columbia

mean maxtmum Listening Pleasure · ...... - thanks to the finest in modern sound · . recording methods and equipment • .• including

Music lovers everywhere know that Columbia LP records mean Glldiotape* more listening pleasure-not in playing time alone, but in superb quality of reproduction. Yet few listeners outside the for the original sound professional circle realize the degree of perfection which-this record quality requires in every step of manufacture and proc­ essing. Ta)noise ratio, distortion and surface noise must measure up to standards which would have seemed entirely impractical a few years ago. But Columbia has found that Audiotape and Audiodiscs are an ideal combination for meeting all of these exacting re­ quirements - Audiotape for recording the original sound and Audiodiscs for the masters from which stampers are made. In ----'! _.-I! ___* fact this same record-making combination is now being used .and ~ with outstanding success by America's leading producers of fine phonograph records and broadcast transcriptions. for the master recording You can get this same sound perfection in YOUT recording .' work, - too - with Audiodiscs and Audiotape. Their superior quality is the result of more than 12 years of specialized experi­ ence by the only company in America devoted solely to the manufacture of fine sound recording media, both tape and discs.

@The exclusive erade-mark oj Columbia Long Playing Records-symbol 0/ hi,hest quality. ~a.de.Marks uColumbia.u "Masterworks," fiilD. @ Reg. U.S. Pat. 00. MaTcas Retutrodas.

AUDIO DEVICES, Inc. · * Trade Mark 444 MADISON AVE., NEW YORK 22, N. Y. · Export Dept.: 13 ~ast 40th St., New York 16, N. Y., Cables "ARLAB"

-f" INCLUDINC

C. G. McProud, Editor Lad~ Haystead, Publisher; may own the nnest pickup, ampliner and Harrie K: Richardson, 4l s~9 cia te Ed itor %J speaker that money can buy • • • yet you'll 9'et Lt:J(d '~(f mer , Prod~ c,t i on /~ Ma~ a ger ~A s~ i1. Cahn, Advert i si '~g Director i~ poor reproduction if your TURNTABLE has ex­ Lucil le Carty, Circulation"' Manager H. N. Reizes, Advertising Manager cessive .wow, hum or rumble!

Editorial Advisory Board Representatives RE K-<:?-KUT H. Thorpe Covington, Special Representative . .. offers a co mpl ete range of 12" Tu rntabl e models Howard A. Ch inn 677 N. Michigan Ave., Ch icago II , Ill. to match yo ur prese nt hi gh·fide lity equipm ent and Sanford R. Cowan, M id-West Sal.,. your own pocketboo k. Not eve ry sound sys tem reo John D. Colvin 67 W. 44th St., New York 18, N. Y. quires the most expe nsive turntabl e ... yo ur turn· C. J. LeBel James C. Galloway. Paci f ic Coast Sales table must be chose n to complement your other co m· 816 W . 5th St., Los Angeles 17, Ca lif . ponents. Eac h RE K·Q· KUT Tu rntabl e carries an engi· J. P. Maxfield Technical Book & Magazine Co. nee ring specifica tion which enabl es you r So un d Dealer 297 Swanst on St., Melbourne, C. 1. to sel ect the appropriate un it. Th e qu ality and work· George M. Nixon Victoria, Australia manshi p of eve ry REK·Q·KUT Turntabl e. from our mos t economica lly·priced mo del. to our highest pr iced profess ional machin e is id entica l . . . price differe ntial is depende nt sol ely upo n the type of materia ls used. CONTENTS FE,BRUARY, 1952 Vol. 36, No. 2 MODEL LP-743 3-Speed 12" T.ul'ntable Inducti on type motor, Audio Patents- R'ichard H . D 01'f ...... 2 designed for smooth, vibration · free operatioo. Letters ...... 6 In s tantaneous speed chan ges without stopp ing Edit0r's Report ...... 8 turnta ble .or removing Audiana ...... 8 di sc ...... $54.95 Net Loudspeaker Damping with Dynamic Negative Feedb ack-Ulric f. Childs . . 11 MODEL CVS-l 2 The Design of Compensated Volume Controls- Part 2- Emerick T oth .. . . . 14 AII- Speed Continuously- The Traveling Mike- N. H. Crowhurst ...... 16 Variable Turntable Plays all records, 6" to I&" Linear-Scale A-F W attmeter- C. G. M cProud ...... 17 and broadcast transcrip· tions. not only at the three Audio in the Home- W illiam C. Shrad'e'Y ...... 18 normal sp ee ds, BUT AT ANY SPEED from 25 to 100 Record Revue- Edward Tamall Canby ...... \ ...... 20 RPM . You play your records New Products ...... 28 at the EXACT SPEED at which they were recorded New Literature ...... : . :...... 29 or at any variation of spe ~ iI or pitch which gives you Relax and L isten ...... 34 ma xi mum pleasure. A "must" for serious music lovers, pro musi cians, AUDIO engineering society SECTION school s, dance studios, etc. . . . . $84.95 Net AES Notes ...... 38 MODELS T-12H & T-43H Employment Register ...... 38 2-Speed 12" Turntables 39 The only 12" dual·speed turn· ~:;:s t~e~:7es . : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :: :, : : : : : : : : : : : : : : : : 39 tables that meet the standard s for speed regulation and WOW Industry People ...... 39 content specified by the Na­ tional Association of Broad­ Advertising Index ...... 1 •••• • • • ••• • • • • • • • • • • • • • • • • • .• • •• • • 40 casters. Recomm'ended for use with ULTRA HIGH FIDELITY Amplifiers and Speaker Systems.

COVER MODELS MOTOR SPEED PRICE Hysleresls A recording session in Studib A of Audio-Video . Recording Company, Inc. T-12 H" Synchronous 78·33V. $119.95 A complete remodeling of the quarters previously occupied by radio Hysler.". T·" H" Synchronous 45· 33% $119.95 station WQXR has made this plant one of the recording show­ 4 Polo places of New York, with faci li ties for tape and disc T.12 Inducllon 78·33Y. $84.95 4 Polo recording, mastering, editing, and most other T·43 Induction 45·33V. $84.95 all ied services. Photo by Jack Sharin. • Delivery limited to very short supply at Hysteresis motors,

See • •• hear • •• compare REK ·O· KUT Turntables .Phonographs and ProfessIonal Disc Recorders at leading MusiC Stores, Audlo·Vlsual Dealers and Radio Parts Jobbers •• • or write for latest, free Catalogue. RADIO MAGAZINES, IN C., 342 MADISON AVE. , NEW YORK 17, N. y,

AUDIO ENGINEERING (title registered U. S. Pat. Oll'. ) Is pubJlsbed montbly at 10 McGovern Ave .• Lancaster. Pa., by REK-O-KUT CO. Radio Magazines. Inc., D. S. Potts, President; Henry A. Schober, Secretary· 'J' reasLir er. Executive and Editorilll Offices : 3 8-0 1 Queens Blvd., Long Is land City, N. Y. 342 Madison Avenue, New York 11, N. Y. Subscription rates- United States, U. S. Possessi.ns and Canada. $3.00 ror 1 year. $5.00 ror 2 years ; elsewhere $4. 00 per year. Single copies 35c. Printed in U. S. A.• \11 rights reserved . Entire EXPOR T OIV ISION -l5B Broadway, New York CIty, USA CANADA ATl AS RAD IO CORP, LT O , 560 King Sf W , Toronto 28, 00t3l10 contents copyright 1951 by Radio Magazines. Inc. Entered as Secon d Class Matter Februa . 9, 1950 at tbe Post Omce. Lancaster, Pa. under tbe Act 01 March 3. 1819.

AUDIO ENGINEERING • FEBRUARY, 1952

J RICHARD H. DORP

ponents at the time of installation to suit them to the auditorium and have the chorus effect so desirable in a good organ and so difficult to attain in an electronic one. The latest U. S. Martin patent, No. 2:563,477, covers a simple monophonic (one­ note-aJ-a-time) instrument. It has the virtues of simplicity and cheapness and entirely aside from any commercial value the circuit should provide a quick and easy way for those interested to build a useful little solo instrument. The circuit of the tone-generator section is shown in Fig. 2. It has only three tubes and could easily be condensed to two (or one and a half) by using a 6SN7 for V, and V •. The oscillator is the simplest kind of two-tube unit. The output of V, is coupled to the grid of V" the plate of which is coupled back to the grid of V,. The only capacitor concerned is C" C" or C" the coupling capacitor between the plate of V, Fig. 1 and the grid of V •. In conjunction with the resistance between . the grid of V. and ONSTANT MARTIN is a Frenchman whose ground, it forms an R-C time-constant developments in electronic musical in­ circuit which determines the freqlJency of struments have earned him widespread oscillation. For a given value of this re­ C sistance, c" or may be selected to ·TRIAD general purpose Power Trans· recognition in Europe. He is not well known Ct, C, produce differing pitch ranges. formers are specially designed, not only in the United States, probably because none of his instruments have been marketed or The resistance in the grid circuit of V. is for replacement, but for use in. elec· to the writer's knowledge, even demon~ divided into several parts and key switches tronic equipment for industry, for PA strated in this country. One of the Constant select the amount of the resistance to be amplifiers, and for amateur gear. Martin organs appears in Fig. I-a two­ grounded and made part of the time-con­ s~ant circuit, thus selecting pitches. Re­ All types are "Climatite" treated, both mat:lUal instrument with several separate OSCIllators per note. These organs are indi­ SIstors R, should be selected by experiment coil and core, for protection against vidually voiced by adjusting stop filter com- to give consecutive notes of the chromatic moisture and for elimination of lamina· scale with a suitable value for C,. Then, tion chatter. Laminations are painted to * 255 West 84th St., New Y01'k 24, N. Y. [Cot£timud on page 4] prevent rust. Although small in size, the high quality materials used in these transformers keep losses to a minimum. B+ R2 R3 B+' Only copper coils for static shields, grounded to case and core, are used. Leads are UL approved for high tem· perature operation. Final tests include checking for correct operation and stressing in all ways beyond maximum B+ operating potential. Beautifully finished in gray enamel with all specifications • T TO FILTERS on decal on the case. a AMPLIFIER Remember! Only Triad Transformers Fig. 2 are "Climatite" treated Available from stock at leading jobbers •

B+

R9

RiO

2 AUDIO ENGINEERING • FEBRUARY, 1952 REEVES

NLY MANUFACTURER OF RECORDING MEDIA SOUNDCRAFT :, 20 YEARS' EXPERIENCE IN SOUND RECORDING ANNOUNC·ES W PRODUCTS. ' • • NEW DEVELOPMENTS. • • NEW ERA IN MAGNETIC SOUND

NEWIMPROVED SOUNDCRAFT TAPE +-1-++-1-1 -H-t-+-t Only Souncraft SPN magnetic recording tape with improved formulation has these new exClusive features. Available in all standard sizes. UNIFORM COATING THICKNESS to produce the ultimate in sound fidelity. PRE-COATED BASE, BUFFED, SURFACE LUBRICATED to assure smooth, curl. (ree operation, uniform output level and high frequency· response, plus years of extra service.

NEW SOUNDCRAFT 5 DRAWER CHEST PERMANENT FILING CABINETS - with five drawers, constructed or durable, lined boxboard, containing 625 or 1250 ft. sizes, at no extra cost. Only Soundcraft can make this offer. BUILD A BmER TAPE LIBRARY. Recordings of fine artists, sound effects, memorable events can now be conveniently filed with the aid of this new cabinet.

NEW SOUNDCRAFT DISCS Only Reeves SOUNDCRAFT Recording Discs offer the latest developments to reduce surface noise to the point of inaudibility and produce the last word in fidelity. INGREDIENTS OF PERFECTION Heavy aluminum bases free of imper. fections, with flawless mirror­ like finish, guaranteed flatness, fine-grain lacquer which has been subjected to micro­ filtration and blended with chemically pure solvents. FACTORY WITH MAN·MADE WEATHER Unlike most discs, Soundcraft Discs.are manufactured in a weather·controlled factory, under dust·free conditions. Temperature and relative humidity are completely regulated. Soundcraft Discs, because of these controlled conditions, are delivered the year 'round.

/ AUDIO ENGINEERING • FEBRUARY, 1952 3 since time-constant is equal to R x C, changing to C, or C, will simply shift the entire chromatic scale to another range without varying the half-tone intervals be­ tween keyed notes. One basis for selecting C. and C, would be to produce tones one and two octaves below or above that of C. Another idea (not mentioned in the patent) might be tcu provide twelve capacitors on the switch so that the instrument would be an automatic transposer which could be set to playa solo in any key while the player may use his keyboard as though the com­ position were written in C, the simplest and most familiar key. .2db 30-20,000 CPS T he plate-supply voltages for the osci lla­ tor will affect the pitch. Voltage for VI is taken directly from the supply through load resistor Rs, but that for V, is taken from a o DB lEVEl ~ 1 5 WATTS voltage-divider network R...-R...-R,. T o coni­ \_ _ ---1----+--\ Tg~~~~~D~~~~~S pensate for tube agin~ and for replacement tubes with slightly different characteristi cs R; is variable and is used for retuning. The ~ OtX=~+==t===E=±=±==+=~ first section of the V, grid resistance, R" is also made variable so that inter-note ~_Q2t=:j~~t=====+=====+======~====~=====t======~====~==i=j intervals can be touched up. This is possible ~ -0.4L::J==~t===~t;==~~====~~====~--L 00 200 500 1M ~---;5M~-~10~M~15~MI~~M2M because it is percentage change in grid re­ 20 30 50 I FREQUENCY 'N C.P.S. sistance that determines the intervals and a change in a portion of the resistance will affect all the percentages. To make it possible to have only one N.o. 80-6. For use in high fidelity No. 80-7. For matching 600 or 150- capacitor in the frequency determining amplifiers. Couples push-pull 6L6's ohm line to a 6/ 8 or 16/ 20-ohm voice circuits the grid of V, and the plate of V. (7500 ohms, C-T) to 6/ 8 or 16/ 20- coil. Frequency response within plus are direct-coupled. This puts the grid VI at ohm voice coil. Center-tapped ter­ or minus Idb. at full rated output­ a positive potential equal to that of the tiary winding provides 15 % inverse maximum power level, 30 watts. plate of V,. To bring the V, grid-cathode feed-back to reduce harmonic dis­ Mounted in compound-filled drawn potential to the desired point the cathode is tortion to a minimum. In drawn steel case, 45lt6" x 3%" X 31 lj16". placed across a voltage divider R .,..R , be­ steel case,45lt6" x 3%" x 311lt6", with Moun.ting studs and pin-type ter­ tween B-plus and ground. The output wave­ mounting studs and convenient pin­ minals same as No. BO-6 illustrated form of the oscillator, not mentioned in the type terminals. above. patent, is probably roughly sawtooth. It should be possible to control the waveshape There's a CHICAGO Output Transformer for Every Full Frequency Use to a large extent by choosing suitable values for R , and R,. Cat. No . Applicatio n i mpedance Max. Power To get rid of clicks and thumps when the BO· I Single Plate to line .... Pri .-15.000 ohms at 0 to 10 rna d·c instrument is keyed, a simple and effective 'Sec.-GOO/ 150 ohms CT ...... +20 dbm .. time-delay circuit is used in the first ampli­ BO·2 P.P. Plates to line .... ' Pri.-20, 000 ohms CT fi er stage V,. A voltage divider R .... R lo 'Sec.-GOO / 150 ohms CT ...... +30 dbm . . across the B-supply places the cathode at a BO·3 P.P. Plates to line . .... Pri.-5,000 ohms CT 'Sec.-GOO /150 ohms CT .. , ...... , .... ·;1-40 dbm .. positive enough value to cut off plate­ tBO·4 P.P. Plates to Line . . ... Pri.-l,500 ohms CT current flow. Tbe cathode is connected 'Sec.-GOO/ 150 ohms CT ...... +43 dbm . . through R 11, RI!, and RIO to a second free BO·5 P.P. Plates to line .. ... Pri .-IO,OOO ohms CT 'Sec.-GOO / 150 ohms CT; IG /8/4 ohms . +37 dbm .. contact on each key switch so that when .the key is pressed the switch first grounds tTertiary winding provides 15% inverse feedback. 'Split and balanced windings. a pitch-determining resistor and then grounds the cathode through its resistors. Famous nSealed in Steel" Capacitors C, and C, make the network a Q two-section time-constant filter, so that HIGH New Equipment Line there is a gradual rather than sudden build­ CHOKES up of plate current when a key switch The units described above grounds the cathode through the resistors. for Dynamic are typical of CT's New Obviously, to obtain much of a delay the Equipment Line featuring Noise Suppression Circuits resistors must be fairly large, which means transformer engineering that RIO must also be large so that the series Two precision-built chokes that's ahead of the trends value 6f R 11-R,r-R" will be enough of a .with inductance values of .8 in circuit design. Get the l::';:~~~§~.. J shunt to lower the bias appreciably. A and 2.4 henrys respectively­ full facts on the complete switch across R" permits the total resist­ accurate to within ± 5 % with line now. Check the fea- up to 15 rna doc. Units have a ance of the delay network to be reduced for minimum Q of20. R emarkably tures, and you'll see why CT is called the faster attack when desired. compact, 11 ~5 1J x 2%" X l UG". "Engineer's Transformer." Check t he An additional diagram in the patent No. Inductance prices: see how little more these advanced shows a complex filter through which the NSI·' .8 h units cost over ordinary transformers. tone may be passed after amplification to NSI·2 2.4 h Write for Complete "New Equipment" Coto/og Tocloy obtain a variety of colors, Any other suit­ able filters, such as those used in the Bald­ win Organ, Minshall-Estey Organ, Organo, Solovox, and similar instruments, could be used. A third diagram shows a vibrato oscillator, but here again any arrangement for varying oscillator plate-supply voltage at about 6 cps will work well. A copy of this or any other patent may be secured from the Commissioner of Patents, Washington 25, D. c., for 254.

4 AUDIO ENGINEERING • FEBRUARY, 1952 MAGNETRON PERMANENT MAGNETS AND ASSEMBLIES

# wlfIL * Die Cast Alum- * SandCastA~ · '~um Jackets Complete ass 11. Permendur em les "With * Celastic Co um,num Jackets n u...,., b ' steel or alu~· ... ases . •.. 1- "ers keepers as s' ll~serts and netized a pecllied. Mag_ required. nd stabilized as

AUDIO ENGINEERING • FEBRUARY, 1952

5 * MAGNECORDIR LETTERS

Li kes Stereos '~, p~~ Sir: I was much interested in the two stereo pairs on pages 20 and 21 of the January is­ sue of lE. I had little trouble in getting the desired results. It seems to me that such pictures could be used to good advantage in a technical magazine. R. E. Kerney, Alburnett, Iowa

Aid to Aunt Minnie Sir : I ran into the problem of simpli fied switching the last time (1·nost recent time ? ED.) I rebuilt the sound reproduction ap­ paratus in our home. The spouse had just learned to switch from phono to radio when I installed TV on the next switch position, thereby confusing her all over again. I have solved the problem on paper, and hope to SOOI1 rebuild again, (See what we 1nean? ED.) incorporating the switch­ ing system described below: Each program source has its own power switch, and its power plug and output plug are inserted into corresponding receptacles in a j unction box having six power recep­ tacles and six audio receptacles. In series with each power receptacle is a low-im­ ... from THE YUKON ... TO IHE WORLDI* pedance a.C. relay with two pairs of con­ .~/ tacts--one pair to energize the amplifier and another to connect the audio output of Magnecorder tape recorders penejratec(the frozen the source to the amplifier input. Tone con­ northland on Exercise Sweetl:;.rja r ·ljoint operation of trols are normal to all inputs, being in the U.S. and Canadian air",,9nd"-'and forces) . Operating input circuit of the amplifier. As far as I know, I shall have to build perfectly at 30 ° belGW zero, Magnecord recorders the relays myself, as I know of no suitable and amplif~.w " s0P p'ied the world with dramatic ones on the market. Also I plan to use a from Alaska and the Yukon. solenoid on each changer-I use three sep­ delay :.~ . pfograms arate ones-to engage the driving mecha­ re;ide radio men also know the dependable per­ nism when the motor is turned on. formance of Magnecorders. One of the hundreds of B. C. Barbee, 854 S. Fredonia, stations relying on Magnecorders is KELA, Centralia­ Nacogdoches, Texas Chehalis, Washington, where delayed programs and "on locations" are handled with ease and confidence. Another Solution Precision and fidelity make Magnecorders the first Sir : choice of radio engineers e verywhere. My method of solving Mr. Snively's problem is to control everything from one panel of the control center, with relays be­ MORE FEATURES GREATER FLEXIBILITY HIGHER FIDELITY ing housed in the lower part of the cabinet. PT7 accommodates 10 1 reels 12" In rock or console, or in its really lifelike tone Qua lity. low distortion, and offers 3 heads, positive tim­ Each unit is switched on and off by a portab le coses, the Mognecorder meet N.A.B. stondards - cnd a t Q ing and pushbutton conlrol. PT7 will suit every purpose . PT6 is moderate price. PT63 shown in rock small 1O,000-ohm DPST surplus aircraft shown in co nsole is available for avai lable w ith 3 speeds {3J/d", mounl offers 3 heads to erose, record relay, using d.c. to prevent noise and to portable or rock mount. 1 7 12", lS" } if preferred. and ploy bock to monitor from the keep a.c. circuits out of the preamplifier. tope while recording. The rectifier is on the line all the time, but draws an infinitesimal current through the bleeder. I use a two-wafer switch, with one wafer switching the audio circuits and another energizing the relays, which feed a.c. power to the amplifier through one.pair of contacts on each relay, and to the in­ dividual input source through the other. Thus the only operation required is to turn the selector switch from off to the de­ sired program source. After the equip­ ment warms up, the radio-for instance-is tuned at the tuner whose dial is lit, and 360 North Mich igan Avenue the gain is controlled at the master loud­ Chicago 1, Illinois, Dept. A-2 ness control on the control center. Send me latest catalog of Magnecord equipment. This circuit has been in constant use for three years without any trouble, and it ha·s Nome._ proven to be very simple for non-technical people to operate. Address Fred Paget, 356 5th Ave., Redwood City. Calif. C ity ...... Zone ...... Stote ...... •••...... ~ ,J (Two sotu.tiolls-yolt pays yer money aJ/d l' I J yolt takes 'yer choice. ED.) / I 6 / AUDIO ENGINEERING • FEBRUARY, 1952 7~ S~ 1Jai«e! , &~ at HUDSON audio-pacific TRUE HIGH FIDELITY AMPLIFIERS Startling Performance at a price everyone can afford! These famous amplifiers give a new concept to the phrase Hi-Fidelity. They have been rated "Sup erb" by so und engineers who know! Leading consumer research organizations have found them' very conservatively rated! The frequency response is from 18 to well beyond 20,000 cycles within .1 Db. Presence effect is truly astounding, reproduction is rich, Famous brilliant and crisp. NOi se, hum and harmonic distortion have bee n reduced to the vanishing point, - 85 Db be ­ (OLLINS MODEL 4 LH -The basic amplifier of medium low 20 watts. ga in for use with tuners and as studio monitors, Harmonic distortion is only '12% at 19.6 watts (40 watts HI-FI eiC. Expressly de signed for tuners incorporating peak), with 40 Db feed back . Hum, and noise measure bass and treble controls and pre-amplifier stage less than 1 milli-volt across a 10 ohm load. TUNER for phono. Tube compliment, 1-6J5, 1-6SN7, High Fide lily AI All the technical skill and craftsmanship possible ~i~Lt~'N!i5~~~eSiZ~ .. ~::~ . x . ~ : ~ .. ~ $64.00 have be en incorporated into Collins Audio products. As a result these units closely approach custom­ MODEL 4 -Same as above with wi th re­ built quality, which is apparent upon inspeclion mote control panel on which and demonstration. COLLINS FM-11 FM TUNER. is Iccated a master level con­ Surpasses the performance of any units in. its price trol, four position selector class. 9 miniature tubes, plus tuning eye and rec­ switch, and a low level listen­ tilier. 19 1/4 x 12 x 7 . ing control designed to allow full round, rich tone at whisper BELL • .. _-.--- levels. Tube '. : "~I). ' compliment I­ MPLIFIER . ,-' WoO 6J5, 1-6SN7, 2- 3-Same as 4 LH but with an addi­ , .\' •. .. . "., 6L6G, 1-5 U4. Size MODEL ~, ' . .",.. as above. tional 1-6J5 for the bass and treble boost , controls which are incorporated. Size same Model 2122 '. - Net price ...... $ 68.00 N~t~~~ce~ .4. ~~ '...... $85.00 Super.b performance and excellent quality at a popular price! for reproducing the extended range of micro­ groove records and FM broadcast! Has separate bass and treble tone controls. Frequency range ±'/4 db from 30 to 15,000 cps when controls are set for flat re sponse . Hum level: - 65 db below rated output of 10 watts with less than 3% dis­ tortion. 4 inputs; Radio or Tuner, Crystal Pickup, and 2 Magnetic Pickups, each with built-in pre­ amplifier. Output impedances: 3.4 to 4 ohms, 6 to 8 ohms and 15 to 18 ohms-matching mo st high fidelity speaker systems.

