JUNE 1952 AU) e 35c EftiGI%EEIRIING

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AT OR www.americanradiohistory.com i

ELECTRICAL TRANSCRIPTION

Behin th fan;ir blue labél of WOR recording studios...

the finest in modern sound recording methods and equipment

Radio stations from coast to coast recognize this label as the mark of a top quality transcription. One that can be de- pended on to meet or exceed the extremely high broadcast ... including standards of sound quality. # To maintain this reputation, WOR Recording Studios, one of the largest in the world, use the finest and most costly tape and disc recording equipment obtainable. And- what's ..11110%411:rw equally important - their engineers have found that Audiotape and Audiodiscs are an ideal combination for P meeting the exacting requirements of broadcast transcrip- ,0/P tion and commercial recording work. This same record -mak- ing combination is also being used with outstanding success by America's leading producers of fine phonograph records. and With Audiotape and Audiodiscs, you can achieve this same sound perfection in your recording work, too. Their consistent, uniform quality is the result of more than 12 years of specialized experience by the only company in America devoted solely to the manufacture of fine recording media, both tape and discs. AUDIO DEVICES, Inc. 444 MADISON AVE., NEW YORK 22, N. Y Expert Dept.: 13 East 40th St., Now York 16, N. Y, Cable. "ARAB"

www.americanradiohistory.com EXACT EQUALIZATION

FOR EVERY STANDARD RECORD Successor to -Established 1917 MANUFACTURER'S CURVE... AUHO. ElNG1liEER1NG± another reason why we ask ... CAN ELECTRONIC CONSTRUCTION BE BEAUTIFLL?

C. G. McProud, Editor and Publisher Harrie K. Richardson, Associate- Editor Edgar M. Villchur, Contributing Editor -chassis Luci Turner, Production Manager S. L. Cahn, Advertising Director Take a look at the under construc- Lucille Carty, Circulation Manager H. N. Reizes, Advertising Manager tion of the Maynard PR -2A ... see the mili- board construction, the Editorial Advisory Board Representatives tary type terminal H. Thorpe Covington, Special Representative silver plated contact points, with every Howard A. Chinn YAYaa 607 N. Michigan Ave., Chicago 11, Ill. UorT Sanford R. Cowan, Mid -West Representative John D. Colvin UREAU 67 W. 44th St., New York 18, N. Y. component clearly visible ... see the com- C. J. LeBel lames SCILATk 5th ISt., Los Angeles 17, Calif.s pact space economy in the layout and parts J. P. Maxfield Technical Book G Magazine Co. 297 Swanston St., Melbourne, C. I. equalization George M. Nixon Victoria, Australia placement ... see the critical assembly, forever protected from dampness CONTENTS JUNE, 1952 Vol. 36, No. 6 and dirt by complete wax sealing -then- Audio Patents -Richard H. Dorf 2 look at its performance record. Check the Letters 6 important features below -each superio' Employment Register 8 virtue adds to your decision that there can New Literature 10 be no other choice than Maynard. Editor's Report 12 62.50 FOB Chicago Planning and Building a Radio Studio- Eugene F. Coriell 15 Handbook of Sound Reproduction -Chapter 1 -Edgar M. Beim- 18

The OTL (Output -Transformer -Less) Amplifier -Victor Brociner and The PR -2A offers these superlative leaturs. Gerald Shirley 21 EXACT EQUALIZATION for the curves of the following record manufacturers -FFRR (Eng ish 24 78). AES, RCA, Col. Covers all curves mkt in Universal Amplifier for Magnetic Tape Recorder -Part 2 -C. G.Mcl'roud standard use. MINIMIZED PHASE SHIFT -- exceptionally dean Ultra -Linear Operation of the Williamson Amplifier -David Hafler and square wave response. Herbert I. keroes 26 INAUDIBLE HUM LEVEL hum less tkar 8 microvolts of equivalent input signal - hiss less Audio in the Home -William C. Shrader 28 than 2 microvolts. MILITARY TERMINAL BOARD CONSIPer Phono Facts -1952 Weil 30 TION on silver plated contacts -easy serviánw -Maximilian each component clearly visible. Low noise i posited carbon resistors and 2% tolerance s Ire. Record Revue -Edward Tatuall Canby 32 mica condensers. SELF -CONTAINED POWER SUPPLY. No oiler New Products 38 connections other than input and output necessary. Price and Product Changes-Radio's Master Reports 47 HINGED BASS AND TREBLE CONTROLS Fre- quency response raised or lowered from a fixed point on curve with enact proportions clearly Advertising Index 48 marked on chassis. REMOTE LOCATION. Distances up to 1000 feet from amplifier allowable with no loss in nigh frequency response. COVER RESPONSE not down at any point more than I db at 7 cps to 80 kcs. Jensen's "Reproducer of the Future" -shown for the first time at the Audio SWITCHING POSITIONS for microthcne Fair in Chicago. This 46 -cubic foot five -way loudspeaker employs four phono and radio inputs. Outlet for AC I ne separate speaker units, three dividing networks, and one acoustic cross- supply to all associated components. in over (between front and rear radiation from the 15 -inch woofer WATCH FOR MORE PRODUCT ANNOUNCEMENTS the upper section). The sub- woofer in the lower compartment FROM OUR FAST-GROWING COMPANY covers the range from 25 to 40 cps and is called a "Trans - Ret lex " -a transmission -line bass -reflex- radiator.

RADIO MAGAZINES, INC., 342 MADISON AVE., NEW YORK 17, N. Y.

AUDIO ENGINEERING (title registered U. 8. Pat. Os.) Is published monthly at 10 McGovern Ave., Lancaster, Pa., by Radio Aagazines, Inc.. Henry A. Schotter. President: C. G. Mfd'rond, Secretary. Executive and Editorial Offices: 342 Madison Avenue, New York 17, N. Y. Subscription rates -United States, U. S. Possessions and Goads, $3.00 for 1 year, $5.00 for 2 years; elsewhere $4.00 per year. Single copia 3be. Printed In U. 8. A. All rights reserved. Entire contents copyright 1952 by Badin Magazines, Inc. Entered as Second class Matter February 9, 1950 at the Post Ocoee. Lancaster, Pa. under the Act of March 3. 1879.

1414 S. WABASH AVE. CHICAGO 5,ILLINOIS

www.americanradiohistory.com The TURNTABLE is the HEART of your HIGH FIDELITY SYSTEM

RICHARD H. DORF You may own the finest pickup, amplifier and speaker that money can buy... yet you'll get HE CLASSIC METHOD of altering poor reproduction if your TURNTABLE has the fre- parallel -T feedback filter, but without the ex- quency response of an audio transmis- other advantages. cessive wow, hum or rumble[ sion system is to insert lumped- constant A basic practical circuit adapted from reactive components -capacitors and induc- the patent illustrations is shown in Fig. 1. REK-C7-KUT tors-in the circuit, in the form of filters Signal from the source is fed to the grid of of one kind or another. There are a number V,, which is a standard phase splitter of .. offers a complete range of 12" Turntable models of disadvantages to this simple- appearing the type in which the cathode and plate - to match your present high -fidelity equipment and practice in many applications. Exact values load your own pocketbook. resistors develop equal output voltages Not every sound system re- for the desired results are not always 180 quires the most expensive turntable ...your turn- easy deg. out of phase. V, and V,, which table must be chosen to complement your other com- to procure and trimming an inductor to may be a pair of triodes in the same enve- ponents. Each REK -O -KUT Turntable carries an engi- the precise value is rarely an easy job. The lope, are excited out of phase by the phase - neering specification which enables your Sound Dealer exact shape of the curve -slope of attenua- splitter signals but have their plates tied to select the appropriate unit. The quality and work- tion or equalization and Q of a rejection or together. If such a circuitt were used in manship of every REK -O -KUT Turntable, from our peaking circuit not always easy to at- the normal manner without filters most economically -priced model, to our highest priced -is the R-C professional machine is identical ... price differential tain in practice, routine though it may be indicated in the diagram, no output voltage is dependent solely upon the type of materials used. in terms of paper design. And at least as would be developed across R,, assuming important as any of these, variation of the the phase inverter to be well balanced and shape and slope of the curve is difficult, V, and V, of identical characteristics, be- MODEL LP -743 especially when the inductance must be cause the out -of -phase signals of V, and 3 -Speed 12" Turntable varied. Control of response at any distance V, would cancel across R,. Induction type motor, from the main circuit is usually impossible The switch and bias cells, together with designed for smooth, vibration -free operation. without elaborate safeguards against line the associated resistor network, is an ar- Instantaneous speed losses and hum pickup because the audio rangement for biasing the two tubes in op- changes without stopping itself must be carried to the control point posite polarities. With the switch in posi- turntable or removing and back. tion 1, disc Net the only bias is that of the cathode $54.95 Ellison S. Purington of Gloucester, resistor R,, and it is the same for both V. MODEL CVS -12 Mass., has patented an extremely interest- and V,. As the switch arm is moved to ing idea along the lines of an electronic position 2, electrons flow in the direction All -Speed Continuously - filter circuit which appears to overcome of the arrows, making the grid of V, more Variable Turntable these difficulties. His patent No. 2,589,133, positive and the grid of V, more negative Plays all records, 6" to 16" assigned to John Hays Hammond, Jr., em- than ground. and broadcast transcrip- tions, not only at the three ploys vacuum tubes in a novel system using Suppose R -C Filter A has a high -pass normal speeds, BUT AT only resistors and capacitors to achieve characteristic, perhaps with the configura- ANY SPEED from 25 to 100 the effects of constant -k, tn- derived, and tion shown connected by dashed lines, and RPM. You play your records other filter types, with turnover and Q con- B is at the EXACT SPEED at Filter simply a large coupling capac- which they were recorded trolled by d.c. bias within a reasonable itor without any frequency discrimination or at any variation of speed range. One of the useful features (not in the audible range. With the bias switch or pitch which gives you pointed out in the patent specification) ap- on position 1 (both tube conductances maximum pleasure. A "must" for serious music lovers, pro musicians pears to he that filter circuits involving equal), there will be a constant relation- schools, dance studios, etc. $84.95 Net sharp cutoffs and single- frequency peaks ship between the signal levels on the two and nulls will not "ring" as will ordinary grids at all frequencies above that at which MODELS T-12H & T-43H L-C circuits used for these purposes. This Filter A begins to reduce low- frequency 2 -Speed 12" Turntables feature is not exclusive with the Purington response. At those frequencies, therefore, The only 12" dual -speed turn- circuit, of course, being true as well of the output of the circuit will be zero (neglect - tables that meet the standards for speed regulation and WOW * 255 11'. Silk St., New York 2-1, N. Y. [Continued on page 4] content specified by the Na- tional Association of Broad- casters. Recommended for use wi h ULTRA HIGH FIDELITY Amplifiers and Speaker Systems.

MODELS MOTOR SPEED PRICE Hysteresis 78.51. T.It N Synchronous 9119.95 T -49 Hyster.sIs N Synchronous 455$91 $119.95 T-12 4 Polo Induction 78$39, $84.95 4 Polo T -4$ Induction 454314 3M.95

Delivery limited to eery short supply of Hysteresis motors.

See ... flea ... compare REK -0 -KUT Turntables, Phonographs and Professional Disc Recorders at leading Mu is Stores, AudioVisual Dealers and Radio Parts Jobbers... or write for latest, free Catalogue. REK-O-KUT CO. 38 -01 Queens Blvd., Long Island City, N.Y.

8.,.,, D,vlsIoN +a ,.sad -a,. N . e1r. U S , CANADA MIS 1,010 CO.. LID . 560 1.n5 SI eW . ld,dld }.. Dnla,.e Fig. 1.

2 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com INC. Memorandum from rJ AUDIO PRODUCTS,

Dear Audio Enthusiast:

and for your inquiry on R -J Enclosures Thank you answer trust that the enclosed literature will we new all your questions concerning these amazing cabinets. available You will note the enclosures will be various models less speaker and can be purchased in of your to accommodate any 8 ", 12 ", or 15" speaker the highs so choice. There is direct radiation of you can use a coaxial speaker to the fullest that also advantage. Of course, if you wish, you can speaker use the cabinets simply to house the bass (woofer) and use an external treble speaker(tweeter). in The 12" and 15" floor models will be available as the unfinished, smooth sanded surfaces, as well the Mahogany and Blonde (Korina) finishes shown in in enclosed literature, but will not be available deviate kit form. Also, it will be impossible to from the sizes, finishes, or construction indicated here or in the enclosed.

We cannot at this time go any further into the principles of construction, or how these amazing listen results are achieved. We ask you to simply distributor to R -J at your nearest top flight parts sound dealer and hear for yourself why engineers or Audio and critics who heard these cabinets at the Shows gave them rave notices. of They should be available approximately the 1st July and if you have any difficulty whatsoever in in locating a source of supply of R -J Enclosures make your territory, please let us know and we will immediate suitable arrangements.

STREET NEW YORK 13, N. Y. R -J AUDIO PRODUCTS, INC. 164 DUANE

www.americanradiohistory.com www.americanradiohistory.com .+Inr '4111.1111, ; t . Reproduces maximum Excellent performance of any speaker, acoustic loading Quality for best without annoying cabinet appearance reprcduction of and construction transients; resonances. Mahogany or gonfla perfect separation of percussive tones. Direct Simple radiation of highs installation of without any loud speaker obstruction. according to user's preference.

Clean, smooth bass Versatile, decorative, equal to inexpensive performance of ordinary enclosures due to small size, good 3 times the size. proportion.

And now suddenly an enterprising friend of mine has looked at natures Future development of the R3 es well wonh immutable" law and found way aubin/ otefully. n is theenure vend roward disproving she 551.11 that good around it, as as you simply please. So bau reproduction regsue, giant sue. simple that it u lust about impossible to believe that nobody in all them HIGH -FIDELITY yea has done it before. His speaker cabinet u tray, just about by enough for speaker to fit made, with inches

to spare. But it gives ban perform- CRITICAL COMMENTS... I grant that for. the ce that challenges the huge monten o,vl perhaps this nee of a is we've always thought were de rkgta.. professof top How largerr -mar ket "importance. rose rthinkange of audio doff Nature, it seems, does know how to the record collector IMMEDIATE, and promoter; Frank the eternal rule has bearbeen dmtyd, even by Robbuu and William Joseph. The INTEREST! ever so little-and anything may happen I "RJ" principle is simple, flexible. AUDIO ENGINEERING adaptable to myriads of special cir cure ataacea L SATURDAY REVIEW in the hi -fi area, the RJ enclosure may prove of trmdous importance to those buyers of high-quality egmipment who have sumptuous but nom- too -big living rooms This was the reason for the crowds and tend, even against engineering advice, around the Audio Fair exhibit marked to order those dreadful combination "arm- chair console" arrangements which sacrifice "1t4 Company." Messrs. It. and J. p'aulre., o,aa.0 I uerie de I,e"1 so mach in acoustical response in favor of (Frank Itobbins and Willinm Joseph oénn"sep connus smaller sise. The RJ enclosure be used á.~...iént`i C." eme, an lied made n vnlinnt effort to find n áa":: chair ee o chairside ta aue'ún.r ` Co aáa ai table blea. Olr á.o:'`Inae"leu,a stool; it buts any chainide r conventional middle ground. Their cabinets awerltaln.. mal aaoNllra et Wlutale jasa. arrangement all hollow-Std gives far su- "under wraps"-wrap.. " - burlap) were pol montre 1 é l'Aeale Fair de New-yerk. perior gustily. With such a speaker the ob- C mole de novent., rabic and only n few inrhm bigger vvlk . nlne i wl.eaunit lation to a separate box is largely nullified. than the speakerr housed. .1 npeedpl ..uepnb4 de . Amplifier and controls-possible emote - they P lemen1 la comparaison avec meilleure. then be stowed away with Use e greatest speaker ,e.11.ailo". in an I{J cabinet is nenlorl hofould case. I see extraordinary possibilities behind by a very small bac but, Io LA RADIO inere, the ultra- custom department. rvtunline I his. il is front -loaded, PARIS ton. AUDIO ENGINEERING The frontal mir chamber is dosed off bol for n sot through which the 't sound comm. .4 slot distribute, sound The RJ f omMar, New York City. rather well. Organ -pipe maker round t'ienom to thin display will probably be !hi. mall lin, rigo mumbling to themselves for month to The but impression of the layman rat leant nta hey have other banner upon hearing music come front opportunity to hear the R -J pskar In THE ATLANTIC action and verify the fact that they this enclosure is that so muck sound can did hear a 30-cpe note coming from cam from no small an object. For ais twallyh. innards of an le-inch aquae box enclosure, called the R -1 alter its in Fleet Introduced publicly through the pages of .E Wt month. the R -J enclosure ventors, is barely larger than the speaker already Y on the way to commercial se it rantains Yet, when fitted with a good reytanes by major manufacturer@ of eu.- Sire for all the way and down, the tom-bullt borne muai< The In. quality speaker this R.I. enclosure is use w tem.. RJ type of cabinet enclosure will give who conducted the R -J exhiba capable of faithfully reproducing basa you excellent bass rayasse and smooth over -all yen literally swamped with sees(- of fundamental tones congratulation on their having reduced down to Ja cycles per response equal to enclosures that simply speaker cabinet Ire without impairing second. that if. the lowest bass toms dwarf it. If this principle works, then we low- frequency repone audible to human ears. ras have our cake aid est it. AUDIO ENGINEERING THE CATHOLIC EDUCATOR AUDIO ENGINEERING

R -J AUDIO PRODUCTS INC. 164 Duane Street New York 13, N. Y.

PRINTED IN U.S.A. www.americanradiohistory.com REMARKABLE ! INEXPENSIVE!

SMALL -SIZE !

(pplaot Pendoel

Maximum Bass-Minimum Space! Use it with ANY loudspeaker!

SINGLE SHELF MODEL No. S.E -U For 8" Speaker). To fit ii bookcase. Finish ;anded, unpainted 11" high z 10" deep x 231/2' long.

3 basic designs, combining finest performance VERSATILITY with maximum deco alive possi3ilities in the home

FLOOR MODEL No. F.15 -M (For 15" Speaker, Mahogany) NO. F15ß (For 15" Speaker, Blonde) N0. F.12 -M (For 12" Speaker, Mahogany) No. F.12 -B (For 12" Speaker, Blonde) Beautiful, hand rubbed Mahogany or Blonde finish. 20" high x 20" long x 16" deep, plus legs. DOUBLE SHELF MODEL No. 5.12 -U (For 12" Speaker). To ft in bookcase. Finish sanded, unpainted 24" high x 21" long x 10" jeep.

R -J ENCLOSURES WILL OUTPERFORM ORDINARY ENCLOSURES MORE THAN THREE TIMES THEIR SIZE!

Hearing is believing! Now available for the first time, R-J moot iuc..\pensive speakers will perform at their best in an Speaker Enclosures have established an entirely new trend R -J housing. With a well -designed speaker, an R -J En- in audio design with thrilling performance from any loud- closure offers clean, smooth bass fundamentals, without speaker in a cabinet only slightly larger than the speaker peaks, down to low organ pedal notes. Distorting cabinet itself. R-J Enclosures are simple, flexible, low in price, and resonances are completely eliminated. particularly adaptable in rooms where space must be con- served. Up to now cumbersome, costly speaker enclosures Remarkable though its bass performance may be, the R -J have been tolerated because their size was considered Enclosure is equally impressive in its unobstructed direct essential for good bass performance. This inconvenience radiation of high frequencies. Excellent acoustic loading has now been effectively overcome by the R -J construction affords best reproduction of transients. principle, which has received the approbation of thousands R -J Speaker Enclosures also offer the advantages of unusual of critical listeners with outstanding results. Long before versatility. Fine furniture construction is used throughout, R -J Speaker Enclosures were commercially available, en- suited in appearance to any room decor. Enclosures are thusiastic comments and inquiries were received from expertly built of heavy, top -quality genuine Mahogany or every part of the world, from home listeners, critics, and Korina veneers, in fine, long lasting hand -rubbed dark or broadcast stations alike. Listeners were fascinated equally blonde finish. Special models are available with surfaces by the outstanding performance of R -J Enclosures and by sanded but unfinished, to fit the shelves of bookcases and their decorative possibilities. other similar installations. Cut -outs fit all standard loud- The R -J Enclosure will permit the user to realize the full speakers, which may he installed quickly and simply potential of any speaker with which it is used. Even the without special tools.

www.americanradiohistory.com Remember the speaker with the champagne glass?

Welcome news to the thousands who heard it at the Audio Fairs, and were amazed, and to other thousands who have written from every part of the world.

Now available for the first time:

SOME CRITICAL COMMENTS WHEN THE R -1 WAS FIRST DEMONSTRATED AT THE NEW YORK AUDIO FAIR.

