NOW for Complete Technical Data. listeners to swallow any other conception. interestingly the National Electronics Conference Diamond's -which compares Be sure to visit us in Booth #1 at with Prokofieff's music for the same-is in a sense deflationary: after Tchaikowsky, any "Romeo and Juliet" music is bound PANORACft RADIO CORPORATION to have that character. It doesn't exult and The music is not particu- 92 Gold Street New York 7, N. Y. lament and strut. larly complex nor very dissonant; it is pre- 46 AUDIO ENGINEERING OCTOBER, 1948

f t wailingly modal, a harmonic tendency noti common among American composers though the English have long been addicted to it. (Play a scale, white notes, from E to E on the piano for the type of modal scale Diamond uses.) Orchestration is rich and conventional. Recording here is good wide range, but not as expansive in its liveliness as the music could stand.

- ryL¢we WARD PROMOTED As a result of increasing company activity in the recording disc field, Don- ald E. Ward has been appointed Ahorn speaker of conventional type may Sales Manager or for Reeves resemble a Racon horn or speaker in Soundcraft Cor- outward appearance. But close examination poration, 10 East of a Racon unit reveals internal differences - New 52nd Street, better mechanical con- York City, it has refinement of design, been announced struction, sturdier materials and other special by A. I.. Travis, Jr., Vice -President. Former- features that represent ADVANCED ENGIN- ly with Audio Devices and Muzak, Mr. Ward EERING. It is these exclusive features that Reeves Soundcraft in 1946 as Assist- joined give you superior performance in any Racon ant Sales Manager. unit. Higher efficiency over wider ranges. distortion. Uninterrupted service. 1 Freedom from The long life that protects your investment.

SOUND DISTRIBUTION RE 1 -RACON RE- ENTRANT TRUMPET -IS llront page 311 Designed to deliver highly concentrated small the vertical coverage angle, and then sound over long distances. Occupies air column of I8 ". Inside by ratio using the horizontal space yet has an again the tone arm of aluminum castings; center section coverage angle. and bell heavy gauge aluminum spinning:. If the coverage angle is unknown, it Very efficient; free from resonant effects. Ruggedly built, practically abuse proof. may be approximated for speech by Length 9 -7/s "; bell diam- moving off the axis of the speaker until eter 8 -3/4 ". Swivel type the articulation is no longer satisfac- wall bracket. tory. Twice this angle off the axis is the coverage angle. In general, speakers of this sort become more directional with increase in frequency" and

loss of articulation is a fair measure of the reduction of high frequency response. This is a very practical solution as well, since it approximates 2 -RACON RE- ENTRANT RADIAL TRUMPET 5R -35R. the actual use of the system. Three Has all of the construction features of precautions must be taken in using RE -35 such as non -vibratory center section, this method of determining coverage heavy aluminum castings, etc. All reflect- angle. The speaker must be mounted ing surfaces of RACON PATENTED far from reflect- ACOUSTIC MATERIAL to prevent reson- in free space enough ant effects prevalent in all large reflecting ing surfaces to eliminate noticeable surfaces. Delivers sound with even intensity impedance on all units: 15 ohms, except is 8 ohms. Special echo; the observer must be at least 25 over a 360° circumference. Length 16 "; dwarf size -which request. feet and preferably 50 feet from the width 17 ", Type SR -60R length 341/2 "; ohmages on width 36 ". speaker and must maintain the same NOW FURNISHED WITH 3- PERMANENT MAGNET HORN WATERPROOF CASING distance as he circles the speaker UNITS All units may now be had with heavy throughout the test; the level at the Highly popular in all types of service. cases, forming a hermetic- Many improvements. Two groups with spun aluminum observer's position must not exceed ally sealed, watertight housing for out- Alnico V Magnets and Alnico Blue Dot Return- door use, at slight extra cost. that of normal conversation. Magnets. Steel parts plated to prevent ing to the 80° speaker, notice that corrosion. Also fitted with corrosion proof Write for Catalog of the sound level increased from 80 db metal or plastic diaphragms. Voice coil complete Racon line. at 160 feet to 96 db at 29 feet from the speaker. To avoid exposing the near- RACON ELECTRIC CO., Inc., 52 E. 19th St., New York, N. Y. by listeners to uncomfortably loud levels, the speakers should be located rSpeakers e;L distance above ground. bora Utl principle of using coverage an- flACON Horns AUDIO ENGINEERING OCTOBER, 1948 47 gle and "throw" to determine a coverage area for a single speaker can be used to locate speakers in a multiple speaker system. In many systems in which the direction of the sound source is not important (for example, a system to cover a parking lot), a number of speakers may be spotted over the area to be covered. The out- RANGERTONE door auditorium or concert bowl is a complex problem. Rettinger and Stevens's have indicated some of the is the ONLY magnetic tape recorder that difficulties involved in an installation of this type, is commercially ready NOW to meet tie although theirs were ag- gravated by the unusually large size new N A B recording speed standards. You of the bowl. can switch at will to any one of the three; Conclusion The above examples do not 71/2", 15" or 30" a second. illustrate all the types of systems that a sound engineer may be called upon * Total harmonic distortion does not to install, but they do illustrate many exceed 1% overall for any frequency of the problems he will face, however. from 100 to 6,033 cycles and 2% The engineer must still undertake his from 30 to 100 cycles. Orher own jobs with good judgment and a willingness to experiment. specifications on request. Experience with actual installations will teach him more than this or any other article. Rader 17. 0. L. Angevine, Jr. and R. S. Anderson, "Facts about Loudspeakers" Part II, Audio RANGERTONE, Inc. Engineering, Vol. 32, No. 3, p. 25. March, 1948. 73 WINTHROP ST. NEWARK 4, N. J. 18. M. Rettinger and S. M. Stevens, "Sound Reinforcement in the Hollywood Bowl",. Audio Engineering, Vol. 32, No. 2, p. 15. February, 1948. r cS'vrane ,lUrue Qeøie4aIo4 RADIO SYSTEMS