The Concertone meets all professional require­ 3-5peed Amplifier with built-in ments, yet is priced Ex­ pre-amp. Range : *20-20,000' cps ± .5 db (± k ceptionally Low! Equali­ db 30-20,000 at 10 watts). Distortion: Harmonic zation conforms to NAB RE(ORD 1.5 %, Intermodulation 2%, at 10 watts. Hum Recommended Broad­ and noise 80 db below output. 4 inputs. cast Standards. Extended No. A1326 ...... Net $48.75 Frequency Response is (HANGER 50 to 15,000 cps ± 2 VM Model 950 GE With magnetic Model Ll. Low Co st Music Amplifier. 8 watts, db at IS" per second cartridge. Top quality Automatic with 2% harmonic distortion. 40-15,000 cps, speed. Tape Noise down 3- Speed Record ~hanger. It plays All Rec­ ± 1 db. 3 inouts. Preamp. Ba ss and treble to random level. Also ords- Ail Spee ds, All Sizes, All Types, and STOPS controls. No. A1325 ...... Net $32 .34 has 7.5" per sec. Speed. AUTOMATICALLY after the last record is played , Pre -al igned heads quick­ regardless of size or type ! It is extremely simp le JENSEN G-610 ly interchangeable for to operate, having only ONE CONTROL KNOB for all Dual or Single Track Recording. Total Harmonic Dis­ functions: Features Automatic Set-Down- Selection TRI-AXIAL SPKR. tortion less than 2% at normal maximum signal level. for all size records. including 7", and Automatic Has 3 dynamically bal anced motors, separate heads for Manual Control (No Adjustments required). Plays Finest loudspeaker ever recording, erase, and playback. Simultaneous monitoring 12-10" , 10-12 ". or 12-7" records automatically- built! IS " 3-way system from tape while recording. Instant choice of 7.5" or IS" 10" and 12 " di scs may be intermixed if same achieves closest ap­ spee d. proach to actual "pres­ per second speeds. Fa st Forward, Rewind, and Reverse ence," Reaches well (2500 ft. per min.) . All controls are interlocked to above 18,000 cps . 3 prevent spillage or tearing of tape. Size of basic GENERAL ELECTRI( SPEAKER individually driven ele­ (ecorder: 22 x 14 x 5" mtg. depth below panel. Use merits with xaver and with any good quality amplifier. Model 5-1201 D control network.J5watts, AVAILABLE AS BASIC UNIT WITH CARRYING CASE OR GE high fidelity 12 " speake r CONSOLE CABINET that is recommended by }M&f4~ $224.91 BASIC RECORDER , 14010: Drive mechanism , power sup­ re sea rch organizations and ply, erase, record, and p:ayback preamplifiers, mounted music lovers. for superior on base plate, ready for installation. $345 00 performance. Frequency re­ AND DEMONSTRATE ALL With Dual Track Heads. No. A16 147, Net • sponse: 50 - 13 ,000 cps. MAKES OF HIGH-FIDELITY Molded cone. Non-warping Model 1401S, same with Si ngle Track Head s, Net $345.00 aluminum foil ba se voice EQUIPMENT FOR PROFESSIONAL OR coil permits power handling capacity of 25 watts. 8 HOME INSiAllAiION. ohms impedance. Superbly VISIT OUR SOUND STUDIOS desig ned and beautifully built to provide true extended ran ge rep roduction. With du st cover. Write for literature on all makes Comf/efe $1-~4 951 S1steIH Complete Set of Cables4' $7.50 extra

AUDIO ENGINEERING • FEBRUARY, 1952 7

;

,/ EDITOR'S REPORT

AUDIANA AGAIN technical articles into more understandable terms, and sooner or later all reader.s will be on the same plane and F ONE WERE to pick up a few letters at random from nobody will have to read AUDIANA any more. those received in this office, one would gather that Of course, there was one reader who suggested that I subscribers wanted (1) more technical articles, (2) those to whom many of JE's articles were not readily more less-technical articles, (3) more record reviews, understandable might better follow his example-he ( 4) less record reviews, (5) more articles about broad­ studied up elsewhere so he could understand us. We cast and other professional equipment, (6) no articles can't agree with his viewpoint entirely-many simply about professional equipment, (7) more articles about do not have the time to learn a new subject from the hearing aids, (8) more articles about home equipment ground up. -in short, more articles about everything. And so, while we are broadening our scope to include Nobody but nobody, to borrow. a phrase from a well more of the interesting aspects of audio, we will still known department store's advertisements, can please continue to carry the technical articles which have long everybody all the time. Maybe if we were able to make been the backbone of JE-those which have put JE in JE into a SOO-page issue every month we could. How­ the compilations of important articles on audio every ever, we are not complaining-we wouldn't have it any year since the first issue. After all, there are many other way. Our only course is to try to give our readers readers who wouldn't be satisfied without something what they ask for-interspersed with what we think new to build every month. Frankly we can't see how they ought to like. they can keep up with the procession of how-to-build-it But one thing that has been more and more obvious articles, but since we have the same inclinations our­ the last year is the desire for some articles which can self, we can certainly appreciate their viewpoint. be understood by the lay reader-and to that end we are reactivating AUDIANA, beginning with this issue as SIMPLIFICATION noted below. Henceforth it will be farther back in the This simplification is in the direction of getting JE magazine, but for its reintroduction it is here, right up each month. Beginning with the March issue, JE will front where everyone will see it. Many readers will re­ be on newsstands in eight of the larger cities of the coun­ member AUDIANA from 1950, with a simplified discus­ try, and it will be simpler for readers to get it. JE will sion of the problems of sound reproduction in reason­ still be available at your favorite jobber's, and perhaps ably non-technical language. Experimentally, the new regular buyers should leave the newsstand copies for AUDIANA will "translate" the more important of the new recruits to this interesting field.

this effect, with the result that it is possible changes of the car. To obtain high quality to predict just how much bass boost must sound reproduction, the cone of a loud­ be added to make the reproduction sound speaker should follow the vibrations of the like the original, even though it may be microphone diaphragm which picks up the I1II AUDIANA III original sound. In other words, there should much lower in absolute sound pressure. Weare generally accustomed to listening be a tight coupling between the two. Design of Compensated to a radio or phonograph at a sound pres­ However, such is not actually the case, and the speaker cone does not follow per­ Volume Controls sure of 70 to 75 db above the threshold of hearing, at those times when we are actu­ fectly because of some flexibility in the The ear does not hear all frequencies "coupling," but acts somewhat like the ally paying attention to the music, and in trailer would if it were drawn by a long with the same sensation of loudness when the average li ving room. At a concert, we they are reproduced at the same sound in­ spring instead of by a solid drawbar-it tensity or pressure. Loudness is a term hear the music at a considerably higher would lag when the car started quickly, used to identify how loud a sound appears level. In order to make the low-level music and it would crowd up against the car when to our ears, in contrast to sound pressure sound just like it does in the concert hall, the latter slowed down. The use of nega­ which is an absolute measurement. For ex­ we must therefore boost the low frequen­ tive feedback in the amplifier tends to ample, if we were to attend a concert, we cies. This may be done by means of the stiffen this spring, but still does not make would hear a certain ratio between the bass-boost control, but it is more conven­ it a solid connection. It is as though the sound produced by a clarinet and that pro­ iently done by so constructing the volume solid drawbar were attached to one end of duced by a bass viol. Each has its char­ control that it is done automatically. Thus a lever mounted on the trailer while the acteristic loudness in the ensemble. But when the volume control is turned well up, other end was connected to the solid part wh.en this same music is reproduced in our we hear the same volume as in the concert of the trailer by a spring. This spring rep­ living rooms, it is rare that it is played at hall, and with the same frequency response resents the resistance of the voice coil of the same volume; and when we reduce the -that is, flat. When volume is turned the speaker, which can not be dispensed volume, we change the apparent character down, the bass is boosted automatically, with. Now if we can connect another spring of the music. The ratio between the clari­ due to the construction of the control it­ at this same point on the lever and extend net and the viol has changed, even though self. The author has discussed the tech­ it back to a point on the drawbar, the flex­ the reproducing system may be ' perfectly nical aspects of this control, describing ibility of the coupling will be greatly re­ flat-that is, with a frequency character­ various methods for -effecting the compen­ duced. istic which is identical with that of the sation as correctly as possible, and in an The circuit connection described by Mr. original sound source. economical manner. Childs does just about that, and a large The actual result of the ear's deficiency part of the fl exibility of coupling between is primarily to decrease the apparent bass Loudspeaker Damping with the amplifier and the speaker cone is ab­ when music is reproduced at a volume level sorbed by the addition of what the author lower than the original performance--a Dynamic Negative Feedback terms "dynamic" negative feedback-the condition which has been noticed by any When we tow a trailer behind an auto­ term dynamic denoting the ability to listener who has heard music reproduc­ mobile, we normally llse a solid connection change in proportion to the amount the tion at a very low level. Considerable sci­ between the two vt!hicles, and the trailer cone deviates from following the amplifier entific effort has been expanded to analyze follows closely t1;e speed and direction signal.

8 AUDIO ENGINEERING • FEBRUARY, 1952 • ~ M ';t- ." WHA1 SPEECH INPUT FACILITIES WILL yOU' NEED ?

:!ii' T' Vision into the future is a major re­ A TES "SA-40 CONSOLE quirement in good audio engineering to determine what speech input facilities will be a~ , ,,,seful five or: ten years from today as now. Certain is the fact that whether: vicleo or radio, broadcasting'of . the future will require more and more studio facilities. We designed the -SA:-40 console with the future uppermost in our thinking. Ultramoder!1 today is the GA'rES SA-40 speech input console, generously equipped to fulfill the needs of video and The SA-40 console is constructed in rugged tilt­ radio stations for many years to come. back cabinet or available in matching ''desk (desk It is known as the console with the extras not illustrated). All amplifiers are individual units, - extra, cir(uit and switching facilities, quickly removable without disrupting remaining extra quality in construction and extra circuits; mixer keys of standard PBX t)tpe for long serviceability for maintenance. i.fe; panel of lifetime anodized aluminum; key If you are planning a studio installa­ knobs in varied colors for circuit identification; beau­ tion today - think of ' tomorrow and then tifully streamlined. Size: 48 inches wide, 21 inches think of GATES. Write for your copy of deep, 14 Y2 inches high. - the Gates EECH INPUT CATALOG and the~stud e 'functio,nalblock diagn!,m ttterein , ,for yourself why, if you are planning for the future as well as today, your ,needs can be best fulfilled , by investing in the GATES SA-40 console of tomorrow.

TERMINAL FEATURE ASSURES FLEXIBILITY Unusual flexibility of operation is provided by bringing many of the circuits such as preamplifier outputs, mixing channel inputs and program and monitoring amplifier inputs to terminal strips, These are, in turn, bridged to other terminals and back The tilt-back-to-service feature of the SA-40 console into the normal circuit. This permits breaking any provides complete accessibility of all components and lerminal connedions. maior circuit in the console for a patch panel extension.

Front view of rack mounted power supply for SA-40 console. Heavy cast aluminum housing 19" wide, 834" high, 11" deep, is designed to provide front

P"~ GAT E 5 .ADI 0 --'''''''' ~ . MANUFACTURING 'NGIN"" SlNC' 1922 . 2700 po, Ik, Avenue, Houston, Texas • Wamer 8uilding, WaShi, 9ton. ' D. C. • International Division, 13 t. 40Ih $1., New York CIty . ' Canadian Marconi C~~any, Montr... I, Quebec L --~- :------~~----,----~--~----~ AUDIO ENGINEERING • FEBRUARY, 1952 9 • What's inside a flladio=f!/leIay station?

Because microwaves travel in straight lines and the earth is round, there are 123 stations on the transcontinental television route between Boston and Los Angeles. This view of a typical unattended station shows the arrange­ ment of the apparatus which amplifies the signal and sends it on.

o NTH E ROO F are the lens antennas, each with its horn tapering into a waveguide which leads down to equipment

ON THE TOP FLOOR, where the signal is amplified, changed to a different carrier-channel and sent back to another antenna on the roof. Here are testing and switching facilities. Normally unattended, the station is visited periodically for maintenance.

ON THE THIRD FLOOR are the plate voltage power supplies for several score electron tubes.

ON THE SECOND FLOOR are filament power supplies. Storage batteries on both floors will operate the station in an emer­ gency for several hours, but

ON THE GROUND FLOOR is an engine-driven generator which starts on anything more than a brief power failure. Anything that happens-even an opened door - is reported to the nearest attended station instantly.

Coast - to - coast &ladio-f!llelatt shows again how scientists at Bell Telephone Labora tories help your telephone service to grow steadily in value to you and to the nation.

BELL TELEPHONE LABORATORIES I mproving telephone service for America provides careers for creative men in scientific and technical fields. f / Loudspeaker Damping with Dynamic Negative Feedback ULRIC J. CHILDS '"

A fu~t~er study of the effect of introducing some current feedback in an amphf.. er to coun~eract. the. varyi.ng i.mpedance of th.e loudspeaker. The au­ thor gives a practical CircUit which IS adaptable to any feedback amplifier.

UDIO E;,GINEERING has gone a long of the speaker and varies from instant OUTPUT TRANS. way smce the day when any ampli­ to instant. SECONDARY Rye Afier with an extended frequency Although Clements chose to call this