The R -J Company, New York City. Visitors to this display will prohably be mumbling to themselves for months to come-at least until they have another opportunity to hear the R -J speaker In action and verify the fact that they ac- tually did hear a 30-cps note coming from the innards of an lt- inch -square box. First Introduced publicly through the magna of .ti last month, the R -J enclosure already is on the way to commercial ac- ceptance by major manufacturers of cus- tom -built home music systems. The in- ventors, who conducted the R -J exhibit. were literally swamped with messages of congratulation on their having reduced speaker cabinet site without impairing low- freoaacy response. SPEAKER AUDIO ENGINEERING ENCLOSURES Pending) (Potent MAXIMUM BASS ! And now suddenly an enterprising ...MINIMUM SPACE friend of mine has looked at nature's "Startling!" 'Small in size, but mighty in performance!" Thousands of critical listeners who "immutable" law and found a way heard R-J Enclosures at the New York and Chicago Audio Fairs actually watched a delicate around it, as simply as you please. So champagne glass, perched on a 20" x 20" speaker enclosure which was pouring forth the throbbing simple that it is just about impossible pedal notes of an to believe that nobody in all these organ. There was no cabinet resonance. No motion in the glass or its contents. years has done it before. His speaker Hearing is believing! The R-J Speaker Enclosure, now ready for the home cabinet is tiny, just after long experi- about big enough ments and development, establishes an entirely new trend in audio design. Listen to an R -J for a speaker to fit inside, with inches hear thrilling to spare. But it gives bass perform- performance from your favorite loudspeaker in a cabinet only slightly larger ance that challenges the huge monsters than the speaker itself! Listen ... and hear the R -J outperform ordinary enclosures more than we've always thought were de rigeur. three times its size! How? R -J Enclosures are simple, flexible, low in price, and Nature, it seems, does know how to SINGLE SHELF MODEL particularly adaptable for rooms where space must produce bass without taking up space. No. S -B.0 (For 8" Speaker) be conserved. Up to now, cumbersome, costly speaker The bullfrog, for instance. Adapt the 11" high x 10" deep x 231/x" long. enclosures have been tolerated because their size was bullfrog's ingenious bass- making sys- considered essential for good bass performance. This tem to the phonograph loudspeaker inconvenience has now been effectively overcome by and you have the essence of the new the R -J construction principle. "EJ" speaker enclosure, so named tentatively after its promoters, Frank Yk1: The R -J Enclosure will permit the user to realize the full potential of any speaker with which it is used. Robbins and William Joseph. The t . ;;1 Even "RJ" principle is simple, flexible, i' the most inexpensive speakers will perform adaptable to myriads of special cir- at their best in an R -J housing. With a well- designed cumstances. speaker, an R -J Enclosure offers clean, smooth bass SATURDAY REVIEW fundamentals, without peaks, down to low organ pedal notes. Distorting cabinet resonances are com- pletely eliminated. This was the reason for th: crowds DOUBLE SHELF MODEL No. S.12.0 For 12" Speaker) around the Audio Fair exhibit marked Remarkable though its bass performance may be, the 24" high a 21" long x R "R-J Company." Mews. R. and J. -J Enclosure is equally impressive in its unob- lo" deep. structed direct radiation of high frequencies. It has (Frank Robbins and William Joseph) excellent acoustic loading for best reproduction of had made a valiant effort to find a transients. middle ground. Their cabinets (lit- R -J Speaker Enclosures also offer the advantages of erally "under wraps" - burlap) were unusual versatility. cubic and only a Fine furniture construction is few inches bigger used throughout, suited in appearance to than the speakers they housed. any room A decor. Enclosures are expertly built of heavy, top - speaker in an R-f cabinet is sealed quality genuine Mahogany or Korina veneers, in behind by a very small box but, to fine, long lasting hand rubbed dark or blonde finish. equalize this, it is front- loaded, too. Special models are available with surfaces sanded The frontal air chamber is closed but unfinished, to fit the shelves of bookcases and of but for n slot through which the other similar installations. Cut -outs fit all standard sound comes. A slot distributes sound loudspeakers, which may be installed quickly and rat her well. Organ -pipe makers found simply without special tools. this out long ago. Here is a brand new concept in speaker cabinet de- THE ATLANTIC sign ...with many, many possibilities for your own home system -now or in the future. It is a treat for Future development of the R -J is well worth your imagination and a treat for your budget. watching carefully as is the entire trend toward disproving the axiom that good See and hear an R -J Enclosure at your favorite bass reproduction requires giant sire. sound department. For literature and further information use the handy coupon. HIGH -FIDELITY FLOOR MODEL r --- No. F-1 5-M (For 15" Speaker, Manogany) R-1 Audio Products, Inc., Dept. AR No. F-15-B (For 15" Speaker, Blonde) 6 . 164 Duane Street New York 13, N. Y. Mahogany. No. F-12-M (For 12" Speaker, Mahogany) Special 15" model available to on No. F-12-8 (For 12" Speaker, Blonde) Gentlemen: accommodate RCA LCIA Loud Speaker. Slightly higher West Coast 20" high x long X 20' 16" deep,plus legs. Please send literature and information on R -J Speaker Enclosures. Name I R-J AUDIO PRODUCTS N C. Address 164 Duane Street New York 13, N. Y. www.americanradiohistory.com City Zone hate ing phase shift for the moment) if the levels at points X and Z are equal, or it may have a finite value if R, and R, are proportioned so that the output of the phase splitter at X is somewhat less than at Z. At frequencies below the point where Filter A takes effect, output of the circuit begins to rise, since opposition signal fed to the grid of V, decreases. The result is obviously a low -pass filter action. The filter is easily controllable. If the switch arm is moved to position 2, the bias on V, is increased and that on V, is de- creased. V. then transmits less opposition signal. Effectively the frequency at which the response of Filter A begins to rise be- comes higher, and the frequency at which the total output begins to drop off does likewise, though the slope does not alter. When the switch is on position 4 (and if this biases V, to or near cutoff), there is no opposition signal at all through V, and response becomes flat. In practice, this would not be likely since it would be un- desirable to operate a tube on the nonlinear portion of its characteristic near cutoff, but the drop point could be shifted to a very high frequency. The explanation given is a simplified one in the sense that it does not take phase shift into account. Actually, there can be no null because of the unequal phase shifts in the two filters. The maximum attenua- tion of the highest frequency must be lim- ited in the adjustments to a degree gov- erned by the phase shift in whatever net- works constitute the two filters. From the above it is obvious how a con- From the highest rote... to the lowest... trollable high -pass filter could be designed. Note that the controlling element is pure d.c. and the leads to the switch could be brought out to a remote control point with- out any effect on performance. Band-pass, band -rejection, and single- frequency filters are also possible. Suppose It's the Sound Tape a filter were desired to reject 5,000 cycles. Filter A might then consist of a high -pass filter with a 5,000-cps cutoff, and Filter B of a low -pass filter with cutoff at the same frequency. The circuit would probably be of Symphony Quality! adjusted so that phase -splitter outputs at X and Z were equal. In addition, a phase - correcting element would be added to each WHEREVER FLAWLESS SOUND repro- filter (for instance, a capacitor across R, in duction is required ... in recordings by Fig. 1) so that at 5,000 cps the two filter concert artists, output signals would have the same phase top -notch orchestras, shift. Then at 5,000 cps the signals on the broadcasters ... "Scotch" Sound Re- grids of V, and V, would be equal and ex- cording Tape is chosen for the job. actly 180 deg. out of phase and the output Recording engineers use more of this would show zero signal, though at sur- all brands combined. rounding frequencies the signal output tape than other would rise again. They recognize many points of su- If the bias switch were then moved from periority in "Scotch" Sound Recording position 1, the outputs of the two triodes Tape: Lower noise level than any other would no longer be equal at the plates and tape Greater output sensitivity than any a signal would appear at the "resonant" frequency. And, of course, sufficient bias TECHNICAL ASSISTANCE on every phase of other tape Better reel-to -reel uniformity inequality would remove the 5,000 -cycle re- commercial sound recording is yours for the than any other tape Erases cleaner than jection completely, making the bias switch asking from your local 3M Service Repre- any other tape No curling or cupping actually a Q control. sentative. Call him today or write us: - ... always lies flat on head Lubricated for A band -rejection filter is obtained by AE -62, Minnesota Mining & Mfg. Co., using a high -pass filter at A or B and a St. Paul 6, Minnesota. longer tape life. low -pass filter at the other, and overlap- The term "SCOTCH" and the plaid ping the cutoffs. The cutoff frequency of design are registered trademarks for the high -pass filter is made equal to the Sound Recording Tape made in low frequency of the rejection band, and U.S.A. by MINNESOTA MINING the cutoff of the low -pass filter is made & MFG. CO., St. Paul 6, Minn. - equal to the high rejection -band frequency. also makers of "Scotch" Brand The method of looking at the band- rejec- Pressure- sensitive Tapes, "Under - seal" Rubberized Coating, "Scotch - tion action is illustrated by the graph of lite" Reflective Sheeting, "Safety - Fig. 2, in which the two directions from Walk" Non -slip Surfacing, "3M" the baseline are the two phases, 180 deg. Abrasives, "3M" Adhesives. General apart, of the outputs of V. and V,. Since Export: 270 Park Ave., New York 17, the curve represents a.c., zero output is N. Y. In Canada: represented by the baseline, while any- London, Ont., Can. thing above or below it represents output [Continued on page 44]

4 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com To You, Belden 's Golden Be Sure of Your Installations ... Use Anniversary Means - product performance :hat can come only from a "know - *Wide -Aired how " that has grown through actual service since the lamodo inception of Radio. -an ability to co-oper- MULTIPLE CONDUCTOR ate in pioneering new wires tomeetorantici- grow- You know what you are doing when you use pate industry's CABLE Belden Multiple Conductor Cables - they're ap- ing needs. titude rated. They are designed to provide desir- In the years that able electrical characteristics, and rigid control assures constant quality. follow Specify Belden Radio Wires. This Belden Program Is- Belden Manufacturing Co., 4689 -R W. Van Buren St., Chicago 44, III. - TO BE CONTINUED

b APTITUDE RATING No. 8453

Stranding . . . 41x34

Insulation Thickness 1 64"

lacket Thickness 1 32'1 O Nominal Diameter .265" APTITUDE RATING No. 8452

O Stranding . 41x34

APTITUDE RATING No. 1454 Insulation Thickness 1 64" APTITUDE RATING No. 8427 Jacket Thickness 1: 32" Stranding . . . . 41x34 Stranding . 26x34 Nominal Diameter .245" Insulation Thickness 1 64" Insulotion Thickness .020" Underwriters' Type SV locket Thickness .040" locket Thickness 1 '32" Nominal Diameter .370" Nominal Diameter .2ái" APTITUD_ RATING No. Interconnecting power ca- 8888

Stranding . . . . 41 x34 APTITUDE WING Po. 8455 ble for all electronic uses. Insolation Thidcness 1:32" Strandng . . . . 16x34 41x34 Nominal )iameer.114x.231"

Insulation Meknes! 1,64" APTITUDE RATING No. 8425 Unaerwr teri Type POT -64

Jacket Thickness 1 32" Standing . :6x34

Nomind Diemen - .280" Irsulation Thickness .020" o lacket Thickness .040" APTITUDE RATING No. 1424 Nominal Diameter .335" Stranding . . 26x34 Irterconnecting power ca- o Insola ion Thickness .020" APTITUDE RATING No. 8426 ble for all electronic uses. Jacket Thickness .040" Stranding . 26x34 Nominal Diameter .305" Insulation Thickness .020" Also used as microphone Jacket Thickness .040" roble. Nominal Diameter .355" Interconnecting power ca- b e for all electronic mes.

The Vaae T.eaTed LIN

AUDIO ENGINEERING BUNE, 1952

www.americanradiohistory.com here's absolute reliability LETTERS Positive Feedback

Sir: for heavy duty With regard to Ulric Childs' article which appeared along with mine in the May issue, I do not believe it is surprising that he found oscillation taking place at around 200 cps. The theory of the circuit predicts that, with increasing positive feedback, audio.. oscillation will occur near that frequency at which the perfortn- once of the amplifier as a negative resistance most nearly equals that of the speaker as a positive resistance. Speakers differ in performance and so do amplifiers. Two speakers might be iden- amplification tical in every respect except the magent size. If one had four times the gap flux density of the other (commercially available speakers vary as much as that), it would have sixteen times the motional impedance at 200 cps. So it is misleading to speak of motional impedances in general as falling to a specified value at a given frequency range. Speakers vary in many other respects. Not the least impor- tant of these is the manner of mounting. With different com- binations of amplifiers, speakers, and baffling, I have encoun- tered initial oscillation at various frequencies all over the audio band. But with any speaker the motional impedance must fall to zero at zero frequency. Then it follows that from zero fre- quency to some specific frequency short of speaker resonance, say to 20 cps, the motional impedance remains less than it is anywhere in the rest of the audible range. If Childs found zero net- circuit -resistance at 200 cps rather than at (say) 20 cps, it was simply because the performance of his amplifier is impaired at 20 cps; it cannot be any other way. (Zero net- circuit -resist- Ulf ance is a necessary, though not sufficient condition for oscilla- tion. Practical circuits involve the use of an amplifier with finite overload point so relaxation oscillations will generally occur, even if sine -wave oscillation does not.) My own findings were that a really wide -range amplifier with a Stephens P -52Lx will oscillate at around 5 cps. On the basis of Child' earlier article there was good reason to suspect that he was running into oscillation at some extreme frequency. Readers will recall that he implied that he had made measurements showing that the point of oscillation coincided with the setting for perfect damping. This could not have been the case if oscillation took place within the audible range, since whenever a speaker makes a loud noise an appreciable motional resistance must be present to absorb the necessary electrical power. So either Childs was meeting oscillation at frequency extremes or his measurements were in error. The latter now turns out to be the case. Not that the error need have been large. In general, the poorer the speaker and the heavier the air loading, the smaller If you've been looking for an audio output tube that's stable the motional impedance (at a given frequency and in propor- under the most severe conditions -completely dependable tion to voice -coil resistance): And the smaller the motional im- - of perfect damp- then this is it! The Tung -Sol 5881 is rugged both mechanically and pedance, the less the spread between the point ing and the point of oscillation. electrically -and directly interchangeable with the 6L6. One final note: there is no mystery as to where the power In creating the 5881, Tung -Sol engineers have made lavish use goes that is delivered to the motional impedance. It is dissi- of the design and production techniques which have proved them- pated in the resistive component; reactances do not consume the selves over the past fifteen years- zirconium coating over the power. Given the value of this component, one can compute with the aid of the equations given in my plate and pure barium getter to effectively mechanical power carbonized metal article. plated wire to minimize absorb gas for the life of the tube -gold However, the shunt reactive component cannot be overlooked. grid emission. These are but a few of the major design improve- The latter will cause additional power to be consumed in plate ments in the 5881. dissipation and in the voice coil. If this circuit is to be used at a setting for near -perfect damping of an otherwise poorly- Tung -Sol produces the 5881 under laboratory conditions, to damped speaker setup, the output stage should be capable of de- assure peak efficiency and maximum uniformity. You'll find this livering considerable reserve power. Even the magic of feedback tube has the stuff to take the whole range of audio service require- will not provide something for nothing. Warner Clements, ments from protracted standby periods to repeated heavy over- Box 969, loads. So, if absolute reliability is essential in your audio circuits, Sherman Oaks, Calif. the Tung -Sol 5881 is a "must." Order it from your regular supplier.

Write for characteristics and performance data The April Cover Sir: TUNG -SOL ELECTRIC J. INC., NEWARK 4, N. Congratulations on 2E's new menu! The dietary deficiencies Sales Offices: Atlanta Chicago Culver City (Calif.) of those of us suffering from electronic malnutrition are rapidly Dallas Denver Detroit Newark being remedied. The dish on the April cover looks especially ap- petizing -low skirt selectivity, no distortion, well balanced out- Tang -Sol makes All -Glass Sealed Beam Lamps, Miniature put, and an excellent set of characteristic curves... . Lamps, Signal Flashers, Pietere Tubes, Radio, TV and Jerome S. Miller, Special Purpose Electron Tubes. 1338 Washtenaw, Ann Arbor, Mich.

6 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com N i CRAFT FOR 3 SOUND REASONS!

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2. Soundcraft 5 Drawer Tape -Chest' You get a permanent filing cabinet for your reels, absolutely free, with the purchase of 5 reels of Soundcraft Tape. The remarkable pyroxylin-coated Soundcraft Tape -Chest, constructed of durable lined boxboard, stores either 5 or 7 inch reels horizontally, each in an individual drawer. Patent applied for.

3. Soundcraft Magna -Stripe''` The most revolutionary development in movie -making since "talkies "! Soundcraft Magna- Stripe is a new method of permanently bonding a magnetic sound track to motion picture film. Now you can make sound movies as quickly, easily and inexpensively as silent films. You can also add Magna -Stripe sound tracks to old silent films, giving them new life as "talkies ". You can erase, and change a Magna -Stripe sound track as easily as you change your mind! **Trademark Reeves Soundcraft. REEVES SOUNDCRAFTCORP. 10 East 52nd Street, New York 22, N. Y.

THE ONLY RECORDING MATERIALS PERFECTED AND MANUFACTURED BY RECORDING SPECIALISTS Please write for additional information.

AUDIO ENGINEERING JUNE, 1952 7

www.americanradiohistory.com Sir: The lovely lady on your April cover haunts me. Herewith permit me to nomi- where the world's toughest transformers nate her as "Miss Audio of 1952." She is so sound of limb and body as to induce heart are a `must" for the toughest installations... murmurs in the male. Irresistibly that old cliche comes to mind : "we could make such beautiful music together." A salute there- fore, to Miss Audio, may she set the tone of the 1952 Audio Fair : "Lovely to look at, thrilling to hear." John Luchaka, 1115 Belmont St., Pittsburgh 21, Pa. you'll find (Even if we did have her phone number, the wouldn't publish such highly classified tfi infnr nut tion. En.) Tape Recording Information Sir: REL specifies and uses CHICAGO I am in the sound recording business, You'll find CHICAGO "Sealed -in- Steel" building my own electronic equipment as- transformers used throughout REL's sociated with disc and tape recording and FM Transmitting and Relay Equipment. reproducing. To further my knowledge, I Absolute dependability is a prime would like to get in touch with someone requirement in all REL equipment -and who is similarly engaged and who would of 7 % -in. tape CHICAGO transformers contribute signif- like to correspond by means icantly to quality, superior performance for economy, either single or dual track, on and long time stability. subjects relating to recording, wide -range disc reproducing equipment, music -both classical and popular -and items of interest in respect to our countries. As I use all European equipment and accessories, such as Pyral, E.M.I., Simon, Decca, etc., much information could be exchanged on perfor- mance relative to American and European equipment. J. I. Tidswell, P. 0. Box 194, Dannevirke, New Zealand. "Let George Do It" Sir: Just a note of sincere appreciation for the editorial in the April issue. I am sure that many FM operators got the same COLLINS specifies CHICAGO "lift" that I did when they read "Let This COLLINS MHF Single Frequency George Do It." The item was an extremely Communications Receiver utilizes accurate blow at one of the obstacles FM CHICAGO "Sealed-in-Steel" transformers has had to overcome since the beginning- for trouble-free, continuous service under public apathy. FMers do not expect sym- the most rugged operating conditions. pathy. pity, or unearned praise. but we would really be most grateful for some sign that listeners do notice our efforts. Indeed where the going's tough- specify CHICAGO "Sealed -in- Steel" I, for one, would be overjoyed to get even a scathing letter from a listener. It would NEW EQUIPMENT TRANSFORMERS show he was interested ! Lest the impres- sion be gained that there are no FM fans, CHICAGO "New Equipment" transformers I can show you stacks of mail we have is discon- (available in 3 mountings) feature one- gotten when a popular program strongest, tinued, but very little is received while it is piece drawn -steel cases -the H -TYPE on. People are funny. toughest, best- looking units you can buy. Hermetic sealing meets all Mark T. McKee, Jr. The one -piece seamless design, enclosing MIL -T -27 specs. Steel bow Radio Station WMLN, cover is deep-seal soldered Mich. an electronically perfect construction, pro- into case. Terminals hermeti- Mount Clemens, and cally sealed. Ceramic bush- (You arc so right. En.) vides the best possible electrostatic ings. Studmounted unit. magnetic shielding, with complete protec- tion against adverse atmospheric condi- tions. For every application: Power, Bias, S -TYPE 4mri0ljNiBKt Steel base cover fined with Filament, Filter Reactor, Audio, MIL-T- phenolic terminal board. 27, Stepdown -ask your electronic parts Convenient numbered solder lisp terminals. Flange- mounted R distributor for CHICAGO "Sealed -in- Steel" unit. transformers. POSITIONS OPEN and AVAILABLE C -TYPE be listed here at no ,Free "New Equipment" Catalog PERSONNEL may With 10" color -coded leads charge to industry or to members of Get the details on CHICAGO'S brought out through fibre insertion in this col- full New Equipment Line - board base cover. Lead ends the Society. For covering "Sealed -in- Steel" are stripped and tinned for umn, brief announcements should be in transformers for every modern easy soldering. Flange - En- circuit application. Write for mounted unit. the hands of the Secretary, Audio your Free copy of this valu- Society, P. O. Box 12, Old able catalog today, or get it gineering from your distributor. Chelsea Station, N. Y. 11, N. Y., before the fifth of the month preceding the date of issue. . Positions Open Positions Wanted * TRANSFORMER and electronics spe- CHICAGO TRANSFORMER cialty firm in upstate New York anxious hire engineer experienced in the de- DIVISION OF ESSEX WIRE CORPORATION to sign of transformers for communication 3501 ADDISON STREET CHICAGO 18, ILLINOIS equipment. Salary $5000 -6500. Box 601, AUDIO ENGINEERING.

8 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com Ci MALL pY NM pECpREr 14,OLYD HIGH Q TOROIDS for use in pWDR now available) Loading Coils, Filters, Broadband data Networks technical Carrier Systems and - (New for frequencies up to 200 K C

For high Q in a small volume, characterized by low eddy current COMPLETE LINE OF CORES and hysteresis losses, ARNOLD Moly Permalloy Powder Toroidal TO MEET YOUR NEEDS Cores are commercially available to meet high standards of physical and electrical requirements. They provide constant permeability * Furnished in four standard permeabilities -125, 60, 26 over a wide range of flux density. The 125 Mu cores are recom- and 14. mended for use up to 13 kc, 60Muat 10 to50 kc, 26Mu at 30 to 75 kc, and 14 Mu at 50 to 200 kc. Many of these cores may be furnished * Available in a wide range of stabilized to provide constant permeability (±0.1 %) over a specific sizes to obtain nominal in- temperature range. ductances as high as 281 Mcnufactured under license arrargements with Nest -rn Electric Company mh, 1000 turns. r n 4127 * These toroidal cores are given various types of enamel and ARNOLD ENGINEERING COMPANY varnish finishes, some of which permit winding with SUBSIDIARY OF ALLEGHENY LUDLUM STEEL CORPORATION heavy Formex insulated wire without supplementary insu- General Office & Plant: Marengo, Illinois lation over the core.