( /rum page 23] meter should be connected in place of the gain -setting resistor, Ro. The antenna coil should be adjusted when all the individual tuners are connected together and to the common antenna terminal, though for flat -topping it will be necessary to make preliminary.' adjustments on the single chassis. If an f -m oscillator and 'scope are not available, the tuner can be aligned approximately by ear, although it is doubtful if the response curve will have the ideal flat -topped shape. How- MODEL 71 SPECI FICATIONS ever, if no other means is available, FREQUENCY RANGE: 5 to 100,000 cycles. fair results can be obtained by this WAVE SHAPE: Rise time less than 0.2 microseconds with negligible overshoot. method of alignment and the quality

OF OUTPUT VOLTAGE: Step attenuator giving 75, 50, 25. of its output should be at least as MANUFACTURERS 15, 10, 5 Generatos peak volts fixed and 0 to 2.5 volts continuously good as that of a conventional re- Sbndud S,gnel variable. Pulse Genet tots ceiver. SYNCHRONIZING OUTPUT: 25 volts f M Signal Generators peak. Geneuton R. F. MODULATOR: 5 volts After the channels sause Were maximum carrier input. are all aligned V ollmeltis Translation gain is Vacuum tube approximately unity- Output imped- and installed, they may be adjusted field to Rudio Noise L ance is 600 ohms. UNF Meters equal audio output levels by selecting Strength POWER SUPPLY: 117 volts. 50 -60 cycles. Capacity Bridges a suitable value for R9 DIMENSIONS: 7" high x 15" wide x 71/2" deep, overall. in each. Prob- Megonm Meters Indlcaton able valises for use in metropolitan PNne Sequence Test lelerision and areas will range from 1,200 to 2,700 FM MEASUREMENTS CORPORATION ohms. Too low a value may cause os- BOONTON © NEW JERSEY cillation, but the proper point at which

48 AUDIO ENGINEERING OCTOBER, 1948 the detector should work is the important parameter, and it is doubtful if it will be necessary to reduce Rn below 1.0111 ohms. Each r -f stage is essentially a tuned -grid, tuned -plate arrangement, and such a circuit is prone to oscillate if external feedback GOODELL is sufficient or if the gain of the stage increases too greatly. The d -c voltage at the cathode of Fa should be from 17, announces 3 important to 20 volts for a 200-volt plate suppl. and this value is dependent upon the r -f signal applied to the grid. new developments in With a number of these channel - it is possible to select stations liy means of a single -gang rotary stvitt L. all filaments being heated whenever audio electronics the system is turned on. For use in recordin: st mhos, however, an additional stage should be added, together with a tube -to -line transformer, as indicated in Fig. 7. One section of the THE GOODELL MAGNETIC NOISERASER Conditions New Tape Restores Old Increases Dynamic Range The Goodell Magnetic Noiseraser consists of a carefully engineered tuned magnetic circuit designed to eliminate all signals and background noise from entire reels of magnetic tape in a few seconds. Recom- mended for use with brand new tape before recording to minimize inherent random noise. A few seconds in the Nois- eraser will completely remove saturation signals. Restores tape to new condition and permits indefinite useful life with minimum background noise.

0.1 MEG SPECIFICATIONS: Aluminum chassis, hake lite top, 7' x1 ': "x8 ". POWER REQUIRE- MENTS: Operates on 110 -1.20 volt. Gû cycles. On -off switch, pilot light and fuse . Mag- netic tuned circuit designed for optimum de- magnetizing fields through the tape. Oil-filled Paper capacitors and double glass insulated Fig. 7. Alternative output arrange- coils insure trouble -free operation. ment suitable for use in recording studios where several channels may be MODEL N -7 $57.60 MODEL N -14 $97.50 used simultaneously. Transformer for 7 -inch reels, net for 14 -inch reels, net matches output tube to 500 -ohm line. THE GOODELL DUPLEX REPRODUCER ARM 12A177 twin triode is used for the de- For Use on Both Standard and LP Records teetor. trhile the other is the audio The new Goodell Duplex Reproducer Arm is designed cartridges or with G.E. or Pickering cortridges in- to carry simultaneously two high quality magnetic stalled. Sapphire or diamond stylii for standard output stage. Some such arrange- cartridges for standard and micro -groove records. records. Diamond for LP micro -grooves. Special con - Change -over is accomplished with one roloting m siderotion has been given in selecting optimum sus ment is required if it is necessary to ber- automatic switching and tracking pre ad- pension structures, compliances and bearings to record more than one station simul- ¡vstment from 15 grams for standard records to 5 provide perfect tracking with minimum stylus and grams for LP micro -grooves. May be supplied without record wear, as well as maximum stability. taneously. Write for prices and descriptive catalog sheet. For a large residence installation. several the low -impedance outputs of The Goodell fixed tuners may be fed at approximately zero level throughout a home. DYNAMIC NOISE SUPPRESSOR AMPLIFIER with a selector switch and an amplifier Here is the perfect monitoring and audition am- at each room where a speaker is de- plifier, ideal for custom built radio -phonograph sired, or any of a number of switch- installations, for schools, industry and ing arrangements may he employed. homes, including every feature desired There is practically no limit to the by the audio enthusiast. The GOODELL applications to which the fixed tuner six -tube version of the Dynamic Noise strip may be put, and their use will Suppressor effectively reduces objec- depend upon the ingenuity of the con- tionable noise while maintaining wide structor. The cost for each tuner range response. should be somewhere around $10.00, MODEL NSA -2 Licensed under Hermon Ilo,mer Scott without pre -amplifier $147.50 without the transformer, and for those Par. Pend. for use o,It m phonograph MODEL NSA -2, and phnnofrnph d'. tmru.n 'seems. $154.50 who wish convenience and quality, with pre amplifier they are considerably superior to more conventional receivers. The Minnesota Electronics Corporation St. Paul 1, Minn. I