range was considered to be "hio-h fidel- kind of feedback posi tive it is speaker­ R1 SPEAKER Ity.. " However, a number of misconcep-h controlled negat'ive feedback and be­ 2,400 tions still remain. One of them is the cause its value is dynamic r~ther than question of loudspeaker damping-re­ fixed, dynamic negative feedback seems duction of overshoot and lag of the to the writer to be an appropriate term. speak.er cone caused by its inertia, Equivalent Circuit especIally at low frequencies when cone excursion is large and the period be­ Fig'/owe 1 is the equivalent circuit of tween impulses is long enough to allow an ampli fier with or without ordinary noticeable wave trains to exist. Because negative voltage feedback. If the nega- only the initial pulse of the wave train Fi g_ 2. Circuit which can be adapted to any is caused_by amplifier output signal, the Ro Rye feedback amplifier to provide improved damping. remainder of the movements create damping be perfect. Using a philosophy signals that are entirely spurious and IDEAL applicable to any closed-loop control are detectable as muddiness in the music. SPEAKER system, no error correction can be had IDEAL AMPLIFIERI AMPLIFIER rv Negative feedback has been widely TERMINALS until an error exists, since the error IGENERATOR) used in the last few years to lower the Go is necessary to create the correction internal impedance of the amplifier so signal. A more cogent limitation is that the signal generated by the loud­ oscill ation, which must exist if damping speaker as a motor during its overshoot is perfect. However, the method can be is shor.t-circuited by the amplifier, and used w ithin limitations to obtain im­ the same dynamic braking effect is ob­ provement in speaker performance Fig. 1. Equivalent circuit of output·transformer which is obvious to the discriminating tained as though a motor a rmature were secondary and loudspeaker in idealized system. listener. shorted with the field excited. However, tive feedback is high, the value of the as Warner Clements pointed out in the Practical Circuit August, 195 1, issue of JE, there is still amplifier internal impedance Ra which a considerable resistance in series with appears in series with the amplifier as Figu.re 2 shows a basic circuit used an ideal generator G , is low, and the to obtain dynamic neg.ative feedback. It the "s ~ or t circuit"-the comparatively a large II1ternal ohmic resistance of the output voltage at the amplifier terminals is similar to the Clements circuit, but voice-coil winding itself. Thus ideal does not vary greatly between load and the net feedback is not positive. Rl and dynamic braking is quite impossible with no-load conditions. R. are a voltage divider across the secondary of the amplifier output trans­ n.egative fee?back of the usual type, The ultimate in this type of design Sll1ce, even wIth a zero amplifier internal would be reached when Ra = 0, but this fo rmer. The feedback lead is connected is impossible by ordinary means. How­ to the junction of Rl and R.. with impedan~e, t?e average 16-ohm speaker vOIce COIl stIll places about 10 ohms in ever, even with a zero internal ampli fie r ground at right of the current feedback the series circuit. resistance, the internal resistance Rvc of resistor R f • The voltage across R. due the voice coil remains and is a severely to the output signal is, of course, in the There are two possible methods of same phase as that from the output achieving perfect or nearly perfect limiting factor in the "s h ort~c ir cu i t" current available for damping. transformer, while the voltage drop d~l11'ping in a loudspeaker. The first, ob­ The ideal solution as a basic idea is across R t due to signal current is in vIOusly only a theoretical possibility, is opposing phase with respect to the junc­ to use a speaker whose voice-coil wire quite simple: the voltage generated by the speaker due to overshoot, for ex­ tion of R. and R f . The net voltage ap­ has no resistance. The second, suggested pearing across the feedback leads is by Clements and explored in greater ample, must be applied independently to the ampli fier in such a way that it will therefOl-e that of R. minus the R, drop. detail by the writer, is to add a new type Because the voltage across R. is much of speaker-control network. T he ampli­ appear at the ampli fier output terminals in the correct phase and at the correct g l- eater, the net feedback voltage is in fier may be made so sensitive to the the same phase as the amplifier output movements of the speaker that when the amplitude to restrain the cone from overshooting. Stated in another way, the voltage. It is fed to an early stage at the speaker tends to overshoot, the ampli fier correct point to make it negat£ve feed­ output is automatically reduced to cancel outJ!lut of the generator Ga itself must be controll ed by the speaker within wide back. the overshoot; when the cone lao-s the When the speaker overshoots it gen­ signal the output is increased to" make li mits of phase and ampli tude so that the circuit will cancel Ra and R vc' T he erates a voltage 180 deg. out of phase the. sp.eaker follow the excitation signal. with the signal voltage. This counter­ This IS done by controlling the inverse speaker then looks back into an im­ pedance of zero 011ms and is perfectly e.m.f. causes a reverse current to pass feedback present in the system so that through R , reducing the existing signal it is largely dependent on the behavior damped. f Examination of this scheme indicates clt-op across R f • The result is an increase '" 1601 First Avenue, Ne'LIJ York 28, N. Y. that under no condition can speaker in voltage across the feedback leads, an AUDIO ENGINEERING • FEBRUARY, 1952 ( 11 j and rearranging to find the ratio using Eq. (1) and converting the resul t­ ing voltage ratio to db. It shows what the e R vo (1 ) output regulation of the amplifier must E R vc+ R a be in order to cancel given portions of Eq. (1) may now be used to find the R vo. Note that to cancel as much as 0.8 regulation with any desired amount of R vo the regulation must be -14 db. The R vo being cancelled out by a negative decibel regulation figure indicates how E: Open·clrcuit Amplifier Ou tput Vo ltoge. value of R a. The result, if expressed in e: Ampli f ier Output Vo ltage Under Lood . much of the negative feedback in the If Ro is nega tive, e > E . db, is a negative number since the loop will be cancelled when the dy­ regulation is opposite in .direction to namic negative feedback circuit is used. Fig. 3. Equ ivalent circuit to show effect of com ­ ordinary positive regulation, i.e., the voltage rises instead of falling when a Thus, to cancel 8/ 10 of R vo , this loop plete cancellation of voice-coil resistance by must have sufficient negative feedback means of feedback. load is applied. IJ one-half the value of R vo is to be from the junction of RIR. of (Fig. 2) cancelled, then to keep net feedback negative even when increase in negative feedback to the a large portion of the negative feedback amplifier, and a drop in amplifier output R a=O.5Rvo voltage. is removed for use in correcting spurious e R vo speaker cone movement. The fig ure for The negative feedback is dynamically 'E= R -05R =2= (- )6 db. controll ed at each instant by the current vc . va negative decibel regulation becomes in­ ordinately large as the damping is in­ through R, in such a manner as to cause For example, if the speaker voice coil the amplifier output to counteract the creased, approaching infinity toward spurious actions of the speaker cone; the perfect damping, as shown in Eq. (2). degree can easily be controlled by vary­ It appears, therefore, that only a limited amount of dynamic negative ing the value of R,-the larger the value I...... the greater the corrective effect. 2 feedback may be used with the very .... v best practical amplifiers, both for the Even '.Vith qui~e s~ all values of R" 4 the effective ampltfier mternal resistance / reason just mentioned and because too 6 may be zero or even negative. An over­ l 1/ great a fig ure causes the designer to run shooting (high-resistance) speaker 8 into severe difficulties with oscill ati on due to phase shift and response non­ causes increase in negative feedback and -, 0 J decreased output voltage ; a lagging linearity at extreme frequencies, even 2 I those far above and below audibility. (low-res istance) speaker causes less I II negative feedback and more output. This 4 The Childs amplifier, which the writer has designed (and redes igns from time contradicts Ohm's Law, according to 6 to time) and constructs for use in better­ which the output voltag-e of a generator 8 II with internal resistance falls as the load I quality custom installations has fi at fre­ resistance decreases. This means that 0 quency response to around 100,000 cps nd down to 10 cps. About 3 db of dy­ the internal amplifier resistance Ra is 2 a namic negative feedback has been in­ negative. It is still amenable to O'hm's 4 Law exc~p t , that instead of causing a corporated in recent models with no 6 voltage drop, Ra c~u s e s a voltage rise, danger of instability and with noticeable the amount of which depends on the -28 improvement in the already impressive performance. Even so, intermodulation value of the negative resistance. - 3 0 The corrective effect may theoreti­ distortion at the IS-watt level rises from cally be raised as necessary to com­ -3 2 0. 1 per cent to 0.2 per cent. pensate exactly for all spurious move­ - 3 4 The graph of Fig. 4 and E q. (1) are additionally u s ~f ul to the designer for ment in the speaker. In this case the - 3 6 measuring the dynamic negative feed­ effective internal resistance of the 8 amplifier would be neg-ative and exactly back caused by a particula r value of R f equal in magnitude to the sum of the - 1 -0 .9 -0.8 -0.7 -06 -0.5 -0.4 -0.3 -0.2 -0.1 in Fig. 2. Each time the value is .!!.2. changed, the open-circuit and loaded Rvo' R f , and any transformer secondary Rye resistance. Regarding the speaker as an output voltages of the amplifier may be ideal zero-resistance generator in series measured and converted to decibels. with the voice-coil wire resistance, the Fig. 4. Curve showing output regulation fo r Reference to the graph will then indicate g~nerat or w?uld look back into a gen­ vary ing ratios of amplifier output impedance to what portion of the speaker's internal ume zero Impedance and, in effect, speaker voice -coil impedance. resistance has been cancelled. Knowl­ would be perfectly braked dynamically. edge of the speaker's resistance will, of Such a condition is, however, impossible resistance is 10 ohms and the internal course, indicate the exact value in ohms of attainment, even in an amplifier with resistance of the amplifier is made - S of the internal resistance of the ampli­ no phase shift at any frequency because ohms to cancel half of Rvo, the amplifier fier. o s c ill a~io n would result, as s ho~n by the output voltage will rise 6 db 'between fo llowmg demonstration. . no load and full load. Perfect Damping Impossible Figure 3 is an equivalent circuit of To obtain complete cancellation of . !he ab?ve proof that perfect damping the ampli fier, consisting of zero-resist­ R vo' Ra = - R vo. Then ~ s In:posslble may be somewhat puzzling 111 view of Clements' statement that it ance generator Ga and internal resist­ e RVQ 1 could be obtained, accompanied by ap­ anc~ R a, connected to a load R vo. As­ E R vo- R vo =0= co (2) summg that Ra has a negative value, as parent verification with an oscilloscope would be necessary if it were to cancel which means that adding any load what­ and a square wave measurement. The ever, no matter of how high a resistance error made by Clements was in assuming out ~ s much as possible of R vo' the amp1ifier output voltage will rise when causes a rise in output voltage to infinity: that an oscilloscope placed across the th~ load is connected. The load voltage which is oscillation. Therefore, it is actual voice-coil terminals could indicate e IS therefore higher in amplitude than obvious that perfect damping simply motion of the speaker. The fo llowing the open-circuit voltage E , and the out­ cannot be achieved. simple demonstration shows that it can­ put regulation of the ampli fier may be But additional figures on this same not do so and that there is, in fact, no expressed in db at 20 log e/ E. Then subject are of interest: How far is it prac­ practicable way of depicting on an tical to go? How ~ l can a speaker be oscilloscope what the speaker cone is E R vo damped with dynamic negative feedback? doing without the use of fairly elaborate e= R~o+ R a The graph of Fi:r;. 4 was prepared by acoustic measurements under ideal COll- ./ 12 AUDIO ENGINEERING • FEBRUARY, 1952 ditions or possibly a special type of bridge circuit. R. In Fig. 5, (A) and (B) show the amplifier output circuit and the speaker, • each equated to an equivalent circuit. Fig. 6. Eq uivalents The amplifier becomes a zero-resistance for further study of G. '" s G '" generator Ga with internal resistance 'scope in dication s. +2V. Ra in series, and the speaker consists of another ideal generator G. with internal voice-coil resistance Rvc. (C) shows the • amplifier connected to the speaker with a 10-ohm potentiometer R in series with (A) (8) the two and an oscilloscope connected between the zero-voltage reference point "wriggles" atop the square wave in re­ time showing an ideal speaker G with (ground) and the arm of R. The pur­ 8 versed phase. With the 'scope across pose of the demonstration is to show that an internal resistance Rvo of 15 ohms zero volts, however, neither is seen, but and the 'scope connected across the for any value of negative amplifier re­ only the square-wave signal put out by speaker terminals. This duplicat.es the sistance we can cause the oscilloscope to the amplifier. If the amplifier is a good show no voltage generated by the scheme used by Clements to adjust one, the signal will appear to have flat feedback. Redrawing the circuit in con­ speaker, giving us a false impression tops. ventional voltage-divider form at (B) that the speaker IS generating no The total resistance around circuit and finding the voltage across the 'scope spurious-movement voltage. The only (D) of Fig. 5 is 5 + 10-5 = 10 ohms. by the same means as before, the 'scope requirement is that we must be able to Since the speaker is generating 2 volts shows -1 volt (Phase B), i.e., 1 volt of place the above-ground lead of the we may say that each 5 ohms drops 1 correction signal. It could be made to oscilloscope at any desired point on the volt (and each - 5 ohms drops l .volt in indicate zero by going into the voice internal resistance of the speaker. Since the opposite phase). coil and connecting it to the tap on R vc it is impossible to do this, we have added If the 'scope is connected between labelled 0 volt, but this is an obvious R, which is merely additional voice-coil ground and point- A it will show impossibility. resistance placed out in the open so we ER + ERa = 2 - 1 = 1 volt positive. It should be clear by now that in order can get at it. Thus the 'scope is showing the for the 'scope to indicate zero, it must The circuit is again a simple voltage speaker-generated voltage (Phase A) be connected at a point where the divider as shown in more conventional less a certain amount of correction volt­ speaker-generated and the correction form at (D). Weare not interested in age (Phase B), the resultant giving no voltages cancel and produce zero. In the amplifier signal but only in speaker­ useful information. case of a perfectly damped speaker this generated voltage, so Ga may be con­ If the 'scope is connected to point B, point would be the terminals of the ideal sidered as simply a short circuited ele­ it shows ERa, which is - 1 volt. This is speaker Gs which are hidden from ac­ ment for the moment. the correction voltage (Phase B) and it cess. It should also be clear that with the The voltage in the circuit appears in will show the "wriggles" in reversed 'scope set at any point it can be made to two phase relationships 180 deg. apart­ phase. Since the points above and below indicate zero by adjusting the amplifier Phase A being the positive speaker­ R are respectively at +1 and -1 volt, output impedance Ra but such an indica­ generated voltage, and Phase B being when the arm of R is at the center, as tion will not indicate what the speaker the negative voltage appearing across shown, the 'scope will show no speaker­ is doing. A third interesting point is the negative resistance of the amplifier generated or correction voltage at all that the 'scope cannot indicate zero when Ra. If the speaker is ringing due to in­ and the square wave will appear to be connected directly across the amplifier sufficient damping on a square wave it fl at. However, the fact that the indica­ output terminals (across Oa and Ra in may be seen on the 'scope if it is con­ tion varies with the 'scope connection is series) which is in practice also across nected across either a: positive or nega­ proof enough that the appearance of a the speaker terminals (G8 and R vc in tive voltage. In the former case the flat-topped wave is no indication of good series) unless the balanced voltages are actual voltage generated by the speaker speaker performance where there is re­ zero at that point, which means that will be seen; in the latter the counter­ sistance between G. and either 'scope amplifier output resistance Ra must be voltage generated by the dynamic nega­ lead. z&o- not negative and not positive. tive feedback will be seen, with the (A) in Fig. 6 is a similar circuit, this Thus when Clements adjusted his ampli­ fier in this way, he adjusted it only for OUTPUT TRANS. zero resistance at the output terminals. The fact that voltage rose with connec­ = ~~ $ tion of a load could only have indicated j~ :;::: that the value of Ra was just enough (A) negative to cancel out only the resist­ ances of the output transformer and of Rt, not that of the voice coil. SPEAKER • VO ICE COIL Adjustment of Circuit Fig. 5. Equivalent circuits for study of The difficulty of measuring the actual (8) : I------~--.,------_I 'scope patterns as a speaker performance is no real handicap, Ro . ~ measure of the ef­ since adjusting the value of R f the + VOLTS. PHASE A -5n tOn . - VOLTS. PHASE 8 fectiveness of output current feedback resistor does not re­ im pedance cancellation. quire such measurement. It is impossible 5n Rve Rve · 5n to approach perfect damping a~Y\Y

AUDIO ENGINEERING • FEBRIlJARY, 1952 '. 13 -' The Design of Compensated Volume Controls

EMERICK TOTH ~'

Part 2. A study of the psychoacoustic considerations involved in designing a control which is practical in construction and adequate in performance.

LL SORTS of separate bass-boost C 1> the upper limit of frequency com­ amplifiers and other more complex Aand expensive schemes are possible, pensation (T=R2C1, and fc= :T at + 1 of course, but the happiest solution ali db from flat response). This circuit is around is probably to combine the low­ simple and approaches a maximum of frequency compensator with the usual 6 db compensation per octave below f c; volume-control potentiometer at the in­ unfortunately, as in F/:g. 6, it has the put to the audio amplifier for simul­ fatal defect of not allowing the volume (01 (bl taneous one-knob operation. This makes control to reach essentially zero output it possible to avoid any large initial in­ level, because R z must have a rather sertion loss caused by the compensator hig h fin ite value (about 1/20 of R 1 ) . circuit and puts its volume control effect This difficulty can be overcome in inex­ to good use by combining it with the pen ive fashion by shunting R ZC1 to ' volume control proper. With appropriate ground with part of potentiometer Rv proportioning of amplifier gain and in­ which then also provides a d.c. grid-re­ put, this combination will also insure a turn path. The result is the familiar (dl degree of automatic bass compensation tapped volume control circuit of (B) , (el which probably represents a reasonable Fig. 7. The response obtained with typi­ compromise in favor of best theoretical cal values in such a system is shown in Fig. 7. Bass-compensation input circuits. average adjustment, as against individ­ Fig. 8. The compensation is not very ual mi suse of an independent control. close to the theoretical values of Fig. 5 generally too low and which conse­ quently seriously limits the rise in volt­ Such a combined control has been used for two reasons: (l) R zC 1 is shunted by in one particular form, as at (B) of F·ig. a fi xed value of resistance (from the age across C 1 at the lower frequencies; 7), in many receiver designs ovel' a potentiometer tap to ground) which is and (2) , the compensation remains fixed at all potentiometer slider· positions be­ period of about twenty years, affording low the tap. some, but usually not enough compensa­ VI R2 V2 O.47meQ tion. This form, as well as that of Fig. 6, Im proved Tapped Bass-compensated is based on the fundamental compensa­ il' Volume Control tor circuit of (A), Fig. 7. As the slider E ci The performance of this type of con­ of potentiometer R 1 moves down toward trol can be improved by a departure R z, the output voltage becomes more from the usual design. Present designs nearly the potential drop existing across generally have the tap at 0 or y.( of the capacitor C l' R z is the limiting factor total resistance relative to the grounded which determines the maximum high­ R6 C3 end, at a point physically about 1/ 3 of frequency attenuation and, together with the rotation from ground. It would be better to have the tap at about 20 per * Radio Techniques Branch, Radio Division II, Naval Research Laboratory, Washing­ cent rotation from ground, with the re­ ton, D. C. Fig. 6. Interstage bass-compensa t ion circuit. sistance of tap to ground equal to at least the value from tap to high end. Each section should theoretically have as close to logarithmic taper as feasible, if approximately constant attenuation ~ of sound pressure in decibels per degree of rotation is satisfactory. L inear at­ 0 tenuation in terms of loudness sensation ~ will require a different taper. The tap­ to-ground section, however, can be ~ Ir--t- -10 db '10 linear without serious disadvantage, i' • ~ :1'-.. since its main function really is to allow I Fig. 5. Theoretical - 20 db the slider to reach ground for "zero" z'2O response of volume con t rol compensated output. Proper choice of values will en­ ~° ~ t~ "'~ :> I-- t: ~ able such a control to have about 40 db :2 l- ~ to 80-db standard . ~ r-.... -3Q db ~ '3 0 f-- ::io~ of attenuation at a substantially constant !i ,.0", "'0 "'~ " rate of db per degree in the first coun­ -- ~g~ § ::»u 5 ---+--t~~~hH-H ---~--+-+-~~~---~ • tel-clockwise 80 per cent sector of rota­ ~ ·40 I-- Sffig ~ __ ~--t-~rt~~-4~O~db~~~~~~~~~ tion. The remaining 20 to 40-plus db, e:>~ ~ f--- ;.. :a. 0: go"' ~ ----t--+--HH-tH+--+-IL If-I-..J-I.+l-II -~ which is usually seldom used, would be "'0: ~ concentrated in the last counterclock­ wise 20 · per cent of rotation. Typical "" '000 FREQUENCY IN CYC LES PER SECOND values might be : Rl = 1 megohm, tapped

14 AUDIO ENGINEERING • FEBRUARY, 1952 compensation in this network, as shown T he curves of Fig. 9 were obtained

0 .9 in the circuit di agram of F ig. 9. H ere with an input generator resistance of / ~ the 0.22 meg. and 150 JLJLf RC com­ about 0.12 megohm, to simulate a me­ bination is used to minimize the drop in dian value of detector-circuit or phono­ o.8 V response above 5000 cps which is evi­ graph-pickup-circuit impedance. (The I dent in Fig. 8. The output loading on generator was about 20,000 ohms in the II both circuits for the measurements was measurements of Fig. 8) . The low-fre­ o.7 a vacuum-tube voltmeter of about 50 JLJA-f quency compensation is quite good, down J input capacitance and 2-megohm resis­ to about - 30 db control setting. Good tance. The I5-JA-JA-f capacitor from the control of high-frequency compensation ~ O. 6 I :r o ,;J'" high end to the slider of the second (0.5 is indicated. The input potentiometer iii megohm ) potentiometer is used to pro­ should have a linear resistance t9-per, '", o.5 JtJ duce the increasing high-frequency boost whi le the input potentiometer should ~ z .:t with increase in control attenuation re­ be log arithmic if linea r sound pressure :! .1 quired by the curves of F'ig. 5. vari ation is acceptabl e. The resistance 4 !J ~o. characteristics of the nominal I-megohm OJ:'" I - linear and 0.5-megohm logarithmic U rl- ~'- o ! o.3 J ~J f- - .:>~~~~ ganged controls used in getting the data I 8"1 1.0 0 for Fig, 9 are shown in Fig. 10. The / 1000-cps input resistance and the at­ ;1 VI o.2 "', ~ tenuation characteristics over the control §, 5'" 0.8 ' 10 I ~ iii ..... /-1< .J' rotation range are shown in F ig. 11. ., "'Go t ... ~ ...~ . T he rate of attenuation in signal voltage o.I 91 '"o " ~O . 6 ~ ' 20 -s>~ or sound pressure is constant within ± 1 J ... 1 ~ ~ ~~t/ .. , J'/.rJ-: , VI ".. .q"k-- db ovel' a range of 40 db, covering Vi ii r-- o j' -r-'" ::> ~ " "", about 65 per cent of the total rotation 20 40 60 80 100 '"a: o. 4ffi~30 f r.. CONTROC ROTATION - PERCENT 5 1= - or about 1 db per each 5 deg. for 200 ' Q.i!O " / degrees of rotation. This curve is also u ,j Fig. 10. D.c. resistance of ganged control ele­ ccO.2 -40 within 3 db of being linear in terms of ments of dual control circuit. / / loudness sensation at 200 cps, which is about the median frequency of maximum 0 I at 0.5 megohm at 20 per cent rotation ' ''''0 20 40 60 eo 100 sound pressure in voice and music. from the low end ; Rz = 27,000 ohms; CONTROc ROTATlON - PERCENT Still better compensation may be ob­ C 1 = 0.0 1 JLf. T he 0.5-megohm resistance tained by tapping R s as shown at ( D) in shunting R C would affect compensa­ Fig. 11 . 1000-cps calibration and input re­ 2 1 sistance of dual compensated control circuit. [Cont'inu,ed on page 30] ti on at frequencies below about 60 cps. A more expensive but generally better way of overcoming the 10," volume limi­ tation of the basic ci rcuit of (A), Fig. 7, is to add a linear voltage di vider sec­ tion ahead of it. Potentiometer R s in EC) accomplishes the purpose of reduc­ • ing th e output to "zero" in the region where R2 prevents the slider of R1 from Fig. 12. Response of reaching ground potential. The limita- ' dual compen s at e d tion on the low-frequency compensation control with gain afforded by this circuit to less than 6 db adjusted for 50-db per octave maximum is primarily due to sound pressure at the reduction in signal cun-ent through 1000 cps. branch R1R zC1 at the lower frequencies . This takes place as a result of the rise in reactance of C1 with decreasing h e­ quency with a proportionate diversion • of current through the lower shunt value 10 • • • • • • 20000 of R s at reduced volume settings. It is '000 '0000 also feasible to include high-frequency FREOUENCY I N CYC L ES P E R S ECO ND

- MEASURED -- ME ASURED -- --- THEORETICAL I JI I - - - THEO RETI CA L I II I REQUIREMENT REQUIREMENT MAX. CONTROL SETTING IM~~ . 'C~ NT R O C SEn:NG o o -~ - 5 db -5 db -.... ~ ~, ~ -IQ db :::t::. -1 0 db r- - I 0 - I 0 - ...... ~ ...... - ~- ~ ' r- -. --...... -. -20 db ..., ....:::. '20 db 0 t- ... -20 I"- ,~ ...... r- --...... l- ...... 1'--..', I'-r- - 3Qdb 1-= I'-.... r-I"- -30 db 0 !!L 0 .22 M"l . It _OUT -30 ~ F-:... IN ~ ~ . .,. , ~~ ~ ~ ~ -4Q db ~ :--~ - 40 db -4 o ~ .._-,~_g~ :;:~~ - 40 .007! ~ TAIN"lPPED AT " f ~f 39,000 0.25 Meg _ - 39.000 r-

• 0 20 0 0 • 0 • 0 '0000 , • 0 • 0 '0000 100 1000 ' 0000 100 1000 '0000 FREQU ENCY IN CYCLES PER SECOND FREQUENCY IN CYCL ES PER SECOND

Fig. 8 (left). Response of typical tapped compensated volu me cO ll t rol. Fig. 9 (right). Response of dual compensated CQllttQl c[[(\.lit.