AUDIO ENGINEERING JUNE, 1952 9

www.americanradiohistory.com the new NEW LITERATURE RCA Victor Division of Radio Corpora- tion of America, Camden 2, N. J. has is- sued a new brochure entitled "The Elec- tron Microscope at Work in Industry." sub -miniature Published to mark the tenth anniversary of the RCA electron microscope the at- tractive two -color booklet is profusely il- lustrated, and describes in detail ten case histories in which the device aided in solving problems of quality control, prod- uct improvement, and new -product devel- opment. Copy may be obtained by writing for Form 2R8195. Address Scientific In- CANNON strument Section. Riverside Manufacturing and Electrical Supply Company, Dearborn, Mich. is of- fering free a brochure describing and pic- PLUGS turing various products the company has designed and produced. Included are wir- ing harnesses and assemblies, cord sets, waterproofed toggle switches, sealed re- lay enclosures, and molded plastic and rubber components. Also described are the company's engineering service and its production facilities. Heithley Instruments, Dept. 205, 3868 Carnegie Ave., Cleveland 15, Ohio, is re- leasing a new eight -page bulletin which describes and illustrates the company's tiny but rugged entire line of vacuum -tube electrometers. Included are instruments designed for a wide range of unique measurement func- Series D Cannon Plugs satisfy a long felt capacity 5 amps.r`High dielectric insulators. tions. Application diagrams cover 17 basic uses. need of the Electronics Industry for a sturdy, Minimum flashover, volts rms. e 1000 The pro- Radio Club of America, Inc., 11 W. 42nd St. New York 17, N. T. is making avail- versatile and extremely compact connector tective steel shells provide an integral mount- able to interested persons an 88 -page booklet titled "Survey of Radio -Fre- for use on miniaturized equipment of all ing flange. The "keystone" shape of the quency Transmission Lines and Wave Guides," by E. S. Winlund. Issued as Vol. 28, No. 2, of the Club's Proceedings, it kinds. These may be mounted as (1) rack shells gives positive polarization with friction contains an excellent historical survey as well as technical data gathered from ar- and panel (2) box (3) wall, or (4) cord con type engagement. ticles published between 1919 and 1936. Included also is an extensive bibliogra- nectars. Junction shells with integral clamps phy. Requests for copy must Include re- mittance of $1.50. protect the terminal ends of the connector Insulation Manufacturers Corporation, 565 Washington Blvd., Chicago 6, Ill. pre- when used as cord or wall mounted units. sents practical information on character- istics, uses, and technical data for Per - Contacts are of the quality you expect to macel pressure- sensitive electrical tapes in a new 16 -page illustrated catalog. Also included is a special section on non -elec- find in any Cannon Plug. Machined from trical tapes. This book should be in the hands of everyone whose occupation calls copper base alloy, gold plated, they accom for use of pressure -sensitive tape, both electrical and non -electrical. Requests modate #20 or #22 AWG stranded wire. Rated should be addressed in care of Publica- Cannon DA Plug and receptacle tions Department. with junction shells. RCA Tube Department, Harrison, N. J. is circulating through distributors a novel, flip -type index designed to place basic performance and mounting informa- tion on RCA replacement radio and TV speakers at the fingertips of dealers and o servicemen. Less than six inches square, o the index provides data necessary for the selection and installation of 22 different types of RCA speakers. Included are such performance data as voice -coil impedance, _ . N- power -handling capacity and resonant 4 frequency, as well as dimensional draw- CO. w ings and me photographs. u W. H. Brady Company, Dept. 86, 1630 E. Spring St., Chippewa Falls, Wis., man- ufacturers of pressure -sensitive tapes and a RADIO markers, is distributing a new 50 -page ó m COMPONENT products catalog illustrating and describ- ing such items as wire markers, pipe _ OR markers, safety signs, reflective signs, N eo INSTRUMENT masks and stencils, and printed roll tape. Nw _ Prices are also included. Production en- _ gineers will find many ways of saving CANNON DA The 4é -scale drawing above shows money wherever labeling is required. 15 contacts the new DA with 15 r22 contacts Copy will be mailed to interested parties and junction shell compared with on request. o AN plug and receptacle having 14 Hato Engineering Co., Mankato, Minn., r16 contacts and cable clamp. Sav- will mail without charge a four -page ing of space outside the supporting CANNON DB folder describing the company's complete unit is 11h'. The saving inside -power ma- 25 contacts is line of a.c. generators, ringing 5/32'. A side view of the DA would chines, motor -generators, rotary convert- make the comparison even more ers, and gas -engine power plants. Kato startling. equipment covers a wide range of power L' and frequency requirements -as a result in of For further information and per. this folder should be the hands CANNON DD every Industrial engineer. CANNON DC formance data request Bulletin D-1. ' 50 contacts 37 contacts Littelfuse, Inc., 1865 Miner St., Des Plaines, Ill., is now issuing the first com- pletely- illustrated price listing of fuses for various electronic applications. The four -page sheet includes full -size draw- SINCE 1915. Factories in Los Angeles, Toronto, New Haven, ings of 25 fuse types, together with blow- CANNON ing characteristics. By matching a blown Benton Harbor. Representatives in principal cities. Address fuse with the corresponding picture on inquiries to Cannon Electric Company, Dept. F -109, P.O. the chart, the jobber salesman or service- man can determine with accuracy the ELECTRIC Box 75, Lincoln Heights Station, Los Angeles 31, California. correct replacement.

10 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com 11 to GATES offers three speech input consoles that fully meet every TV need. Whether you select the ultra complete SA -50 dual channel equipment, the much used SA -40 single channel console, or the very popular 52 -CS studioette, you can be certain of top quality through advanced GATES engineering. For 30 years now, GATES has been build- ing fine speech equipment - and for TV there is nothing finer!

GATES SA-50 Dual Channel Console .. .

Nine mixing channels, dual program amplifiers, dual V. U. meters, 10 watt monitoring amplifier, self -con- tained cueing amplifier, five preamplifiers with room for two more where required. Complete remote, over- ride, cueing and talk back facilities. Extremely low cross talk combined with high gain. Deluxe equipment all the way!

GATES 52 -CS Studlioette .. .

All GATES consoles have the same top quality com- ponents. This popular, modestly priced console is made possible by combining functions through key control. Many TV stations will prefer to use several 52 -CS Studioettes instead of a single larger console. Every progressive TV engineer will find it worth while to investigate the 52 -CS Studioettel

GATES SA -40 Single Channel Console .. .

Perhaps the most used speech input console in TV and radio today. Nine mixing channels, wide circuit selec- tion, low cross talk, high gain and extreme ease of servicing are but a few of the SA -40's many features. GATES will gladly send detailed circuit data on request.

r *I' - " °' GATES Speech Input Catalog -Tours for * the asking is a 44 -page catalog on GATES speech equipment plus a new 12 -page brochure on remote control apparatus. No 1'* * obligation, of course. Why not write now.?

GATES RADIO COMPANY SALES OFFICES manufacturing engineers since 1922 2700 Polk Avenue, Houston, Texas Warner Building, Washington D. C. International Division, 13 E. 40th St., New York City Canadian Marconi Company, Montreal, Ouebec QUINCY, ILLINOIS, U. S. A.

AUDIO ENGINEERING JUNE, 1952 11

www.americanradiohistory.com EDITOR'S REPORT

PICKUP POWER verted Musician's Amplifier is noticeably superior to the original design. We were quite skeptical; it must be Auto PEOPLE are wont to congregate and talk -end- admitted, when we first read the original article about lessly it seems. But what audio man wouldn't talk the U -L amplifier, because it appeared that the meas- as long as he could find anyone to listen to him? urements were not indicative of too great an improve- And with so much talk, new ideas and interesting con- ment in distortion, and resulted in a relatively small troversies are sure to be brought to light occasionally. increase in power output. Furthermore, the U -L's de- Such a point arose during a conversation with a signers claimed that it "sounded better " purely sub- reader recently during some otherwise aimless surmis- -a jective reaction. Anyhow, we tried it out and it does ings about phonograph pickups. The question there -is sound better. any difference in the loading on a pickup when the grid The conversion to the Jusician's Amplifier was sug- resistor of the first tube is of a low value, as compared to gested to us by Bill Shrader, who has been writing for the same pickup working into an unloaded grid? lE lately, and the change was so simple that it can be The subject developed following a discussion of the made in less than ten minutes you get the ampli- usual practice of shipping microammeters with a short- -after fier on the workbench. Mel Sprinkle and Dave Sarser, ing wire across the terminals to damp the movement so the designers of the Musician's Amplifier, have favored that the shaking up that such an instrument gets during us with an article on this conversion-which will appear handling and shipping will not injure the moving coil next month. However, we haven't given up on the cross - and its bearings. Since a certain amount of power is coupled phase splitter -but we are beginning to wonder consumed by the low- resistance shunt, and since this if this device is as good as claimed. power is generated by the movement of the coil in the meter, it follows that the moving coil is damped criti- TAPE EXCHANGE cally, or even more ; consequently, the pointer is not bounced around as much as it would be without the \Ve are asked from time to time if we know of anyone shunt. who has good tape recordings to exchange, and while Getting back to audio, it was suggested that perhaps we know a number of people who have good tape ma- a pickup would perform better without the loading re- chines, we haven't found most of them willing to part sistor, although the resistor serves another purpose pri- with their most prized recordings. And on this same marily. Considering a hypothetical pickup which gen- subject, we are amazed at just how many private indi- erates a signal of, let us say, fifty millivolts -about the viduals- people who have no professional use for the maximum of any current magnetic pickups-and deliv- highest quality of equipment -have two, repeat two, ers this signal to a 27,000 -ohm grid resistor, we find studio model tape recording consoles. There is a definite that the power consumed in the resistor is slightly less advantage to having two machines -such as the ability than one -tenth of a microwatt. Next we began to get to make dubbings, or to record a long program without curious about the amount of power consumed by moving breaks, or any such tricks as one might care to dream the stylus against its spring, and it was suggested that up, but we can't help but be surprised at how many of while some power is required to accelerate the stylus our acquaintances have two such machines. from rest to its maximum velocity in one direction, Oh well. some people go in for trout fishing. part of this power is returned to the record by the spring. Then we got bogged down in mathematics and CATALOG DELAYS the subject. Any solutions will he appreciated. changed Several readers will undoubtedly be disappointed in not receiving the Recording and Phono Equipment ULTRA -LINEAR AMPLIFIER booklet which was offered in the May issue and again Some months ago, we mentioned trying out our own this month. When the requests come from ?E's sub- version of this circuit which is rapidly growing in pop- scribers, it is not difficult to look up their addresses in ularity. We must admit that our patience gave out be- case they write on a postcard and omit this important fore we succeeded in making the cross -coupled phase information -but when newsstand buyers request the splitter work to our satisfaction. However, we did con- booklets, we have no crystal ball to give us the mailing vert our Musician's Amplifier, and we are "hobbling" address. If you experience too much delay in receiving along on that for the present. your copy, please write again-and please include your "Hobbling" is actually a misnomer -since the con- address in full.

12 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com T H E B ' S . .. B E E T H O V E N . B RA H M S A N D L,/

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es. Bach, Beethoven and Brahms are now better than ever -we don't mean we've improved their music. hut we do mean we've improved the reproduction of their recorded music.

It's the new. improved Pickering Cartridges that give credence to this claim. Yes. Pickering Cartridges are different. They're improved. They're better than ever. Pickering patented Cart- ridges with Dynamic Coupling* are superior in every way. by providing ..

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constant stylus contact with the record grooves over the entire audio dynamic coupling spectrum (10- a0.0oo cps) assures { full frequency response full transient response NO RESONANCES NO MISTRACKING NO GRINDING OF GROOVE WALLS

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AUDIO ENGINEERING BUNE, 1952 13

www.americanradiohistory.com THE TRANSISTOR

A picture report of progress

FIRST TRANSISTORS were of this point con- NEW JUNCTION TRANSISTORS, still experi- MUCH HAD TO BE LEARNED, especially tact type (picture three times life size). mental. also use germanium but have no about the surface of germanium and the Current is amplified as it flows between point contacts. Current is amplified as effect of one part in a million of alloy- wires through a wafer of germanium it flows through germanium "sandwich" ing materials. Transistors promise many metal. These transistors are now being -an electron -poor layer of the metal be- uses -as amplifiers, oscillators, modula- made at the Allentown plant of Western tween two electron -rich ends. This new tors ... for Local and Long Distance Electric, manufacturing unit of the Bell transistor runs on as little as one -millionth switching ... to count electrical pulses. System. They will be used in a new selec- of the power of small vacuum tubes. tor which finds the best routes for calls in Long Distance dialing.

tiny amplifying device first announced by Bell Telephone Laboratories in 1948 is about to appear as a versatile element in telephony.

Each step in the work on the transistor ... from original theory to initial production technique ... has been carried on within the Laboratories. ASSEMBLY PROBLEMS, such as fixing hair- MOIST PAPER AND COIN generate enough Thus, Bell scientists thin wires to barely visible germanium current to drive audio oscillator using demonstrate again how their wafers, have been solved through new junction transistors. Half as big as a skills in many fields, from tools and mechanized techniques. Fin- penny matchbox, an experimental two - ished transistors withstand great vibra- stage transistor amplifier does the work theoretical physics to production tion and shock. Engineers see many of miniature -tube amplifiers ten times engineering, help improve opportunities for these rugged devices in larger. telephone service. national defense. BELL TELEPHONE LABORATORIES

Improving telephone service for America provides careers for creative men in scientific and technical fields.

www.americanradiohistory.com Planning and Building a Radio Studio

EUGENE F. CORIELL Major. USAF

Part- 1. Beginning a series devoted to the design and construction of a broadcast or record- ing studio following every step from preliminary planning to the first program on the air.

SOUK F:H OH LATER, many broadca.t en- gineers find themselves faced with the design and construction of radio studio facilities. It is the purpose of this series of articles to consider some of the problems involved in the planning and the construction of a studio instal- lation. The points to be covered include: physical layout; acoustics; heating, ven- Control Room "B," Armed Forces Infor- tilating and air -conditioning; construc- mation School, Fort tion problems; cooperation with the Slocum. New Rochelle. architect and the contractor; electric N. Y. power considerations; audio block dia- (Official Defense gram; jack -field and rack layout; con- Department Photo) duit diagrams, running sheets and inter- connection sheets; and tests of the com- pleted installation. These articles do not cover transmitter facilities, and they be- gin at the point where the location of the studio building has been selected. The writer wishes to acknowledge the pioneer writings in this field by Mr. John D. Colvin of Commercial Radio - experienced in the studio field. His years who will draw the building plans. Some- Sound Corporation, whose articles in in this highly specialized area are good times, the chief engineer is given this AUDIO ENGINEERING for July, August insurance against the costly mistakes of programming information in final form and September, 1947, were the basis for inexperienced design in structural mat- and in good time, and sometimes, alas, several studios built by the author and ters and physical layout. Equally im- he isn't. What started out as an an- are also the basis for much of the audio portant, the architect is familiar with nouncer- and -separate- engineer arrange- layout portion of the present series. local building codes, safety require- ment with anonuncer- operated turnta- It should be emphasized that the saf- ments, and the laws governing public bles in the studio is sometimes switched est-and in the long run the cheapest - buildings. Such provisions almost al- in the middle of construction to a one - plan is to work closely with an architect ways apply to studio structures. How- man combination arrangement -with a ever, since the architect is apt to rely on recording rack and two cutting tables *Technical Officer (Radio), Armed the station engineer as his contact with squeezed into the control room for good Forces Information School, Fort Slocum, writer feels the measure. Changes like this give the en- y. Y. the management, the data herein on layout and construction gineer ulcers, run the costs far above should be helpful to the audio man on the estimates, and often result in an un- his first station -building job. satisfactory layout. The number, size, layout, and equip- Assuming that the programming poli- ment of studios and supporting facilities cies are set, the first thing to do in plan- ought to depend on the programming ning the physical plant is to list the policies of the station. Is the station to various spaces required, including offices ride the net most of the time? Or will and other non-technical areas. These the station be a big network -owned or latter are not, strictly speaking, the comparable independent operation? If chief engineer's responsibility. However, the former, there probably will be the as probably the only technical man of barest minimum facilities for local live the station management, group, he will originations. If the latter, the pro- almost certainly find himself the follow - gramming will require facilities for full - up man -if not the layout man -for the scale musical and dramatic presenta- entire construction project. tions, involving big studios, critical The spaces involved include some or acoustic and spatial considerations, mas- all of the following: Studios, control ter control, etc. rooms, master control, news room, audi- Somewhere between these extremes torium, sound effects storage, recording are the programming arrangements for room, disc and tape libraries, shop, sup- any station. The chief engineer should ply room, clients' booths, offices for Fig. 1. Layout of AM -FM studios of WDSU, final story on executives and for personnel of pro- New Orleans. Note that all audio spaces except have the complete and Studio C are completely surrounded by corri- programming policies early enough to gram, sales, continuity, traffic, and book- dors, storeroom. etc., effectively isolating these permit him to work up layout sketches keeping departments ; wash rooms and spaces from outdoor noise. and construction notes for the architect dressing rooms, employees' lounges,

AUDIO ENGINEERING JUNE, 1952- 15

www.americanradiohistory.com lobby, reception rooms, corridors, ma- of a standing cast or orchestra conduc- activity, occasional recordings can be chinery room for air -conditioners, blow- tor (on his podium). made on equipment sandwiched into the ers, pumps, etc.; transformer vault, and The control room is laid out to scale control room. This is especially true if miscellaneous storage ... and let's not on cross section paper and only after a only tape equipment is involved, since forget space for stairs, elevators, closets lot of cut and try does an apparently this can be mounted in a modest of and sound locks. area firm layout dimly rack Studio sizes appear through the panel space. Disc recorders require are determined by the erasure smudges. requirements of staff, talent, audience It's a good idea to test floor space, suction gear and often pri- and equipment. For musical studios, this layout by making up a rough three - vacy'where the harassed technician can there are also optimum values of room dimensionl scale model. This mock -up swear feelingly while performing the volume given in the literature.' As re- will give a good check on such points wizardry sometimes needed to get a gards studio proportions, avoidance of as ease of entrance and exit, adequate clean, quiet cut. On the other hand, if standing waves calls for an approxima- working space in front of the gear and recording is to be a major station activ- tion of certain ratios among height, maintenance access in back of it, con- ity, it deserves adequate space and con- width, and length. These ratios cannot venient placement of the equipment venient layout. Space is required for always be attained because of space where it can be reached or watched both tape and disc recorders, including limitations, and in the case of small with minimum arm- stretching and neck- spares. Utility playback turntables and speech studios are sometimes not im- twisting, and the amount of waste space tape editing gear must often be accom- portant. However, for music studios, involved. This last is quite important, modated, along with racks containing these ratios should be followed as closely especially in high -cost metropolitan amplifiers, radio tuners, limiters, distri- as possible. These points will be dis- areas, since rental varies as the square bution panels, and other control equip- and construction costs as the cube. cussed in greater detail in the section ment. There is sometimes need for a on acoustics. Most of the control room factors Control room size cannot always be above also apply to master control, along with some other Some- determined accurately until both the considerations. 2x4 STUDS equipment and its physical disposition times, this room is to be visible to visi- in the room are known. This is because tors as a show- piece, in which case a I" ROCKWOOL BLANKET there is always some waste space due to whole wall must be given over to a large door swing, stairs, observation windows, observation window. More often, the operating and maintenancq clearances, master control is simply one of the PLASTER etc.; and various layouts of identical studio control rooms with a switching BASEBOARD equipment involve consolette alongside the studio different amounts of board. 2K4 PLATE such waste space. The layout is deter- This is almost always the case with FLOOR mined by the number of studios served, smaller stations and sometimes requires I2 I V2" PUNCHED window locations, sight lines into visual contact with all studios. In larger -.I h- CATTLE HAIR FELT studios and other adjacent rooms, pres- plants, master control sits in remote SIDE ELEVATION PLAN ence of producers and other dignity far from the bustle and tension non- techni- Fig. 3. Detail -stud wall. cal personnel such as guests; presence of studio control rooms. In addition to of staggered of turntables and their expected use as switching and monitoring facilities for desk and always need for storage space a transcription players, disc jockey facili- incoming and outgoing lines, master for tape and disc supplies. Since tape ties, or standby gear; number of racks, control sometimes contains standby turn- is sensitive to magnetic fields, tape stor- type of console, and possible use of a tables, spare amplifiers, and test instru- age facilities should not be located near floor raised above that of the studio. ments such as gain set, audio oscillator, 60 -cps power distribution transformers This last is prompted by a desire to etc. However, the presence of this gear or other sources of a.c. fields. The lay- bring the eyes of the seated control need not unduly complicate the room out should provide space for simultane- operator and producer level with those layout since most of these items are used ous operation of most or all the equip- infrequently as compared to the operat- ment, and adequate space for mainte- ' J. P. Maxfield, "Liweness in broadcast- ing equipment and can therefore be lo- nance access behind the racks. A com- ing," Western Electric Oscillator, January cated in spaces not adapted to the latter. mon arrangement is to place the record- 1947. If recording is not to be a major ers along opposite sides of a room longer

0.5 TYPE OF CONSTRUCTION WEIGHT IN LBS AVERAGE DB PER SQUARE FOOT ISOLATION FOR 0.4 128-4096 CYCLES 0.3

SINGLE 2.4 STUD, PLASTERED 0.2 ON BOTH SIDES 19 30 0.15

STAGGERED 2 4 STUD. ONE-INCH ROCKWOOL BLANKET FASTENED 0.1 LOOSELY TO INNER EDGES OF 009 o 0B ONE ROW OF STUDS, PLASTERED u 0.07 ON BOTH OUTER SIDES 23 50 006

°I 0 05 SINGLE SOLID 6- CINDER BLOCK .04 PLASTERED ON BOTH SIDES 45 48

0 03 DOUBLE SOLID 6' CINDER BLOCK. 6'AIR SPACE BETWEEN, 0.02 PLASTERED ON BOTH OUTER SIDES 90 60 0.015 SINGLE HOLLOW 4- CINDER BLOCK PLASTERED ON BOTH SIDES 30 39 0.01 001 0.015 002 0.03 0.05 007 01 0.15 02 03 04 05 o.

Fig. 2 I left I. Wall isolation (transmission loss) values. Data are based on high -grade workmanship in construction. Fig. 4 (right). Curve showing the values of -log.)1 -a) of the Eyring reverberation -time formula in terms of a.