AUDIO ENGINEERING OCTOBER, 1948 49 -)

used on AM, but, at the present time, I have no AM tuner and do not need any. For listening to records, I use a crystal pick -up which directly gives me a push -pull output: I unsoldered the grounded end of the pick -up and connected a second wire. The best bet for the music lover, at least in New York City, is to listen to FM transmissions of live concerts. NOTES ON A 6AS7G AMPLIFIER A "disadvantage" of the high quality 111í a few months, I have been Another feature of the amplifier is amplifier is that the listener perceives using a 6AS7G amplifier. The that it is push -pull throughout. No the difference between records and live F final stage is practically identical inverter is used. The problem is solved music much more than on an ordi- tu the final stage of the amplifier by grounding the middle point of the nary radio. described in the March issue of discriminator. The de- emphasis cir- Triodes, to my ear, sound better AUDIO ENGINEERING. Previously, I was cuit is split into two symmetrical re- and cleaner than anything else. I have using a pair of 2A3s. The 6AS7G sistors and two symmetrical conden- listened to directly -coupled push -pull is superior to the 2A3s in every re- sers. I have never seen it done, but 6L6 amplifiers, with plenty of feed- spect. The amplifier is used most of it is, it seems to me, the most natural back, and my conclusion is that a the time with un FM tuner, in New solution. A similar solution could be beam power amplifier always lacks York City. It has relatively little pre -amplification. My experience has been that most amplifiers have too much pre -amplification, with a corresponding increase in hum, distortion and noise. With a single push -pull triode stage, using two 6C5s, I have too much volume for a large living room. The interstage transformer i. an UTC LS 22. The amplifier is so silent that, the tuner being cold, the ear has to touch the speaker before the slightest noise can be perceived.

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Enclosed find S for Center channel to keep magazines fastened in Binders position.

Round corners, hinges and backbone fully rein- Name forced. Address Rich 61uá Dupont Fabricord -stainproof and washable. City State $2.50 postpaid. Foreign orders add 50c.

50 AUDIO ENGINEERING OCTOBER, 1948 something that a triode amplifier- the plate current up to a mil or so, or the grid impedance at various settings. with 2A3s or, it fortiori, a 6AS7G,- taise the plate voltage to 40 or more. Knowing this, the set -up can be ad- has. The triode amplifier gives a nitwit Read the output. voltage. Insert enough justed for maximum power step -up. more natural sound. while even the secondary load to bring this voltage It is obvious from the above data best beam power amplifier gives the down to half. The plate impedance is that there is an advantage in operat- impression that the sound is "recon- then matched. The secondary load ing at high plate values. The gain is structed". The only instrument I use (say 500 ohms) times the square of the no more in ternis of voltage in and is an old voltmeter; hence, I cannot step -up ratio of the ransformer (for out, but always bear in mind the give figures on distortion for my am- example, 4) gives s.tlfttl ohms plate input circuit consumes power. As plifier, but, to my ear, it sounds better match. the plate voltage rises the input im- than any commercial or custom ampli- lit the general case, the load is now pedance goes up markedly, so we gain fier I have heard until now. The re- watched for all plate voltages so long in power ratio by the lessening grid production, on voice or music, is re- as it. is above 30 or so. Incidentally, driving power. The higher grid im- markably clean and natural. the treated crystals will often work pedance also makes it easier to cascade -Join VAN HEI.IF:NIroBT wolf at 100 volts on the plate. units. As the plate voltage is varied, check [Continued on page 521

ALTEC MOVES Offices of Altec Service Corporation SÑd4cIe WARD LEONARD and its manufacturing subsidiary, Altec Lansing Corporation, have been moved from 250 West 57th Street to 161 Sixth Avenue, VITREOUS ENAMEL New York 13 as of June 28th. The new telephone number is Algonquin 5-3636. 44cd The warehouse and electronic laborator- VaauQatea Pzoteete ies now at 533 West 57th Street will be moved to the new address on July 23rd. The Altec Service and Altec Lansing of- RESISTORS fices will occupy the entire eleventh floor at 161 Sixth Ave. The New York district of Altec Service Corporation remains in their present offices at 250 West 57th Street eVide441 GERMANIUM CRYSTALS /0444/- `T iiit° aa44 a, goúQugg I tom page 33] fi method has three conditions. At zero bias, zero signal, and no plate voltage, The extra service you get from Vitrohm Resistors is due in great measure to the the impedance is 600 ohtns. With 0.5 exclusive vitreous enamel coating de- volts positive bias, but no plate voltage, veloped and produced in the WARD it falls to 150 ohms. f==. LEONARD laboratories. This Vitreous enamel forms a perfect bond with the As the plate voltage is raised front core, the wire and the terminals . zero, the grid impedance rises from 125 quickly conducts away generated heat the winding. to as high as 2,000 ohms. This is with . insulates and protects It is the ideal armor against mechanical plate loaded and signal in both grid and electrical breakdown. Vitrohm Re- and plate, and with signal effective sistors are available in a wide range of etiOldo your across the grid of 0.1 volt rms. types and sizes for every need from WARD LEONARD Distributor. This series of apparently unrelated facts is so unusual that it takes quite Radio & Electronic Distributor Division a while to readjust our thinking and WARD LEONARD ELECTRIC CO. attempt to design the equivalent net- 53 -R West Jackson Blvd., Chicago 4, III., U.S.A. work for the crystal. One single help ullNrl-uulurí- FOR CATALOG D -130 is that apparentl, reactive components SEND Gires quick, helpful data and can be neglected at 1 lie, so we appar- information on the many stock lItt, ently must build our equivalent cir- types and sizes. Also includes cuit with meshed one -way non -linear Radio Amateur Relays. resistors and an occasional two -way resistor. Apparently feedback inside the crystal also takes a rather unusual form. ECIJZZ +tC With the setup of Fig. 1, after you have established which contact controls the other, and have overshot and WARD LEONARD tapped the crystal, remove the sec- RELAYS RESISTORS RHEOSTATS ondary load from the output trans- ID former. Put in an 0.1 volt signal, run