AUDIO ENGINEERING • FEBRUARY, 1952 15 , / / .' MIKE LEAD JOCKEY PULLEYS Fig. 3. Diagram of the complete pulley and cord system. (Spindles and other mountings, and also the ----+,.-~ tensioning arrangements, have been omitted in the FAR SIDE interests of clarity.) MAIN PULLEY PULLEY 'I--~'H-' SYSTEM BLOCK

'\~ONTROL ~"'~ The Traveling Mike

N. H. CROWHURST '"

Difficult stage arrangements can be covered easily from the wings

FLOOR with this device; the author gives adequate constructional details. MOUI

EVERAL WAYS can provide the neces­ sary coverage for sound reinforcing Sequipment in theatres and hall s where sound pick-up is required from a Fig. 1. General view stage of considerable area. Most of them of the traveling mike system. The mono­ involve the use of several mikes, but rail is supported at there are disadvantages in having its ends by the verti­ several mikes "live" at the same time: cal "trestles," and One mike, appropriately placed can intermediately by the work with higher gain setting than a steel wires from over- group arrangement before acoustic head anchorages. feedback sets in; and the multiple arrangement suffers some loss of quality when the artist occupies an "overlap" in the areas served by two mikes. The traveling mike here described overcomes these disadvantages, and is simple to construct. ribbon type. T his particularly suits combined with the ability of rotating The mike used is of the bi-directional dialogue pick-up, if placed between the the mike head, enables the mike to be * 82, Canterbury Grove, London, S.E .. 27, artists so they speak to opposite sides placed at its best position quite readily. England. of the mike. The traveling arrangement, It is very useful in picking up dialogue from two people walking across the stage, and for various other scenes where the centre of interest moves about. Instead of the actors' having to modify their acting to suit the mike arrange­ ment (which they seldom do stlccessfully CARRIAGE anyway), the mike can be moved about WHEELS to suit the act. Everyone who · has worked in this field will immediately appreciate the advantage. The mike is fixed in height. The height chosen will depend to some extent on the height of the stage proscenium. The mike must be high enough to clear artists' heads safely, also it should not protrude too far below the overhead curtaining, so as to divert attention ROTATION when moved. Generally the best height CONTROL will be related to the size of the building. CORD Travel across the stage is achieved by mounting the mike on a carriage run­ MICROPHONE SPINDLE ning on a mono-rail. T wo "endless" cords running alongside the rail provide the means of control. One pulls the car­ riage back and forth along the rail, while the other rotates the mike by means of a Fig. 2. The microphone carriage with one side-cover removed to show the arrangement of the differential rotation control system. [C ontinued on page 35]

16 AUDIO ENGINEERING • FEBRUARY, 1952

! j ( Linear-Scale A-F Wattmeter

C. C. McPROUD

Am plifier testing with an intermodulation distortion analyzer is considerably speeded up if power output can be read di­ rectly in watts on a linear-scale meter. One solu tion is pre- sented by the author.

NTERMODULATION -DISTORTION testing Fig. 1. In termodula ­ has been proven of sufficient value tion analyzer built by I that most amplifier ratings are con­ the author for use sidered incomplete unless they are in­ in makin g series of amplifier distortion cluded. Since }£ begins a series of measurements, and in equipment reports in the next issue, such new equipment development. measurements were deemed necessary, and accordingly a simple 1M analyzer was constructed, following the design 16 OH MS employed in the instrument described over a year ago.1 However, as will be • apparent to anyone who has had occa­ sion to make a number of such measure­ ments, the amount of 1M distortion Fig. 2. Meter scale should be plotted against power output for use on 2-amp. in watts, and the conversion of an out­ C. E. thermocouple put voltage to watts involves consider­ meter with calibra­ tions di rectly in able fig uring, in addition to close inter­ watts output fo r B- polation of meter indications to take and 16-ohm loads. readings at I-watt intervals. Accord­ ingly, some means to indicate watts di­ rectly, preferably on a linear scale, was desired, and efforts made to develop one. Power output from an audio amplifier • is generall y calculated by using the formula E2 in Fig. 1, and the meter scale is re­ P=1? ,------...e !~~~:ZU;R OF produced full size in Fig. 2, so it may be SECTION cut out and transferred directly to a with E be ing meqsured across the load similar meter in case its use is con­ resistor, R, which is selected to match 1 sidered desirable. the output of the amplifier under test. The circuit of the input section of the SAl '5" While an a.f. voltmeter could be cali­ ~ "~9~:'" 75W 1M analyzer is shown in Fig. 3. With a brated in watts, the scale would be 7" full-scale range of 64 watts, and to avoid crowded at the upper end. overloading of the load resistor, it seems After discussing the problem with M ~~F obvious that the latter should be a 75- C. J. LeBel, who suggested that a watt unit, and this size is available as thermocouple ammeter had a deflection an adjustable 15-ohm resistor. The re­ proportional to the square of cu,rrent, Fig. 3. Input circuit of I M analyzer modified to sistance of the meter (with its internal the use of such an instrument was in­ accommodate load resistor and direct-reading shunt) is of the .order of 0.2 ohms, so vestigated. Consider for example, the a.f. wattmeter. it may be neglected. Since a 20-ohm re­ use of a 2-amp. thermocouple meter. sistor would not pass 2 amps. within its Power may also be calculated by the Meter- Scale Configuration rating, and since a 16-ohm resistor was formula P = [2R, so a 2-amp. meter Inspection of various r.f. meters not available, a lO-watt, I-ohm resistor would give a full-scale indication shows a range of scale distributions. was added, as shown, to make a total of equivalent to 4R watts. Thus for an 8- For a truly li near current-square scale, 16 ohms. The switch is marked with ohm load resistor, full-scale deflection the mid-point of the current scale for a three positions- HI is for use when would represent 32 watts, and fo r a 2-amp. meter should be 1.414 amps., measuring 1M distortion in voltage 16-ohm load it would represent 64 watts. and one instrument ' fo und to meet this amplifiers where a load resistor was not These ranges are about what would be requirement is the 3-inch General Elec­ to be used, and 16 and 8 ohms where it suitable for measurement of the output tric instrument. A new scale was hand was required. This arrangement pro­ power in ampli fiers usually considered drawn, with calibrations of 0 to 64 watts vides the greatest facili ty of measure­ desirable for home music systems. above the scale ,are, and 0 to 32 watts ment, as proven by considerable use of below the arc. The ' two scales were the instrument. 1 John M. van Beuren, "Simplified inter­ 'One caution is important. Thermo- modulation measurements," AUDIO ENGI­ labeled 16 ohms an? 8 ohms respectively. NEERING, Nov. 1950, p. 24. The completed 1 ~ analyzer is shown [C ontinued on page 37]

AUDIO ENGINEERING • FEBRUARY, .1:952 J 17 • Audio In the Home

William C. Shrader*

Converting a commercial radio-phonograph into a custom inst allation usually st arts with the replacemen t of the record player and the addition of an FM tuner. The author tells you how.

USTOM BUILDI NG has become quite a complex fi eld. F or purposes of simplicity, we shall di scuss only C one of its aspects in this article-the transltion fro111 commercial to custom sets. T here is a large business in thi s fi eld alone, if it is handled properly. For instance, it is estimated that it will take about 15 years for a complete transition to LP, Typical home installations made by the author, again showing that the finished appearance is limited only by the imagination of the homeowner. even at the present rapid rates. Many people who would like to have a custom install ation already have expensive con­ soles representing a considerable in­ vestment which they do not want to "write off" entirely. Commercial con­ soles, designed for the mass market, put a premiulll on space, but it is imperative that their owners recognize these pieces of furniture as a g reat asset. Custom equipment- a tuning device, a n ampli­ fie r, and a three-speed record player -may be housed in such a cabinet with a modest cash outlay. S ince it is the de­ si re f0 1- three-speed record players that leads most music lovers to improve their existing sets, and since, in turn, a better record player creates a desire for im­ provement in the other components, we shall begin with a discussion of thei·r installation. The Garrard RC80 and the \iVebster- * Preside/If, S hrader lVIa ll llfacfwr ill IJ Co. , IlIc., 2803 .11 Stree t, N .W., Washington 7, D. C.

18 AUDIO ENGINEERING • FEBRUARY, 1952 Chicago 106-27 are the most satisfactory changers now on the market. Each model is designed for magnetic pickups-which are most desirable-but these require . more amplification than is available in an ordinary I~a dio. To overcome this, a pre-amplifier may be added. This some­ times sounds relatively easy. However, we have encountered several difficulties in installations of this type, notably the hum induced into magnetic pick-ups from the stray magnetic fields around power transformers and chokes. Turn­ table motors used in some changers do not lend themselves to use with extended range cartridges at all because of their excessive rumble. Fi her and GE make the most sati s­ factory pre-ampli fiers at a low price, and both may be obtained with built-in power supply to simplify install ation. By at­ taching the electric cord to the same out­ let as the record changel·, no additional power is drawn hom the transformers, and hum problems are minimized. In at­ taching magnetic pickups and pre-ampli­ fi ers to an existing radio, all networks of resistors and capacitors must be removed from circuits between the phono input of the radio and the volume control. \!\T here price is an im portant consid­ eration, two GE cartridges-one for L P and one for standard, and each properly weighted-make an excell ent combina­ tion, preferably with a diamond stylus for the LP. Shrillness may be encount- ered on LP records unless each cartridge is terminated with used with the GE pre-ampli fier, a voltage divider should be proper resistance. It is suggested that the GE LP be termi­ install ed between the compensator and the pre-amp. Such a nated with a 6800-ohm 1/ 2-watt resistor, and that the stand­ voltage divider should consist of a 3900-ohm series resistor ard be terminated with a 15,000-ohm 112-watt resistor. In and a 6800-ohm shunt resistor to ground installed inside the plug-in heads, such as used with both Garrard and Webster, these can be mounted right across the cartridge. pre-amp for the standard cartridge, as shown in F ig. 1. For Pickering cartridges work best with their own pre-ampli­ the LP, an additional I·esistance of approximately 3900 to fi er in combination with the 132-E Record Compensator, and 5000 ohms- dependi;lg on personal taste and the other com­ with this set-up, no terminating resistors are necessary. ~T h e n ponents in the system-may be placed across the pickup head. W ith the exception of the Audak, dual cartridges are un­ desirable, and while the Audak works perfectly well with the Garrard, it is not recom­ mended with the Webster be­ cause of hum from the motor. T he Audak is the most suc­ cessful of the dual cartridges because of its weight compen­ sations. Although it could work with two grams on LP, eight grams are required for proper operation on 78's, and this is considered the optimum weight for a du~l cartridge. It is a very convenient car­ tl·idge, especially fOI· house­ wives. A table of termination resistor values is furn ished with each cartridge, and these values may be used quite suc­ cessfully with either the GE or Fisher pre-amp. The Pick­ ering pre-amp has insufficient [Contn1ted on page 27]

AUDIO ENGINEERING • FEBRUARY, 1952 19 "So-called Audio Engineer"

HE MAI N PURPOSE of this letter," writes "scene" and focus the ear on the trans­ two symphony orchestras (a marvelous a radio listener to m e, "is to tell you mitting process itself which, ideally, should disc indeed, thoug h quite unnatural). Thow happy I felt when you advised be aurally transparent and therefore in­ "So as you see, I'm qui te excited, and my your listeners to 'open up your treble audible. house, always a haven for my friends, will and bass' for the Schubert Octet L P. be even more frequented by those that want It so happens that the day before Thanks­ Ultimate Consumer to hear their Ansermet Sacre de P rintemps giving, my long-anticipated hi-fi set was But let's not get ultimate objectives and the like. Unfo rtunately, the neighbors delivered and I have been in ecstacy ever mixed up with the means towards those ob­ shan't relish my new found joy." since." jectives. T he obj ective of better audio is, Bet you were fooled by that. T he man is A happy sort of letter to introduce this no technical expel·t on music at all and his 99 per cent of the time, better music. And reactions are definitely not in professional month's discussion and I'll 'quote more of it th fi nal judgment on better audio is to be in a moment, because this "hi-fi" convert fo und in terms of music itself. And so I musical terms. He's a listener, like you and expresses himself in a way that should be continue to quote from the above letter. me, a person who knows a lot of music by interesting to all audio makers and elec­ T his is what I mean by judgment in mus ical ear and has picked up plenty of hi s own tronic experts-here is a fin e laboratory terms of an audio improvement. knowledge concerning listening; his musical specimen (if he will pardon me) of the very "Part of the joy (of my new set) is due to repertory is big. But he's an amateur, in best type of audio fan, the person who takes your recommendation of the -- amplifier. both music and audio; and that, may I sug­ up audio specifi cally because of what it can I had my set assembled and cabineted at one gest is our man ! H ere is the true user of do for his music. of the jobbers so numerous in New York audio, to his and audio's greatest advantage. I t still is a source of astonishment to me . . . (and) since T hursday I have enj oyed Less Lovely Side how many people are crazy over audio, yet my records more than ever . . . and my quite apathetic towards the music it makes If all letters were as enthusiastic as the possible, or even downright antagonistic FM programs. My outfit, actually a modest above, the audio business could congratulate towards it ! F or every comment I get on one in the atmosphere of $500 speakers and itself on a job beautifully done. I'm bound, ultra-expensive amplifi ers, is superb to me. then, to quote a sael representative from a improved musical effects, I run into dozens Aside from the very adjustable -- ampli­ about improved highs, better transients, less lovely side of the audio area. This sort fier, I have an - - FM-AM tuner (N.B. of thing goes on entirely too much. It ought middles, lows, and so on, with never so I'm not giving plugs; th e ma.ke is not sig­ much as a mention of the music in which to be stopped, for the good of us all. Granted these improvements (or faults) occur. One nificant in this situa.tion! E. T.e.), qu ite there are plenty of potent problems in put­ might, not knowing the si tuation, conclude satisfactory if a bit thin on the bass end, ting fancy technical equipment in the hands that many an audio fan played nothing on a defect remedied somewhat by the amplifier of those who know nothing about it. Never­ his expensive equipment but audio tone bass-boost. T he speaker, which I chose out theless, we cannot stand, any of us, for records. Not true, of course. And yet how of four or five I heard, is a IS-in. --, the taking-advantage-of-babes-in-the-woods many people think of the music? giving sublime results. that is epitomized by this miserable hi-fi "Of course I hear passages on my favorite addict. I am acutely embarrassed, as will records which I never knew existed, espe­ Walking Audio Analyzer every reader be. cially triangles, bells, and the like. The "I am a lover of good music and it means I'll have to admit immediately that the Polka from the Age of Gold by Shosta­ a lot to me to hear such music properly audio engineer's business is audio, and the kovitch, on that early 7-in. Columbia, is my reproduced, over FM and phonograph· more technical the better. I have long since pet demonstration record to shock friends records, since my wife and I don't go to any found that my ear is not too trustworthy with. And it feels so good to hear the sting­ live performances. as to analyzing specific distortions, dips, ing 'cello stab, in the first movement of the "Many months ago, being a layman and not peaks, etc. because I have too much trouble Lonelon di sc of Beethoven's 15th Quartet. knowing better, I bought some expensive detaching myself from the music in oreler T lie Scherchen H aydn symphonies, and high-fidelity equipment from a so-called to listen specifically to the sound itself. I Mass in B Minor are extraordinary . . . audio engineer. I realized too late I was tend to hear thr01tgh the sound, on into the they shocked me on myoid set, but they spending too much for my pocket. I sold all music, which definitely interferes with my throw me to the floor with this one. the equipment save a 16-in. turntable with efficiency as a walking audio analyzer. I "Toscanini's Brahms' First on V ictor's magnetic cartridges and a diamond styli. hear the perfo rmer, or more often the com­ 78's is monstrous in sound and my friend The table was held for me by this 'audio poser himself. Like looking through a clean brought his old 78 of T oscanini's Beethoven engineer' until recently in his store room. - or dirty- window pane, transmitting a 5th which rocked the walls, 8H and all. It is supposed to be a new machine yet the clear-or distorted-scene. It is clearly an Naturally, Bach's 3-" and 4-Harpsichord speed regulator was fo und to be on the expert art to be able to ignore the musical Concer ti on HaydJi Society are bril­ blink. I can't get satisfaction from the above liant-er than e v e 17 ~ and Weiss-Mann's person . . . the manufacturer has not * 279 W. 4th St., N ew York, 14, N . Y . Italian Concerto on Allegro sounds like [C ontin1~e d on page 22J

20 '. AUDIO ENGINEERING • FE BRUARY, 1952

\ In musin e

" ... In music, listening quality is every­ thing. My CHROMATIC POLYPHASE has that quality to a degree that I never thought possible. More than that-for the first time, I am now able to test-play a soft master-disk without damaging the groove-walls. I am thrilled with its performance.... By the way, after many comparisons, I have decided to use the AUDAX H-5 CUTTER exclu- sively. . . ." So says the violinist (Toscanini's NBC Symphony) of "Musician's Amplifier" fame. For years Weil has been preaching that, in reproduced music, listening quality is everything, just as it is at the actual concert. If the quality is present, you can be sure the range is there, too. Al­ though the reverse is seldom the case. Never before such EAR-QUALITY, such FAITHFUL REPRODUCTION, but •.. after all the reams are written about kilocycles and other laboratory data­ is everything when the chips are down-YOU and only YOU can decide what sounds best and most pleasing. Therefore ... SEE and HEAR POLYPHASE and-You be that ~s what the man says ... the judge.

with the new Compass· pivoted Audax arms and for Record Changers.

Write lor editorial reprint on POLYPHASE Principles Be sure to obtain a copy 01 PHONO FACTS from your distributor

One single magnetic unit plays all home AUDAK ~OMPANY records- Sapphire or Diamond styli. 500 Fifth Avenue New York 18 Special models for radio stations, including " Creator 0/ Fine Electro-Acoustical apparatus Vertical-Lateral units. for ovpr 25 years."

AUDIO ENGINEERING • FEBRUARY, ·1952 . >..1 2'

I ' answered a card I sent them on this several months ago. "At present I have a high fid elity set which I bought from another so-called audio engineer many months ago. To this day the FM is much too bassy and not enough highs whi le the phono is fine on the hi ghs but there is 11 0 bass response from it. I stopped dealing with the 'audio engineer' involved because, over the months, he kept promising to adj ust the set and each time he either had some excuse for not coming or just didn't come at all. ,. I also found that he was dishonest with me. H e claimed that the set had a -­ amplifier, but the name is nowhere in sight on the amp or preamp, and neither looks exactl y the same as the present model on the market. H e claimed he sold me a -­ changer with a synchronous motor. I fo und out there is no such animal. install. Designed for eye as well as ear "All in all, I have been over-charged for a set that is not working properly. I am in all other audio loaded down with surplus expensiv e equip­ ment to sell, and I have been victimized by is you r assurance of the best. un reliable and dishonest 'audio engineers.' Can you put me in touch with a reli able 'audio engineer' who won't charge too much to make the necessary adjustments on my set ?" T he tragedy of the above (and plenty of AM-FM TUNER sim ilar cases) is that, obviously, there is 303A - Th e best tun er money can b uy for nothing seriously wrong with his equi p­ audio qua lity .. . wide ba nd su per het AM ment except perhaps the turntable speed · .. supe rlative FM . .. equalized p hono pre­ control. "Too much bass," "thin highs"­ amp ... extra i nput for tape ... record cross­ these are, nine times out of ten, simple over freque ncy selection . . . equalization for matters of equalization or matching; a LP records ... bass and tre bl e rise and d roop magnetic cartridge is being used unequal­ · . . built in power supply .. . mo re de pe nd­ ized, or a crystal is fed into a preamplifi el­ able fe atures than any other tuner. intended for magnetics, etc. P erhaps there is more trouble here, but I doubt it, having REMOTE AMPLIFIER run into this so,-t of thing a thousand times before. A-433A - Beauti ful compact control unit for P erhaps the man is a g rouch. (I don't use with phono whe n 303A tune r is not re­ know him.) P erhaps he is all wrong and his quire d. Contains VR phono preamp and all "engineers" all right But the evidence of the novel features found in th e 303A tu ner doesn't m ake it look that way, and I can · . . three inputs .. . perfe ct for re production assure you this letter stands fo r thousands from records, tope a nd ro d io tu ners. of other helpless users who, because of utter lack of any audio knowledge, are agonizingly POWER AMPLIFIER frustrated for months on end when a half A-333A - Th e power ampl ifier without a hour's tinkering could fi x things up to their peer .. . 15 walls with less than ';' % har- eternal delight. Not a good situation at all, monic distortion .. . 27 walls with less than quite aside from the possible fraud being 5% d isto rtion ... fl at from 20-20,000 cycl es perpetrated by the "so-called audio engi­ · . . complete ly controlle d from 303A tu ner o r neers" involved. T he turntable mentioned A-433A remote a mp li fier. here is very expensive and should definitely operate correctly and professionally, to the LOUDSPEAKER best of my knowledge. The company's cus­ tomer liason must be singularly poor. The. 6048 DUPLEX - Still th e fi nest loudspea ker changer, one of the standard makes, does of its type e ve r produce d .. . proved superior have a "synchronous" motor like every tim e a fter ti me in critica l listening tests by other fi xed-speed model; the implication is expe rts and layme n . . . six other mode ls from that the engineer was suggesting that the 8" to 15" also available. motor was something very special-a 820A - Alte c's new corner speaker system ... hysteresis motor, for instance-and he prob­ th e conce rt g rand of th e loudspea ker world ably asked a special price for it on that ... two 15" woofers ... h ig h fre qu en cy scor e. If so, it was fraud. The' w hole deal, spea ker and multice llular horn in a f urniture as they say, smell s. cabine t provide fa mous Allec " Voice o f the Th eatre" quali ty fo r the hom e. Better Business

SEE YOUR DEALER T he weakest aspect of the whole rapidly OR WRITE TODAY expanding honle audio industry today is thi s twin business of installation and mainte­ nance. T he large di stributors have done an excellent job, mostly mail order style, in simplifying installation and the initial choice of components ; se rvice is evidently possible through them too, if the ailing equipment can be mail ed or shipped. The big-city radio outlets with their new sound-salons and the like also seem to do a good direct-sale job on the choice and installation end at least. But the "so-called audio engineer," a lone­ wolf operator (or even, perhaps, a com­ pany) , taking advantage of customer en­ thusiasm and ignorance, is a real menace to 22 AUDIO ENGINEERING • FEBRUARY, 1952

'\

, '1 \ , ~~ 5" 0 SCI L LOS COP E KIT . ' New "spot shape" control for spot adjustment - to give really sharp focusing. • A total of ten tubes including CR tube and five miniatures. • Cascaded vertical amplifiers followed by phase splitter and balanced push-pull deflection amplifiers. • Greatly reduced retrace time. • Step attenuated - frequency compens oted - cathode follower ver- tical input. • Low impedance vertical gain control for minimum distortion. • New mounting of phase splitter and deflection amplifier tubes neor • CR tube base. • Greatly simplified wiring layout. • Increased frequency response - useful to 5 Me. • Tremendous sensitivity .03 RMS per inch Vertical .6V RMS per inch Hor. • Dual control in · vernier sweep fr equency circuit - smoother acting. • Positive or negative peak internal synchronization. • Multivibrator type Wide Range Sweep Generator. A brand new 1952 Heathkit Oscilloscope Kit with a multitude of out· standing features and reall y excellent performance. A scope you'll truly like and certainly want to own. The kit is, complete with all pares including all tubes, power crans­ former, punched a nd formed chassis, etc. Detailed instruction manual makes assembly simp le and clear - contains step-by-step instfucci ons, piccorials, diagrams, schematic, circuit de- script.ion and uses of scope. A truly out- standIng value. $43 MODEL 0-7 50 SHIPPING WT. 24 L8S. •

~~VACUUM TU BE VOLTMETER KIT • New styling - formd case for beauty. • New truly compact size - Cobinel 41/a" deep x 4· 1/16" wide x 73/8" high. • Quality Simpson 200 microamp meter. • New ohms battery holding clamp and spring clip - assurance good electrical contact. • Highest quality precision resistors in multiplier circuit. • Calibrates on both AC and DC for maximum accuracy. • Terrific coverage - Reads from '!2V to 1000V AC, V2V DC, and .1 to over 1 billion ohms resistance. • Laroe, clearly marked meter scales indicate ohms, AC Volts, DC -Volts, and DB - has zero set mark for FM alignment. • New styling presents attractive and professional appearance. The 1952 Model H eathkit Vacuum Tube Voltmeter! Newly designed cabinet combines sey le a nd beauty with compactness. Greatly reduced size to occupy a minimum of space on your work-bench. Cover.s a tre­ mendous range of measurementS a nd is easy to use. Uses only Quality components including 1 % precision resistors in multip lier circuit for greatest accuracy, Simpson 200 microamp meter with easy to read scales for fast and sure readings. All parts come right with kit, and complete inserunion manual makes assembly a cinch. MODEL V-5 SHIPPING WT. 5 LSS.