16 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com than it is wide, with equipment racks and perhaps a supervisor's desk at one 200 end. The recording room should be free from building vibration. It should be ,00 IIIIIIIIIUI111111IS!I.I,, air -conditioned. dust -conditioned, and held to close limits of temperature and s0 -Nl /_:íß %ial/!Zí humidity. - %dull)) The maintenance shop should not be Piíi .00PríO ó %:Ao;; ;ems' .0001 Mill 2 _.._ short -changed as to size. Stations grow, p 1í/'11 _ and the bench space in some dark corner Fig. 7. Preferred stu- deemed adequate for a part -time repair dio dimensions and 2 .,!.ss,1i11111111111 i:::::1r 1r 1r 1. 6 man becomes painfully inadequate for ratios of width and ß:1i i::1N1 1.1 1...::: 10. .111111.11. 11 1111 emm..uu the full -timer plus spare parts, sound length. (Chart /MMEB1111i =IMMESH1.. 11111MMEO\91. - 2 3 9 6 769 2 3 9 6 2 3 9 6 69 courtesy RCA). 9 effects gear, and the usual miscellany Ow 10.0(0 moo° 1,000,000 VOLUME IN CUBIC FEET that generally finds its way there. By CNART the time you get around to enlarging TYPE ROOM H: w : L DESIGNATION the shop, any spare space that may once SMALL 1 : 1 . 2 5 : 1 . 6 E : D: C have been there will have been comman-

F : : B deered for the librarian-or for the AVERAGE SNAPS 1 : 1.60: 2.5 D ladies room was somehow that omitted : LOw CEILING 1 : 2.50: 3.2 G : C B from the original plans. Spaces not so far considered are LONG 1: 1.25:3.2 F: E : A mainly the non- technical areas -news rooms, offices, etc. These are not or- dinarily the responsibility of the chief traffic should be kept separate from that The point is that much of a station's engineer, as mentioned earlier, and are of outsiders such as salesmen, studio growth can be anticipated and can there- best left in the hands of the architect, audience guests, etc. The radio portion fore be considered in the evaluation of except perhaps for one point': In the of the AM -FM -TV studio facilities of the space before rental or purchase, and process of fitting together the various WDSU shown in Fig. I illustrates many allowed for in the original layout. It is and sometimes conflicting claims of the of these principles. also generally wise to allow for expan- programming, technical, and clerical de- In figuring floor space layouts, re- sion when installing the initial electrical, partments, something has to give. May member that dividing walls and air -con- heating and air -conditioning services. it not be the audio spaces. This pious ditioning column ducts have thickness. These should provide not only for ade- hope brings up some general layout For example, a double six -inch cinder quate future capacity but also for acces- principles. block wall containing a six -inch air sibility of taps. It's a lot easier to con- Put offices, corridors and other non - space and finished on both sides with nect future facilities to a duct turn -off audio spaces between the outside walls plaster and baseboards takes up the bet- brought out through the wall for the and the studios, to minimize transmis- ter part of two feet. In this connection, purpose and capped, than to tear into sion of outdoor noises. Keep doors of make sure your layouts show the archi- finished masonry and ductwork. It's true adjacent studios as far apart as possible tect the finished dimensions of the vari- that such initial planning for the future to reduce transmission between them. ous spaces, labelled as such. The writer is expensive, but it's a lot less so than For the same reason, studios opening on once nearly wound up with a control the cost of unforeseen and therefore un- opposite sides of a corridor should have room glass twelve inches out of position controlled growth. their doors staggered rather than di- because he failed to allow sufficiently for rectly opposed. differences between rough and finished Acoustics Visualize the traffic patterns of the dimensions, and between nominal and The acoustic phase of studio planning layout. It should be possible to go to actual measurements. can best be handled by a professional or from any studio or control room Plan for future expansion, and for the consultant in this field. Some acoustic without going through others, and with- direction it should take. This latter may materials firms offer design services, out brushing shoulders with those you take some doing, but you don't have to materials, and installation as a package meet on the way. Entrances to studios be a prophet to know that the separate deal. Any extra costs under this ar- and control rooms should be sound locks recording room in your future is going rangement can be considered as insur- whenever space permits. In so far as to go in the space you've allowed for ance against unsatisfactory acoustic possible, the principal flow of house it-that is, if you have allowed for it. [Continued on page 39]

SURFACE AREA 1024 CYCLES FT. COEFF. TOTAL 50. CALCULATED I I 1111111 1 111 ABSORPTION VALUES IDEAL RELATION BETWEEN SABINS REVERBERATION TIME B FREQUENCY r 2.0 (CURVE B "I 2.0

CEILING (MATERIAL'ALPHA" I 476 .30 143 1.5 WALLS (MATERIAL'ALPHA') 870 .30 261 1.0 60 (00 1,000 8,000 GLASS 30 .04 1.2 FREQUENCY CYCLES FLOOR (MATERIAL 'BETA') 476 .05 23.8 1.4 I.4 TOTAL SURFACE S 1852 1.2 OCCUPANTS( 5) 01 4 SABINS e6. 20 TOTAL ABSORPTION (SABINS) 449 .O VALUE OF o IN ETRING FORMULA .24 .8 I. TOTAL ABSORPTION / S L.61,11-21 .27 .4 OPTIMUM REVERBERATION TIME FOR L69,(1- 019 507 ¢ RADIO BROADCAST STUDIOS 1,000) (CURVE A'1 ROOM VOLUME In CUbk leer V 4760 m ,2

1 I 1 11111 REVERBERATION TIME - SECONDS .46 O o .049V á STUDIO VOLUME CUBIC FT. I09,(1 -6) 5 1,000 IO 000 100,000 1,000,000

Fig. 5 (left). Sample calculation of 1000 -cps reverberation time for studio 10 ft. high, 17 ft. wide, and 28 ft. long. Calculations are of sliderule accuracy. Log term was taken from Fig. 4. Fig. 6 (right). Morris -Nixon curves. The optimum reverberation time at any frequency can be computed by selecting the 1000 -cps value from Curve A and multiplying it by the appropriate value of K from Curve B. Redrawn from the Johns- Manville pamphlet "The Control of Sound in Broadcasting Studios."

AUDIO ENGINEERING JUNE, 1952 17

www.americanradiohistory.com Handbookof Sound Reproduction

EDGAR M. VILLCHUR

Beginning a fundamental reference work on all the aspects of sound re- production, which will run serially until completion. The author covers the sound wave -the basic form of all sound -in this first installment.

Chapter 1 -The Sound Wave

IT IS, of course, necessary to under- Although the graphical representa- DIRECTION stand the nature of that which we tion of a sound wave physically resem- of vIeRATION DIRECTION Or RAVE TRAVEL wish to reproduce before discussing bles the pictorial representation of an a--o the technique of reproduction. ocean wave the two should not be con- sound wave is an impulse, travel- fused. An ocean wave is of the trans- IAN The 1111111211111111111111 11111111 ling in an elastic medium, which causes verse type (see Fig. 1 -2) ; the medium the particles of the medium in its path particles vibrate in a direction perpen- to vibrate longitudinally, that is, back dicular to the direction of travel of the PEAK and forth along the path. In the course wave itself. It is for this reason that the NORMAL of this vibration the particles alternately pictorial and graphical representation of PRESSURE crowd together and spread apart, creat- the ocean wave have the same form. But 1 & in . cal ing areas in the medium which are since particles vibrating due to a sound DISTANCE turn compressed and rarefied. wave do so in a line parallel to the di- FROM SOURCE .% The source of sound is what Newton rection of motion of the wave, sound TROUGH called a "tremulous body "-a vibrating waves are called longitudinal. Motion de- of particle, as its string, diaphragm, or some similar the actual referred to Fig. 1 -1. (A) Pictorial representation of vibrat- vice-which first pushes away the par- excursion, is very small. Ocean waves ing source of sound (without overtones) and

ticles of air (or other medium) onto may displace the water dozens of feet, the pressure state of the medium. 1 B I Graphical their adjacent neighbors, then draws but most sources of sound have a maxi- representation of the pressure state of the back to form a slight vacuum. The area mum excursion of a small fraction of an medium. of intensified pressure exerts a repelling inch, and by the time the sound wave force on particles further from the has reached our ears the molecules of source, making them in turn to crowd air are only displaced by an amount together and the pressure area to move; which is in the range of / billionth of the rarefied air exerts a pulling force an inch to about .01 inch. on neighboring molecules further from the source, and in bringing these in to Definitions fill the partial vacuum creates a new The following expressions may be Fig. 1 -2. A transverse mechanical wave trave - rarefaction further out. Thus the wave defined in terms of the graph (B) of ling along a rope. The solid arrows indicate moves away from the source, but the displacement of particles in the rope, the Fig. 1 -1. dotted arrows displacement of the source. particles of the transmission medium Amplitude- Values represented along only vibrate, and never get anywhere. the vertical scale of the graph, which in but periodicity is not to be confused If we were to analyze the pressure this case stand for pressure in the me- with of the regular repetition of sounds in- state the medium a short while after dium. Amplitude may also refer to the volved in rhythm. the source had started to we Non -periodic sound vibrate amount of displacement of the vibrating has no pitch and is called noise. might see a condition such as that il- source or of particles of the medium. Phase-The relative position of the lustrated at (A) in Fig. 1 -1. This figure Maximum amplitude refers to peak or wave's graph along represents the pressure state pictorially. the horizontal axis. trough values; instantaneous amplitude Two waves are said to be in phase if The condition of the medium may also refers to the value at any desired instant their peaks and troughs occur be symbolically at the accurately represented or point. This characteristic is most same point along the horizontal axis. by a graph, as at in Fig. 1 -1, where (B) important in determining the sensation They are said to be 180 deg. out of points of maximum pressure are re- of loudness. phase if the peak of one occurs by peaks, points of at the corded the graph Cycle -One complete series of same horizontal point as the trough of normal pressure by crossing of the hori- changes, in this case of medium density. the other. zontal axis, and points of maximum rar- The part of the graph between points Wave Form shape of the wave. efaction by the -The troughs. a and b represents one cycle. This characteristic determines, for the The pressure and rarefaction areas of Frequency -The number of cycles oc- most part, the sensation of timbre or Fig. 1 -1 are moving away from the curring per second. This characteristic tone color. A sound wave of the pure source. We could install a pressure me- determines, for the most part, the sen- sine form of (A) in Fig. 1 -1 is unusual ter at any one point in the path and we sation of pitch. in nature, and has a very dead and un- would find the alterations recorded with Period -The time taken up by one interesting tone. respect to time. It is therefore possible, cycle. The period is the reciprocal of Wave Length -The physical distance and it is customary in representing the frequency, or numerically equal to in the medium between the peak or sound waves, to use units of time rather one over the frequency (1 /f). trough of one cycle of the wave and the than of distance as the horizontal axis. Periodicity -The repetition of a num- corresponding point on the next cycle. In other words we represent the activ- ber of successive cycles, each having There is a definite relationship be- ity of the medium at one point over a the same period, as in Fig. 1 -3, so that tween the wave length and the fre- period of time, rather than concern our- a definite frequency may be assigned to quency; in a given medium the higher selves with a whole section of the me- the sound for a certain duration: A the frequency the shorter the wave dium at one instarit of time. periodic sound has pitch, and can be length, and vice versa. * Contributing Editor, AUDIO ENGINEER- hummed or played. It is the kind of The distance covered by a moving ING. sound from which music is constructed, object is equal to the speed of the ob-

18 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com ject multiplied by the time elapsed. The wave length, the period, and the speed of sound may therefore be related as follows : = vxp =V /f INTRODUCTION where 7. = wavelength P= period V= velocity of sound f = frequency The Field of Audio The same curve as that of the graph (B) in Fig. 1 -1, besides representing changes in medium density exactly as POPULAR INTEREST in high -fidelity reproducing equipment has they occur, also describes the motion of become such that use of the technical jargon associated the vibrating source which created those with audio matters is now widespread. Audio talk is often changes, or displacement of the vibrat- accompanied by impassioned opinions and preferences champion- ing particles of the transmission me- ing one or dium. Points of maximum pressure or another brand, technique, or circuit. Devotion to the rarefaction become points of maximum cause concerned, however, is often not matched by an equal de- particle displacement away from or to- gree of erudition. wards the source. It is obvious that The earliest audio engineers were the musical instrument mak- greater pressure will be associated with ers. They were concerned with making devices which could pro- greater displacement. The relationship duce pleasing sounds, sounds to evoke particular emotional re- between displacement and frequency is sponses. Along with the trade secrets handed down from one gen- not as obvious, but assumes great im- eration of craftsmen to the next were personal or traditional pref- portance in the study of mechanical de- erences for a particular tonal quality. For example, violins made vices like loudspeakers and pickups. by Guarneri tend to have a more robust sound than those of Oscillatory Motion Stradivari. Lupot's instruments are known for their brilliance. Musical instruments have individual characters which may ap- As the frequency of a vibrating peal to one performer or listener more than to another. source goes up the amount of displace- In an industry devoted not to the production of desirable ment required to generate the same amount of sound power decreases pro- sounds but to the duplication or re- production of sounds already portionately, and, vice versa. the lower created there is far less room for personal preferences. It is true the frequency the greater the excursion that certain problems must involve personal factors. The problem required for the same power radiation. of how close the fidelity of sound must be in a given reproducing The actual average velocity of motion system, or the problem of striking a balance in a necessarily im- of the vibrating source thus remains perfect system between one type of distortion and another, can- constant for constant sound pressure, not be solved without taking taste into consideration. But most no matter what the frequency, a condi- questions of concern to the audio technician must be answered tion which will be referred to in a later chapter as "constant velocity." This is without benefit of such preferences. another way of saying that for equal Nevertheless there are many popular legends and old wive' sound energy at any frequency the me- tales in the sound field. If we were to eavesdrop on the conver- chanical energy of the source must re- sation (seemingly as esoteric as jive talk) between two audio main the same. initiates, we might hear the following: The relationship between displacement "I just got a new woofer- tweeter." and frequency, for the same sound "What's the fidelity ?" power, may be expressed in simple form as follows: "30 to 13,000." _K This conversation reflects two of the most common miscon- ceptions in audio: one, that frequency response is practically synonymous with fidelity ; and two, that frequency response is where T; = maximum displacement (during the period) adequately described by a number or pair of numbers represent- f= frequency ing limits in the frequency spectrum. It will be seen that there K= a constant are many factors, each one at least as important as frequency re- The graph of oscillatory motion of sponse, involved in the fidelity of sound reproduction, and that a source of sound or of particles of the frequency response itself is a characteristic which cannot be de- medium may be expressed in a "wave scribed as simply as our audio friends have done; the variation equation." A form of this equation, and between particular limits. for instance, often assumes more im- its derivation, appears in Appendix I. portance than the limits themselves. It describes displacement mathemati- is hoped cally where we have here discussed it It that the following pages, in addition to serving as a descriptively. reference for various aspects of audio, will aid in providing an Electromagnetic waves such as light informed and rational approach to the field of sound reproduction. and radio waves differ in many respects The subject matter will be divided into four main parts. Part 1 from sound waves. They do not create will deal with the basic nature of sound. Part 2 will discuss, in changes in medium pressure but in the light of Part 1, the requirements of faithful sound reproduc- "field" condition; the medium which tion, and will include a brief historical survey of the phonograph. transmits them, if such exists, is un- Part 3 will treat those aspects of recording known; which most directly their frequencies are much influence reproducing procedures. higher than those of sound Part 4, constituting the bulk (light fre- of the work, will quencies, for instance, are on the order cover reproducing methods and components. The component parts of the reproducing system be of billions of times higher) ; the speed will discussed of electromagnetic waves is about 186; in the same order as they are normally designed, from high level 000 miles /sec, almost a million times to low level. Thus room acoustics and loudspeaker systems will greater than the speed of sound. Yet the be considered first, and pickups and preamplifiers last. design of the universe is such that the same wave equations apply to both types

AUDIO ENGINEERING JUNE, 1952 19

www.americanradiohistory.com TABLE I VELOCITY OF SOUND Fig. 1 -3. (A) Per- iodic wave energy (A) Velocity (sound a 'l 1'7'' 1'I, 1'1' Material ft sec of flutes ' Aluminum 17,000 composed of a funda- Mahogany (with the grain) 13,100 mental and harmon- Brick 12,100 ics. (B) Non -periodic wave energy, com- Salt Water 4,935 posed of many ran- J 1 Cellulose Acetate, molded 4,300 frequencies. lB) Hydrogen 4,170 dom Air, 20° C. 1,130 Air, 0° C. 1,085 Chart of approximate sound velocities in dif- where ; = displacement from rest position. essary to adjust the units involved. Divid- ferent media. For data on other materials see: ing Vmax by w accomplishes the required H. F. Olson, "Elements of Acoustical Engineer- K constant power, (at a given and unit conversion,2 and the equation becomes: ing," 2nd ed., p. 8. in a given medium). w =2x x the frequency. a as = sin tot t= time elapsed since source was at w of energy, and sometimes problems con- rest. cerning radio waves may be aided in Vanaa /w, which is limas expressed in cm./ solution by the use of parallels in sound. The sine function determines the wave radian, is numerically equal to maximum Experiments on antenna radiation pat- form of a pure sound without overtones. displacement, and is the correct term for K. terns, for instance, have been conducted The expression wt, or 2nft, is the number The wave equation can be amended with of degrees (in radian units), that have slightly to apply to a vibrating particle of a slummy antenna radiating sound been passed through in time t. The fre- the transmission medium at any distance instead of radio waves. quency, in cycles per second (cps), multi- from the source. The equation describing 1 The vibrations of a source of sound plied by the time, in seconds, gives us the the behavior of such a particle is: are of two types, free and forced. number of cycles that have occurred Free during Vu,ur vibrations occur when the source, after time t. 2n is the number of radians in one to being given its initial stimulus, is al- cycle. Thus : lowed to vibrate at its own natural fre- wt = 2x radians /cycle x f cps x t sec. Where x = distance of particle from source quency, called the resonant frequency. = 2nftradians or 360 ft deg. (with both particle and source A plucked string, or a cymbal respond- at normal rest position) Multiplying t by to converts the units of c = velocity of sound ing to the blow of a drumstick, vibrate the horizontal scale from seconds to de- freely. The reeds of musical instruments grees. It is this conversion which allows us The expression x/c is the time required or of human vocal chords also vibrate to use the sine function to give us the rela- for the wave to reach the undisplaced par- freely, even though a stream of air is tive height of the graph at time t. ticle (distance divided by velocity). This is of particle's continually directed against them, as the The height of the graph represents the t =0 for the beginning the steady displacement from rest relative to the ex- cycle. x/c subtracted from the total time pressure of the air merely gives t, is the time elapsed since the par- them the mechanical treme values for any sine function, 1 and elapsed, energy required. -1. The term K will determine the absolute ticle went into vibration, as illustrated in and does not determine the frequency value of this displacement. Since maximum Fig. A. of vibration. displacement on the relative scale is equal If we apply this equation to a particle When a source is subjected to forced to 1 or -1, K must be numerically equal to in the source itself x/c is equal to zero and vibration, on the other hand, not only the actual value of maximum displacement is eliminated, returning us to the original is mechanical energy imparted to it, but in centimeters or inches. equation. The above equation is therefore control is exerted It is obvious that, for the same frequency, a general one, applying either to the source over the frequency medium. and other characteristics of its oscil- the greater the velocity of the source or or to particles of the latory motion. The loudspeaker and the particle the greater will be the displace- ment. Van ,, :, the maximum velocity, may 2 It can be shown, with the aid of more phonograph pickup are examples of de- thus be used as an index of K, but it is nec- vices subjected to forced vibration. in advanced mathematics, that l'aia:, in cm./ common with all mechanical systems radian, would be numerically equal to the radian = 57.3 deg. maximum displacement: they have resonant frequencies of their 11 own, but they must vibrate in imitation Let tot= B radians of many different voices, and, like good (cm) = K sin B Amax = K (sin B)r,ur =K actors, must submerge their own per- e sonalities in favor of the characters they 1 V (cm /radian) = = K sin B =K cos B de play. I de ['max = K(cos e)aaas =K Appendix I A A Vmax = t;nagr 'd Vmar is converted from cm. /second to I The Plane Wave Equation cm. /radian through dividing by w. Multi- VII I The displacement of a plying units of time by w, as we have seen, vibrating source d b--- of sound (assuming no harmonic modes of changes seconds to radians: vibration) at any instant of time is: Vmar cm. /sec. Vmaa cm. /radian Z= Ksi.atut. Fig. A to radians /sec. to

SCHIFINO PROMOTED HARVEY REPEATS FINK JOINS PHILCO One of the audio industry's pioneers, Fur the second consecutive year, Harvey Donald G. Fink, editor of hleitro,,ics, is A. G. "Tony" Schifino, has been elected an Radio Company has been chosen "Jobber joining the Philco Corporation, in an officer of the Stromberg- Carlson Company of the Year" for the New York Metropoli- executive engineering capacity effective with the title of general manager of the tan area, in an annual election conducted by June 1. Sound Equipment Division. Parts Jobber magazine. Voting participants In leaving Electronics Mr. Fink is re- First with Stromberg- Carlson in 1929, are manufacturers' sales managers and fac- signing from the only job he has ever held. Mr. Schifino operated his own business tory representatives. He assumed his first assignment with the from 1931 to 1940 when he rejoined the In acknowledging the unique honor, magazine immediately after graduating company as sound engineer. Harvey E. Sampson, president, stressed from M. I. T. 18 years ago, and has been His latest promotion coincides with the the important part audio and sound equip- on its staff continuously except for a leave introduction of the new Stromberg- Carlson ment had played in solidifying relations be- of absence during the war. line of high -quality custom sound equip- tween the company and its suppliers and He is a member of JTAC and NTSC, as ment. customers. well as many professional societies.