AUDIO ENGINEERING OCTOBER, 1948 5i Control After Flashover after a 45 second series of tests, it the fundamentals. We also have sev- The trea.ed crystal shows control at again becomes operative. eral other holders we shall show in the high plate currents about one -third the The writer regrets that full time next issue. time. For instance, with a thousand on this job has produced more confu- It is obvious that circuit design is ohms in series with 130 volts, de., sion than light. However, we have had going to be difficult. We must come grid variation of one volt gives plate as a primary objective the production up with simple circuits of few com- variations 41) (i0 from to tua, a dc gm of a mechanical design we have con- ponents. One very promising feature of 20,000. This lasts about 45 seconds fidence in and that allows instant is the apparent unlimited life of the on an average, and then the assembly change of crystals without extensive unit, which can easily be Made shock- heats up and becomes inoperative. manipulation of catwhiskers. We now proof. There are many applications where the are lucky enough to have such a struc- We have done considerable work us- amplifier is used at a low duty cycle ture, so our number of tests ncr hour ing the overshoot characteristic as a and for short periods where this would can be greatly increased. We feel this basis of working a relay and also for be usable. subject is important enough to war- counter circuits, but since this is au On cooling off for about a minet,-. rant n real basic effort to understand audio engineering magazine ,this application is beyond our scope. COMPONENTS At this stage we must do much BETTER spade work, particularly in understand- BUILD ing the basic apparent disadvantages BETTER of the unit, so we can either design around them or turn them to advan- REPUTATIONS tage. It is well to remember that the field of electronics is so extensive that we have endless possibilities of taking these unusual characteristics and put- tin!: them to work. . .. S P E C I F . EFlilenca No product is better than its weakest CM 20 Available in "A", "B". "C" and component. That's why more and more "D" characteristics SPEAKER ENCLOSURE manufacturers specify El -Menco - the 2to 1500 mmf. in tolerances down to ±1% or .5 mmf. at 500 D.C. mica capacitor that always gives customer working voltage Wont page 251 satisfaction and builds better reputations. 6 -dot color coded 'Whichever is greater CM 35 Available in "A", "B ", "C ", "D" resulting in a clean break. A rough clean- CM 15 (93z" x 1/2" x 3/16 ") and "E" characteristics ing powder, mixed with sand and water is For Radio, Television, Other Electronic Minimum tolerance 1% remain- Applications swished around in the half tube 500 D.C. working voltage ing, thus cleaning the inside. Clean var- 2 to 420 mmf. capacity at 500v DCA 6 -dot color coded nish is then poured into the tube and the 2 to 525 mmf. capacity at 300v DCA ARCO ELECTRONICS rods dipped. Withdraw the rods slowly, Temp. Co- efficient ±50 parts per million Sole DisMbertan for Jobbers and Retailers sold at the same time, blow softly and per degree C for most capacity values. in United States and Canada. evenly at the point the rod emerges from 6 -dot standard color coded 135 Liberty St., New York, N. Y. Write, on firm letterhead, for samples and catalog. the varnish. This removes all air bubbles. Stand rods on end to dry. THE ELECTRO MOTIVE MFG. CO., Inc. Willimantic, Conn. The next step is to mount the speaker baffle and to finish the inside of the a heavy blanket STATEMENT OF THE OWNERSHIP, MANAGEMENT, CIRCULATION, ETC.. cabinet. Fold about QUIRED BY RE- THE ACTS OF CONGRESS OF AUGUST 21. 1912, AND MARCH 3, 1933 four times and place newly painted, of AUDIO ENGINEERING, published monthly at New York, N. Y., for October 1, 1948. but dry cabinet, front side down on Slate of New York tt County of New Tot it to do the following work: Before me, a Notary Public in and for the State and county aforesaid, personally op- Around the edges of the speaker Peered John H. Potts, who, having been duly sworn according to law, deposes and says that he is the Editor of AUDIO ENGINEERING, and that the following is, to the best of his aperture and inside, screw the four knowledge and belief, a true statement of the ownership. management, etc., of the aforesaid wood strips which are 4-inch tile': publient on for the date shown in the above caption, required by the Act of August 24, 1912, as amended by the Act of March 3, 1933, embodied in section 637, Postal Laws and Regula- anti 14- inches wide. These give great- tions. to wit: er rigidity to the front section and I. 'l'lurt the names and addresses of the publisher, editor, managing editor and business manager are: Publisher, Sanford R. Cowan, 1620 Ocean Ave., Brooklyn 30, N. Y.: Editor: also set the speaker battle and grill John H. Potts, 1737 York Avenue, New York, N. Y. ; Managing Editor, C. G. McP.roud, back about -inch to allow room for Peter Cooper Road, New York, N. Y.; Business Manager, S. R. Cowan, 1620 Ocean Ave., '/a Brooklyn 30, N. Y. the back mold to fit into place later. 2. That the owners arc: Radio Magazines, Inc.. 342 Madison Ave.. New York 17, N. Y.: John H. Potts. 1737 York Avenue, New York, N. Y.; and Sanford R. Cowan, 1620 Ocean Tack the grill cloth at the four cor- Ave., Brooklyn 30, N. Y. ners first, then tack along edges every 3. That the known bondholders, mortgagees, and other security holders owning or holding 1 per cent or more of total amount of bonds, mortgages, or other securities, are: None. three inches. Cut round hole of prop- 4. '''hat the two paragraphs next above, giving the names of the owners, stockholders er size in Celotex speaker battle, and security holders, if any, contain not only the list of stockholders or security holders as they appear upon the books of the company. but also. in cases where the stockholder or se- mount speaker firmly and fasten baffle, curity holder appears upon the banks of the company as trustee or in any other fiduciary relation, the name of the person or corporation with speaker, against grill cloth into for whom such trustee is acting, is given ; also that the said two paragraphs contain statements embracing affiants full knowledge awl the four wood strips mentioned above, belief as to the circumstances and conditions under which stockholders and security holders who not with eight wood screws. The speaker do appear upon the hooks of the company as trustees, hold stock, and securities in n capacity other than that of a bona fide owner: and this affiant has no reason to believe that mounts just above center of the baffle any other person, .association, or corporation has any interest direct or indirect in the said stock, bonds, or other securities than as so stated by him. board to decrease the possibility of any (Signed) JOHN H. POTTS, Editor. .... resonant effect. Sworn to and subscribed before me, this 8th day of ieptember, 1948. (Seal). HARRY N. REIZES, Notary Public. For complete sound -proofing, Kinnsul In 'the State of New York, Residing in Kings County, Kings Co. Clk's No. 634 Reg. No. .trips cut to fit all sides of 612 -R -9; N. Y. Co. Clk's No. 779, Reg. No...... Term Expires March 30, 1949. are the inside of the cabinet except the front and