AUDIO ENGINEERING • FEBRUARY, 1952 23

\ \ the whole fi eld. So too, in a way, is the well meaning friend, amateur, or professional, who "loads down" the bewildered music Here's why lover with stuff he doesn't know how to use, CANNON then dodges the problem of making it work. those in the know Incalculable damage is done to audio's reputation by every instance of this sort of -demand PLUGS operation. It's not for me to organize some sort of guidance in this area, but I strongly suggest that a "better business" committee or All contacts machined from equivalent be set up in the audio industry solid bar stock and to begin the huge job of regu1ating, by con­ electroplated with silver. structiv.e suggestion or by force, the un­ Insert retaining screw threads into metal barrel scrupulous or misguided activities of those instead of plastic • .• inserts who sell audio too fast and too loosely. can be removed easily. Unethical practices are rampant, I'd say, and opportunities are at hand everywhere, Full·floating, non·twisting what with "high fidelity" a big appeal and socket contacts provide alignment, relieve strain knowledge of it abysmally low. But plain on contacts. confusion is even more widespread. W here can a purchaser of good equip­ Solder cups hand tinned ment go for regular maintenance and repair inside only, make clean service? W hat sort of guarantee should soldered conn ection s. apply to composite outfits sold as complete packages? What service policy, if any, should an outlet offer on its components? H ow can we educate the average radio re­ pair man to the point of servicing our better equipment ? H ow can we placate him on the loss of a "list" sale when equipment is brought in from outside sources- as is al­ most universally the case these days? Im­ portant questions, if the name "audio engi­ neer is to remain in public good standing. Speech on LP Romeo and Juliet-Margaret Webster Pro­ duction. Eva Le Gallienne, Dennis King, Richard Waring. Atlantic # 401 (2) Improved rubber bushing serves as Shakespeare almost complete and an excellent cable relief, gives added insulation and weatherproofing. production, in the t raditional manner, the top Improved patented latch locks roles good, the recording wide range and plenty connectors for added safety. clear. Modi.fi ed Tcha ikowsky for bridges and back­ ground, plus simple sound effects. Romeo and Juliet Excerpts; Soliloquies from If you talk to sound technicians anywhere you'll find Hamlet; Five Sonnets. John Gielgud; Pamela Cannon Type P connectors are the accepted standard of Brown. Decca DL 9504 Gielgud is highly dramatic, with much vibrato, quality ... taking a beating day in day out where frequent heavy breathing; some will be annoyed, others changes in circuits are required on all kinds of jobs up will enjoy ! Excel1 ent speech recording, minus music. Very smooth. to 30 amp. capacity. The close attention to important details called out in The Only Jealousy of Emer. Yeats. Pa ula Bauersmith, Gerald E. McGonagill, etc. the above illustration is typical of the care used in the Music by Lou Harrison. Verse choir. design and construction of all Cannon Plugs--the world's Esoteric ES 506 most complete line. Frankly, this gives me the willies. The lyric, sing-song I rish dramatics, so special and rare an The above type series is distributed through expression when done right by the Irish, is here selected franchise distributors. The line is fully (to my ear) absurdly exaggerated, in the worst manner of a "speech class" hard at work I Stage described in the Type P Bulletin. Engineering English is scarcely understandable. N ice, if rather bulletins describing each of the many basic li mited m usic by Lou H arrison (might be better in a more proper atmosphere) . Speakers mostly types of Cannon Plugs will be sent on request. too far from mike; some blasting, and highs are lacking (tape alignment ?) . Type P insert arrangements include 2-3 -4-5-6 and 8 contacts. All con­ Selections from "The Temple" (Herbert). tacts are 30 amp. capaci ty except 14 Poems of Emily Dickenson. Read by those inP-8layoutwhich are15 amp. Austin Warren. " Idiom LP Full scale layouts, front vi ew pin in­ EO-LQC 12957/8 H ere's an interesting technical e..xposi tion on sert, engaging side, shciwn at right. the nature of monaural reproduction t This 'Poetry is read by a "perfessor," clearly a man well used t o reading poetry out loud, known and revered for it, effective at it. But m onaurally, via the mike, CANNO N h is technique is astonishingly wrong, the poetry ineffective and hard to understand. The "live" lecturer uses a whol1y different ELECTRIC speaking technique than the radio speaker. Sudden and large changes of volume fo r dramatic empha­ Since 1915 P-B sis, excellen't live, are death to the mike, as are CANNON ELECTRIC COMPANY slurrings·over of weak syllables, the tendency to LOS ANGELES 31. CALIFORNIA mumble the less important parts of sentences. Diction must be far more even, via the mike. Factories in Los Angeles, Toronto, New Haven . Repre· E mphasis must be achieved entirely in the pitch sentatives in principal cities. Address inquiries to Cannon level and in the significant pause. TIlese things a Electric Company, Department B·I09,P. O. Box 75, lincoln Heights Station, Los Angeles 31, California. * 809 Amherst, Ann Arbor, Mich. J I 24 AUDIO ENGINEERING • FEBRUARY, 1952 platform lecturer does not know (unless he catches on almost instinctively, as some do). On this score, the perfessors must learn from the radio announcers, like it or not. The old order ' changeth.

Abraham Lincoln. (Poetry and Prose of Lincoln, Markham, Rosemary Benet, Vachel Lindsay, Whitman, read by Walter Huston, Carl Sandburg, Agnes Moorehead. Orson Wells. (Some with Music). Decca LP DL 8515 ..• And here is the necessary rejoinder. Words by, and concerning, Lincoln, read in this case by a galaxy of big professionals, expert in the craft of miked speech. Old Sandburg is a law unto him­ self, but that law included an instinctive under· standing of the mike. Huston, Moorehead do usual standard effective radio·style job (the professors won't like it much, nor do I). Orson Welles, though, is a genius at mike tech· nique. If you want to study genius in this special area, try his "Second Inaugural' here. Welles con­ ceives for this reading a tired Lincoln, exhausted by the long war, not far from his own death. To do this, ' '1e1les must slur over words, read in tired voice, trail off his sentences- i.e. disobey all the rules. Yet so perfectly does Welles know his mike that his reading here is completely intelligible, to the last word, and tremendously effective. This is the "art" of mike technique-to simulale an effect, act it, rather than to do it literally, as does Austin ' '1arren in the preceding listing. i Ask one of our 3M Service Engineers Spanish Through Pictures. I. A. Richards, Ruth Metcalf, Christine Gibson. With Pocket Book. **Ed. Svces. LP (2) FOR A "SOUND -SOLUTION" TO Second in an excellent LP series of 'modern language courses. Reader says sentences, you reo peat, while looking at same in print in accompany· ing Pocket Book (same title); meaning comes ANY TAPE RECORDING PROBLEM through silly but effective little iine drawings. Much skillful repetition of key words, phrases, gradually extends your vocabulary. LP allows lots Eighty 3M engineers in the If you are using sound record­ of time at low cost. (How can anyone buy one of the old 78 language courses any more I) Recording field- backed by twenty tech­ ing equipment in your radio sta­ below par, but adequat e for intelligibility. I've tried this set- and by golly, it works. (See also nical experts in the 3M labora­ tion, laboratory or business, call "French Through Pictures, U same system; more tories- stand ready to help you upon the 3M Service Representa­ to come.) with any tape recording problem. tive in your community. He'll be The Quick and the Dead. vol. i, The Atom This 3M Service Organization glad to help you make better re­ Bomb; 'vol. ii, The Hydrogen Bomb. Bob works daily with radio station cordings-more easily. If you are Hope, W . L. Laurence, etc. (NBC Broad­ casts) RCA Victor LM 1129/ 30 engineers, electronic engineers contemplating the use of tape' re­ Another LP issue of broadcast documentary. and industries using tape record­ cordings, he'll be glad to analyze These were long educational programs on NBC, here transcribed over four LP faces. Hope, the ings in process or quality control. your requirements and aid in the dope, makes foolish comments, asks dumb ques­ selection of equipment. tions; W. L. Laurence, top journalistic expert on The same electronic and engineer­ A· and H'stuff, does the narrating. Laurence is ing know-how that produced and Call him today-or, if you pre­ folksy, sincere, gives first·hand information, but he goes terribly slowly, the whole production- for perfected the famous "SCOTCH" fer, write directly to Minnesota radio--is astonishingly slow·paced. Excellent stuff, but for record buyers it could be a lot more adult. Sound Recording Tape offers you Mining & Mfg. Co., Dept. AE-22, And about half as 'long, maybe. Nevertheless, don't technical assistance on every St. Paul 6, Minn. No obligation, overlook these as unique educational material. phase of sound recording. of course. Robert Frost Reading his own Poems. Two 10" LP's (Also 78's). Here's why more recording engineers ~,* * Nat. Council of Teachers of Eng. Replacing an old but good set of shellac 12·in. . use "seOTe HII Sound Recording Tape records, these LP's have improved quality. But , than all other brands combined: Frost, being older-though he still does an excel· lent job- tends here to read without full convic· • REEL TO REEL UNIFORMITY-con­ tion; he's been reading these things for so many trolled coating assures consistent output. ' years, t ime after time. Frost knows his mike, instinctively, and it shows at once. You won't • THINNER CONSTRUCTION-resists miss a word here, and the material is thoroughly temperature and humidity changes. enjoyable, beautifully natural, homespun, un­ affected. Technical wo),k by RCA. • NO CURLING OR CUPPING-tape lies flat on recording head unaffected by humic;lity. Illegitimate Sonnets; Clinical Sonnets. Mer­ rill Moore reading his poetry, with com­ • UNIFORM TAPE SURFACE-no "drop­ mentary. ****Harvard Vocarium LP outs" on recordings due to surface irregu­ The term "SCOTCH" and the plaid design are registered trade-marks for Sound Recording Tape A genuine experiment in pure LP and signifi­ la rities. m ade in U.S.A. by MINNESOTA MINING & can t for all ,~ho follow progress in recorded art. MFG. CO., St. Paul 6, Minn.-also makers of The poet here Had libs u his commentary, at ease, • LONGER TAPE LlFE-speciallubricat­ ((Scotch" Brand Pressure-sensitive Tapes,tt Under­ ing process reduces friction. seal" Rubberized Coating, nScotchlite" Reflective ** Educational Services, 1702 K St., N . W. Wash­ Sheeting, "Safety-Walk" Non-slip Surfacing, "3M " ington 6. D. C. • GREATER SENSITIVITY-more output Abrasives, t(3M" Adhesives. General Export: Minn. *** 211 W. 68th S t., Chicago 21. on your present macl;line setting. Mining & Mfg. Co., International Division, .270; **** Bus. Mgr., Harvard Univ. Libr., Cambridge, Park Avenue, New York 17,N.Y . ln Canl!-da: Mmni Mass. Mining & Mfg. of Canada, Ltd., London. Canada.

AUDIO ENGINEERING • FEBRUARY, 1952 25 /

\ , informally, taking h is time via LP, ill lect ure

style. The " poems, II slightly eccentric, verge on the unprintable, arc amusing and sophisticatcd­ BRII.I.IANTLY it's hard to tell them from the commentary which, ENGINEERED in the manner of peopJe·pleased*with-the·sound·o f­ their-own-voice, is considerably longer than the poetry ! All in a ll a slightly crack-pot b u t can· PRECISIO'N vincing demon stration of what can be cl olle with BUII.T tape and LP that could never be conceived in the old 78 m edium. Pla y it over first before you try it on the ladies.

Readings from the Bible. Ronald Colman. RCA Victor LM 124 The V oice of the Preacher, actor- st yle, Colman is an old hand at mike work and these are perfect as to intelligibility with top recording to help. But the rather affected st age style will not p1ca;e those who would prefer a m orc genuiue and si mple di g nity in the read ing of s uch texts.

Certrude Stein read by Certrude Stein. Dorian LP DR-331 For an yone who has been intrig ued by the why a nd wherefore o f this t·emarkable lady. th ese re· COl-dings should be priceless. S tein's operatic No. 1401 works, as set to m usic by Virgil T llomson ("'Four Saints;" " The ~ioth e r of Us All") gave us a st ron g feeling t h at there was som ething vocal and nutsical about Stein's writing, that she made most sense when heard. not looked at on the page; music's own ki nd of repetition and development of M~C an idea. b y g radual a lternation , seemed to be in TAPE RECORDER hel- words themselves. N ow, with S tein reading, g iving her own intonations, emphasis, it's ea ~ y to A precision built and thoroughly tested instrument, capable of recarding hear the same. As read by her, the stuff makes the full range of audible sound with full dynamic sweep and freedom from enornlous ly m ore sense than it does on the printed Only the most e xpensive professional equipment rivals the tape handling ability of the Con­ pag e. \ Vorth a "try, an yhow. The record ings were certone recorder - and none equals its versatility and simplicity of operation. Especially issued in limited ·ed. back in 1935 ; no w , on LP. designed for the most discriminating user, the basic recorder comprises a 14 11 x 22" rigidly a re much improved, wi th intellig ible high s t o 6000 ribbed, cast, aluminum plate carrying the tape mechanism, dual track heads, a shock mounted or so, plenty for speech. chassis containing erose amplifier. record amplifier, and playback preamplifier, a power sup· ply chassis, mounting lugs for auxiliary equipment, and all necessary switches and controls, ready for use. We ight: 30 Ibs. Easi ly connected to your existing high fide li ty amplifier system. Monitoring directly from the tope while recording gives a constant check and control on what is being recorded while it is being recorded. A much prized program is never lost unknowingly, any departure from proper operation is immediate ly heard . High speed rewind in either forward or reve rse direction, firm, positive braking and fully inter~ EDMUND T. FLE'WELLINC locked controls assure rapid handling w ithout damaging tape. A special circuit controlling a cathode "eyell gives accurate indication of the proper record level for best results. A special locking button prevents occide ntal e rose of recordings. The Concertone magnetic tape recorder uses any standard reel from the tiny five inch to the professional NAB 10'/2 inch ree l, together with instantaneous choice of 71/2" or 15" per second tape speeds, permitting matching freque ncy response and le ngth of program to operating cost.

• Broadcast studio quality complies with • Hi g h speed rewind, forw ard and reverse NAB standards. - 2500 feet in 60 seconds. • Separate h e ads for high frequency • Single or dual track optional. erase, record and playback. • Size: 2211 x 14" x 5" mounti ng depth • Simultaneous monitoring from the tape be low panel. while recording. • Freque nc y response: -+- 2 db from SO to • Prealigned heads quickly interchanged 12 ,500 cycles at 15" / sec. for single or dual track. -+- 2 db from SO to 7,000 cycles at • Instantaneous choice of 7.5 or 15 inch 7 .5" / sec. per second tape speeds. • Total ha rmonic distortion: Less than 20/0 • Plays standard 5 inch, 7 inch and NAB at normal maximum signal level. 10'12 inch reels. . • Playing time: Up to 2 hours on dual track. CARRYING CASE CONCERTONE TAPE NO. 501 RECORDER ACCESSORIES Thi s handsome fitted ' custom case quickly and easily converts the basic REEL ADAPTERS r e corder #401 into a No. 801-Accurately machined to hold NAB compl e te system for lOY, reels on standard RMA spindles of portable use. Suppl ied ,ecarder • • • . • • • • • • • • • ., 300 with built-in monitoring amplifi e r # 603, and NAB 10%" REELS eight inch high fidelity E dmund Thomas F lewelling, oi A h­ speaker mounted in detach- 8250 No. 802 - Low inertia 2500 foot reels, able cove r. Convenien t to burnham, Mass., died Sunday, December 30, flanged an both sides. Conform to NAB carry; 24" x 15" x 12". Weight: 15 Ibs. 1951 foll owing a heart attack and auto­ recommended standards •• . , •• • 2 50 CONSOLE TRAY AS ILLUSTRATED 14.95 mobile accident of two days befo re. 111'. Flewelling was 64. TERMINAL VALUES ON QUALITY In telephone and electrical work since PLASTIC RECORDING TAPE 190J.-wben he was 16--Ned, as he was DOMESTIC PROFESSIONAL known to his friends, naturally gravitated 600 FT...... $1.35 600 FT ...... $1.65 into radio in the early 20's, with his first 1200 FT ...... 2.10 1200 FT ...... 2.70 contribution in this fi eld the super-regenera­ tive circuit which revolutionized radio re-· 2400 FT...... 4.20 2400 FT...... 5.40 ception in those days. In later years, his short-wave adaptors made possible the re­ ception of foreign stations on standard broadcast receiver s. Since 1938, he had been doing research wurk on wind generators and in the fi eld of audio, in which he brought out within the last two yea,-s a low-frequency speaker enclosure.

26 AUDIO ENGINEERING • FEBRUARY, 1952 AUDIO IN THE HOME [from page 19] ''I'm IOUI" IInt/rulled.. gain for GE or Audak cartridges, but it works exceptionally well w ith P ickel- ing 6uI 'he SWIITEST milee ,OU ever oWlle"'" cartridges_ Most record changer manufacturers and pickup manufacturers supply suffi­ ci ent information about templates for motor-board cut-outs, some even furnish­ ing a mounting boa rd to replace existing wood work. If a record changer installa­ tion is made properly- keeping the above considerations in mind- the music lover will be -gratified by the results_ The replacement of othel- parts of the con­ sole are more difficult and require more thought and consideration_ T he next improvement which the music lover will seek nowadays is a good FM tuner, largely because of the many good music programs which a re broadcast exclusively on F M_ Many southern cities affected by electrical storms (to cite an example of depend­ ence on FM for static-free reception) are now able to get noise-free reception

Remove lead from here I Pickup jack

Fig_ L Method of connecting two resistors at input of GE pre-amp to prevent overload when using with Pickering cartrid ge_ even from moderately powered FM transmitters, whereas SO-kw AM sta­ ti ons failed to cut through the noise_ Experience with most early post-war commercial sets and many F M tuners then available has left a bitter taste for many people_ It is unfortunate that so many sets were sold that had no advan­ tage over AM because of poor design_ Yes, the Turner Model 99 Dynamic is the biggest problems encountered in greatest mike in the world for taking hard The knocks ... yet retains the perfect response and FM tuners a re excess ive drift, insuffi­ sensitivity that make the 99 an outstanding cient sensitivity, and frequent break­ microphone value. downs, all of which make the FM re­ ceiver no better than medi ocre table­ Indoors or outdoors - for ham rig, broadcast remotes, sound installations - the Turner 99 TURNER model sets_ The better F M tuners have Model 99 none of these faults, however. Another is completely dependable. For mobile public cause of poor FM reception is the fail­ address systems, paging systems, communica­ ure to install a proper antenna_ tions or recording machines, the 99 delivers Vve could consider the Meissner 8C crisp, clear results. Its smooth response is not or 8CC the minimwm quality for an FM affected by heat, cold or humidity. Response: 60 - 9000 c.p.s. tuner. With a proper antenna, this set Specify the Turner Model 99 D ynamic micro­ Level: 52 db below 1 volt/ dyne! will perform reasonably well, and will phone with confidence. Use it with pride. sq. cm. at high impedance remain stable after a 15- to 20-minute Order from your Turner dealer or represent­ Impedance: 50, 200, 500 ohms warm-up perod_ The latest models of ative, or write direct for complete micro­ or high impedance B rowning, Radio Craftsmen, and Brooks phone literature. List Price ______$36.00 seem best suited to custom install ations_ It is difficult-if not impossible- to in stall an F M set in some console models because of a lack of space on the con­ THE COMPANY trol paneL In some cases, the set may be TURNER mounted in the r ecOl-d storage space, IN CANADA: but then it is not handy to tune_ The Canadian Marconi Co., Ltd. 929 17th Street N.E. Toronto, Ont., & Branches problem of mounting an additional tuner Cedar Rapids, Iowa will be discussed in this column next EXPORT: Ad. Auriema, 89 Broad Street month, along with suggestions about New York 4, N ... Y. replacing the existing amplifier portion of the radio set in the never-ending- ques t for better sound reproduction_