20 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com The OTL (Output -Transformer -Less) Amplifier

VICTOR BROCINER° and GERALD SHIRLEY

A survey of the possibilities for circuit arrangement to avoid the use of the output trans- former in audio amplifiers -which may readily become the next big development in audio.

or adapted to the OTL amplifier ; others, of OTL amplifiers, however, is going to perforce, will be abandoned. The sub- be paced mainly by the speaker manu- ject of critical or optimum damping as facturers. The experimenter, designer related to speaker efficiency and acoustic and /or manufacturer of OTL amplifiers loading has lately been receiving at- is going to have to base his choice of tention from various sources, and as output tubes (and their number), single - further research brings the problems ended or push -pull operation, direct into clearer focus and begins to provide coupling or shunt feed, etc., on what the needed answers, it can be assumed voice coil impedances are made available Fig. 1. Symbol proposed by the authors for that both conventional audio systems by the speaker manufacturers, and on h'gh- impedance loudspeakers IA) compared and those using OTL amplifiers will whether center- tapped and perhaps with symbol used for conventional speakers I benefit. multi- tapped voice coils can be made; The chief and probably the only real also. possibly, secondary coils for feed- YEARS audio engineers and ex- victim of the technological progress back circuits. The tube manufacturers perimenters have been talking about represented in the development of the too can contribute to OTL progress by F the wonderful improvement in sound OTL amplifier is the OT- output trans- trying to develop higher perveance out- that might be obtained former- itself. In a way this is ironical put tubes, both triode and beam -power, reproduction im- if the output transformer could be which require lower values of load eliminated and the speaker' directly pedance for proper matching. coupled to the output tubes of an In this article we will consider some amplifier. Recent developments in cir- of the various circuit configurations pos- cuit and speaker design have at last sible with OTL amplifiers. It is hoped combined to make this idea a practical that this will stimulate others in the reality. audio field to start thinking about OTL In a paper presented at one of the amplifiers, building them and writing technical sessions during the 1951 IRE about them. It is important to find out Convention (March), a new single - as quickly as possible what are the opti- ended push -pull output stage was first mum values of voice -coil impedance and described by Sinclair and Peterson. In the best configurations so that the speak- also accurately er manufacturers can get started on the their paper the authors Fig. 2. Conventional single -ended output stage forecast the early advent of OTL amp- problems which they will have to solve with low- impedance speaker (A) and with when they try to meet the specifications lifiers, pointing out that the only re- high- impedance speaker and no output trans- maining problem was the development former (BI. theoretically required. Incidentally, the of speakers having impedances of sev- problem of designing speakers with suf- eral hundred ohms. Then at the 1951 because the improvements that have ficiently high impedance is made less Audio Pair (November) the first been made in output transformers in the difficult in certain cases where horn line of high- fidelity last few years have been startling in- loading is employed, as this method models of a new into speakers with high -impedance voice deed. and are, in fact, one of the chief causes the impedance looking the voice coil to appear considerably higher. coils were demonstrated by a leading factors in the aforementioned overall And almost si- improvement in amplifier performance. Before beginning consideration of cir- speaker manufacturer. would like propose multaneously an article describing a Today there are transformers available cuits, the authors to OTL system was published in which deliver full rated output from 8 a new circuit symbol to represent a complete high- impedance speaker, as shown at one of the technical journals.' or 10 cps to 100 kc or more within a most of the articles on am- couple of db, and with excellent square (A) in Fig. 1. The chief difference be- Though the one used to plifiers which appear in the literature wave response at 20,000 cps. One is tween this symbol and are annoyingly redundant, the fact is tempted to ask, "How close to perfec- depict a conventional, low impedance speaker, is in the number of turni that in the last five years the art of am- tion must we get ?" The answer. of (B), plifier design and construction has ad- course, is, "As close as we can !" The used to represent the voice coil. One vanced to a degree which few engineers elimination of any link. in the chain of would have believed possible in 1941 elements which makes up an audio re- or, for that matter, in 1946 at the con- production system brings us that much clusion of active hostilities. Some of closer to the goal of perfection. these advances will be incorporated in In the past. improvements in overall amplifier performance were made with- * Brociner Electronics Laboratory, 1546 out any particular reference to speaker Second Ave., New York 28, N. Y. performance. The amplifier was treated and it could he IV. St., New York as an independent entity, ** Tele-.'e.r, 474 238th given speaker or 63, N. Y. assumed that with any speaker system, the better the amplifier 1 Fletcher and Cooke, "Cathode -follower loudspeaker coupling," Electronics, Nov. driving it, the better the reproduced Fig. 3. Capacitance- coupled output stage may resistor (A) or choke (B). 1951. sound. Development and improvement be shunt fed using

AUDIO ENGINEERING JUNE, 1952 21

www.americanradiohistory.com specifications will not cost as much as a vide a large value of filtered voltage top quality output transformer, it will which will in turn require a large value nevertheless be an item of expense and of Red to limit the bucking current to a certainly cost considerably more than a proper range which can be closely ad- resistor B of the same power rating. justed by Rea. Incidentally, if a gradually R2. R2B Using either a choke or resistor for rising bass boost in the speaker is de- shunt feed, there is still the coupling sired one can select the proper value of e capacitor to contend with. Any leakage C. for the "crossover frequency" de- means that d.c. will be flowing through sired. If flat response is wanted, it is the voice coil, though this could be mini- only necessary to make C. large enough. i mized by connecting the other end of the voice coil to B + instead of to ground, Single -Ended Circuits which in the case of plate -loading would The next case to consider is that of a Fig. 4. D.c. bias on high- impedance speaker place a lower d.c. potential across the single -ended output stage with power may be avoided by use of second B supply to capacitor. Besides being frequency dis- taken off at the cathode instead of the buck out d.c. from main B supply. criminative at the low end, the capacitor plate. The d.c. resistance of the voice is also a significant item of expense. For other prefatory note : in all the illustra- coil would be a convenient source of the tions where a single output tube is a 500 -ohm voice coil something around self -bias for the output tube, but 100 as shown, it can be assumed that a plurality lf is a minimum requirement. Elec- pointed out earlier d.c. cannot be tol- of tubes could be used in parallel trolytics are unsatisfactory for such ap- erated in the voice so as to plication, coil. Again we can lower the value of the load impedance and paper units of such large resort to shunt feed, using a resistor or required in cases where speakers of suf- choke and a coupling capacitor. As in ficiently high impedance are not avail- the case of plate loading, each has its able. Similarly, push -pull stages can be advantages and disadvantages. If a elaborated into push -pull parallel opera- choke is used, as at (A) in Fig. 5, it tion to accomplish the same result. would ideally have a d.c. resistance such as to provide the proper self -bias for the Circuit Arrangements output tube. The circuit illustrated at (A) in Fig. Using a resistor for shunt feed is not 2 is a conventional single -ended output always as simple as might first appear. stage in which the low impedance speak- That is, one cannot simply use a resistor er is coupled to the output tube through which gives the correct self-bias for the a transformer. Assuming that this tube tube, because its impedance will be con- is properly matched with a 2000 -ohm siderably lower than that of the speaker load and that we have available a speak- Fig. 5. Two methods of feeding high -im- load. This would mean not only a severe er whose voice coil has a 2000 -ohm im- pedance speaker from cathode of output stage. mismatch-since the total a.c. load pedance, the question is how to hook would be much smaller than it should them up. The simplest and most direct capacitance are expensive. Distortion be-but also the greater part of the re- method would be that shown at (B) in can also be introduced by hysteresis in duced power output would be dissipated Fig. 2. Unfortunately, however, this the coupling capacitor. Direct coupling in the resistor instead of in the speaker. method cannot be used because the d.c. seems to be a desirable goal, from the One solution to this difficulty is shown flowing through the voice coil of the standpoint of both cost and performance. at (B) in Fig. 5, where another resistor speaker will induce a magnetic field One method of making direct coup- Ra is connected between the bottom of which will react with that of the speak- ling possible-admittedly somewhat un- Rk and ground. (Rk is chosen for correct er's own field, and cause the voice coil economical-is shown in Fig. 4. Here biasing of the output tube.) If Re is to move to a new reference or zero -volt- there is an extra power supply which is made two or three times as large as the age position which is offcenter both completely isolated from the main power speaker load, then the mismatch is re- mechanically and magnetically. In this supply. The directions of current flow duced to tolerable proportions and the condition it is no longer a linear trans- through the voice coil are opposite and speaker will absorb the greater part of ducer and will produce distortion. the d.c. magnetic fields are cancelled the power output. Note, however, that as To keep d.c. out of the voice coil, we out. A certain amount of useful power Re is made larger, the B supply must be can use shunt feed as shown in Fig. 3. output will be dissipated in R...- R.a -C., increased proportionately to make up for Each method has advantages and disad- but this loss can be kept to a minimum the voltage lost across R.. Actually, this vantages. The resistor feed introduces by having the second power supply pro- is very similar to what happens when no change in frequency response at ei- ther end of the spectrum. On the other hand, being in parallel with the load for a.c. means that useful output power ---- will be wasted in the resistor. This loss can be reduced by using a larger re- sistor, but this means that a larger B supply must be provided. If carried too far it will also result in power loss e caused by clipping due to the low ratio of a.c. load to d.c. load. By using a B+ choke of sufficient inductance to permit B+ shunt feed of the output tube, the 11 1r' -- amount of useful power dissipated in the shunt -feed element can be reduced to a negligible value and the need for an oversize B supply eliminated. But to e maintain good response at the low end the inductance must be very large, dis- -- tributed capacitance must be kept to a minimum to preserve performance at 1A) (9) (C) (D) the high end, and it must be so con- structed that there will be no core Fig. 6. Four possible circuit arrangements for push -pull output stage. Note that all such circuits saturation. While a choke meeting these require tapped voice coils.

22 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com the resistor and load are in the plate circuit. Incidentally, in the circuit of (B), direct coupling between driver stage and output tube could be employed, 20 20 given proper selection of driver tube and plate -load resistor. In general it appears that single -ended operation is not too well suited to OTL o design because of the necessity for keep- ing d.c. out of the voice coil. Suppose, however, that a high -impedance speaker were to be constructed in such a way that with nothing connected to it the voice coil would be out of the gap a short distance in the forward direction. Then with the correct amount of d.c. flowing through the voice coil -i.e., the plate current of the output tube-it Fig. 8. Possible circuit arrangement for two -way speaker system driven by cathodes of output stage would move back into the gap to the same position occupied by voice coils in push -pull output stage3. +, one version of normally constructed speakers The which is shown in Fig. 7, we find that authors of this paper are not in complete we can go back to an 'untapped voice agreement as to whether this type of coil since no d.c. is flowing through it. construction would result in a condition This circuit may well turn out to be the of mechanical non -linearity in the cone standard hi -fi output stage of the future If it did, however, it should -and the not too distant future at that. suspension. from be possible to control this effectively One of its outstanding advantages of efficient the OTL standpoint is that its load -im- through a combination requirements are only one speaker design -i.e., a strong magnetic pedance of same tubes field concentrated in the gap and tight quarter those for the voice coil a suit- when connected in a conventional push - coupling to the -and example, a ably low source impedance. Even with pull arrangement. For pair inexpensive speakers there might be a of 2A3's which normally drive a 5000 - in performance over that ob- ohm load now require only a 1250 -ohm net gain Peterson circuit, tained with low- impedance speakers load in the Sinclair- which are coupled by output trans- and if four 2A3's are used, the require- frequency range ment drops to 625 ohms which is already formers. The overall avail- would certainly be extended, and if there reasonably close to the presently caused by possible able 500-ohm speakers. For a single were any distortion Fig. 9. Optional circuit arrangement for two - non -linearities, this distor- 6AS7, the optimum load would be about mechanical way speaker and cathode- coupled output stage. might be tion might turn out to he no worse and 280 ohms. In fact, this figure one of those theoretically ideal standards of voice -coil impedance mentioned same reason, direct- coupled cathode earlier which the speaker manufacturers loading of a push -pull OTL stage is also might agree on and begin manufacturing practical. If the d.c. resistance of the as soon as possible. voice coil is not sufficient to provide the The OTL system shown in Fig. 8 is correct bias for the output tubes, then the one mentioned in the second para- bypassed resistors would have to be in- graph of this paper. The audible results serted at the cathodes, and as shown at were claimed to be superb, though the (B) in Fig. 6 one of them could be made circuit does not appear to be a very adjustable to permit balancing the zero - efficient one, to put it mildly. Eight signal plate currents. A cathode loading 6AS7's requiring 2000 Ina at 200 volts circuit using beam -power tubes is shown delivered only 6.32 watts into the at (C). speaker system. (A single 6AS7 can adaptation of a new circuit 7. arrangement for single - A possible normally deliver 8 to 11 watts.) Much Fig. Output coupling by Hafler and ended push -pull output stage. development reported greater efficiency could, of course, be Keroes2 is shown at (D) in Fig. 6. It obtained with high -impedance drivers, possibly less objectionable than the dis- was shown that the combined advantages -tapped woofer were be and if a center tortion introduced by cheap output of triode and tetrode operation could the circuit of Fig. 8 could be im- available, transformers. Some experimentation by obtained by providing a screen -load re- as shown in Fig. 9. The of the arranged the speaker manufacturers with this pedance of about 18.5 per cent values of L and C in the dividing net- type of construction would certainly plate -load impedance and closely coupled work would of course have to be re- seem to be warranted. to it. This was accomplished by bringing of the network of the calculated. Elaboration out taps on the primary winding for a three -way system would not be Push -Pull Circuits output transformer to feed the screens. circuit difficult. Also, the whole business could We now turn our attention to push - Transference of this ingenious be transferred upstairs for plate loading pull OTL design. Figure 6 shows a to OTL design depends on the ability if in which woofer to design desired, case the push -pull plate -loaded OTL stage at of some speaker manufacturer center -tap would be connected to B+ (A). The screen grids have been drawn and build a multi- tapped voice coil. re- instead of to ground. If for any reason in dotted lines to indicate that either It can be seen that the minimum center -tapped voice coils are not prac- for -coupled, conven- triodes or beam power tubes can be used. quirement direct tical, the circuits suggested in this para- (If the latter are used, the screens could tional push-pull OTL circuits is a cen- be fed in the conventional manner or as ter- tapped voice coil. However, when [Continued on page 45] done in the Williamson arrangement.) we go to such unconventional circuits as the Sinclair- Peterson single- ended, Here direct coupling is practical because 3 Sinclair and Peterson-IRE Group equal and opposite direct currents flow Paper. through the voice coil, and their mag- 2 Hafler and Keroes, "An ultra -linear + General Radio Experimenter, Oct. netic fields cancel each other. For the amplifier," AUDIO ENGINEERING, Nov. 1951. 1951.

AUDIO ENGINEERING JUNE, 1952 23

www.americanradiohistory.com Universal Amplifier for Magnetic Tape Recorder

C. G. McPROUD

Part 2. Continuing the description of an amplifier which combines the ability to serve in professional applications as well as in a high- quality home music system. This installment details the circuit design.

HE PRELIMINARY DESCRIPTION of this went for professional applications, it is amplifier covered only the basic phi- provided with low- impedance inputs, T losophy of the design, without giving with 30 ohms being chosen so as to sim- any of the details of the various circuits plify equalization in the playback chan- employed. In this article the complete nel. The input transformers have a circuit principles will be described. relatively high step -up ratio, and it is probable that more modern units would Input Switching give better performance. However, one With only two input amplifier stages, of the units chosen was already in the it seems obvious that only two separate "junk" box, so it was natural that an- inputs could be accommodated at once. other of the same type would be secured. In order to increase this facility even In order to provide for a high- imped- slightly, input switching was arranged ance input for any purpose for which to permit a third circuit to be connected it might be required, the secondary of to either of the transformers. A key the input transformer is connected to switch was chosen which is, in effect, the contact of a closed- circuit jack, J two separate DPDT switches, since two the spring being connected to the grid circuits are made in the neutral position Fig. 6. Schematic of complete equalizer as of the switch, and each throw of the used with this amplifier and the Presto RC -7 switch affects only one of the circuits. recorder. The outside contacts are paralleled and capacitor connected to the third microphone jack. serves a frequency -correcting The two pairs of arms are connected to element. The plate of V.,b is coupled the transformer primaries, in- to the mixing network- composed of and the R,,, R,8, and and plain side contact pairs are connected to the R19 the gain first and second jacks respectively. Thus control, R22. The mixing network cou- (0) IBI ples the high -impedance radio input. in the center position, jacks 1 and 2 are connected to the two transformer when used, to the main gain control IA) as primaries; when thrown to either side, Fg. 5. Equalizer configurations: orig- without appreciable effect on the input i ally designed; and i B i as finally used for the the third jack is connected to either the stages. When two microphone stages 15 -in., sec. speed. The 7.5 -in sec. equalizer is are being used, they are coupled to- first or second transformer, as desired, like IA). opening the other input circuit. In use, gether and to the gain control by the same mixer network. the associated mixer dial is turned to of V,. The 3.9 -meg resistor Ra provides The output V6 minimum, the switch thrown, and the a grid return if crystal microphones amplifier, and Vs, will dial then turned to the required position. be recognized as being similar to the should be used. The first stage employs Williamson When the selector switch, SW is in a 6AU6 to provide sufficient amplifier input circuits, and gain, and it serves equally well in the REC position, two playback heads it is coupled to the gain control in the this application. provides considerable gain, is may both be plugged in, one into J= and grid circuit of the second stage. The Vsa and one into and either machine may latter is directly coupled to Vab as a phase split - J be one half of a 12AX7 (the other ter, which used for playback by throwing the half occupies a similar position in the drives the two halves of VS switch to the desired position. second with as a push -pull output stage. Feedback channel, separate cathode from Since it was desired to use this equip- and plate resistors). The small by -pass the plate of Vsb stabilizes the out- put amplifier. It would have been pos- sible to derive the feedback from the secondary of the output transformer, but this would have necessitated grounding P. MOO the secondary, which is not desirable if SO Id the equipment is to feed a balanced line when acting as a remote amplifier. The tape recorder selected, a Presto RC -7, was designed to be fed from a 500/600 - ohm transformer, and also some such impedance is desirable to feed a remote line. Actually, the equalization for the recorder is quite simple with this type of feed.

i..t , Output Switching When used to feed a recorder, the 600 -ohnt winding of the transformer Fig. 3. Complete schematic of the amplifier chassis, repeated from last month. is fed directly to the output jack, Ja,

24 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com ground being obtained in the recorder entire winding when the meter is con- chassis. This ground is common to the nected across half of the secondary. The power supply, so the circuit is properly isolating pad has a loss of 6 db, so the grounded when so connected. level at its output would be +4, as indi- When used to feed a remote line, a cated by the switch. Similar calcula- dummy plug with terminals 2 and 4 tions and a voltage divider between the shorted is placed on J8 (which is a male center tap of the secondary and a tap Jones plug, actually) so as to connect of lesser impedance permit adjustment the "high" side of the output circuit to of output to obtain additional levels of the output switch. Swa. In the center or 6 and + 8 at the line terminals. OFF position of this switch, the amplifier For recording, the +4 position pro- is terminated by RS3; in the LINE posi- vides a level of 2.6 volts to the input of tion, the amplifier is connected to a the recording equalizer ; the +6 position 6 -db pad composed of resistors R3R to provides 3.6 volts, and + 8 provides 4.5 Rb2, inclusive, and thence to the line volts, giving a range of recording lev- jack, Je, through two sections of Sw,; els. A small arrow is placed on the in the cue position, the monitor jack is panel at the + 6 position to indicate that connected directly to the line jack so step is normal for recording. For piano that cue may be heard from the station recording, a lower level is desired, and on the monitor phones. The output the + 4 step gives a cleaner tape. To switch is the same type of switch used date, no recording application has been for the input switching. found where the + 8 position was found desirable. VU Meter Switching In order to provide for measuring the The Recording Equalizer output level when feeding a remote line, The recorder unit of the Presto RC -7 or for the correct recording level, as contains the bias -erase oscillator with well as to measure bias and erase cur- suitable resistors to provide indications Fig. 7. Equalizer in its shield can is mounted inside the re- rent in the recorder, the VU meter on the standard VU meter and two leads corder case, and supports bracket for idler and capstan switch. SW, has six positions. Two are brought out for this purpose. In ad- sleeve. positions provide for measuring bias and dition, the "constant- current" resistor erase E E in as is currents, and are labeled and of 500 ohms is this unit, a trap exact values by removing turns. The respectively. bias voltage out of Three output levels are circuit to keep the toroid coils used were wound in provided of two -+4, +6, and + 8-being so the recording amplifier. One side the sections, making it arranged level quite simple to ob- that the fed to the line at recording circuit is grounded in this tain the correct values. Complete equal- the output of the 6 -db isolating pad is unit, and this provides ground to the izers of this type are used in the Magne- indicated at zero on the VU meter. The entire output circuit since the same corder, and are available in separate particular recorder used requires a volt- power supply feeds both sections. shield cans with a plug -in base, as age de- applied to the input of the recording Therefore, in order to place the record- scribed in an earlier article,' and could equalizer of approximately 3.5 volts for ing equalizer in series with the line, it undoubtedly be employed full with the modulation (which is presumably is only necessary to bring out. three Presto recorder if the effort of making the equalizer is considered too trouble- some. However, the equalizer as con- structed functions satisfactorily and oc- Fig. 8. Curves of cupies relatively little space. playback equaliza- In tion: (A) that due most professional -type circuits, the high- frequency to feeding the 500 - equalization is pro- ohm playback head vided in the recording function, while into a 33 -ohm resis- the low- frequency equalization is pro- tor; IB) total equali- vided in the playback function. The zation obtained with high- frequency equalization compen- addition of feedback sates for gap width, and requires a boost network around first of about 18 to 22 db at 7500 and 15,000 stage. cps for the low 70000 and high speeds respec- ,0000 tively. The curve is FREOUENCY IN CYCLES PER SECOND of the equalizer ad- justed by the relative inductances in the series and shunt elements of the equal- 10 to 15 db below tape overload). The leads to connect to the equalizer- input, izer, and the amount of equalization is sixth position of the switch is marked the "high" line from the amplifier; out- adjusted by the resistor network. It will EX, and connects the meter directly to put, to the "constant- current" resistor be noted that the series resistors used terminals 1 and 6 of J8 (with the 3600 - followed by the trap and the recording are each of 510 ohms, and that the shunt ohm series resistor which is always in head ; and ground. resistors are employed to make final ad- the circuit). Another plug, with leads The equalizer configuration originally justments for frequency response. This connected to these terminals, may be planned was that of a constant- imped- construction does not follow accepted used to check the meter from an ex- ance network, as shown at (A) in Fig. network practice rigorously, but the ternal source, or to permit measure- 5. Final adjustments of frequency re- performance seems to justify the means ments on other circuits with the same sponse required a few changes, and the taken. Frequency response is within ± 1 meter. final configuration for the 15 -in. /sec. db from 30 to 15,000 cps at 15 in. /sec., For a given voltage across the meter, equalizer for a flat frequency response and within ±2 db from 30 to 8000 cps the voltage across the entire secondary from input to tape output is as shown at 7.5 in. /sec. will be twice that value if the meter is at (B) ; the 7.5 -in. /sec. equalizer fol- The entire network, with the switch, connected across half the winding, and lows the original configuration. The is assembled in a Bud Minibox, 2% x if connected across less than half the complete equalizer is shown in Fig. 6, 2% x 4, and is installed inside the re- winding, the total voltage would he still with the necessary switching for the corder case, as shown in Fig. 7. The greater. Thus with a meter indication two speeds. [Continued on page 34] of 0 VU, which corresponds to a level The coils used for these equalizers of +4 clue to the meter calibration, a were made from unmounted toroids, ' C. G. McProud, "The interview ampli- level of + 10 would appear across the with the windings being adjusted to fier," AUDIO ENGINEERING, Dec. 1951.

AUDIO ENGINEERING JUNE, 1952 25

www.americanradiohistory.com Ultra- Linear Operation of the Williamson Amplifier

DAVID HAFLER and HERBERT I. KEROES

The Famous "Williamson" can be improved simply by replacing the output transformer and making a few minor changes in other components. The results are well worth the effort and expense.