52 AUDIO ENGINEERING OCTOBER, 1948 held into place with the use of small ALTEC LANSING NOW "PACKAGES" nails, backed with cut -out pasteboard washers. The back is held tightly in ALL COMPONENTS FOR A COMPLETE place by the use of six wood screws fastened into the one -inch square wool HIGH FIDELITY RADIO -PHONOGRAPH strips wihch run across the inside of SYSTEM TO BUILD INTO YOUR HOME the top and bottom sections of the enclosure. The cable from the speaker terminals comes out at the bottom center, about an inch from the back but outside the base board. Examination of the photograph of the back molding pieces shows how 3/16 -inch holes are drilled in the two shorter pieces which support the grill rods. The molding is press -fitted into place. The rods also are held into One of many alternative placements of place by friction. elements. In actual installation, speaker After six weeks of operation of this will be concealed by decorative fabric. new cabinet, all who have listened The Altec Lansing Custom -in -Built Music full range of sound (35 to 16,000 carefully to high -quality program ma- System consists of the famous Altec Lan- cycles). It achieves the highest faith- terial have agreed that the reproduc- sing Duplex speaker, a special Altec fulness to living sound now known to tion is extremely good. Lansing amplifier, a newly designed TRF science. Built -in Altec Lansing Day- Altec Lansing AM -FM tuner, and the light Television can also be included. Webster '70 record changer with GE vari- Complete installation instructions able reluctance pick -up. Integrated with come with each system. INTERNAL RESISTANCE the structure of your room, this system A brochure will be sent on request. transcends completely the inherent acous- [from page 201 tical and electronic limitations of conven- simply be 10 ohms. This means that tional radio- phonographs, eliminates radio "furniture," and reproduces the ALTEC p = .5. Let us assume that we ob- ori- serve a drop of voltage from the 161 Sixth Avenue, LANSING ginal 5 volts to 3.6 volts: the voltage New York 13, N. Y. 3M/5 = 1161 N. Vine Street. custom -in -built reduction q is consequently Hollywood 38, Calif. .72. Substituting the values for RI, home music system q and p into equation (4) results in R = 12.8 ohms. This amplifier, as far as internal resistance is concerned, therefore approaches the condition of an amplifier with triodes in the output stage, where we could expect an internal resistance of 10 ohms, when looked "back" at from the 20 -ohm out- .4 put tap. ATIEOPIORS In the ease of a straight pentode amplifier, it will be found that the reduction of the load resistance to one half will reduce the voltage across the load very nearly in the sanie ratio. which is easily understandable, since TYPE n pentode is essentially a constant cur- 850-A 850 -B rent generator, rather than a constant voltage generator. This means that q is very nearly equal to p, which results in I? = oo. It may be of interest to use equation (4) to calculate what q will be observed in the case of a triode amplifier designed in such a way that the The units illustrated represent a complete load resistance is equal to twice the redesign of our older precision attenuators for plate resistance. Under this condition, the internal resistance observed when loboratory standards. Flat for all frequencies in the audio looking back into any of the output range. Reasonably flat to 200 k.c. up to 70 db. should taps of the output transformer Bulletin sent on request. be equal to one -half the load resist- e ance for which the particular tap is designed. Looking back into a 10 -ohm Manufacturers of Precision Electrical Resistance Instruments we see 5 ohms. 1NC.bi tap for instance, should too, A7oR1E5, 337 CENTRAL AVE. JERSEY CITY 7, N.J. If, for the purpose of our test, we

AUDIO ENGINEERING OCTOBER, 1948 53 make R1 equal to the rated load re- The ONE -STOP sistance value of the particular tap. Store for for instance 10 ohms, and then reduce PRODUCTION EQUIPMENT it to one -half of this value by placing 10 ohms parallel to it, p will be equal to .5. With R = 5 ohms, and R1 = 10 ohms, equation 5 can be solved for q as follows:

A great step forward ... Atlas Alniro.V -Plus 1 -e 1 -e Superefficient (5) 5 -10 x magnetic assembly. Fnergy per .5 2e-1 unit volume over three times as great as any and therefore e - .75 used before. Traditional Atlas Quality and Fidelity to Precision are incorporated In all This shows that in an amplifier with these new developments. Keep step with a properly designed triode output stage Sound Advancement with Atlas Sound. the voltage should drop to 75 per cent, when we change the load from the rated value at any given output tap to one -half of this value. If it should 35mm or 16mm Single System drop further, the amplifier is not as Soundfilm Cameras Auricon Pro TV TPISV New Atlas good from the damping point of view Special Kit - Self blimped 16mm soundfilm recording ramera AlnicoV -Plus Dual as a well -designed triode output stage. actR as furnished Projector. to ABC, CBS, NBC. WIIC PD -8V New Also in If the voltage drops less than 35' >. and Iradin-n TV film producers. Allas larger model TP24V. ('omplete to the Nth detail 82213.50 stream- the internal resistance is less than lined Hi- Fidel- Coming Soon - Auricon Pro TV ity AlnicoV- half the load resistance. Transcription Recording Camera Plus Driver Makes 16mm Soundfilm off-the-tube for 24 f.p.s. Units. showing. 1200' magazine runs for full half hour. ATTENUATORS Write for details! ERRORS IN SPECIALS FROM OUR $250,000 HU -24V All Atlas STOCK 1 341 /rom page B. Maurer compl. I 6mm recording Speakers have new system unbreakable Alnico 82275.00 - The attenuation error given by equa- Comb. 16mm film phonograph & tepe V-Plus Driver Units. To obtain the recorder 1996.00 tion (6) is in nepers. 200 Watt (reline) sputa with heavy result in decibels, the right side must yoke 57.50 PD -SV All Adas be multiplied by 8.68. By substituting B & H 35mm Unit Eye PD -V Driver Shuttle 660.00 Units ore com- (7) in (6) we obtain the following ex- ArriOei motorized Newsreel Camera, 4 pletely lenses, worth $1250. Complete 695.00 Mag- HU -15V pression which relates the decibel error netically Maximum Cinephon motorized Newsreel Camera, Shielded... efficiency as repro- to the resistance error and nominal 4 lenses. worth $1800. Complete 1295.00 ducer, utmost per- STUDIO, EDITIN(. & LAB. SUPPLIES entirely Her- formance as micro- loss of the network. Lights, mikes, booms, dollies, lensed, background etically phone in talk-back. process. moviolas, etc. Send for Catalog Sealed. (8) Sturelab -7- thousands of good buys. 00- -6.60 N11 Se 0e S. CINEMA SUPPLY CORP. The N may obtained quantity Dept. AE 602 W. 52nd St. New York 19 from the nominal loss of the network by the following identity; PRECISE AUDIO PD -3V All Atlas (9) N - Ant,lo9 D20 MEASUREMENTS PD -V Driver Units have full Equation (8) is shown in nomo- phenolic Pickering and Company, Inc., leaders in un- 2. breakable dia- graph form in Fig. The nomograph the audio field, make available to the phragms. is directly applicable to II and Tee, industry their specialized facilities for attenuators, and in each case Z. is to audio measurements. Write for MICROPHONE We are prepared to make guaranteed STANDS -Atlas is branch and New Illustrated be considered as the series measurements on any type of audio outstanding in Qual- a Catalog. ity, completeness Z. as the shunt branch. If for Tee equipment for manufacturers, dealers, or and variety of mod- network the two series branches are consumers. els produced_ assumed to have different tolerences. PICKERINCi & CO. Inc. or if this condition is assumed in the Oceanside, N. Y. two shunt branches of a II network the network may be broken into two \'EItTICAL LATER:V. dual REPRODUCERS RECONDIT.ONED 34°4Atlas Sounds L sections and each section individ- Types 9 -A, 9 -B, D- 93306, MI -4856, 4875 -C value. Per- MI. + purpose out. sec- Complete Stock of New Replacement Parts powerful ually treated. The error for the L Sect for for All Types MULTIPLE ai- tion will be one -half that of a full Tee lcoltapsible This service is being used by leading SPEAKER model) or indoor radio pod array or IT composed of two such sections. stations and wired music companies from SUPPORT (illustrated) - coast to coast. Factory Prices prevail. Quick, con The value of the nominal attenuation STAND I Portoble. VIBRATION SYSTEMS, INC. renient, Practical. applied to the chart should therefore 1040 W. Fort St. Detroit 2o. Michigan be taken on the basis of that of a full section. Several interesting applicatit'us of WANTED the chart will now be considered. For G. I. Flyer motor model 818 example, let it be required to find the double field two speed; also a 20 db Blue Flyer 1 AAn -39th STREET maximum decibel error of type model t. Contact BROOKLYN Y. 1 18, N. Tee attenuator which is variable in W. Dacosta, Sound Dept., WOR db steps and is constructed of 2% re-