AUDIO ENGINEERING • FEBRUARY, 1952 27 / NEW PRODUCTS

• Portable Transcription Pla yer. Man y needs of s chools and indus t ry will be served by the Model T R -1 6AM combina ­ l tion record player a nd public a ddress sys­ tem. Ca pa ble of handling recor ds of any ?hta size u p to 17 'P. in. dia meter, the player provides operation at 33 1/3, 45, or 78 A fULL r .p.m., w ith a specia l control to provide ACOUSTICAL LENS

FOR LOUD includes pow er supply and n ecessary hard­ war e. Each u n it is m oun ted, wired, tested, a nd a lig ned a t the fact ory, a nd the same SPEAKER t ubes u sed during t est s a r e included with t h e kit when s hi pp ed, thu s elimina ting t he USE! n eed for intricat e a lig nment proced ures on t he pa r t of the u ser. Avera g e sens itivity of t h e completed t u ner is 6 t o 10 micro­ Distributing a volts. P hysical dimens ions a r e 6 x 8 x 17 uniform sound ins. Collins Audio P r od uct s Compa ny, P. wave over the entire O. Box 368, W estfi eld, N. J . au dio spectrum, the new lim Lansing acoustical lens • Miniature Audio Oscillator. Alt h ou gh smooths out the high frequencies a nd tiny in s ize, t he n ew Waveforms Model gives them a mellowness that your ear 510-B extended-r a nge a udio oscilla tor e qu ~ l s in performa n ce many of its stan d­ wilt appr eciate instantly. Unlike the minor variation from the thr ee sta n da r ds. a rd-s ize count er-pa r t s. A pr ecision instru­ multicellular horn, which performs well Weig hted 10-in. t urntable contr ibut es to m en t in every r espect, it m easures but at some frequencies but beams or dis­ stability. Cir cuitry of t he u nit includes 6 x 4 'P. x 5 % ins. Frequ en cy ran ge is 18 a built-in scratch s uppressor. Sta nda rd cps to 1.2 m c in five overlapping ran ges. tri butes very poorly at others, t.h e new equipment inclu des a GE var iable reluc­ Consta nt output is with in ± 0.5 db f r om lens is not sensitive to frequency . . . be­ tance pickup. P ow er ou tput is 10 watts. 18 cps to 100 kc. Distortion is less t h an cause the band width is wider than the Ma n ufactur ed by Newcomb Audio P rod­ 0.2 per cen t over most of u seful r a nge. u cts Co., 6824 Lexington Ave., H ollywood, a udio spectrum with which it is used. Calif. The new lens and horn is for use with the l im Lansing # 175 high frequency driver. • Wide Range Speaker. Exceptional dis­ pers"ion of h igh frequ encies is a feature TRANSPARENCY CAN BE MEASURED! of the new University Diffu sicon e-1 2, a Transparency or absorption 30-watt speaker designed t o fi ll t h e n eed for a hig h-qua lity u nit in t he modera te in the new l im L a n s ing price fi eld. High-frequency r esponse ex­ acou s t ical l e n s ",,-----~ tends beyond 13,000 cps. The "diff usor" is measu red in the same way element provides dual horn loa ding of t he as it is in the optical lens . speaker apex, thus s ubstan t ia lly in crea s ­ ing efficiency in t he upper frequen cy - by index o f r e fr ac ti o n. r a nge, a t t h e same time a ffording radia l The percen tage of sound abo pr ojection and aperture diff u sion to a s s ur e sorbed by the J im L ansing acoustical lens is less than the percentage of light absorbed _ ~ by the f inest optical l e n s. W-I-D-E D I 51 RIB UTI 0 NAN GLE ! The angle of distribution which is con­ trolled by curvature' and index of refrac­ tion can now be varied bet\veen wide limits ... permitting the sound to be con­ centrated or diffused. The 175 DLH shown above (driver, lens an d horn) Ou tpu t v oltage is calib r ated, a n d t h e u nit distributes over a 90° soli d angle. is equipped with a 300-deg. vernier-drive dial. Opera tion with bala nced output is a fforded by mea ns of T ype T-10 m a tching tra n sf ormer a va ilable as accessory. Manu ­ fa ctu r ed by Waveforms, I nc., 333 Sixth ~ Ave., N ew York 14, N. Y. LIGHT OR ===-==== SOUND WAV~ES~ ______• Corner Cabinet Kit. A n ew kit which, SPREADING w hen a ssembled, m a kes a n acou stically OUT EVENLY--: correct folded-horn cor ner spea ker en­ closure, is t h e newest a ddition to the line of cabinetry manufactured by G & H r eception of the spea ker's entire r ange of Wood Produ cts Co., 75 N. 11th St., Brook­ ~ reproduction irrespective of the listener 's lyn 11, N. Y. Sold t hrough jobbers u nder loca tion. Con stru ctiona l features inclu de t he compa ny's tra de n a me Ca binart, the A graphical diagram or ray tracing of a 1'h lb Alnico V magnet a nd a 2-in . voice the opera"tion of the Jim Lansing acous. coil. D es ign of the Diffu sicone-12 permits tical ne gative lens show ing foca l point r eplacement of eit her the diaphra gm­ and diffusion angle as controlled by ba sket assembly, or the magnet, in a curvature and index of refraction. matter of minu tes with a n ordinary scr ew Ask your audio dealer driver t he only r equir ed tool. A complete for demonstration of ca t a log sheet, including cabinet dra wings, the new Jim Lonsing w ill b e s upplied free on r equ est to Uni­ acoustical le ns today! versity Lou dspeakers, Inc., 80 S. K en s ico Ave., W hite P la ins, N. Y.

• Pre-Fabricated I'M-AM Tuner Kit. A r ecent departu re in t uner desig n is the new Collins "Pre-Fab" model. Ma de up of fou r w ired a nd tested s ub -assemblies, the t uner is eas ily complet ed by t h e u ser wit h substantial s a vings <}:yer the cost of a finis h ed unit. The Pre-'Fab k it is comp osed of (1) t h e FM r .f . tuning circuit, (2) t h e FM Lf. a mplifier, (3 J the AM t uning cir­ f , R S T I N FI N £ SOU N D cuit, and (4) t he cnassis assemb ly which

28 / AUDIO ENGINEERING • FEBRUARY, 1952

\ , kit is remarkably low priced, and is sup­ plied complete including c u t lumber, grill cloth, hardware, plastic wood, acou stic insulation, and assembly instruction s .

• Vacuum-Tube Voltmeter. Hiclwk's new Model 215 VTVM affords laboratory ac­ cu racy in an instrument d esigned essen­ t ia lly for field engineers and service tech­ nicians. Featured with the 215 is a new dual-purpose a.c.-d.c. probe with built-in Elwitching. D.c. range is to 1200 volts with ...~ ...Z :JO Z• o III Z til Z III • n~ III• ~ U ... is ~z o> n I- •:JO U'" :I> III n !il o WAVEFORMS MODEL A-20-5 Z • GARRARD RC-BO • '"~ Music lovers and critics have acclaimed Professional engineers attest to the out- : ~ this superb amplifier as One of the finest standing performance of this superb, tI ~ at any price. Inputs-Radio, Magnetic completely automatic record changer. is ~ phono, 2 others. All with individual COn­ Heavy-duty constant speed motor assures n quiet operation-without u wows." Re- ;: • trois. Controls-Selector, Volume, Bass, movable spindle permits removing records ~ ~ Treble Cutoff. Frequency Response--l 10 megohms input r esistance. Readable without damage. Three speeds. Auto- ~ resistance range IS 0.2 ohm to 1000 meg­ ~ db from 20 to 20,000 cps. Power rating matic shut-off. Special heavy duty spindle ... ohms with center scale rea ding of 10. A.c. a:I -20 watts. for 45 rpm records. Z ranges are to 1200 volts rms, and 3200 volts peak-to-peak. Input impedance is 30 U• $199.50 NET (less pickup) ...... $39,00 •;:: megohms shunted by 150 mmf. Frequency Z NET response is 40 cps t o 3.5 mc. Crystal probe Z ------~ is available as accessory to extend u seful ~ • 0 ra nge to 250 me. For complete information o ~ write to The Hickok E lectrical Instrument c Co., 10617 Dupont Ave., Cleveland 8, Ohio. '"a:I ...... • • o~ ~ o

'" ." NEW LITERATURE • n ~• o e( ... • Tube Dpeartment, :B.a.1M0 Corporation of H ... America, H arrison, N. J. is now distribut­ o •." ing a revised edition of the booklet titled a:I :z: Preferred Tube Types for New Equipment c Design. Advances in tube design and ap­ Z• :JO plication a r e reflected in m a ny revisions III ... in this la test version cif a n old standby in g •." the engineering field. Copy will be mailed a:I o free on request. • AUDAK CHROMATIC ~ ~ DIAMOND POLYPHASE DL6 : • Freed 'l'ransfonner Company, 1718-36 Weirfleld St.t Brooklyn (Ridgewood) 27, •• Thousands of AUDAX owners are ac- ELECTRO-VOICE cZ N. Y. is puolishing in booklet form a s ummary of the many types of tra n sform­ I- claiming POLYPHASE reproducers as in- COAXIAL MODEL SP - 12 3: ers a nd test instruments manufactured by ~ dispensable where absolute quality of 0 t he company, also information on the tech­ :::E performance is desired, And nOw the new All the magic of high-fidelity reproduc- 2 nicalliterature the compa ny has available. Will be mailed free on request for "Freed t CHROMA TIC POLYPHASE with its tion is yours with the RADAX SUPER- ~ Facts." ;;: near-infinite compliance of record groove TWELVE loudspeaker system. Coaxial in z ~ and stylus, gives to the listener a new principle, both low and high frequencies ~ • American Standards Association, 70 E. Q thrill in music reproduction.••• Never reach respective speakers thru an ex- i: 45th St., New York 17, N. Y. h as approved o before such Ear-Quality. • . . Never t remely effective crosSOver network. :; and is distributing The American Stand­ Ii before such absolute fidelity to the orig- Power Rating-25 watts. Frequency Re- -­ ard Graphical Symbols for Single (One) Line Electrical Engineering l!>iagr ams, ~ ina!. It is truly "the standard by which sponse--70 to 13,000 cps. Impedance-- :: Z 32.1.1.-1951. Offering for the fi rst time a • others are judged and valued." 8 ohms. 3: Single volume in which a r e combined single-line dia grams for u se in both ~ NET (with changer adapter) .. $43.51 NET ...... $52.92 • power a nd communication work, this ~ Standard coordinates the diagrams con­ ::J tained in the American Standard Graphi­ e( . cal Symbols for Electrical Power and Con­ NEW BIC CATALOCUE: You'll want our new trol, Z3'Z.3-1946, and for Telephone, T ele­ til• 48 page Catalogue featuring the latest and finest graph a nd Radio Use, Z32.5-1944. The e( Standard contains 81 sections covering in equipment for audiophiles. Just drop us a ...l­ n symbols for almost a ll electrical circuits e( postcard, or phone: :z: in the field of power and communication. n Copy of the Standard may be obtained COrtland 7-0315 :I> from the Association u pon remittance of n o $1.40. • ~ • Antara Chemicals Division of General m Dyestuff Corporation, 435 Hudson St., New aI Y ork 14, N . Y. has assembled in booklet ... form the complete story of the Company's ... uniq u e product, Carbonyl Iron P owder s. :JO This booklet r epresents industrial pub.­ lishing at it s best. L avishly illustrated a nd printed in many colors, it covers t h e manufacture a nd usage of iron powders in s u ch a manner tha t even the most tech­ nical aspects of the subject are under­ standable to the engineer whose sphere of specialization m ay lie in oth er fi elds . A necessity irrespective of what your inter­ est may be in electronic engineering. AUDIO ENGINEERING • FEBRUARY, 1952 / 29

~--~------~------~------~------/------VOLUME CONTROLS [from page 15]

Fig. 7 and adding compensator R4 C2• T his is equivalent to combining ( B ) and (C). Proper choice of va! ues and tap WILLIAMSON HR-l 5 AMPLIFIER can yield a closer approximation to the • KIT THE COMPLETE LINE OF • theoretical curves of Fig. S. • Apparent Loudness and Room Noise • The compensated control has the ad­ • vantage of making listening easier in the • presence of room noise, as indicated by • Fig. 12. A good deal of the power in • (Exclusive wilh HARVEY in· Ihe N. Y. area) • music and speech is concentrated in the Transformers for all applications - Military and • frequency region below 500 cps. Com­ • lifi I It Civilian - High Fidelity Audia Components, pensating a sO-db-level program so that The famous Williamson HR·15 amp ,er c rcu "Trijets" _ Hermetically Sealed Midgets, Power . . . now available with the original Partridge I • transformers built to Williamson's specifications. 1 Transformers, Replacements, Filters, Reactors, it approaches the over-all frequency re­ • Build this kit in 3 hours or less, and enjoy sound etc •••• for the laboratory, Industrial, serVicing, sponse of an uncompensated 80 db aver­ of a quality you never heard before. The HR.15 Is I· public address and radio amateur fields. Com· age level can make the program sound a 2·Chassis power amplifier for use with tuners 1 plete stocks of all components are on hand for • • or other front ends having own volume and tone I immediate delivery. much louder without increase in annoy­ controls. All American triodes, 2·6SN7GT, 2·807 ance factor, because proper balance of • or 6BG6G in PP output, 5V4G rectifier. Response 1 The TRIAD "HS" series of transformers Incor. bass and treble prevents the shrillness ± .5 db, 10·100,000 cycle s. Output impedances I porates hum·bucking coils arid nicket·alloy mUl­ • 1. 7 to 109 ohms in 8 steps. Absolute gain 70 .8 1 t iple shielding for minimum pickup. They are • which often characterizes the uncom­ db. 20 db. of feedback around 4 stages and the small in size, light in weight, rigidly supported pensated low-level reproduction of voice • output transformers. Kit is Complete with Tubes, I and hermetically sealed. Note actual appearance Punched Chassis, Pre·wired Resistor Board, Sock- I or music. This makes background music ets , Genuine Partridge Output Transformer. and I cif the "HS" case, on left, to meals or conversation much more All Necessary Pa rts ...... $76.50 and the telescopic view • on the right wh ich ·illus­ • soothing and pleasant. As Above, with CFB Transformer ...... $90 .00 1I trates the construction PARTRIOGE Outout Transformers available of this particular type. Both the noise and the signal curves • separately WWFB, as used in above kit..$26.00 I • in Pig. 12 have loudness sensation as it CFB Transformer, Hermetically sealed ...... $40.00 I Write for latest appears to the mind as the ordinate. The • HR·15T A Williamson Kit with all TRIAD TRANS· I catalog. usual method of presenting room noise FORMERS •.. including power transformer, , • chokes and specially desi gned output trans- , over the audio-frequency range in terms former which is completely sealed in tar. ± 2 of sound pressure or intensity level tends • db. 10·100,000 cps. Harmonic Distortion less I • than .1 % - 10 watts output. Output impedances I NOTE: In view of the rapidly changing market • to give a misleading impression of the 4·8·16 -ohms ...... $69.50 conditions, all prices shown are subject to annoyance effect of the low-frequency • Both HR·15 and HR·15T Kits available with KT·66 1 change without notice and are net, F.O.B., N.Y.C. noise components. Fortunately, the ear • . tubes for $3.60 extra. 1 responds so poorly to low frequencies at the usual room-noise intensity levels that noise such as the hum of 60-cps home Popu/ar- Priced machinery remains tolerable. L istening at lower than concert-hall GARRARD levels also pays another dividend besides 40

THREE-SPEED RECORD PLAYER ...z tl30 ..... v An excellent unit designed to sat­ ..ffi V .isfy the increasing demand for a z .....7 ~20 three· speed, single record player. a: Same sturdy construction and preci· ~ V <5 ...,.,. V sion parts as the famous GARRARD .J'O :! Record Changer. Four pole, heavy V e V duty silent motor. Hum·free ! o Rumble·free! Heavy weighted turn· 50 60 70 80 90 too SOUNO PRESSURE - db table. Interchangeable plug.in heads for crystal Or magnetic cartridges. Fig. 13. Harmonic distortion of the human hear­ ing system at 1000 cps. Includes harmonics Model M, less cartridge, only. $24.50 from second to fifth, inclusive. Model Me, with dual stylus crystal turnover cartridge, only ...... $29.65 toleration by family and neighbors. As Fig. 13 shows, the harmonic distortion of the human hearing system is quite VISIT THE low, less than 3 per cent, at a low ·sound NOTE. I. .io. .1 Ih. pressure level such as SO db.3 At 80 db, AtJDI~TORIUM however, the harmonic distortion is high, about 22 per cent, and intermoclulation distortion components will be present in Her, 3 Derived from data on subjective tone measurements; Moe, I. A cous. S oc. Am., 1942. ( / 30 AUDIO ENGINEERING • FEBRUARY, 1952

\ unpleasantly large quantity on complex tones. Many people react unfavorably to hi gh sound levels ; it seems rather likeiy that this is due not just to distorted re­ production but also to distortion in the individual hearing system. If this is so, no amount of improvement in amplifier and speaker fidelity, or other elements in the reproducer chain will sell loud sound to some persons.

Amplifier Power Requirements It is desirable to have fairly de fi nite knowledge as to the maximum output power requirement of the amplifier needed for the maximum average level of 80-db sound pressure on which the lJass compensation previously considered is based. Since the low frequencies will be attenuated much less than the' middle and high registers at low-level settings schooled in of the volume control, the amplifier will Iha l o nly wide. full Ira nsienl reo wh ich en ables Ih e

"~a V ~ . ~ ,/' ...- :,..' P ickering en g ineer s a n d designers h a\'c but o ne objec­ ~ . ,. , . - l ive •• • 10 produce producls Iha l will please Ihe m usie . lover s' insa tiahle HppCli tc for the flawless rccrea lion o f ~ .' r ecorded l11lls ic • • . for the u tlllo st in qua lity insist u pon {.. t.o ~ .... ' v":;'i.,,:{:>uf.. P ick ering Audio COll1 p On c nts . ~ ¢f."' ~ " ~-" ~ ~ ~ ~ ... " Pickering Cart ridges tran smit I..-' 1-- ' all of the sounds in t he musi· ~OI cal spectrum, w it hout loss of ~ 0: '" definit ion. just as a fine opt i· ., cal lens passes all of Ihe col· ors i n the rain bow. I..-' '" For literature address Dept. A T"is Audio

Fig. 14. Loudspeaker power input required for Wit" A various sound levels, base

4 Based on data for aCQustic power re­ 2667 N. HOWARD STR EET quired at 80-db sound intensity; Olson, R CA R eview, 1936. Philadelphia 33, Pa. 31 AUDIO ENGINEERING • FEBRUARY, 1952 / / J db average maximum sound pressure level, the input to the audio ampli fie r ,UU(.",~nuH/~ ~~~~~~~ from radio receiver or phonograph­ pickup circuits should be adjusted to ARE produce about that output level at m,L'{i ­ mum output position of the compensated A volume control. The curves of Fig. 14 will facilitate such adjustment if a stand­ ard frequency record (or audio oscilla­ tOl') and output voltmeter are available. T he proper input level is not a highly cr itical matter, since record and radio program average levels and balance vary. If the loudspeaker conversion effi­ Th ere 's a b ig factor show ing up i n th e tape recording world , • . i t 's the tragedy o f High Ma intenance Costs. ciency is 1y.; or 6 per cent instead of 3, Many radio stations a re confronted with the daily it will produce only plus or minus 3 db ritual of recorder ch eck ing and adiusti ng. Added to error in setting up the level by the the t i me cost of such insp ect ion s is the co st of fre­ curves of F ig. 14. A good memory fo r quent parts replacements and loss of program tim e. the way music sounds in the concert hall In contrast, Ampex users find th eir equi pment will may not be as unreliable a criterion for operate continuously eighteen hours a day w i th but adjusting the input level as it may seem i nfrequent insp ect ion , Upkeep and replace ments are offhand. a l most n i l; heads have remarkably lang life. Ampe x p erfor mance is con stont over long per iods of contin ­ Conclusions uous operation. Long l ife w ith low maintenance i s T he author has found that a compen­ assured i n each Ampex recorder by high manufac­ CO lllplele Specificalio1lS 011 Request turing standards and complete test of each machi ne sator designed on the basis discussed Shown is the t im e proven Mode l 300 Console. b e fore shi pm ent. It all adds up to one sure fact­ above is a very desirable addition to a Throughout the fie ld of professional audio A mpex quickly pays for i tsel f aut o f saving s from recording this ma ch ine is the recognized wide-band radio-phonograph system in lower operati ng costs and added d ep endabi lity. leader. home use. It appears to make music much more tolerable to the casual lis­ tener. Because of this factor of less an­ noyance to other people who may not -:;":; ' ::::::~t::;~""_~8'-- X particularly wish to listen, proper auto­ ~~ Redwood City, California ~k\ AI Tfl)IE matic bass compensation should always A X E0 D eal ers in Plinci pa l Cil ies be considered for inclusion in enter­ 72 ~\~ ~ W1llr' tainment receivers intended fo r. either civilian or military use. T he military types generally operate in rather noisy locations with many persons present, and especiall y need the advantage shown in Fig. 12. The fo llowing conclusions have been reached as the result of this study: We're not saying you MUST have AUDIO ENCINEERINC (a ) Automatic bass compensation, as obtained by combination with the vol­ BUT our many readers think of AE as a sort of institution, a necessity ume control, is highly des irable in enter­ for their interests and way of thinking. Our readers tell us they tainment receiver and phonograph equi p­ think AE is worth its price and much more. ment. (b) Because they are plotted on the IF YOU ARE interested in Audio (and who isn 't, these days?), you basis of an arbitrary change in sound know that AE IS THE ONE MAGAZINE DEVOTED TO AUDIO pressure or intensity level between AND NOTHING BUT AUDIO. curves, the so-called Fletcher-Munson curves do not provide a direct indication IF YOU WANT to know what is happening in your field of work or how of loudness sensation. If a control having to better the sound equipment you are making for your living room, a linear rate of change in db of loudness you'll want every issue of AE. sensation is desired, curves of apparent loudness sensation in the mind versus SO, before you miss one of the stimulating issu es we have scheduled for sound pressure level should be used to , the spring months ahead, fill out the coupon below. That one back determine resistance taper. issue you n eed most is generally sold out. (c) An average maximum sound pres­ sure level of 80 db at the listener's ears ~------, appears to be a desirable value for de­ AUDIO ENGINEERING signing compensating circuits fo r enter­ 342 Madison Avenue, New York 17, N. Y. tainment purposes. Enclosed is my 0 check 0 money order for $ .. . .. " . .. for my subscription to AUDIO (d) It is feasible to include fi xed ENGINEERING FOR 0 1 yr 0 2yrs. hi gh-frequency compensation to correct Subscription Price in U. S. A. and Canada and Pan American Union : 1 yr-$3 ; 2 yrs-$5. fo r deficiencies in various parts of the Add $1. per year for foreign subscriptions. reproducing system to a reasonable de­ gree, because the high-frequency re­ Name (please print) ...... , ...... • ...... sponse of the human hearing system is Address ...... , ...... substantially constant at the sound pres­ sure levels of interest. City ...... Zone . .... State ...... (e) Most engineers interested in sound reproduction and many musicians, Occupation (give title and department ) ...... , ...... if allowed to listen at the levels they prefer, will need r eproducing equipment / L ______Firm name ...... • ...... ". _ of about 10 times the power rating re­ quired by the general public.