FROM T11h. TIME an article' appeared describing the principle of Ultra - Linear operation of power output tubes, considerable interest has been evidenced in the application of this new circuit improvement to the famous Williamson amplifier. The William- son circuit has been publicized in several arrangements including at least one commercial one, and the configur- ation is undoubtedly the most popular The authors' Ultra - high -quality audio circuit ever devel- Linear amplifier com- oped. For many people there is little bined with the power necessity to attempt to improve this supply on a single basic amplifier circuit. Its listening chassis. quality is excellent; it is easy to con- struct; and it provides top quality at a cost comparable with units which cannot measure up to its capabilities. The one category in which the Wil- liamson amplifier is significantly de- ficient is with regard to efficiency and power- output capabilities. Peak power output is less than 15 watts, and it takes a 450 -volt supply at approxi- mately 130 ma to achieve this power screen grids of tetrodes are energized '1'0 -300, which was designed for use output. If this limitation can be over- from a tap on the primary of the out- with tubes of the 6L6 type. Its 6600 come without deterioration of quality, put transformer. This connection, on ohms primary impedance therefore, is a change in the original design is justi- which patents are pending, modifies the also correct for 5881's and 807's in the fied. If simultaneously it is possible to operating characteristics of the tube. Ultra- Linear hook -up. In addition, KT- improve the amplifier both in measure - Proper location of the tap results in 66's can be used without deterioration able aspects and in listening quality, optimum input -output linearity simul- of quality as the slight mismatch is in then a change is not only justifiable, it taneously with efficient operation, a favorable direction with respect to is mandatory. power capabilities approximately double distortion characteristics. Therefore, It is difficult to improve on some- those of a triode connection, and low - this transformer can be used with the thing which is really good. There are impedance output such as is offered by tube types normally used in William- some audio enthusiasts who will scoff triodes. In short, it permits better per- son amplifiers without compromise of at the idea that the Williamson circuit formance than either triode or tetrode characteristics. It is of interest to note can be improved. However, it has been connection of the tubes, and this is sub- that the change in impedance to 6600 five years since Mr. Williamson pub- stantiated in comparative listening does not violate Mr. Williamson's de- lished his circuit ; and in the course of tests and distortion measurements. sign considerations. The modified tube five years, there is little which can The unique merits of the Ultra -Linear characteristics of the Ultra- Linear con- maintain supremacy without change or stage are particularly applicable to the nection require this impedance if we renovation. When a basic circuit im- Williamson circuit. The mating of the wish to preserve operating conditions provement -the Ultra- Linear output two seems to have been inevitable. The similar to those of the original amplifier. stage arrangement -came along, it was simple substitution of an output trans- In other words, the tubes are still natural to see how it could fit in with former with primary taps for Ultra - matched for minimum distortion rather the basic Williamson circuit. Linear operation and a few minor than for maximum power output. The The Ultra- Linear output stage is not changes in circuitry, which will be transformer, therefore, can be placed in a triode stage as is used with the Wil- discussed below, combined the basic the circuit directly and the screens of liamson circuit -nor is it a tetrode or circuits into an amplifier which prac- the output tubes connected to the ap- pentode stage. It combines the ad- tically everybody agrees is an improved propriate taps as shown in Fig. 1. This vantages of both triode and tetrode by version in all respects. Obviously, we eliminates the two 100 -ohm screen using an arrangement in which the must gain improvement if we substitute stopper resistors of the original circuit. a morelinear output tube and use a The plate and screen leads of the trans- Acro Products Co., 369 Shurs Lane, Philadelphia 28, Penna. transformer which exceeds the origin- former are color coded to avoid phasing i Hailer and Keroes, "An ultra -linear ator's stipulations for performance. difficulties. amplifier," AUDIO ENGINEERING, November The original Ultra- Linear circuit Several additional circuit changes 1951. utilizes a transformer, the Acrosound have been found beneficial for optimum

26 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com 6SN7 OR 7N7 807, Seal, 04 KT-66 \á0.1u1, 0 25 0 25 i- 6SN 7 On I T. 7N 7 O 6 40,000\ 560 -O b

o MATCHED PAIR \ ", 0 25 I.000 0.25 F ACnO TO- 300 e TO PREAMP MATCHED PAIR 10 - Et-/CP, 50mo

400-0-A00 200 ma

5V 2A 9-105 +50mo /m1d0n

0 - +O 2 500 WE 117 IT 5 VAG

é 6.3V DA

Fig. 1. Schematic of the Ultra- Linear Williamson amplifier. The components in dotted circles are those which are changed from the original circuit in making the conversion of an existing amplifier to Ultra- Linear operation.

performance. One of these is the all taps of a tapped secondary winding. creasing one pair of coupling capacitors change in value of the feedback resistor The amplifier, as converted, now from .05 of to .25 of gives a five -to -one to 10,000 ohms in order to maintain 20 surpasses the original with respect to ratio of time constants for the two db of feedback. In the Ultra -Linear response, distortion, and transient char- pairs of networks and increases the stage the gain of the stage is greater acteristics. In addition, it was con- low- frequency stability margin at nom- than for a triode stage. In addition, the sidered desirable to make certain other inal increase in cost. change in primary impedance changes slight changes which primarily increase The insertion of a 10,000 -ohm para- the proportion of voltage fed back. the stability under feedback conditions. sitic suppressor in the input grid and Thus the feedback is increased unless The low- frequency time constants of a 100 -I+µf capacitor across the feedback the feedback resistor is changed to the original circuit's interstage coupling resistor adds to the high- frequency compensate. The readjustment of this networks were the sane for both such stability margin and eliminates a slight resistor to the desired value then per- networks. This is not particularly de- ringing in the vicinity of 200 kc. mits the added gain of the Ultra- Linear sirable in a feedback amplifier since a One last optional difference from Mr. output stage to increase the amplifier given frequency loss is accompanied by Williamson's original sensitivity. circuit lies in the It can now be driven with maximum phase shift. Separation of use of a bypass capacitor across the a little 1 over volt as compared to almost the time constants permits less phase cathodes of the output stage. This has 2 volts required for the original amp- shift for the same frequency loss. In- been found beneficial in lifier. both the Ultra The feedback is taken from the 16- olmt tap regardless of the speaker con- nection. This tapped secondary arrange- ment is extremely convenient when Fig. 3. Square -wave shifting to speakers of different im- performance at 20 pedance as it does not require a change cps (left) and at 50 in the value of the feedback resistor. It kc (right). is made possible by special transformer design (on which patents are pending) which permits equivalent response on

Linear conversion and in the triode Williamson2 at high levels of operation as distortion at the overload point is diminished. Fig. 2. Curve of in- termodulation distor- There are no changes required in tion vs. effective the remaining stages nor in the power sine -wave -power out- supply. Most of the publicized versions put. [Continued on page 43]

2 Keroes, H. I. "Building the William- son amplifier," Radio and Television News, December 1950.

AUDIO ENGINEERING JUNE, 1952 27

www.americanradiohistory.com Audio in the Home

William C. Shrader*

A review of a number of the more popular amplifiers used for home systems, with pertinent pointers on performance of each. A truly useful compilation for anyone about to select this important component.

THESE ARTICLES have thus far dis- even the leading brands of amplifiers. fier, it is the only truly remote controlled cussed an inexpensive group of Two other problems that often arise in amplifier, having only one cable going components, and ways in which the installation of custom systems are to the remote control box. The record they might be used to improve standard pick -up of TV signals. and hum from player and the FM tuner may be located commercial consoles. We shall turn our ground loops. The adaptability of com- with the power amplifier, while the front attention now to those who wish to im- ponents to one another is also quite im- end is at some distance on an armchair, prove such a system or who are inter- portant. An amplifier that starts boost- for example, for convenient adjustment ested in obtaining sets that give good ing at too high a frequency may give al- of volume, tone, record compensation, sound and can afford four to five hun- together too boomy and barrel -like re- and switching between radio, phono, and dred dollars for equipment. Improve- sults when used with one type of TV. It has one of the most versatile ments in an existing system almost cer- speaker design but may be perfectly record compensating controls of any am- tainly would be done a step at a time, satisfactory when used with another plifier and has been endorsed by some of and those who are just beginning to pur- speaker. Changing either the amplifier the nation's leading music critics. have the option of buying units in or the speaker would result in reason- chase Brook 12A3 and 10C3 steps or all at once. The end result for ably satisfactory matches. each group would be the same. Analyzing the leading brands of am- Brook amplifiers were among the first Since the amplifier has to magnify plifiers in alphabetical order, the writer available with remote controls for ease very small voltages and build them up feels that the suggestions hereunder may of installation. They have an all -triode to loudspeaker volume without distor- give greater satisfaction and prevent circuit which many audio enthusiasts tion, and be able to compensate for vari- difficulties from arising. consider the only way of getting true ous recording characteristics, room fidelity of reproduction. It was the first acoustics, hearing deficiencies (Fletcher - Altec A -433 -A Preamplifier and A -333 -A amplifier in the high -fidelity field to be Munson effect), and the inability of the Power Amplifier produced on a large scale with adequate loudspeaker to radiate much power at The extremely good design of this tone controls, and it has been used for extremely high or low frequencies, it is amplifier facilitates ease of installation. many years for demonstrating quality considered the heart of the system. The preamplifier is also readily adapt- speakers and pick -ups at various audio Although it is always desirable to have able to other power amplifiers such as shows. The difference between the 12A3 the speaker mounted in a compartment the many versions of the Williamson and the 10C3 is that the former is a 10- separate from the other equipment, this circuit, notably the Musician's. If used watt straightforward circuit, whereas is sometimes impractical. Where they with Audak pick -ups, however, it is sug- the latter uses automatic biasing so as are together, microphonics due to tubes gested that the feedback capacitor, Cr, be to get 30 watts from the same output or to some kinds of pickup cartridges doubled in value. If whining or singing tubes. often give rise to complications with noises are apparent it is suggested that the 12AX7 and 12AU7 tubes be selected Leak "Point One" carefully for a minimum of this trouble. This model is advertised as Britain's 2803 M Street, N.W., Washington, Motorboating may occur if the output finest amplifier, and is highly endorsed D.C. tubes in the power amplifier are not well by the National Physical Laboratory of balanced. If this occurs, raising the Britain, which is similar to our National value of the feedback resistor will re- Bureau of Standards. It does not employ duce it. the famed Williamson circuit, but in units tested the distortion is as adver- Bell 2145A tised. namely 1 /10 of 1 per cent. This Although the same in outward appear- amplifier, using oil filled filters and ance as the 2145, this is actually a new coupling capacitors, has obviously been model, since the latter has been com- well thought out and is beautifully wired pletely redesigned. An all- triode ampli- [Continued on page 421

The amplifiers discussed by the author include, at left, the Altec Lansing A -333 -A with the A -433 -A preamplifier, and below from left to right, the Bell 2145A, the Brook 12A3, and the Leak "Point One."

28 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com -.3t produces near perfe.:n_, L tree sound enthu-

siast . . . the White horn -loaded speaker, and the new, crossover ampli- fier, designed and built for White Sound, by Maynard.

of research and laboratory tests bring you this smooth -flowing peak -less sound without resonance, to mar its perfection. Two, separate, high -fidelity amplifiers (on one chas- sis), operate into this entirely new speaker that doesn't have to sit in a corner!

the kind of sound that engineers dream of ®attaining; no peaks, no hangover or ringing effects. This amazing system results from perfect damping -and the frequency response of the speaker is flattened and variations in its impedance during operation. unaffected by ,14.24>EL cf5 - /B vvu of this smooth -working team tJ cs.a /79. 00 fEU. 77Ik' /AG, is uinerent xou o -,' n cars will tell you that this is the finest electronic sound yet heard. If you are a perfectionist, there can be no other choice than this decisively superior combina- tion. Its listening result is pleasant beyond imagination!

RTTEf1TIOf1 CUSTOM-SOUIID IFISTRLLERSI Here iS d system that offers interior decoratcrs d perfect solution to placement problems. Heretofore, corner -type speakers have made it necessary to violate principles of good room arrangement. The White speaker and associated amplifiers can be used anywhere in the room! no need to be forced into using corners. Speaker may be placed where good design ordains it should be placed -parallel to the architectural lines of the room. THE WHITE HORII- LORDED SPEAKER Acoustical compliance of room volume taken into account in .esign of system Back -loaded horn, coupled to speaker through unique phasing device Flare cutoff of horn 20 cycles fine foot horn, uniquely folded into non -resonant cabinet within 1% of true exponential forming Air mass loading on speaker, twenty times the mass of the speaker, resulting in a virtually mass -less driving system Beau- tiful custom finishes. Available in either blonde Korina wood, or dark Mahogany with hand -rubbed finishes. Bonded formica real -wood tops, alcohol and cigarette proof Dimensions: 35,/2" high, 23" wide, 20" deep. 595.00 FOB Chicago. THE WHITE SOUFID, MAYFIARD- EFIGIFIEERED CROSSOVER AMPLIFIERS no crossover network used in output or ampuners - non -resonant, istortion- ree RC crossover network at the input of each section of the dual amplifier Sharp cut (18 DB per octave) Crossover network Separation of high and low frequency channels result in minimum of intermodulation distortion- so low, that it can be considered at the vanishing point Dual amplifiers make it possible to USE high and low freiduency speakers of different impedances, with efficiency differences of speakers compensated for exactly, by individual gain control in each amplifier Perfect damping. Frequency response of the speaker is flattened and unaffected by variations in speaker impedance. Amaz- ingly clean transient response Less than .1 of 1 % intermodulation distortion from below 3 cps to 100,000 cps. 250.00 FOB Chicago.

- ...

^-9 ci . /NC.

fTIAYIIARD ELECTROf1ICS, Inc. WHITE SOUnD, Inc. 1414 SOUTH WABASH AVEf1UE 105 WEST MADISOFI STREET CHICAGO 5. ILLIFIOIS CHICAGO 2. ILLIf1OIS

AUDIO ENGINEERING JUNE, 1952 29

www.americanradiohistory.com Neither Phono Facts 1952 MAXIMILIAN WEIL

If you wish to obtain the fine performance inherent in vinyl LP and 78- r.p.m. discs, you must give them the care they deserve.

ONE OF THE IMPORTANT FACTORS in the public acceptance of LP discs is the fact that they can be played RECORD CARE with a total absence of surface noise, I. Never use a cloth -moist or dry commonly referred to as "scratch," that -to clean your records. has plagued music lovers for over 50 2. Never rub the record surface with anything. permit the low distortion years. It is of vital importance, there- and high effi- 3. When in doubt about using any- ciency of fore, to prevent these discs from becom- ing scratched or marred in any way. thing on a record -other than the stylus itself your- Extensive investigation shows that the -just ask ULTRA LINEAR self if you would use it on a operation of the audio output stage. This owner himself unwittingly inflicts me- valued soft lacquer disc. new circuit principle and the Acro- chanical injury -scratches -on his rec- 4. Correct any stylus -record mis- sound TO -300" output transformer are ords. alignment. designed to work together for the high- Anything rubbed over the record will 5. Check stylus alignment every est quality sound obtainable. The com- scratch the groove walls by rubbing the time the head is installed on the bination provides optimum performance dust and grit into the delicate glossy sur- arm after being removed for re- resulting in the most linear output face. In the case of a very dusty record. pair, stylus replacement, or for stage which can be made with conven- any other reason. it is better to fluff the dust off a tional tubes. with 6. Never play a record with a stylus loosely waving handkerchief. When out of alignment. The specifications and proven perform- playing, a stylus attracts the dust, lint. 7. If your amplifier has no provision ance of the and other matter lodged in the grooves. for proper de- emphasis, the Therefore, brush the dirt off the stylus maker of your pickup will sup- after each play-it will help keep your ply the proper circuits for opti- records clean. mum results. riC24Q"(441 * Oddly enough, no protective envelope that would entirely protect a disc from TO -300 dust and grit has yet make it the world's finest output been introduced. a way that it does not touch the record transformer. surface. Arm misalignment can be cor- Stylus Alignment rected easily by shimming up one side of the base with a piece of cardboard. It is highly important for the stylus to be exactly vertical on the record, when De- emphasis viewed from the front, and that it does not lean against the groove wall. Last, but not least, is the question of An investigation on this condition ex- circuitry -principally that of the correct tended over a period of eight months and de- emphasis network. If LP records are included pickups and arms of all makes. played with a "flat" high end, there is Out of 74 installations, 39 were found to likely to be some noise. Most users are of the belief be badly out of alignment ; that is, the that the record material stylus was tracking the groove at an alone is responsible for the normally angle- either right or left. An excellent lower noise level of LP's, but such is not method of checking stylus alignment is entirely the case. While the record sur- offered for consideration : faces are naturally quieter, a large part of the noise reduction is due to the use Place a small unframed mirror (such of pre -emphasis in recording, and the as may be found in a lady's handbag) -on corresponding de-emphasis in playback. the turntable. Now, lower At the stylus point In practice, pre -emphasis boosts the $ to contact the mirror in the - Leading same gentle recorded level of $24.7te Distributors manner you would use to lower it onto the high frequencies by a record surface. Standing squarely in a predetermined amount-16 db at 10,000 ,/response - 1 db 10 cps to 100 kc line with the front end of the arm, so that cps according to the standard LP curve, / 20 watts undistorted 20 cps to 30 kc you do not see the sides of the arm, you or 12 db at 10,000 cps with the AES will /40 watts undistorted 30cps to 20 kc be able to see the stylus alignment curve. In playback, therefore, a similar easily, because the mirror causes ,/ Unequalled square wave the an- amount of high -frequency attenuation performance gle of misalignment to appear twice as - ,/ Resonance free response great. de- emphasis -must be employed to re- store the frequency ,/ Uniformity of characteristics As there must be some play in the response to normal. However, since scratch is composed ,/ Compact, rugged construction mounting of a pickup in order for the parts to fit together readily, frequent mis- largely of the higher frequencies, the ul- Complete data and characteristics of Ul- alignment is to be expected. A slight shift timate effect is that the scratch is re- tra Linear Circuits and Acrosound Output of the Transformers on request. reproducer-clockwise or counter- duced. Combined with the quieter record clockwise -will cause the stylus to lean materials, the de- emphasis serves to re- Patents Pending against one of the walls of the groove. duce to The arm also may scratch an almost inaudible level. be out of alignment. Properly played -and carefully taken ACRO PRODUCTS It should be adjusted so that it has no CO. tendency to swing to the right or to the care of -LP records offer the best 369 Shurs Lane, Roxborough, left when the pickup is supported in such quality and a minimum of noise, but Phila. 28, Pa. since the surface is somewhat softer than Chief Engineer, Audak Company, 500 shellac's, the element of care takes on a Fifth Ave., New York 36, N. Y. greater importance.

30 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com You can Hear Them All AT HARVEY and Choose the Finest

SUPER SPECIAL McINTOSH AUDIO AMPLIFIERS MEISSNER 8C A novel and unique circuit design FM TUNER has been teamed with carefully selected components and superb Covers full 88 to 108 me FM bond. construction to bring you amazing efficiency and clean, crisp power. is less than 1% from 20 Flat response: ± 2 db from 50 to 15,000 cps. reproduction at peak Distortion cps. Full bandwidth response is 10 to 200,000 cps with Sensitivity: 40 microvolts. to 20,000 phase shift kept to negligible values. A Dynamic range in excess 8- tubes: 2 -6AG6, 2 -6BA6, 2 -6C4, 1.6AL5, and 1- 6X5GT. of 70 db and extremely low impulse distortion contribute to the Can be fed to phono terminals of any radio or TV receiver, or wonderful listening quality of these amplifiers. high impedance input of any quality amplifier. SOW-2 50 watt amplifier (peak: 100) $249.50 Self -contained power supply... all in handsome Walnut cabinet. - 20W -2 - 20 watt amplifier (peak: 40) 149.50 List Price $91.15 At HARVEY ....$4350 AE2 -A - Preamplifier 74.50

AUDAX POLYPHASE CRAFTSMEN C500 AUDIO REPRODUCERS AMPLIFIER A top -performing, practically distortion - One unit with two replaceable styli operating free amplifier, constructed by a leading with one pressure (8t/2 grams) for all three record manufacturer, and based upon the fa- speeds: 331/2, 45, and 78 rpm. Provides excellent mous Williamson circuit. Total distortion coupling between stylus and record groove, with is less than 0.1% at 10 watts, and fre- high stylus compliance and minimum moss. Response is from 20 to quency response -!' .1 db, 20 to 20,000 cps, and ±: 2 over 10,000 cps., with an output of about 20 millivolts. Needle -talk db, 5 to 100,000 cps. Furnished with (5) tubes, including $9950 practically nil. KT -66 output tubes.

DL -6 Head with Microgroove Chromatic CRAFTSMEN 300 EQUALIZER -PREAMPLIFIER Diamond and Standard Sapphire $41.70 A remote, completely self -powered unit with full -range bass and L -6 Head with two Sapphire Styli 20.70 treble controls for tuner, phono, and other signal material; phono KL -4 Head with Sapphire Styli 20.70 preamplifier for all variable reluctance pick -ups, with 5- position (for record changers.) equalization switch; low -pass filter; loudness control; and cathode - PLUG -IN RECORD CHANGER ADAPTERS follower for long line to main amplifier. Measures: 15 x 9 x 3 inches. for Garrard $2.70 for Webster $2.70 $7950

WILLIAMSON The HR -1 5 AMPLIFIER KIT MAGNECORDette A truly professional tape recorder for The famous William- the discriminating listener... designed son HR -15 ... still ac- for home use. knowledged the lead- er ... in kit form, with Quickly interchangeable speeds: 71/2 or 15 inches /sec. original Partridge Rewinds 1200 feet in 40 seconds. Transformers. Assemble this kit, and in 3 hours or less, enjoy the Frequency response: ± 2 db, 50 to 15,000 cps. at 15 inches /sec. finest sound you ever heard. Operates from a tuner, phono pre -amp, and 50 to 7000 cps. at 71/2 inches /sec. crystal pick -up, or other signal source. Has absolute gain of 70.8 db Dual AC motors for operation and rewind. with 20 db of feedback. Frequency response: ± .5 db, from 10 to Maximum flutter 0.3 %. 100,000 cps. Kit is complete with 5 -tubes (1.5V4G, 2- 6SN7GT, and Housed in an attractive cabinet, natural or walnut. 2- 807 or 6BG6G), 2- Punched Chassis, Pre -wired Resistor Board, Sockets, Genuine Partridge Output Transformer WWFB, Assembly $38500 Instructions, and All Other Necessary Parts $76.50

HR -15, as above, but with Partridge CFB Output 101/2 INCH REEL ADAPTER MECHANISM Transformer (Hermetically Sealed) $90.00 Attaches simply and easily to all MAGNE- PARTRIDGE OUTPUT TRANSFORMERS -Available Separately CORD and MAGNECORDette Tape Record- [414', and high WWFB $26.00 CFB $40.00 41' ers. Permits playback, rewind, speed forward with 101/2 -inch reels. In stock HR -15T WILLIAMSON Kit- Furnished as above, with TRIAD for immediate delivery $35.00 Transformers and Chokes $69.50

NOTE: HR -15 and HR -15T Kits may be had with VISIT THE AUDI O- NOTE: In view of British KT-66 Output Tubes for $3.00 additional. TORIUM. Come the rapidly changing and visit our new market conditions, sound department... all prices shown ore all these items and subject to change HARVEY'S New Catalog of High Fidelity many more a work- without notice and F.O.B., Dept. A6 ing display n at all are Net, Equipment is now AVAILABLE! Write times. New York City.