54 AUDIO ENGINEERING OCTOBER, 1948 sistors. The maximum error will occur with full attenuation and when the series resistors are 2% high and the MAKE YOUR OWN shunt resistor 2% low. The quantity m/n then equals .96 and the decibel WIRE-WOUND error will be -.28 db. Next, let us consider a ladder attenuator for this application, also with RESISTORS! 2% resistors. The maximum decibel error will occur when all of the series This kit contains all the resistors are 2% high and the shunt necessary components for the construction of wire - low. For one step of 1 db resistors 2% wound, non -inductive re- the error will be -.018 decibels and sistors. You can make meter for the twenty steps the total error multipliers, shunts and non - will be -.36 db. It is thus seen that inductive resistors for use the error exceeds of the Tee in audio circuits. Values total that up to 100,000 ohms may type attenuator. It is very unlikely, be easily wound by follow- however, that all of the series resistors ing the instructions. will be high and the shunt resistors The large variety of wire low. Instead, it would be expected that sizes and alloys that are the resistors will vary randomly about included in the No. 200 No. 200 Resistance Wire Kit Resistance Wire Kit makes List Price $6.50; Net Price $3.90 the nominal value for each step, and it invaluable to laboratory that the maximum total error will not technicians and experimenters who find the need to make resistors on the spot. differ greatly from that of a single Resistors of 5% accuracy may be made without instruments by referring to the ''Ohms step. On this basis, the ladder would Per Foot" marked on each wire spool label. accuracy provide a greater overall than CONTENTS: 8 spools of assorted resistance wire, bobbins, fiber cards, eyelets, pigtail the type Tee attenuator. leads, emery paper, cerise wax and complete instructions. calls for operation If the application JOBBERS INQUIRIES INVITED at low temperatures, it is of interest to make another comparison. In type Tee attenuator,, the resistors are aunt CINEMA monly all of wirewound construction, ENGINEERING COMPANY and the wire used has a more or less 1510 W. Verdugo Ave. Burbank, Calif. uniform temperature coefficient of re- Cable Address: CINENG BURBANK sistance. In the ladder attenuator, however, the construction would consist of a wirewound series element and carbon composition shunt resistors. At a temperature of -40° C., one could expect the type of carbon resistors used FEATURES t to change by + 15 %. The correspond OF ASTATIC'S ing change in a nichronte wire series FL -33 PICKUP element would be -1 %. The ration min then equals 1.16, and the error per step will equal + .075 db. The total error Five - Gram Needle 1 Pressure; will be -I- 1.5 db, and at the twenty Attar& FL -33 PICKUP the actual attenuation decibel setting FOR COLUMBIA MICROGROOVE RECORDS Permanent Sapphire will be 21.5 db. It is thus seen that 2 Needle with .001" Tip Here is no mere version of what a pickup for use over a Radius; when operation temperature with Columbia Microgroove Records should be -but range is a consideration, the Tee atten- the actual playing arm designed to meet the precise uator will provide a much greater ac- requirements of Columbia's new recordings. This Approximately O n e- 3 curacy than the ladder. new Asiatic Pickup is manufactured to meet the Half Volt Output; specifications by Columbia, to insure maximum quality performance of the Columbia LP Microgroove Frequency Range 30 Record. Available. then. in the Astatic FL-33 Pickup 4 to 10,000 c.p.s.; and LP -33 Crystal Replacement Cartridge , is the DISTORTION MEASUREMENTS ultimate of Microgroove companion equipment . alone capable of getting the most out of LP Records. Novel Design at Base Ruin page 18J 5 Eliminates Tone Arm ALSO AVAILABLE is the LP -78 Cartridge that fits re- Resonances and Assures eluse together and analyzing the the FL Arm, but having a .003" radius needle for Perfect Tracking; sultant intermodulation products with playing 78 RPM Records. a wave analyzer seems to offer a more LP -33 Cartridge, with FL FILTER: For best performance approach to the problem of dis- 6 replaceable, perma- direct with high qualify speakers. Controls nent sapphire needle. tortion measurements when the trans- pickup response - Eliminates high fre- fer characteristic is a function of frequency peak. quency. It is apparent. however, that further experience with intermodula- LISTED IN RADIO INDUSTRY RED BOOK tion distortion measurements is required before standardization of tech- Manufactured CORPORATION Astatic Crystal Devizes C O w rr f AUT. O H I O niques cart be attained. Under Brush Development Co. Patents

AUDIO ENGINEERING OCTOBER, 1948 55 SPECIAL PROBLEMS Introducing ADVERTISING 9h.autd demand INDEX QCLu I It11'ON SPECIAL TREATMENT

Engineering judgment is often Allied Radio Corp 56 the most important ingredient Altec Lansing Corp 53 in the design and construction Arnold Engineering Co 5 Astatic of apparatus for your specific Corporation, The 55 Atlas Sound Corp needs. We specialize in 5I - Audio Development Co 8 one -of -a -kind instru- Audio Devices, Inc Cover 2 - Audio Instrument ments for electronic or Co 56 Brook Electronics. Inc 46 electro- acoustic measure- Burnell and Co 11 ments Camera Equipment Co 12 -recording or reproduc- Chicago Transformer Corp 7 ing systems with special Cinema Engineering Co 55 Daven Company, characteristics The....Cover 3 Electro Motive Mfg. Co.. Inc. 52 A HIGH FIDELITY - subminiature audio Electro- Voice, Inc 35 reprod,3c_: .h,t General 50,5 a new standard o.` efficiency in for Radio Company 9 electro- acoustical conversion. Small enough Laboratory, Production Testing, LeBel, C. J 38 to fit into the home console. Sound output Magnecord, Inc. 45 ample for studio and industrial require- Broadcast Station, Recording ments. Has 120° horizontal Maurer, J. A. Inc 10 angular dis- Studio, or Discriminating Home Measurements persion. Special fyp3 cone speaker for low Corp. 1 48 reproduction, and cellular speaker for wide Minnesota Electronics Corp 49 angle coverage. High frequency response, Newark Electric Co., Inc 56 and many other exclusive features. Fre- Audit. Instrument Company Panoramic Radio quency range 50 fo 12.000 cps. Size II "x Corp 46 15 "xlI" deep for 6 waft operations, size Pickering & 1947 Broadway, New York 23, N. Y. Co., Inc 41. 54 15 "x21 "x12" deep for 20 watt operations. Presto Recording Corp 37 Write for free catalog Professional Directory 38 RACON ELECTRIC CO., INC. Racon Electric Co., Inc 47. 56 52 E. I9th St. N. Y. 3, N. Y. Radio Corp. of America emomeepepenowilk (Broadcast Equip. Div.) FREE! send for our Cover 4. Radio Corp. of America FREE (Sound Equipment Div.) RADIO BUYING GUIDE 26. 27 Rangertone, a complete and diversified lineup of Inc. 48 Reeves Soundcraft electronic Corp 39 equipment Scott, H. H. Inc 44 sound systems Shure Brothers, Inc 43 recording equipment Simpson Electric amateur Co 6 gear S.O.S. Cinema television Supply Corp. 5t components and kits Sylvania Electric Radio replacement parts in Products Inc 1 Everything laboratory and service Supplies test equipment Tech Laboratories 53 and Electronic Save time and money -buy direct from United Transformer Corp 2 Service distributor! Consult the NEWARK radio U. S. Expert buying guide, featuring all standard makes Recording Co 38 Complete, value-packed listing of 20,000 essen- Vibration Systems. Engineers 'lsiais for expert servicemen, engineers. Inc 51. for Radio soundmen. Ward experimenters, teachers, Leonard Electric Co 51 hobbyists, amateurs, Wells, Winston 24 HOUR SERVICE 38 ON ALL ORDERS Western Electric Co 12 You'll find everything you need in radio and Free technical advisory service! electronic equipment for laboratory and Westfield Metal development work. in the new I80 -page Products ALLIED Catalog. Rely on one dependable JUST MAIL THIS COUPON TO CET YOUR CATALOGUE Co., Inc .1.1. source for the world's largest stocks -thou- Newark Electric W Co., Inc., Dept. I I 0 R 51 sands of parts, tubes, tools, books, test in- 242 West 55th St., New York 19 Wrigley, struments, sound apparatus -ready for in- William Jr. .. Co 10 Please send me the FREE Newark Buying stant expert shipment at lowest market prices. Guide of line radio equipment. Write today for your FREE copy of ALLIED's newest Buying Guide. Name WANTED Address 12 seta of ARCS equipment City as follows: State 1125 Receiver (1.5 - 3 mc.) MT7A Receiver ALLIED RADIO Rack MT5 Receiver Base T18 Transmitter (2.1 - 3 mc.) MT69 Transmitter Rack MT88 ALLIED RADIO CORP., Dept. 17.K -11 3 Gen 610133 Transmitter Base M137 Modulator DY8 133 W. Jackson Blvd., Chicago 7, III. 115e 1s S1 171 Ilal Dynamotor MT76 Modulator Rase MT77 oil 373 sai Males ser O Send FREE New ALLIED Catalog. IJEWARK tera0 Relay Base T17 Microphone HS33 Headset MurOtlrarani E39 Receiver local tuning knob PLI52 Plug Nacre I1D10 t1IlIYISION Mao ssll3er.R1MIK1.r. PL147 Plug PL151 Plug PL148 Plug 9377 MVO 14I1Seet Mal u. Plug. Box 90, Audio Engineering. Address