32 AUDIO ENGINEERING • FEBRUARY, 1952 \ \ LOUDSPEAKER DAMPING [from page 13] sible to go. The obvious approach is merely to increase the value of R, until the amplifier oscillates and then back off to a safely stable value. After this has been done the actual values attained may be measured and calculated as shown. An alternate method of measurement is interesting and useful because of its simplicity and because it may be used to measure either positive or negative values. An a.c. bridge and a resistor are required. The resistor should be somewhat larger than the negative re­ sistance the amplifier is e;xpected to have and should be measured first on the bridge by itself. Then the resistor, bridge, and amplifier are connected as shown in Fig. 7 and the bridge is bal-

R .-<>-1- AMPLIFIER -f-o"'Ww-c>f- INPUT OUTPUT B~i~GE

Fig. 7. Set-up for measuring output impedance. anced. The input terminals of the ampli­ fier are shorted and the only signal source used is that within the bridge. The resistance of the amplifier is the indication of the bridge at balance minus the l'esistor value. If, for example, the resistor is 10 ohms and in the circuit of Fig. 7 the bridge balances at 5 ohms, the amplifier internal resistance is 5-10 = - 5 ohms. A positive internal amplifier resistance may be measured simply by connecting the bridge to the amplifier without a re­ sistor and accepting the indication of the bridge at balance. All of the information given in this article has been checked extensively in the laboratory by independent empiriCal means. The writer has tried to be care­ ful to include no assumptions that can­ not be proved without so labelling them. While it is true, as has been pointed out by Clements, that improved speaker damping demands a price in some de­ creased bass, the bass heard before was at least partly due to underdamping and consisted in some part of spurious cone excursions which tended to "compen­ sate" for poor speaker-air coupling at low frequencies. The writer has not been PRESTO able to detect any bass deficiency but has noticed a decided increase in clean and crisp sound. It is felt, in any case, that allowing a speaker to overshoot to compensate for deficient acoustic coupl­ ing between cone and air is a poor way of trying to get something for nothing. The writer wishes to acknowledge the aid of Richard H. Dod in organiz­ ing this material for publication and in acting as a reverberant sounding board against which to throw the ideas de­ veloped on dynamic negative feedback.

AUDIO ENGINEERING • FEBRUARY, 1952 / / I I Unique new sound showroom makes selec tion of equipment an adven ture in comfort, with literally hundreds of combinations available to the listen,er's fingertips from his easy chair.

'EWEST rendezvous in New York for the hi-fi enthusiast is the elaborately • Nequipped sound studio of Hudson Manager Max Baum e demon ­ Radio and Television Corporation at 48 strates various amplifier­ West 48th Street, in tFle heart of the Radio tuner-speaker combinations to City area. Unusual facilities have been pro­ a customer, using the small remote-control unit. This de­ vided here for prospective customel's to make vice saves salesmen a lot of comparative tests of tuners, ampli fiers, speak­ walking back and forth . ers, turntables, and phonograph pickups. •

The studio occupies the entire mezzanine Aoor, in­ dependent of the parts-sales floor on street level. Wall­ to-wall carpeting, comfort­ able chairs, and attractive lamps eonvey a "home at­ mosphere." Each equipment position is equipped with input and output jacks and a d.c. operated relay for turning power on and off. Combinations of one input, two amplifiers, and two speakers are set up from a control box in the living-room 5ection. Actual switching is performed by telephone-type steppin g switches, and A-B tests between the two amplifiers or speakers are made by relays, also actuated from the control box. Equipment volume controls are pre­ set, and final adjustment of vo lume is made with a master pot at the remote position. This arrangement is popular with customers because it enabl es them to make their selections easily. As many as 25 inputs, 50 amplifi ers. attd 50 speakers can be accommodated, although there aren't that many available models.

Left : Control cabinet for selector system, wi th cover removed to show inside wIring. Below. left to right: Main bank of tuners and amplifiers along a 22-foot wall, and group of speakers in derylon ­ stration cabinets of uniform size and finish along far end of studio.

I ,/ 34 / AUDIO ENGINEERING • FEBRUARY, 1952 BUFFALO RADIO SUPPLY 219-221 Cenesee St., Dept. AE TRAVELING MIKE Buffalo 3, New York [from page 16] MACNAVOX HICH FIDELITY ·RECORD DEMONSTRATORS speciall y constructed differential ar­ Sensational Bargain in Brand New rangement. Record Demon~trators (Built for Columbia by Magnavox) Ordinary curtain rail serves for the • Amplifier designed in required mono-rail , and fin e steel wire conjunction with the speaker to be capable supports it throughout its length. Sup­ of bringing you the ports a re combined with brackets for the storm and fi re of Wagnerian opera or pulleys that carry the mike lead across, the light and lilting four such pull eys being shown in F ig. 1. melody of Debussy with equal realism. Noti ce that the ra il shoul d be erected • Oversize 12" speaker with a sli g ht a rch, to prevent a tendency • In c lined s ounding board to buckl e with the weight of the mike HOW LEADING NETWORKS • Console Cabinet ap­ loading. The best degree of arching can proximately a yard wide and a yard high be set up when installing the system, by USE ~cll~@lJ' CONVERTERS • 2 tone controls-Treble and bass- as well as running the carriage across ( with the 2 vol ume controls • A pickup that is as gentle as a beam of winter mike attached) and adjusting the tension Photo above shows NBC's Tommy Bartlett sunshine. Intended to leave records in new of the support wires until mechani cal and Universal Recording Company engineers condition after demonstration. making transcriptions from STORAGE BAT­ ·78 and 33 1/ 3 R.P.M. Originally 3-speed motor, stability is achi eved. TERY POWER by means ot Carter Fre­ but Columbia had the sleeve on the motor shaft for 45 R.P.M. removed, so the R.C.A. The movement of the carriage must quency Controlled Converter. doughnuts could not be played . This can be be noiseless, as any sounds made by it re- installed for a few cents, if you must play Wherever 115v. line voltage is not available, the doughnuts. will be picked up by the mi crophone. or hard to get, Carter Converters supply • No ex pense was spared in producing these T hi s is achi eved by using pulleys with dependable AC power to make on location record demonstrators. They were Columbia's recordings. Operates from storage batteries. weapon in the savage, relentless and costly war rubber tires fitted as running wheels for . . . Used by leadillg networks, broadcast between the titans in the record business that the carriage, and also for the small stations, and program producers. only ended when R.C.A. started to market 33 J / 3 R.P.M. records under its own label. wheels of the differential arrangement. • This buy of a lifetime is guaranteed to please Take care in construction that a ll Recommended by Brush you. Price only $60.00. A quick sellout is cer­ tain at this low price. Rush your order. wheels run freely and without being able • Extra special-We have a few of the Magnavox and Magnecord amplifiers alone, brand new and identical with to touch the chassis so as to scrape when the amplifier in the record demonstrator listed traveling. One model operates above. These have the separate bass and treble both Brush and Mag­ tone controls and volume control, and are com­ Figtwe 2 shows the construction of necord equipment. plete with tubes. Believe it or not, we are the differential in the carriage. When Deli vers clean 60 letting these prizes go for a $20.00 bill each. cycle AC power. Re­ A bargain like this would be unbelievable be­ the carriage traverses, the stationary quires no filtering. fore Korea. let alone now. rotation control cord causes the differ­ Frequency control feature compensates SUPER HEAVY DUTY HICH FIDELITY ential pulleys to rotate in opposite di ­ for 10% input volt­ rections. The microphone spindle is age variation. Avail­ UNIVERSAL OUTPUT TRANSFORMER able tor 6, 12, 24, by famous manufacturer anchored to the differential hub, so no 2~ 32, 64, and 115v. Flat within 1 db to 20,000 cps. Han- rotation of the mike is caused by this Ov innut Toltas:e. - dies up to 125 watts without distor- Size 8'A,'!i xilj8"x7'h" tion • Hermetically sealed with 2500 traverse action. But when the rotation high. Wei&ht only volt insulation and porcelain stand- control cord is moved by its contl'ol­ 15* lbs. Pertorm­ :::-.-\___ off terminals' 6%" high and 14 Ibs. ance Guaranteed! net weight • 18 voice coil imped­ knob, both di ffe rential pulleys rotate in anceOJs available. plus 500 ohm line' $80 .00 value for only $20.00. the same direction, thus rotating the di fferential hub and the microphone 35 MM SLIDE PROJECTOR 6' ENLARCER coupled to it. It is im po rtant· to see that Most terrific photographic bargain of the 20th Century. A fine precision instrument with many the microphone mounting is pos i ti vely features! anchored to the spindle, and the spindle • Takes roll film or 2 x 2 slides. • Automatic slide changer furnished. to the differential hub, as these points • No more waiting for enlargements to enjoy your take the full weight of the mike. candid shots. All you need is a flat surface to project on. Payout and hauling in of the mike New Converter catalog • Has 300 W 110 v Bulb, but 6v, 12v, 32v, or lead is coupled to the carriage movement Illustrates entire line of n ov available. Carter Rotary Convert­ • 8 precision ground, optical glass elements in by means of the large pulleys in the main ers for recording, TV, optical system. (No other projector on the mar­ sound projection and ket has more. I pulley system shown in Fig. 3 at the transcription players. • Automatic rewind of film. control side. T hese are locked to the SELECTOR CHf RT In­ • Heat absorbing filter. cluded tells model to • Use as an enlarger for same shaft so that the mike cabl e moves suit your purpose. Mail making your own with the traverse cord. The mike lead Coupon now for your prints of any desired copy and name of size. is kept in cl ose contact with its drive nearest Carter distribu­ • Plano-Convex Menis­ pulley by means of two jockey pulleys tor. cus Condenser. • 5" focus Brown- Violet mounted in side th e cover of the pulley coated, double anas­ system housing. T his arrangement en­ t igmat F:3.5 lens. • Swivel projector head • Projector tilts to any sures that the connecting cable requ;res ®rstl@11 MOTOR COMPANY angle on base' Brand new Society for Visual Edu­ no attention. A bracket is fi tted to the cation Model AAA Projector. Supe r Special $50. side of the mike carriage to drop the 2648 N. MAPLEWOOD AVE., CHICACO 47 Similar SVE Projector for strip film only w ith mike lead into the bearer pull eys pro­ regul ar 3 " focus lens $35.00 ,.. ------, vided at intervals along the rail to carry Carter Motor Co. 35 MM 150 W Projector similar to above for strip 2648 N. Maplewood Ave., film with regular 1" focus lens $15.00 it. Figure 4 shows how this works. Chicago 47, III. Please .end eataloc #349 with information NEW IS-WATT UTILITY AMPLIFIER KIT Operation on Frequency Control Converters. and name Kit Model SA, high quality wide range 18-watt of nearest dlatributor. public address amplifier. Matched component Control is two handed, but the mike parts. 3-stage, high-gain circuit with push-pull Name •. .•.•• .. . . •...... • . • .• 7C5 output tubes. Dual inputs for mike, phono stays put in any position so that the or radio tuner. Fully shielded. hands are not "tied to the contl·ols." T he StaUon or _ Universal output to match COliDpany ••••• •• • •• • •• • • • • • • • •• • •••• I - any speakers used. Fully vari­ right hand controls .:otation of the mike able tone control on output. head by means of a- knob carrying a Addre. . • ...... • . .• .•••..••••.•• '1/ fi3 0 Complete with all parts and ~. . ' tubes. Super Special. $16.95. pulley for the endless ~ord operating the '------AUDIO ENGINEERING • FEBRUARY, 1952 35

/ •

differential "gear" in the carriage. T he ~ef t hand moves the car riage back and forth by l"aising and lowering a con­ veniently placed handle. T his handle, in conjunction with the rail terminal fixi ng and the fl oor fixture below it, carries a pulley system to step up the foot or so of vertical movement (which is all that is comfortably available by moving a '7~ IO-PfUtHd PtVetaJie handle vertically) to a horizontal move­ BATTERY-OPERATED TAPE RECORDER ment covel"ing the whole length of the rail. Fig~tre 3 clarifies this system, all The MAGNEMITE * offers convenience in field record­ ing never before attained. This unusual unit measures except the essential pulleys and cord only 11112 x 8 V2 x 5112 inches. and weighs only 9% lbs.• being omitted in the interests of clarity. including its self-contained batteries which last 100 A ll types of cord are slightly elastic, operating hours. At 1 % in. per sec. tape speed. 2 full although a cord giving a good combina­ hours of recordings are accommodated on standard tion of strength with pliability should be 5 in. reels of tape. Constant tape speed is assured by means of a governor-controlled spring-wound motor that runs 15 minutes per winding. Recordings can be made up to a distance of 100 feet from the microphone. Net The same instrument may be used for playback Factory through earphones or external amplifier. For any Com plete with batteries, recording. in or away from home. office, or studio, it's microphone, and earphones . the MAGNEMITE * ! • Trade Mark Reg . Order today, or write for additional literature to: AMPLIFIER CORP. of AMERICA 398 Broadway • New York 13, N . Y.

Over the past few years AUDIO ENGINEERING has proven itself a valuable reference work worthy of a permanent place in your library. Many of the 1951 issues are exhausted, but you may have a complete bound volume of new copies for the entire year.

Order Now Limited Supply

Fig. 4. End view of the microphone carriage on $8.95 a section of monorail, to illustrate how t he mike-lead clamp lays the lead in the bearer pulleys mounted on the rail supports. Radio Magazines, Inc., chosen. It should be quite flex ible, yet 342 Madison Ave., New York 17, N. Y. with little stretch. T he residual possi­ bility of stretching means that precau­ tions must be taken to prevent cords from leaving their pulleys when tension in one place due to movement allows slackness to appear elsewhere. Lengths of tension spring are included in each cord to take up this slack automatically. 9205 DEGAUSSER Three springs are required, one fo r each of the three lengths of cord in the system. Demagnetizes magnetic tape and film The "endless" rotary control cord has to erase r ecording and residuals. one, to connect together its two ends. Accommodates 5400 foot reels of * /I T he other two cords make up an "end­ tape; 1000 foot, 35mm. Write f or less" arrangement, with the carriage in­ Catalogue. Net P rice ...... $ 69.50 serted at one point and the control handle virtually at another. (Actually there are two ends, anchored at top and bottom of the pulley multiplying system, but if the drive were direct, i.e. so both CINEMA ENGINEERING ,COMPANY handle and car riage moved at the same 1510 WEST VERDUGO AVENUE, BURB , NK, CALIFORNIA rate, the system would be truly "end­ less"). A spring is inserted in each of Export Agenh: Frazar & Hanlon, Ltd. • 301 Clay Street'; San Fran: isco, Calif., U. S. A.. i the ends of these cords, one to take up / 36 / AUDIO ENGINEERING • FEBRUARY, 1952

I r slack when pulling one way, and the other for pulling' the other way. In the traverse opel'ation, even with springs, it would be possible to produce slack by pulling too hard in either direc­ "''''''10M tion, so one spring would extend more HEADQUARTERS FOR than intended and the other would com­ pletely relax. To avoid this possibility, SOUND EQUIPME'NV limitiflg wires are added to the back and forth springs, so that they cannot be overstretched to the point where the other cord would become slack. This arrangement gives particularly smooth operation, with freedom from trouble. MAXIMUM QUIET AND All the parts, with the exception of the EASE: Special Full-Grip, Velvet ... Action clutches, inner·lined with various pulleys, the monorail, and the wear-proof locking collels, function smoothly at slight pressure. yet cannot creep. jam. mike itself, are constructed quite simply rasp. jolt or jar. from sheet metal, using a little amateur MAXIMUM WEAR: Heovy oversized tinsmith's art. The pulleys, rubber tires, tube assemblies ore super~hrome plated for highest durability. and their spindles can be purchased as spares for any of the variety of toy con­ MAXIMUM VARIETY: Floor 01 tobie, boom, orchestra or collapsib le types-there's on struction kits on the market. ATLAS stond for your every studio need . • COMPARE ATlAS at your distributor TODAY. See why more studios specify ATlAS than any a,hut line. Write NOW for FREE latest Cato­ log 551- LlNEAR·SCALE ATLAS also leads in the manufacture of public address loudspeakel'l and accessories. r AF WATTMETER MAGNECORD [From page 17J RECORDER (:, AMPLIFIER Complete Systems from $564.00 couple meters are not designed to stand much overload-probably not over 10 i per cent at the most. Therefore, care should be taken to set the load switch to 16 ohms for a few initial measurements In Canada: AUo! Radio Corp., ltd.• Toronlo, On'. to make sure that the amplifier could not exceed the 32-watt level-with the leads THE being connected to the appropriate 16- ohm output terminals. If it is found that the amplifier output does not exceed 32 IDEAL watts, the leads may be reconnected to the 8-ohm terminals and the more ex­ HORI panded scale employed. NEWCOMB Applications AMPLIFIER Complete line of High Quality Amplifiers, Most steady-state measurements are from small single units to large rack made with a fixed output power for a j installations period of time; i.e. the duration of the measurement is sufficient for the sluggish action of a thermocouple meter. 'Such a meter would not be suitable for any kind of program material because it ... For use in 2·way and 3·way speaker sys· is incapable of following the audio en­ terns baving low cross-over frequencies. Espedally adapted to corner speakers using velope at all closely. In making a series folded low· frequency horns of the Klipschtype. of 1M measurements it has become ; Acoustical cut·off: 330 cydes. Dispersion standard practice to start at the maxi- . angle: horizontal 90'; vertical 60'. Non·res· onant, accurate, metal casting. Permanent mum output power expected-or de­ non-warping, unit construction .. . superior termined by measurement-and to pro­ to assembled horns. gress downwards in power output, mak~ Model H' 330 H. F. HORN-List Price $108 With adaptor for Electrovoice TIO and T25, ing distortion measurements at I-watt Lansing 0175, Stephens PI5 and 108 . AdaptorJ intervals. This avoids the need for re­ l1t'ailabl, for other driv'f unitL ducing the calibration setting before making each successive power-output in­ ELECTRO-VOICE SPEAKERS _~I'" crease. By starting at the top and re­ From $27.00 to $756.00 NET '''I' I ducing the power watt by watt. the calibration control is advanced slightly PRE.AMl' LIF I ER EQUALIZER List: with each change in output. A100 $55; A100P Spot checks with an a.f. voltmeter , (Self power~'ElECTRONICS linear scale have made the additional in­ 1527 CHESTNUT ST. BROCINER lABORA/ORY vestment for the met,er well repaid in , PHILA. 2, PA.· RI-6-8388 1546 SECOND AVENUE • NEW YORK 28, N. Y. time saved.