AUDIO ENGINEERING JUNE, 1952 31

www.americanradiohistory.com EDWARD TATNALL CANBY

TI EkE'S NO nourrr about it, high fidelity same, distorted or not. Good records sound has become the major fad. By which I "The Well- Produced good on good equipment; on cheap equip- mean that everybody is talking about it, ment they sound about the sanie as the poor second hand, and only a few know anything Commercial Record" ones. about it, first hand. A fad is a fine thing in a way (though if we like it, we call it 2. "Emphasis of the extreme registers in the hi -fi often leads to a neglect of the something else), but it has its disad- cellent -but surely not the acme of hi -fi. solid middle and gives a special tone picture vantages, principally in that misinforma- An original tape is the only true hi -fi which may be agreeable to some listeners tion and false attitudes spread faster than medium that can he held as potentially we who are in the business can stop them. but is quite removed from the reality of better than the best "well- produced" sounds Some of the most zealous promoters of discs. as we hear it in the concert hall." unintentional misinformation are among Moreover, it is to the best of my ex- Two bogies here. First, the old, old those who just have to know better, the perience a myth that good recordings -the business of equalization. How many times record critics. best "hi -fi" the industry can produce - must we all repeat -if a recording is Granted that a musician with reasonably sound worse on ordinary machines than played with the proper equalization the sensitive ears has plenty to be hurt by in those that are in some way tailored to sound will be properly reproduced; middle what goes on in the name of high fidelity. fit cheap machines. It's true that the higher and all. Virtually all our records have pre - Even so, if that musician is a writer who range on a good record cannot be heard on emphasized highs, a rising curve. The rea- must judge music as found in recorded the usual machine. But neither can the Sons for this should be clear enough to all form, he should be able to discount some lower end, and the resulting balance is record reviewers by this time. (In fact, of the nasty noises or explain them at least, usually correct for the listening ear. The it is this very pre -emphasis of the highs to help the neophytes in the direction of only way that I know to tailor a record that makes the good record sound passable more pleasing sound. Some items I've read for low -fi use is to add a false peak some- on the average machine with its deficient recently from those in this category where. in the middle -high range, to give a high response.) On good equipment serve only to promote more confusion. The sort of brilliance, that may pass for clarity. wrongly used (as is too often the case), the critic's ink is wasted if his righteous feel- A peak of sorts in the 3000 -5000 range good record may shriek and screech. But ings concerning bad audio are tied to would do the trick perhaps. But is this that is the operator's fault, not the record wrong explanations of the cause for the really practicable or necessary ? maker's. Nasty sounding highs, of the sort trouble. For popular records, perhaps. But I that hurt musician's ears and Mr. X's, are One gentleman, a fine musician and suspect that even there, what counts in the decidedly not a necessary part of our high composer and an aimable soul, has written sound is the presence of brassy, sharp close - fidelity. an unsigned, piece for "Listen" magazine to mid -highs in the actual music. Close-up called "Low Brow or Hi -fi" which de- trumpets, saxes, etc. The mike set -up does Reality? mands an answer from this department. as much as any peak. And in the classical And secondly -never think Like plenty of musicians, this writer, whom area, with larger, more blurred sounds, that a good we shall call Mr. X, doesn't too much like bigger orchestras, record is supposed to give us a concert a necessarily more dis- hall sound. The record, remember, all the excitement over Hi -Fi. He is quite tant pickup (it's in the style) is mon- you can't do aural, not binaural. It gives us an right; a lot of it can be chalked tip as much in this respect without distorting the over -enthusiastic hot air. But the three music itself. impression of naturalness and it may pro- vide stimulating and highly pleasing listen- points he makes in explanation of the I honestly don't think that a "well pro- trouble are not to my mind the sort that duced commercial ing. But it will never give us the "reality" record" is one with an of concert hall sound sometimes, in help clarify things. exaggerated peak in its curve, around the -and middle. If not, my opinion, the recorded sound is prefer- 1. "High- fidelity records reproduce sound then there can be only a able to the concert hall difference in the pre- emphasis -not a peak one, anyhow. The best only on special hi -fi equipment. . . . two are, at this point. on an equality, and Actually, the average machine sounds better but an all -over rise in the highs. And for that, we must all learn to evaluate recorded with the well -produced commercial record within conventionally accepted limits, music in its own we can compensate by proper ternis, as an experience than the special hi -fi disc." equalization that is not a mere substitute for the con- in our quality equipment ; on cheap ma- cert hall. Here is an old myth that will not die. chines a sharp pre- emphasis produces a Good reproduction, hi -fi, do These days, a "well- produced commercial sort of peak anyhow (the highs falling off will never record" is not to be distinguished any harm to the sound captured on a from above a certain point). recording. a "hi-fi" record, unless we count as hi -fi The plain fact is that a lot of poor rec- only those specially made discs in limited ordings, notably those with distortion in Audio and Music pressings, mostly of sound effects and them, sound better on cheap machines than other non- musical noises. They are ex- on fancy equipment. Distortion is sup- 3. "The search for hi -fi often masks other pressed on the cheap machine; good and features in a record such as music and * 279 W. 4th St., New York 14, N. Y. bad, all of the records come through the [Continued on page 36]

32 AUDIO ENGINEERING JUNE, 1952

s. www.americanradiohistory.com When POLYPHASE was first introduced, One single, superb- quality magnetic unit our ads gave detailed laboratory data. By plays all home records ... and it costs no now, however, POLYPHASE fame has more than ordinary pickups. spread far and wide- principally by word Replaceable Sapphire or Diamond styli. of mouth -and the thousands of POLY- Available with the new Audak compass - PHASE owners themselves speak. . pivoted arms and for record changers. "Never before such LISTENING QUAL- The new AUDAX literature gives details, ITY, such FAITHFUL REPRODUC- and includes the now world- famous CHRO- TION." But, you and only YOU can decide MATIC Diamond POLYPHASE -send what sounds best and most pleasing. There- for it. fore, see and HEAR the Chromatic POLY- Be sure to obtain a copy of Electronic PHASE and -YOU be the judge. PHONO FACTS from your distributor. AUDAK COMPANY 500 Fifth Avenue New York 36 "Creators of Fine Electro- Acoustical apparatus for over 25 years."

www.americanradiohistory.com TAPE RECORDER THE ,New [from page 25j cable from the recorder -oscillator chas- sis is plugged into the receptacle on the equalizer box. If the recorder is to be AMPEX SERIES 400 -A used with its regular Presto amplifier, the two terminals of the plug are shorted. The ground lead is carried MAGNETIC TAPE RECORDER through the shell of the plug, and serves to ground the equalizer can. A bracket for the extra idler and capstan sleeve is mounted on the side of this can to keep them out of the way -their normal posi- tion is on the top panel of the recorder. Two other changes were made in the SOLD AND SERVICED BY recorder -oscillator chassis. Since the os- cillator tube used in the regular chassis 1 was a 6Y6, which has a recommended maximum voltage rating of 200, it was thought preferable to replace this tube with one designed for a higher voltage, Audio Ec Video because it works with a 300 -volt plate T ' P R O D U C S C O R'P O R A T I O N supply. A 6V6 was tried, but insufficient erase voltage was obtained, so the tube 730 Fifth Avenue New York 19, Y. N. now in use is a 5881, which draws the PLaza 7-3091 same cathode current and provides the i same h -f output if the screen resistor is changed to a 2 -watt 47,000 -ohm value. LITERATURE This decreases the heating in the re- describing this new portable equipment will be corder case, since the 5881 draws 0.9 rushed to you as soon as we receive your request. amps as compared to 1.25 for the 6Y6. In all fairness, it must be AUDIO & VIDEO also sells and services the fine noted that no products of the leading trouble had been encountered from the audio equipment manufacturers. We will be glad to supply you with use of the 6Y6-it was just a whim that descriptive literature, prices and delivery information. prompted the change. The second change involved the in- CHECK those manufacturers whose products you would like details stallation of a padder capacitor having about. Use this advertisement as a convenient coupon. a range from 200 to 600 µµf in parallel with a 200 -µµf fixed capacitor in place of Exclusive in Canada the 560-µµf capacitor (Cl0, in the .... In Washington, D. C Presto diagram) to provide for varia- Canadian General Electric Co., Ltd. Gallant Engineering Company tion of bias and erase current. Again, in 212 King Street, West 261 Constitution Avenue fairness, it must be noted that little Toronto, Canada Washington 1, D. C. change in performance has been noted, but the designer's whims must be satis- fied. AMPEX STANDARD RECORDERS The Playback Channel AMPEX SPECIAL RECORDERS The second channel is almost identical Alive McIntosh with the first when the selector switch is in AMP Cinema M. M. 8 M. Tape the position, except that no is Ed tall provision made for a high- impedance Pickering input. However, in the REC position of Ele dro -Voice Pre -Recorded Tape the switch, the input stage is connected Fa'rchild Disc so as to provide the remainder of the Langevin Recording Rates low -frequency equalization necessary Lansing Telefex for playback from the tape, and to feed the playback signal out to the radio circuit through J. Most of the low- frequency equaliza- AUDIO & VIDEO PRODUCTS CORPORATION tion is provided by working the 500 -ohm 730 Fifth Ave., New York 19, N. Y. playback head into a 30 -ohm resistance termination across which the input Please send me literature and details concerning the products of transformer primary is bridged, as those manufacturers whose names I have checked above. shown by curve A in Fig. 8. The re- Name quired total equalization is shown in curve B. The small amount of addi- Title tional equalization is obtained by the Ccmpany Name feedback network composed of C R,, Mail Address and R with some high -frequency cor- rection obtained from C15 and Rd,. This circuit is similar to that used in the Magnecorder amplifier, and previously used by the writer in the Interview Am-

34 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com plifier. However, the amount of equali- zation is considerably less in this ap- plication. The entire equalization set -up for this amplifier was determined in the following manner: With the selector switch in the AMP position, both input channels were equalized to provide flat response to the top of the main gain control, Rz,. Then the output amplifier was equalized for flat response to the secondary of the output transformer -this being neces- sary if the amplifier were to be used as a remote pickup unit. Then the playback channel was adjusted for flat response from a standard tape obtained with the Presto recorder. Finally, the recording equalizer was adjusted to provide flat playback from the tape recorded through this unit. It will be noted that considerable high- frequency compensa- tion was required. It is believed that this is occasioned by the use of ex- tremely small shielded wire which car- ries high -impedance circuits through- out the amplifier. This will become evi- dent when the construction is described. Again judging from results, it should be stated that the over -all IM distortion of this unit from microphone input jack to tape output -thus including the tape itself-is less than 3 per cent at full re- cording level, i.e. with a 0 -VU indica- tion on the meter with the level switch set at + 6, and with a playback output of 1.0 volts. The second stage in the second chan- nel is identical with that in the first channel, but the output connects to one section of Sw,. In the NOR and REC po- sitions of this switch, the output of the second stage is fed to the grid of a cathode follower, V,. Sw, in this grid circuit permits choice of monitoring di- rect from the mixing network, or from the tape playback. The 5879 was chosen because of its low heater drain, and be- cause it was less microphonic than the 6C4 which has the same drain. The seemingly high value of cathode resistor for V, was determined by IM measure- ments. In the original construction, a 10,000 -ohm resistor was used, but .IM distortion in the playback channel was over 4 per cent. As the value of this re- sistor was increased, the distortion de- creased to less than 1 per cent. (In Fig. 3 of last month's article, resistor R,, in the grid circuit of V , connected to Sw,b, should have been marked R49, and its value listed in the Parts List as 0.27 meg, % watt. R is correctly listed as 1000 ohms, and is located in the feedback network of V,. This error has been corrected in the ac- companying schematic.) The output of the cathode follower stage is fed through a coupling capaci- tor C, to the closed circuit jack J, and thence through the REC position of Sw,e to J5. In the NOR position of Sw the two terminals of Je are shorted, and in the AMP position they are connected to the output pad. Thus a single jack serves two purposes. AMPEX ELECTRIC CORPORATION Advanced Series 400 -A In connecting this unit to a radio sys- Redwood City, Ca.ifom a tem, usually at high impedance, it is write for Bulletin A -211 suggested that the output of a control unit be fed into terminal 2 of J5, and

AUDIO ENGINEERING JUNE, 1952 35

www.americanradiohistory.com that the input of the power amplifier be connected to terminal 3. Thus this unit intercepts the circuit between the con- trol unit and the main amplifier. This increases the length of shielded leads, but should cause no frequency discrimi- nation if the output of the control am- plifier is of sufficiently low impedance, such as would be obtained with a cath- ode follower. Then in the NOR position FREE! - 68 page catalog section of the switch, the recording amplifier is from Radio's Master. out of the circuit; in the REC position the incoming signal is fed to the mixing Catalogs the products of 30 manufacturers the playback of Recording, Phono Equipment and network, is recorded, and Accessories. from the tape is fed back to the line and the main amplifier. Sw, permits choice Complete descriptions, specifications and of direct or recorded playback monitor- illustrations of such products as: ing. There is a loss in level of approx- Recorders, Phono Motors, Turntables, imately 10 db, which can be compen- Record Changers, Cartridges, Pickups, switch in the Tape, Needles, sated by an additional Discs, etc. main amplifier if desired. The concluding part of this series will cover the construction of the amplifier, the power supplies used, and the various possibilities of hooking up the unit for its many applications.

Mt ;77rr rara a RECORD REVUE fty [front page 32] W performance. It leads the collector down the garden path where his tastes become e quite changed." point in its Represented in RADIO'S MASTER 6B I certainly agree with this pose Recording 6 Phone Equipment main premise. "The business of Audio is Booklet ore the products of the follow. column has often before sug- ing music." this manufacturers: gested. Highs without music (and low ALLIANCE MANUFACTURING CO. AMERICAN MICROPHONE CO. lows similarly) are of precious little in- AMPEX ELECTRIC CORP. terest to this reviewer. It is true enough "'AMPLIFIER CORP. OF AMERICA hi -fi enthusiast finds the mus- THE ASTATIC CORP. that many a AUDAK COMPANY ical values of his reproduced sound about AUDIO DEVICES, INC. the last thing to interest him. Many an BELL SOUND SYSTEMS, INC. in audio, it might be remarked, °rdN Ì BERLANT ASSOCIATES engineer aetRr BOGEN CO., INC. has a similar viewpoint. CLARKSTAN CORP. But I'd suggest to Mr. X that to think DUOTONE CORP. people away from ELECTRO -VOICE, INC. that hi -fi is leading the FAIRCHILD RECORDING EQUIPMENT CO. musical values is unwise. I honestly do 8" 10j,. GARRARD SALES CORP. not believe that anyone "unlearns" music. GENERAL ELECTRIC COMPANY knowledge and GENERAL INDUSTRIES CO. One hangs onto whatever JENSEN INDUSTRIES, INC. love of music one has to begin with, and MAGNECORD, INC. the practice of hi -fi either leaves that MASCO ELECTRONIC SALES and un- M. A. MILLER MFG. CO. knowledge largely untouched MINNESOTA MINING & MFG. CO. affected-or it changes it for the better. NEWCOMB AUDIO PRODUCTS CO. can learn an enormous amount about ORRADIO INDUSTRIES, INC. One PENTRON CORP. music from listening to records well re- You can obtain without cost or obligation, a complete PERMO, INC. produced -or one can learn very little, 68 page catalog section as reprinted from the Industry's PICKERING 6 CO., INC. is most likely the normal state of PRESTO RECORDING CORP. which Official 1100 page Radio's Master, 16th edition. RECORDISC CORP. affairs with the majority of phonophiles. RECOTON CORP. I am fairly sure that most newcomers This section catalogs in detail the products of the REEVES SOUNDCRAFT CORP. to our so- called high fidelity will outgrow leading Recording and Phono Equipment manu. REK -O -NUT COMPANY SHURE BROS., INC. the rabid stage, where highs and equip- facturers - all in 1 handy booklet. It is complete V.M CORPORATION ment specifications mean everything and with descriptions, specifications, and illustrations as WALCO PRODUCTS, INC. music very little. Most of us go through it written by each manufacturer. Whether you buy, sell, WEBSTER -CHICAGO CORP. WEBSTER ELECTRIC CO. for awhile- longer or shorter -as a sort or specify these products, you will find this booklet Thee nufacturers not included of growing stage. Some poor souls never extremely helpful. in Radio's Master section. Separate get through it at all. But then lots of catalog sheen will accompany each MAIL TODAY booklet. people never grow up at all, either. The This offer is -fi newcomers made possible trouble is that the rabid hi make the most noise and do the t. by a special always AUDIO ENGINEERING most hurting. The agonized musician arrangement be- every few tween AUDIO 342 MADISON AVE., NEW YORK 17, N. Y. probably doesn't notice that ENGINEER- months the turnover is complete-a new Please send me, without cost or obligation, the 68 page Recording batch of noise -makers have discovered ING and the & Phono Equipment and Accessories Booklet reprinted from publishers of hi -fi and the old batch has graduated into RADIO'S Radio's Master. intelligent, adult, sensible listening for real MASTER. Be Name musical values. sure to get And don't anybody think that high your Address copy now. fidelity, rightly used, doesn't enhance Fill course does. For it in the coupon City Zone State musical values. Of it Is, after all, by its own name, more music below and mail. per given sound.

36 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com KEY o Outstanding recorded sound for the type of music. Unresonant, deadish acoustics. you "talk test "... b Big bass. b0 Big brass. Close -to highs with sharp edge. C Some distortion in highs. r Flattish high end; needs boost Peerless over normal LP playback. I live, rather 0o talk distant mike pickup. m Recorded hum. " From older 78 disc originals. t' Piano is excellent. Some surface noise. r Tym- pani, drums, percussion excellent. Violin (or other) solo very close -to. ' Bass seems weak; will need boost over normal PEERLESS ì 240.0 LP playback. Y Lacks highs. " Fuzzy spots -imperfections. all tests demonstrate

Modern -good for hi -fi. PEERLESS at Black Maskers (1923) . Sessions, The Square Wave Amer. Recording Soc. Orch., Hendl. superiority Amer. Recording Soc. ARS -11 music, reminiscent Acid, powerful early -twenties Comparative square wave tests on trans- of "Le Sacre," the Scythian suite, etc., but with formers shown all over the country have demonstrated Peerless Sessions' high pressure, almost violent expression. Cow Ismut NS.1 Wonderful hifi, but it'll hit you between the eyes. superiority ...Now Peerless emphasizes another very important property of transformers as shown by the "exciting current test." alp Rawsthorne, Piano Concerto #2 (1951). Curzon; London Symphony, Sar- An output transformer's ability to deliver plenty of clean, low- frequency (the goal of every music lover) is inversely proportional to the gent. London LPS 513 Competitor No. 2 power Neo- Rachmaninoff stuff from a leading British amplitude and distortion of its exciting current. composer, little known here. Romantic slant makes handling is illustrated for easy listening, but I'd rate it as musically PEERLESS superior low- frequency power capacity overstuffed and derivative, though basically well in these comparative oscillograms. written. Competitor No. 3 Write for complete data. Elliott Carter, "*l' Piano Sonata (1945 -6) ; Cello Sonata (1948) . Beveridge Webster, pf.; Bernard Greenhouse, cello, Anth. PEERLESS Makas, pf. Amer. Recording Soc, ARS -25 DL t on Electrical Products The piano sonata is an outstanding example of good piano recording. Difficult, nervous, complex 1P and jittery music, strictly an art sonata and 9356 Stott Mom. Dive. scarcely for the novice listener -but the fine Beverly Hills. Calif. sound and expert playing may carry you away. 161 Sixth Avenue The cello sonata, a similar idiom, is less accessible Competitor No. 4 New York 13, N.Y. to average listening. But both are major works for those who can take 'em.