56 AUDIO ENGINEERING OCTOBER, 1948 itlrE INI ENGINEERING - LEADERSHIP

MINIATURE ATTENUATOR - TYPE LA -130 30 STEPS - DIAMETER 13/4 INCHES Here are 4 of the more recent DAVEN "firsts ".

30 step fodder network, provided in a min- DAVEN long has been known for leadership .. . imum of space . .. recommended for use in high quality equipment requiring a wide ronge of attenuation, where mount- particularly in engineering and design. Many of ing space is limited. our developments have been incorporated in the

equipment of broadcasting stations, sound record-

ing studios, and electrical laboratories throughout

the world, and are specified as standard.

will be The DAVEN Engineering Department glad to discuss these new developments with you. Your inquiry is invited on the units shown or on any other DAVEN product.

...frequencies read directly to 100,000 THE CO. cycles with an accurocy of - 2% of full

scale in use . . . 7 ranges available .. low input voltage requirements .. - large. easy -to -read illuminated meter ...mounted DAVEN on o 5y. relay rock panel. 191 CENTRAL AVENUE NEWARK 4, NEW JERSEY

TRANSMISSION MEASURING SET - TYPE 10A C

Direct reading instrument .. . meters and ATTENUATORS WITH BUILT -IN CUEING CONTROLS range controls can be used associated . . . eliminate the need for auxiliary independently as VU meters . . . range switching mechanisms . . . also available + 26 to -100 DBM . . total attenuation with built -in switchette for a pilot light 111 DB in steps of 0.1 DB. . may be obtained on Ladner, "T", (Pot. Pend.) Potentiometer, and Ladder Fader controls. (Pat. Pend.) F

.:.makes your RCA turntable a high -fidelity recorder'

The RCA Recorder mounted ao a Type 70-D1 turntobie

THIS IS THE NEW improved t. -type stu- sert space between recordings without Type 72 -DX is complete with -fidel- dio cutter. It is high designed specifically breaking groove continuity. Plenty con- ity recording head, mounting base, to give you instantaneous rest- high -quality venient, too, for making starting and post, suction nozzle, and compensator. recordings with your present "70 Series" finishing spirals. turntable For prices and details, see your RCA -at surprisingly low equip- It's dependable driver ment cost to you. -No slippage Broadcast Sales Engineer, or write or "knocks" ... because power coupling Dept. 7J, RCA Engineering Products, - It's uniquely 9exible -With this pro- is made to the center of your turntable Camden, fessional N. J. attachment you can record at through a vertical shaft spiral gear and a 96, 112, or 136 lines per inch -and at three -pin driving flange. No cutter car- speeds of 331/3 or 78 rpm. You can re- riage riding on the feedscrew ... because SPECIFICATIONS cord outside-in or inside- out- without the carriage is supported on a metal tube changing gears or lead screw. You can that encloses and protects the feedscrew. Input Impedance to Cutter.. 15 ohms, nominal adjust the stylus cutting angle and cut- No groove grouping .. . because the Frequency Response: ting depth during recording. head rides smoothly along a tubular en- type 72 -D 3 db, 50- 7,500 cps It's simple to operate -A new im- closure that protects the feedscrew. type 72 -DX ' 2 db, 50- 10,000 cps proved cam -operated lowering device Here, we believe, is the finest cutter Sensitivity (groove velocity 6.3 c.n /spc. helps you lower the cutter gently to the yet designed for high -quality studio re- 0.00079' -peak to peak) at 1,000 cps: record ... eliminates stylus damage-and cording . - - at modest cost. Type 72 -D is type 72 -0 +30 dbm (1.0 watt) deep cuts caused by sudden dropping. A complete with a standard head, mount- type 72 -DX +30 dbm (1.0 wan) spiralling hand crank enables you to in- ing base, rest -post, and suction nozzle.

BROADCAST EQUIPMENT RADIO CORPORATION of AMERICA ENGINEERING PRODUCTS DEPARTMENT, CAMDEN, N.J.

In Canada: R C A VICTOR Company Limited, Montreal