AUDIO ENGINEERING • FEBRUARY, 1952 37

/ ) ., AES News '-CLASSIFIED- HE Third Lecture Series given by the Rates: 10c per word per Insertion for noncomme rcial Audio Engineering Society began on advertisementsi 2Se per wo rd fo r commercial adver .. TJanuary 10 with H . H. Scott's paper on tlsements. Rates are net, and no discounts will be "The Amplifier and its P lace in the Cir­ allowed. Copy mu st be accompanied by remlttlnee In cuit-A Survey." W. R. Ayres gave the fUll , and must reach the New York offlee by the flrlt second lecture on "Power and Voltage Am­ of t he mont h preced I na t he dat e of IlSue. plifi ers" on January 17, and attendance was around 100 at both meetings. FOR THE FINEST Audio system, or im­ Availability of lecture notes on this se­ prove your old one, call Arthur Levine, OLln­ ries will be announced at the close of the ville 2-8615, six-week course. THE AUDIO EXCHANGE buys and sells high-fidelity sound systems and components, The Los Angeles Section of the AES Guarant.eed used and new equipment. Cata­ recently elected new officers. Those chosen logue. Department A.E, 159-19 Hillside Ave., to serve during 1952 were: Boyd McKnight, J ama ica 32, New York. OL 8-0445. Chairman; Richard Hastings, Vice-Chair­ FOR SALE: Meissner 9-1091 FM-AM tuner, man; Carl Shipman, Secretary; and W est­ Thordarson T-31 Wll volume-expansion ampli­ ern Vice-President Howard , Tremaine as fier. Excellent condition; together or sepa­ rately. Martin Stark, 550 Piaget Ave.. Clifton, Secretary. N. J., or NYC telephone PLaza 7-1200, Ext. F rank Gilbert of Stephens Manufactur­ 5867 9 a.m, to 4 p.m. ing Company provided the technical por­ tion of the December 18th meeting with a SAVE $7.50! PICKERING D-140S Diamond Stylus LP Ca rtridges only $28.50 each. Bra nd talk and demonstration of a new, small new, in sealed boxes, gua ranteed. Skalamera, wireless microphone for use in T V and 435 E, 74th St., New York 21, N. Y. BU 8-2057. movie studios. ELECTRO-VOICE "Slim-Jim" P rofessional Mi crophone, Model 655, cost $120-your price $95, Matched pair Pickering Diamonds, 78 and 33, cost $60-your price $45. Gray Model 602 , Professional Equalizer, cost $49.50-your price $35. Above are all brand new, unused. Money orders only, to,b. New York. R. Bennis, Box CF-1, AUDIO EN GI NEERI NG. REVERBERATION-time meter, like-new condition. $150 for quick sale. Box CF-3, AUD IO E NGI NEERI NG. POSITIONS OPEN and AVAILABLE TAPE RECORDE&--Twin-Trax. ACA Model PERSONNEL may be listed here at no 540C. Automat ic two-way ~witchove r system charge to industry or to members of for two raclt-mounted Twin-Trax chassis. 4-in. VU meter, dubbing amplifier for copying tapes. the Society. For insertion in this col­ 1 year old, excellent condition. Original cost umn, brief announcements should be in $842: will accept best offer. Mr. Salerno, 65 'ronnele Ave" J ersey City, N. J . Henderson the hands of the Secretary, Audio En­ 5-6365. gineering Society, P. O. Box '12, Old WILL TRADE OR SELL: Sync. Wire re­ Chelsea Station, N. Y. 11 , N. Y., before corder mechanism, 50 spools wire recorded the fifth of 'the month preceding the with concert mUSIC, J ensen J AP-60 speaker, date of issue. Winchester 52 target rifie, Colt match-target SOUND WHERE pi ~to l , .22 cal. a mmo. Supel' Ikontll BX Camera. • Positions Open • Positions Wanted Want Magnecorder or Concertone Tape Re­ corder. Agress, 260-63 75th Ave., Floral Park, * Salesm an. F or leading dis tributor of N. Y. YOU WAN T IT! s ound a nd recording equipment in m etro­ politan New Yorl{ a rea. Box 201 , AUDIO FOR SALE : Presto RC-I 0-14 recorder (three motors, hystereSis motor on capstan) , ENG INEERING. 900-A-l Amplifier, revolution counter, all on RepaIrman for tape and disc recording ­ 4-f t. rack on casters; E-V 645 microphone, * stand. 30 hrs. use, all in excellent condition. equipment. Full or part time. B ox 202, $1100 value for $725. Phone TRafalgar AUDIO ENGINEERI NG. 9-7290, New York City, or Box CF-l, AUDIO * EngIneer s. White Sands Proving E NG INEERING. Ground needs several eng ineers of all PRE STO 900Rl Tape Recording Mecha­ types immediately, with ex cellent oppor­ nism, $325. RCA 15-watt P .A. amplifier, $60. tunities In research , engineering, a nd de­ Transcription arm with new G,E . cartridge, velopment on guided missiles Il.nd com­ $25. Pilotuner and Presto AM tuner, $90. Reco-Art, 1305 Market Street, P h iladelphia, ponent part s . Classifications needed are: Pa, civil or mechanical, GS-ll, $5940; elec­ trical, GS-9, $5060; mechanical, GS- 5, WORLD'S FINEST Recording Wire ! Lear $3410 ; electronic (general), GS-9, $5060 ; Premium, gua ranteed 6 db better HF response. Hear the diffe rence ! List $7,50, wholesale m e<;hanical GS-7, $4205; electronic (2) ~ 4.5 0; your cost only $3,95, or $3,00 in quan­ GS-ll, $5940 ; mechanical, GS-ll, $5940 ; tity. 2327 Arthur St" Los Angeles 65, Calif. COB-ll COBRA HORN engineers, a ll options, GS-5 and GS-7, $3410 and $4 205 ; All salaries on per an­ This horn projects sound only where it is 0 n u m basis. Send Standa rd Application needed. Distribution pattern of 40° x 120 Form 57, obtainable at any Post Office of results in a concentrated beam of sound with­ Civil Service Board of E xaminers to Civil­ out " up in the air" losses and ground reflec­ P ROFESSIONAL ia n P ersonnel Office, Ordnance Corps, a t tions. CUTS COSTS .. . The elimination of White Sa n ds Pl'oving Grou nd, L as Cruces, DIR~crORY reflex members in its "straight" horn design New Mex ico. increases sound output and reduces amplifier input power up to 40% . Also excellent as a • Recording Tec}mIcIan : Has good, a ll­ I mid -range or high frequency horn in 2 and arou nd background with medium-sized 3 way wide range systems. Heavy cast alu­ organization. Experienced in standard minum throughout. Cut-off is 250 cps and pitch a nd micr og roove recording, and Custom-Built Equipment thread size is standard. Dimensions 17-%" 1;ap e. E xcellent tape editor. V ersed in H, 22- 14" W, 13- %" D. Free catalog. s tUdio supervision , microph one tech­ n iqu es , monitoring, and a ll techniques re­ la ted t o mastering and dubbing. Works U. S. Recording Co. In Canada: 1121 Vermont Ave., WaJhiactoll 5, D. C. Dom inion a well with recor ding artists. Know s music; Sound can r ead a nd edit from s core. Sidney C. LIncoln 3-2705 Equlpments, 'lfacon gUuc a. inc. Silver, 107 B rooklyn Ave., Brooklyn 16, Ltd. 52 Easl 191h Slreel, New York 3, N. Y. N . Y. P h one HYacin t h 3-0491. .I 38 'AUDIO ENGINEERING • FEBRUARY, 1952 Review

Make Music Live, by I rving Greene, J ames R. R a dcliffe, and R obert Sch arff. 256 p p. New York : Medill McBride Company, Inc., $4.50. Covering t he entire subject of high­ fi d elity reproduction of music in th e Ideal for home, this is t he fi rst volume dir ect ed BROADCASTING THE ARM WITH A HAND entirely to the non-technical r eader w h o RECORDING has succumbed to the desire for better Has "SLIDE-IN" Cartridge Holder soun d q uality. The fi rst four chap ters PUBLIC ADDRESS "- "The ultimate in micro· This fine quality Clarkstan tone arm a r e devoted to a description of the four basic elements of the h ome syst em­ phone quolity," says features a special Slide-I n cartridge Evan Rushing, sound holder that accommodates all standard record player, tuner, amplifi er, and loud­ engineer of the Hotel cartridges, including the General Electric speaker- followed by a discussion o f New Yorker. RPX-050. It's easy to use ..• no solder­ speaker enclosu res and their require­ • Shout right into the ing . .. just twist a thumb screw and m ents for good r eproduction over the new Amperite Micro­ the cartridge is securely held in place. entire a udio spectrum. The next fi ve phone-or stano 2 'eet Positive electrical contact is assured by chapters cover the planning of the ph ys­ away-reproduction is always perfect. silver plated spring-loaded plungers. Ac­ ical layout of the h om e system , w ith practical ideas for installa tio n in book­ • Nol affected by curate balance is easily maintained by a any climalic condilions. quick weight adjustment that gives shelves or other furniture elem en ts, a nd a no ther chapter covers t he construction • Guaranteed to with­ proper needle force. The Clarkstan Arm stand severe "knocking plays all LP, microgroove and standard a nd fininshing of home-b uilt cabinetry. T he author s a lso discuss tape r ecorders, around." records. Comes in two sizes for records primarily from t he v iewpoint of the per­ up to 12" and 17". See your jobber or son w ho plans to assemble thes e u nits " Kon/ak" Mikes write for bulletin 172-F # in to a present or project ed system. A Model SKH, $12.00 212 Transcription A r m for 16" list t ranscriptiom ...... Net $ 19. gO g lossary of hi-fi terms is included, a long KKH, list $18.00 212G Transcr iption A r m same 88 212. but d otted to accommodate GE with chap t ers on High-Fidelity R ecords cartridge RPX·0 5 0 as well a. all by W a lter Grueninger , r ecord reviewer otho" .. . ..•.•. . •...... • Net $111.90 213 R i car d arm for 1 2" r ecords .. Net $ 18.90 for Consumer s' R esearch, Inc., a nd on 2130 Record arm same as 2 1 3, but I lotted to accommodate GE car- T onal Quality a nd F idelity b y Peter t rtdgu RPX-Q50 as well as aU H ug h R eed, editor of American R ecord otbe" •• . .• . .•... .. ••• . . Net $18.110 Guide. T he book is r ecommended reading for ElECTRONICS DIVISIOM 8A the non-technical person w h o w ishes to assemble a home music system for his own use, since it is full of valuable point­ ers on "how-to-do-it." It is n ot a book w h ich would interest the technically m inded individual who enjoys b uilding If you are moving, you must notify our Cir­ the various com ponen ts-the k ind of p er­ culation Department at least 5 weeks in son w ho is usua ll y called an "audio advance. The. Post Office Dept. does not hobbyist." forward magazines sent to wrong destina­ tions unless you pay additional postage, and we can NOT duplicate copies sent to you once. Please give old and new addresses. RADIO MAGAZINES, INC. 342 Madison Ave. B. S. Fenton, vice-presiden t, announces New York 17, N. Y. appointment of Floyd J. Van Als tyne as Jobber Sales Manager of Permofiux Cor­ poration.... Arthur Priest ann ounces resignation as ad manager of L eonard Radio, Inc., New York- f u ture plans will WHY WILL YOU CHOOSE be announced soon... . George SUber, president of Rek-O-K ut, combining busi­ ness with pleasure in trip to F lorida wit h YOUR 'APE RECORDER? family (Ed's note: How he do dat ?) . .. . Dr. Balph Bown, director of research for Plays all speeds Bell Telephone Laboratories since 1946, elevated to vice-presidency.... Herman Holstein has resigned as a d mana ger of Lafayet te Radio, I nc., New York.... Kenneth Boothe, Audio & Video Prod­ GARRARD' ucts Corp. exexc, announces appoiritment of Paul Baran as fie ld engineering repre­ sentative. . .. Promotion of Philip Barnes to director of t he sales division of Weston J!7~" E lectrical Instrumen t Co. is a nnounced by Earl B. Mellon, president.... Dr. A. W. WORLD'S FINEST Friend has joined Magnetic Metals Co., Camden, N. J., as director of engineering and r esearch .. . . U. S. Chamber of Com­ RECORD CHANG'ER merce has expanded radio and t elevision services with George H. Sandefer, for­ merly of NBC, as director . . . . New manager of sales operations for RCA Tube Department is Douglas Y. Writ e. for FREE literature Smith, elig ible for a quarter-cen t ury pin in RCA service. . . . B. N. Kellogg, Jr. is GARRARD SALES CORP., Dept. 2E new advertising director of Burlingame 164 Duane st., New York 13 No matter which of these features you consider Associates, New Yorlt .... Henry C. Boe- Gentlemen: • most important. You'lI find them all-and many mer, V.p. of F ederal Telephone a nd R adio 1 am interested in learning what to look for when • Corp. since 1950, has been elevat ed to vurchaslng a 3 -speed record changer. P lease lend • other exclusive features-in our Twin ~ Trax Recorders. presidency . ... Frank Bobbins a nd Bill me, without ob llo\:atLon. your FACT SHEET. • Se nd today for your free copy of our Catalog Joseph, co-inventor s of the R -J Speaker, I #5272. 'Reg. U. S. Pa t. 011 . are litera lly swamped with· letters asking NAM. E • . •.... . ••.. .. • • •.• • . . . •• •.• • • • • when and where t he unit will be avail­ ADDRESS • . ..•• .•.• • · • •• •. ••• • • •• •••• ••. able .. . . Budy Boza,k, president, R. T. AMPLIFIER CORP. OF AMERICA Bozak, Inc., Buffalo, N. Y. , beaming at • CITy •..•. . ..• ZONE • .• STATE...... = results of recent testis of Bozak speaker 398 Broadway I New York 13, N. Y. by a well-known Eastern university. •... ------_.------_._-_ ..•

AUDIO ENGINEERING • FEBRUARY, 1952 39 A SUGGESTION / PARTRIDGE ADVERTISING WILLIAMSON THERE IS SOME TRUTH OUTPUT TRANSFORMER INDEX Built to the original specification • De-luxe model now available from stock f rom . 11 Air-Tone Sound & Recording Co. 37 important R adio Stores t hroughout t he U .S.A. (Price $26.00 duty paid) Altec Lansing Corp...... 22 This t ransformer is now accepted as the most Amperite Co., Inc ...... 39 efficient in the world. According to " Audio En­ Ampex Electric Corp...... 32 gineering" (N ov. 1949), there is no U .S. equiva­ Amplifier Corp. of America ...... 36, 39 lent. Thousands already sold in the U.S.A. Arnold Enginellring Co., The ...... 5 Partridge CFB Output T ransformer, accepted a. Atlas Sound Corp...... 37 without rivai. Series leakage induct .10 mh., primary shun t induct, 130 H ., with " C" core 'IN OUR ADVERTISING Audak Co...... •...... 21 construction and hermetically sealed. (Price Audio Devices , Inc...... Cover 2 $40.00 duty paid) The Following Stores are among those now While we were in America, we had the Stocking Partridge Transformers. chance to have the performance of the 215 Bell Telephone Labs, Inc...... 10 Harvey Radio Co. Inc. Terminal Radio Corpo- 103 West 43rd Street, ration, speaker in its Boffle measured by one of the Bogen, David Co., Inc ...... 40 New York 18. ' ~~wC~~~~n~~ Street. best equipped sound laboratories in the U. S. A. Brociner Electronics Lab ...... •... . . 37 Electronic Wholesalers Gates Radio Company, As you will know, we are rather averse to p'ub­ Buffalo Radio Supply ...... 35 Inc. 2700 Pol k Avenue. 2345 Sherman Ave., Houston Texas. lishing response curves, since they can be so Washington. D. C. Wholesale Radio Pa r-ts easily misinterpreted, and we wondered just Cannon Electric Co...... 24 Radio Electric Service Co. Inc. Co. 311 W. Baltimore St., what this opportunity would reveal. Carter Motor Co...... 35 5 North Howard Street, Baltimore 1. Maryland. Chicago Transformer Co...... 4 Baltimore 1. Maryland. Sole Agents in Canada : Our own measuring equipment is not truly Atlas Radio Corporation, Cinema Engineering Co...... 36 Gates Radio Company, 560 King Street West, reliable. beyond 15,000 cps, and our previous Quincy. Ill inois. Toronto 2-B. Clarkstan Corp...... 39 curves have not shown what happens beyond If you are unable to purchase Partridge trans­ Classified Ads ...... 38 formers in your city, write to us and mention the 10,000. This time we were going to get a story name of your dealer. up to 20,000. What would that story be? Fullest data, including square-wave tests. distor­ Daven Co., The ...... • • .. . . Cover 3 tion curves, etc. , together wit h list of U. S. We had estimated what the speaker actually stockists rushed Air Mail to you. did beyond 15,000, which accounts for our claim EI -Tronics, Inc ...... PARTRIDGE TRANSFORMERS LTD. 31 TOLWORTH , SURREY, ENCLAND that it will show up extremely well against. any multiple unit system, but could we prove it by Garrard Sales Corp ...... 39 measurement? The unqualified answer is " Yes." Gates Radio Co...... • ..... 9 We are much gratified that independent meas­ urement has confirmed what we thought the Hartley, H. A. Co. Ltd...... 40 speaker really does, and we are even more Harvey Radio Co ...... 30 gratified at an astonishing set of polar curves Heath Co. , The ...... 23 which show that the 215 has a truly wonderful Hudson Radio & Television Corp. 7 forward spherical radiation. These curves are being embodied in a tech­ Lansing Sound, James B., Inc ...... 28 nical report covering the performance of the Leonard Radio, Inc...... 29 215 in detail. A copy will be sent to everyone on our mailing list, and anyone else who is in­ Magnecord, Inc...... 6 terested-and every high-fidelity enthusiast Milo Radio & Electronics Corp. 38 will be interested to find what can be achieved Minnesota Mining & Mfg. Co ...... 25 at so little cost-need only send us a postcard bearing the words "215 report please", and your Partridge Transformer Ltd ...... 40 name and address. Pickering & Co., Inc...... 31 We are often told that our advertisements Professional Directory ...... 38 tend to carry conviction, and that is gratifying to us. Nevertheless, we are glad to be able to Racon Electric Co., Inc. . . . : ...... 38 prove to you that everything we have said about Rek-O-Kut Co...... 1 the 215 speaker is true-by measurement. Reeves Soundcraft Corp...... 3

The Hartley-Turner 215 Speaker Sonocraft Corp...... 33 IS the greatest bargain in the world of high-fidelity. Terminal Radio Corp...... 26 Triad Transformer Mfg. Co...... 2 Price only $48.00. Import duty $6.00 Trimm, Inc ...... 33 Turner Co ...... 27

H. A. HARTLEY CO. LTD. U. S. Recording Co...... 38 152, Hammersmith Road . United Transformer; Corp...... Cover 4 London W.6, England

AUDIO ENGI'NEERING • FEBRUARY, ' 1952 • Are all bran • Specifications

• Naturally, n • the right to

• Brands of Resistors vary as widely in the completeness of a line and in per­ formance, as do brands of any other product.

DAVEN originated the first pie-type, wire wound Resistor more than a gen­ eration ago. Since that time, DAVEN has designed and manufactured Pre­ cision Wire Wound Resistors of every conceivable type to meet the increasing demands of the electronics industry.

SUPER DAVOHM RESISTORS are noted for their high stability and accuracy un­ der extreme temperature and humidity conditions. DAVEN Resistors are made in accordance with JAN-R-93 specifica­ tions and are in use in all types of Army, Navy and Air Force electronic equipment.

DAVEN has developed special small precision Resistors for use in minia­ turized assemblies. All types of mount­ ings, sizes, tolerances and temperature coefficients are available from a large variety of standard types. That's why DAVEN can fill your precision Resistor needs .

. Take advantage of DAVEN's ad­ vanced engineering. and manufac­ turing techniques to help with any Resistor problem confronting you.

Visit DAVEN at the I;R.E. -"Show. Booths 94B & '95. ' /

The UTC t ype H Q permalloy dust t oroids are ideal for all a udio, car rier and supersonic applications. H~A coils have Q over 100 at 5,000 cycles . . . HQB coils, Q over 200 at 4,000 cycles . .. HQC coils, Q over 200 at 30 KC ... HQD coils, Q over 200 at 60 KG ... HQE (mini­ ature) coils, Q over 120 at 10 KC. The toroid dust core provides very low hum pickup . . . excellent sta bility with voltage change .. . negligible inducta;nce cha nge w ith t emperature, etc. P recision adjusted to 1% tolerance. Hermet ically sealed.

HOA, HOC, HOD CASE 13/16"Dia. x 1 3/16"High

In~uctanc e Net Inductance Net Inductance Net Type No. Value Price 'Type No. Value Price Type No. Valu, Price HOB CASE mhy. $7.00 HQA·16 7.5 hy. $15.00 HQC·l mhy . $13.00 I Sls"x 2 Sls"x 2 I 12" High HGA·l 5 HGA·2 12.5 mhy. 7.00 ~ HQA·17 10. hy. 16.00 HQC·2 2.5 mhy. 13.00 HQA·3 20 mhy. 7.50 HGA·18 15. hy. 17.00 HQC·3 5 mhy. 13.00 HQA-4 30 mhy. 7.50 HQB·l 10 mhy. 16.00 HQC-4 10 mhy. 13.00 HQA·5 50 mhy. 8.00 HQB·2 30 mhy. 16.00 HQC·5 20 mhy. 13.00 HQA·& 80 mhy . 8.00 HQB·3 70 mhy. 16.00 HQO·l . 4 mhy . 15.00 HGA·7 125 mhy. 9.00 HQB·4" 120 mhy. 17.00 HQD·2 1 mhy. 15.00 HQA·8 200 mhy. 9.00 HQB·5 . 5 hy • 17.00 ' HQO·3 2.5 mhy. 15.00 HQA.s 300 mhy. 10.00 HQB·6 1. hy. 18.00 HQO-4 5 mhy. 15.00 HQA·l0 .5 hy. 10.00 HQB·7 2. hy. 19.00 HQO·5 15 mhy . 15.00 HQA·ll .75 hy. 10.00 HQB·8 3.5 hy. 20.00 HQE·l 5 mhy. 6.00 HQA·12 1.25 hy. 11 .00 HQB·9 7.5 ' hy. 21 .00 HQE·2 10 mhy. &.00 HQA·13 2. hy. 11 .00 HQB·10 12. hy. 22.00 HQE·3 50 mhy. 7.00 HOE CASE HQA-14 3. hy. 13.00 HQB·l l 18. hy. 23.00 HQE-4 100 mhy. 7.50 1/2"x 1 S/16"x 1 3/16"High HQA·15 5. hy. 14.00 HQB·12 25. hy. 24.00 HQE·5 200 mhy. 8.00

These U.T.C. st ock units take ca re of most STOCK FREQUENCIES common filter applications. The interstage (Number after letters is frequency) filters, BMI (band pass), HMI (high Net Price '$25.00 pass), and LMI (low pass), have a BM I·60 BM I·1500 LMI·200 BML-400 FILTER CASE M nominal impeda nce at 10,000 ohms. BMI·l00 BMI·3000 LMI-500 BML·l000 13/16 x 1 11 / 16, The line filters, BML (band pass), HML BM I·l20 BMI·l 0000 LM I·l000 HML·200 BMI-400 HMI·2OQ LMI·2000 HML·500 15/ 8 x 2112 High '(high pass) , a nd LML (low pass) , BMI·500 HMI·500 LMI·3000 LML·l000 a r e intended for use in 500/600 ohm circuits. BMI-750 HM I·l000 LM I·5000 LML·2500 All units are shielded for low pickUp BMI·l000 HMI·3000 LM I-l0000 LML·4000 (150 mv /gauss) and are hermetically sealed. LML·12000