Ture Rangstroem, Symphony #1 in C sharp minor (1914). Stockholm Concert Ass'n, Tor Mann. London LLP 514 A hugely scored symphony in the best gloomy northern tradition, out of Sibelius-I'd call it a passel of cliches, and a poor work. Might do for a Garbo picture, some day. (But evidently this composer is much thought of in Sweden.) Prokofieff, "Love for Three Oranges" suite Dynamotors Genemotors

1 1921) ; Russian Overture, Op. 72 Radio Berlin Symphony, Rother; (1936). EFFICIENT Berlin Philharmonic, Steinkopf. DEPENDABLE ... COMPACT... Urania URLP 5005. Here coupled with the early and brassy L? Racine/ Pow "Oranges," now being heard more frequently, is a heretofore unknown (in the U.S.A.) big overture, a good- humored, tuneful, heavyweight product of Carter DC to AC Converters, Dynamotors, Genemotors, P.'s Sovietized days -along with "Peter" and Magmotors, and Inductor Alternators )inverters) ara "Lt. Kije" and not unlike them. A good contrast, both well played (the "Oranges" very well), both mode in a wide variety of types and capacities adapt- well enough recorded-the "Oranges" would be Recorder Converters able to communications, laboratory, and industrial ap- outstanding except for the overloading, some- where along the line, that is an occasional Urania plications, of many kinds. Widely used in aircraft, hallmark. marine, and mobile radio, geophysical instruments, laboratory work, ignition, timing and many other uses. Copland, Music for the Theatre (1925) . e Moross, Frankie and Johnnie (1938). Amer. Recording Soc. Orch., Hendl; solo- ists. Amer. Recording Soc. ARS -12 C@P M oß013 The American Recording Society issues this Chicago 47 and the others above via the Ditson Fund, non- 2648 N. Maplewood Ave., profit; for such a venture the results are terrific Inductor Alternators Saler Onces in Principal Cities technically (where so often we find dreadful work) and most interesting musically. Copland's 1925 shocker isn't quite as jazzy and wicked lure as in the old Victor recording, one of my pets -a MAIL COUPON FOR CATALOGS rather academic performance, this, though good. "Frankie and Johnny" you will find unusually Please send catalogs containing complete in interesting. Much of this type of concertized folk formation on Carter Rotary Power Supplies. music is dreary and pretentious musically, but this is well written and ultra- dramatic. Three Name sopranos sing the story, to the old tune set in Address fancy modern style. This is the music for the Trade Mark ballet, which has appeared on and off for many Registered City State seasons. Magmotors

AUDIO ENGINEERING JUNE, 1952 37

www.americanradiohistory.com NEW PRODUCTS

P-J Speaker Enclosure. Long-awaited Constructed of % -in. Philippine mahog- commercial introduction of the R -J cabi- any plywood the new cabinet is available net was scheduled at this writing to take in two models-one with space for instal- place at the Chicago Audio Fair. An ex- lation of a 12- or 15-in. speaker, the other hibit of experimental models was shown with space for record storage. All parts, at the Audio Fair in New York last fall. such as changer- drawer slides, hardware, First Introduced through the pages of Æ, glue plastic wood, and grill cloth are sup - the R-J enclosure departs entirely from plied. Assembly and finishing instructions conventional concepts of design in that it are also included. Complete details will be supplied on request to the manufacturer. Medium- Priced Ampex Console. Fea- tures that have long been inherent in more expensive models are included in the new Ampex console -type tape recorder de- signed for the moderate -price market. A professional, dual -speed recorder, the new Model 401 -3 includes push- button control of all functions, built -in microphone pre- amplifier, and 15,000 cps response at 71/2 ins. /sec. tape speed. Model 402 contains half-track heads while Model 403 is a single -track unit. Both models feature a newly -styled cabinet which, through use the new Permoflux low -cost corner cabi- net, designed primarily for housing the Permoflux Royal Eight speaker but equally suited for improving performance of any other 8 -in. unit. Handsomely finished in a choice of mahogany or blonde, the console measures only 25 in. high x 20 in. wide x 11 in. deep. Permoflux Corporation, 4902 -A W. Grand Ave., Chi- cago 39, Ill. does not depend on large cubic displace- ment to achieve low -note reproduction. Portable Turntables. Variable speed Large enough only to enclose the speaker control, permitting adjustment from 25 it contains, the R -J virtually eliminates nno w osrpmadl to 10 p the factor of cabinet resonance in audio aboveCO atn o f 7 8, 45 and reproduction. Performance is governed 33 1/3 r.p.m., is featured in two new port- entirely by characteristics of the driver. able turntables designed for moderniza- High- frequency tones are direct- radiated tion of sound systems, manufactured by and low- frequency response is governed Califone Corporation, 1041 N. Sycamore largely by cone resonance. Although exact technical specifications are not available, tests have shown the R -J capable of re- producing pedal notes of an organ with startling realism. The R-J is presently available for 12- or 15 -in. speakers in a choice of mahogany or blonde finish. R -J Audio Products, Inc., Dept. HR, 164 Duane St., New York 13, N. Y. of a unique tilt -hinged mounting of elec- tronic and top -plate assemblies, permits quick, easy inspection of any component Xiaiatnrised Phone Ping. Measurable while the recorder remains in operation. reduction in operating space of audio - Complete details and specifications are equipment is effected by the new "imp" available by writing Ampex Electric Cor- phone plug recently introduced by Carter poration, Redwood City, Calif. Studio -Type Audio Console. An entirely self- contained a.c.- operated unit, the new Altec Lansing 230B audio console Is de- Ave., Hollywood 38, Calif. Designated as signed for two -studio broadcast -station Model 8V with turnover -type crystal cart - use, as well as for professional recording ridge, and 8V -P2 with dual -stylus mag- studios and extensive public -address sys- netic cartridge, the units are housed in tems. There are four separate preampli- attractive leatherette-covered carrying fiers, two booster amplifiers, a line ampli- cases with conveniently recessed controls. fier and a monitor amplifier all mounted on a single chassis. Also included are Preamplifier -Equaliser. Remarkable flex - miniature plug -in power supplies for ibility is inherent in the new 120 -A pre- plates, filaments, relays, and signal lights. amplifier- equalizer recently introduced by The console is equipped with six mixing Hermon Hosmer Scott, Inc., 385 Putnam potentiometers, four of which are con- Ave., Cambridge 39, Mass. Designed for nected through switching keys to eight use with most existing power amplifiers, low -level microphone or turntable inputs. the 120 -A contains a record compensator The remaining two are connected with for virtually all recording characteristics. four line inputs. Thus is provided a total Treble roll -off and bass turnover equaliza- of twelve inputs, any six of which can tion are provided on eight positions, in- be mixed simultaneously. An emergency cluding five for standard 78's and three Parts Co., 213 W. Institute Place, Chicago for LP's. Continuously- variable tone con- 10, Ill Tip and sleeve of the new plug trols provide boost and attenuation of fit standard 2-conductor jacks, but the both treble and bass. Loudness control molded Bakelite handle has been reduced compensates for characteristics of the to approximately half normal size. human ear. Provision is made for using the Scott Dynaural Noise Suppressor as Pre -Built Radio -Phono Cabinet Kit. an accessory. Frequency response is sub- Ample space is provided for standard stantially fiat from 19 to 35,000 cps. All makes of tuners, amplifiers and record heaters are d.c. operated to minimize hum. changers in a new custom -cabinet kit The 120 -A is housed in a beautifully -fin- recently announced by Modecraft Cabi- ished hardwood case, the rear of which is nets, 64 Robbins Road, Arlington, Mass. open to permit instant removal of tubes for inspection or replacement. Descriptive literature will be supplied free by the manufacturer.

switch permits instant replacement of the line amplifier with the monitor amplifier. System gain is 100 db. Frequency re- sponse is ± 1 db from 20 to 20,000 cps., and signal -to -noise ratio is 74 db. Not- withstanding the 230B's many functions, dimensions are only 9 3/8 in. high x 36% in. long x 17 in. deep. Manufactured by Altec Lansing Corporation, Beverly Hills, Calif., nd distributed by Graybar Elec- tric Company. Corner Speaker Enclosure. Exceptional low- frequency response is achieved with

38 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com RADIO STATION [from page 17] conditions which are often expensive to Leonard correct later. However, there are some basic acoustic principles, and if the sta- s° HIGH FIDELITY tion engineer knows them, he will be able to cooperate intelligently with the SOUND EQUIPMENT acoustic consultant, architect, and con- tractor. ,: AT ITS FINEST There are two general problems in wow broadcast acoustics. One is to minimize + sound leakage into and out of studios; sss 4ll` ai A section of the world's largest Sound the other is to control the quality of the Equipment Studios - at LEONARD'S. sound to be broadcast. Perhaps the best way to minimize sound leakage from ú outside the building is to build the > WHEN YOU LEND AN EAR studios in a relatively quiet area free ó from heavy traffic, airplane noise, etc. YOU SHOULD GET IT BACK WITH INTEREST Noises from other parts of the building caused by heavy machinery like printing Yes -when listening, you bave a right to enjoy more interesting, more presses, pumps, etc. are best avoided by pleasurable sound reproduction. You will get it with these new units -at LEONARD'S! buying some other building. This is not u an attempt to be facetious, but simply a NEW HIGH FREQUENCY UNIT restatement of the old proverb that pre- < The JENSEN RP -302 High Frequency Unit vention is better than cure. Having z makes a 3 -way system from your present taken such precautions, the next thing is u coaxial, or a 2 -way from a single unit. This to consider isolation "super -tweeter" mounts flush on baffle or the sound value of Ñ panel in 1- 11/16" hole. Features include: various wall structures. Other things Smooth clean "highs" from 4,000 cycles being sY to 18,000 cycles. equal, the sound isolation value - o Small horn disperses sound to 120 degrees or transmission loss-of a single, solid in horizontal and vertical planes. o Impedance 16 ohms. Power rating 30-40 wall is proportional to its mass in z watts speech with music signal when com- pounds per square foot of z bined with A-402 Crossover Network area. Air- (illustrated below). borne sound impinging on one side of Ó Compact- Overall diam. 2 -3/4 ". length sti 3 -5/B ". Model RP -302 .. NET $32.93 a wall sets the wall in vibration, thereby m imparting sound energy to the air on The JENSEN ST -901 H -F Balance Control is the far side. The more a variable 16 ohm L pad to adjust level of massive the par- RP -302 High Frequency Unit .... NET $3.38 tition, the less it will vibrate under a 8 A-402 JENSEN Crossover Network JENSEN RP -302 given level of airborne sound. é Divided into two channels (hi -pass, low pass) However, there are practical limita- with crossover at 4000 cycles. The RP -302 companion unit connects to hi- channel, your tions to increasing the mass, namely é present system or speaker to low channel. weight and cost, and recourse can be 180° constant resistance. Impedance 16 ohms ó input. Attenuation, 12 db per octave outside had to another technique -that of in- 0 pass band. 4 -3/8" high, 3 -1/2" wide. 3 -1/4" terposing a barrier in the path of vibra- deep. Model A-402 .... NET $6.61 tion through the wall. The most com- z mon method is to use a double wall ELECTRO -VOICE Model 18W enclosing Very-Low Frequency Driver an air space. Contrary to Extends bass response to 32 cps. 224 sq. in. popular opinion, air is not a good sound cone area. 5 -1/2 lb. Alnico V magnet. Power insulator but it offers a high mechanical input 20 W above 30 cps, 30 W above 100 cps. W Impedance: 12 impedance, t ohms nominal, 8 or 16 ohms so to speak, to the transmis- optional. 18 -1/4" diam. baffle opening JENSEN A -402 ELECTRO -VOICE 18W sion of vibration. Such a wall is called 16 -1/2 ". Model 18W .... NET $7938 z a discontinuous structure. W Aside from wall vibration, airborne sound leaks into studios through 0 cracks New WEATHERS One -Gram and holes and is overcome by calking all W openings, and sealing masonry pores by é Capacitance Pick -Up (Cartridge Form) plastering. Some airborne sound also o At last -realism without noise, 20 to 20,000 cycles. Will 0 improve and replace any modern phonograph pickup. enters through electrical conduits and One gram stylus pressure with ideal tone arm. ventilating ducts ; these problems will be 4 Two grams pressure on modern Webster changers. discussed under the sections on electrical Y Very low pressures with other players, including RCA 45 work and air -conditioning. 4 changer. o Will operate properly with most good arms. The table in Fig. 2 quality tone shows the ap- Oscillator requires 6.3 volts ac at 0.3 amp, and 250 volts dc proximate transmission loss of various at 2ma. Available from most amplifiers. types of wall constructions. Since the Complete with ultra -flexible cable, oscillator unit, tube, net $37.50 degree of isolation increases with fre- 3 installation hardware and instructions quency, these figures are the averages WEATHERS power supply, complete with power cord, for 117 volts, 60 cps. net $14.50 for the range of 128 to 4096 cps. Note u Mail Orders filled promptly. Write Dept. A6 for immediate, courteous service. that the table applies only to airborne sound, and not to solid -borne sound such g Send now for our FREE 48 p. CATALOG of as impacts. However, the latter are ithe latest and finest in Sound Equipment. greatly minimized by the discontinuous COrtlandt 7-0315 or double -wall type of construction. In this connection, the staggered stud wall the hou.e in the table gives remarkably good iso- built on 69 CORTLANDT ST., lation as to both SERvh F; airborne and solid - Leonard NEW YORK 7, N. Y, borne noise, considering its relatively

AUDIO ENGINEERING JUNE, 1952 39

www.americanradiohistory.com low cost and light weight. This type of structure is shown in detail in Fig. 3. One factor affecting the degree of iso- lation needed is the type of program- AMPERITE Ampex + Terminal ming. Ideally, all studios should have Studio Microphones dividing walls of 55 or 60 db transmis- = unrivaled sion loss so that any type of show could at P.A. Prices be produced in any . studio without fear Performance of mutual leakage. However, it is some- Service! times necessary to cut corners to meet Ideal for Reliability ... a budget and the wall isolation can vary BROADCASTING with the type of program material. Be- RECORDING tween studios used only for conversa- PUBLIC ADDRESS tional -level speech such as newscasts "The ultimate in micro- and dramatic shows, 45 db might be a phone quality," says Minimum value. The same figure could Evan Rushing, sound be accepted for the wall between the engineer of the Hotel studio and the New Yorker. DIRECT FACTORY DISTRIBUTOR control room where a Shout right into the FOR ALL higher loss would be difficult to achieve new Amperite Micro - AMPEX RECORDING EQUIPMENT because of the leakage and vibration of phone-or stand 2 feet presents the observation glass. Between two away -reproduction is studios used for music always perfect. or dramatics, 50 Not affected by db is a minimum Models the NE W and 55 or 60 db is any climatic conditions. better. Guaranteed to with RBLG -200 ohms Another factor affecting the needed stand severe "knocking RBHG -Hi -imp. isolation is the ambient level. This is the around." List $42.00 AMPEX sound level that exists when no program is going on, and is due to leakage of "Kontak" Mikes '403' CONSOLE outside noise, building vibration, ven- Model SKH, list $12.00 tilation system hiss, etc. The networks Model KKH, list $18.00 TAPE RECORDER like to keep this down to 25 db (meas- Our,r' ured on the "A" scale of a standard p Introductory Write for SpecialS folder. sound level meter). Special illustrated However, this is and 4-po9 sometimes an expensive ideal to attain Offer: and in the writer's experience, an am- bient of 30 to 35 db can often be toler- AMPERITE Cmpany Inc. ated in practice. 561 BROADWAY NEW YORK 12. N. Y. lip to this point, we have been dis- Canada. Atlas Radio Corp., Ltd., 560 King 5t. W., Toronto cussing the first half of the acoustic problem -leakage of sound into and out of studios. The other half of the prob- lem is the control of broadcast sound originating in the studio. One of the most important factors here is reverbe- ration. There are few real -life situations (except outdoors) wherein reverbera- tion is not present to some extent. The naturalness of reproduction depends in large measure on the degree to which Here is the "standard setting" the reproduced material contains the 400A in a beautifully designed reverberation the listener associates gray modern console cabinet. with the corresponding real -life situa- tion. t= Dimensions: 231/4" wide, 301/4" $A95 77 i high, 253 /e" front to back. Reverberation is Available in full -track or half.' Reverberation not the same as echo. The latter is a separate and distinct ASSEMBLE THIS HANDSOME track models. ` repetition of the original sound. Rever- Remote control assembly optional beration is the extent to which the sound Philippine Mahogany at extra cost. lingers after the source has been cut off, and is quantitatively expressed as "re- audio cabinet verberation time." This is the number Brochures and engineering data AND SAVE % THE COSTI on all AMPEX equipment of seconds required for the sound to die away to one millionth of its original in- I At last, a cabinet worthy of available on request. the finest equipment at a price tensity, a reduction of 60 db. It is com- you never dreamed possible! Visit our audio department by Separate changer, tuner, ampli- puted the Eyring equation fier compartments ample for all zz` rY fora recording and play- popular makes. Accommodates .049 1V 12" or 1 S- speaker. 38"' h. x _IN. bock demonstration. T 36'/ " w. x 183/4" d. overall. -loge (1-a) S Slide-out changer board cut out for Webster, Garrard or blank. where V is the studio volume in cubic M nus grille, Control panel blank. Pre-cut and feet, and S is the total surface area in- w th big rec. sanded 3/s" Philippine Mahogany o d storage plywood. End grain veneered. /Terminal cluding walls, floor, and ceiling. The space. All hardware, glue, assembly term a is the noise absorption and finishing (blond or dark) in- coefficient $45.95 Complete structions, etc. Dealer inquiries for a given frequency. It is obtained by l inv'ted. Radio Corp. multiplying the number of square feet of Send 25% down (balance and express each type of surface by its coefficient at charges C.O.D.) or write for details. 85 Cortlandt St., New York 7, N.Y. a specific frequency, adding the products modecraff pre -built cabinets WOrth 4 -3311 so obtained and dividing the result by 64 ROBBINS ROAD ARLINGTON MASS.

40 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com the total surface area of the room. The coefficients for various acoustical mate- rials are given in the trade literature2 Mer who know Q for the standard frequencies of 128, 256, 512, 1024, 2048, and 4096 cycles. The McPROUD, log of (1 a) to base c can readily be C. G. for Top TV Performance! obtained from the more familiar log10 by Editor of AUDIO multiplying the latter by 2.3026. How- Engineering, writes: been obliged to assemble several ever, as soon as a is known, the logarith- "I assembled my more Techmaster receivers for mic work can be avoided altogether by own home TV re- friends who marvelled at the pic- off value of the log term picking the from a standard Tech - ture quality of my own. In each Fig. 4. ceiver from the curve, at the case the results were excellent and in master kit. I was amazed Consider the sample calculation with which have provided their owners with Fig. 5 in which for simplicity, only speed and simplicity three surface materials are involved. this coulci be done. So gratifying uninterrupted top quality, Given a studio 10 feet high, 17 feet was the result, that I have since trouble -free performance ... " wide and 28 feet long. The walls and ceiling are covered with material "alpha." The floor is covered with ma- More leading engineers and chosen Tech - terial "beta." One of the short walls con- technicians have Exp :rtsEverywhere Concede the Superiority master Kits and Techmaster tains thirty square feet of observation than any Technaster 630 Type Circuit! Chassis, glass and five persons are assumed to of the other television. work in the studio. The calculated time The TECHMASTER 630 has been at 1024 cycles is 0.46 seconds. advanced in every detail of modern en- Reverberation time can be made to gineering and design. Every component vary with studio volume for a given is the best available, and rigid factory frequency, and with frequency for a and test standards are your assurance given studio volume, as shown in the of years of trouble-free performance. Morris -Nixon curves, Fig. 6. Curve A Prefabricated unit construction, factory shows desirable times for various studio pre.adjusted and pre -aligned make as- volumes at 1000 cps. Curve B shows the sembly an utterly simple and enjoyable ratio between the time for any given procedure with a minimum of tools. frequency and the time for 1000 cps. For example, in a 10,000 -cu. ft. studio, the time at 1000 cps is 0.6 seconds from See your local Jobber for model details illustrated literature. curve A. Curve B shows that for 100 or unite direct to factory for cps, the time is 1.7 times 0.6 or 1.02 seconds. 0 TECH- MASTER PRODUCTS COMPANY 443 -44S BROADWAY, N. Y. 13, N. Y. The Morris -Nixon curves apply prin- cipally to music studios and provide relatively live acoustic conditions. Music is almost always heard in real life with considerable reverberation, and natural- ness requires that this be provided in its transmission and rêproduction. On 4ltcae44:.t .' the other hand, speech studios require ea 44/t ' .--TUBE CHECKER / i somewhat less reverberation than is in- u>io GEN KIT dicated in the curves. Reverberation t_IT . '- - y295o times of 0.35 to 0.6 seconds are common for speech studios. The writer suggests that for speech, curve B remain flat at Hcathkits are con',. the 1000 cps value as the frequency de- pletely engineered instruments creases below that point alterna- supplied unassembled Every kit g.es -or and easily. All dr K- tively, that it drop slightly 1000 together smoothly below ing, punching, and paint ng has al- cps-and that it rise only slightly above ready been done for you. 1000 cps. For very small announce Its easy and fun tc bui d a Hea h. booths, a flat curve is suggested, with a kit. All parts are furn shed and are of value under 0.4 seconds. highest quality for y :ars of troulie- free, dependable operation Another factor affecting tonal con- Save mont-y by censtructing yam. trol of broadcast sound is the degree of own. All expensive w ring and asscn. diffusion or dispersion of reflected bly costs are completely cllminateel BAT EBY- sound. When there are few reflecting Detailed construcricn man. al ELIMI,LATO, where each wire and paths between the source micro- shows clearly KIT ... . and the part goes and tells axanly how to phone, there is little diffusion, and as a build the kit. Write for free cataLsg. result, there may be standing waves whose frequency depends on the dimen- sions of the studio. Sometimes reflected sound arrives at the microphone simul- taneously with-and in or out of phase It VACUUM TUBE with -the direct sound. This results in _ VOLTMETER KIT reinforcements and reductions and a $245? studio so plagued is said to have "live" and "dead" spots. This condition may be particularly bothersome in a studio which has two equal dimensions, since HEATH COMPANY BENION ARBOR :5, M I C E I G A N 2 "Sound Absorption Coefficients of Ar- Rear IINIIRMRTONU CONY. chitectural Materials." Booklet of Acoustic III I SEW VICTIM n 1141 Materials Association. Bulletin X, 1948.

AUDIO ENGINEERING JUNE, 1952 41

www.americanradiohistory.com there can then be two sets of standing TWIN -TRAX* waves of identical frequency. When all Official Magnetic Recorder three dimensions are equal, trouble is TAPE RECORDER almost a certainty. of 1952 OLYMPICS One way to avoid standing waves is to select room proportions which are Selected in International Competition neither square nor cubical, and which by the Olympic Purchasing Commission do not permit the ratio of any two dimensions to Because of their quality, be a whole number. Most dependabil- authorities suggest ratios among height, ity and exceptional value, Twin -Trax* width and length based on the one -third Recorders were selected exclusively by the or two -thirds octave principle. For ex- Olympic Purchasing Commission for all on- ample, consider a ceiling height of 12 the -scene recording and subsequent feet. Two -thirds the octave of 12 is 8 rebroadcasting of this year's Olympic which, added to 12, gives 20 feet as the Games by sportscasters and commentators preferred width. Recommended dimen- of every nation. sions for various design conditions are given in the table in Fig. 7. Note that Recorder, Yes, engineers of Finland's Olympic Pur- the dimensions reflect different ratios o Twin :trot.' °t own the gre chasing Commission circled the globe for different sizes and shapes of studies. too, can for of You, chosen Pproject while visiting Standing waves can also be reduced rely recording you'll every manufacturer of tape historic by using splays and serrations in walls est and °Aso TwinTro recorders and testing their products, to c -with thousands and ceilings to provide multiple reflec- our select the tape recorder that would not broadcast dotions, tion paths. Another way is to build op- n Cogeocies an only guarantee perfect recording, but give i o9 loversvers posite walls out of parallel, and to slope government music their them critical maximum value for their money. the ceiling with respect to the floor. schools, also made recorder. who hove Multiple reflection paths not only reduce tuck exclusive Write today for Catalog »5324, containing complete "officiel the likelihood of standing waves Mark Reg. specifications and direct factory prices. but also ''Trade provide a smoother decay curve. In AMPLIFIER CORP. of AMERICA 398 -4 Broadway New York 13, N.Y. studios having good diffusion, micro- phone placement is less critical. Construction details on the acoustic features described will be discussed in the next issue. The writer would like to acknowledge the kind assistance of Mr. Authoritative M. J. Kodaras of Johns- Manville, and and Messrs. George M. Nixon and H. M. Gurin of the National Broadcasting Company in furnishing some of the Enlightening... acoustic material herein.

Acknowledged the Leading Publication AUDIO IN THE HOME in the Field of Sound Reproduction Li) ow pays 281 -a condition found in only our most I f you are novice, hobbyist, experimenter, or engineer ... if you professional equipment. As in the Brook are a lover of music and in pursuit amplifiers, tone controls are in steps and ... of sound, undistorted are rather abrupt. It has extremely low . . . Audio Engineering will be your faithful, reliable com- distortion and a high damping factor, panion all the way. You will find no more pleasureable and and when used with the proper speaker stimulating reading and cabinet would be considered one of than there is in AE; absorbingly interest- the best amplifiers available in this ing material, valuable and authentic data, workable detailed in- country today. structions... all comprehensively and yet practically presented. The Musician's Amplifier This is the American version of the "What to Do" and "How to Do" will guide your every move famed Williamson amplifier that was through this thrilling experience we call Audio. first published in Wireless World in April and May, 1947. Much credit should be given to David Sarser and Melvin Each new issue brings New Ideas, New Slants, and Latest Devel- Sprinkle for being the first to adapt this opments ... month in and month out ... twelve times a year. famous circuit to American components.' There have been many Williamson cir- BE SURE to get your copies REGULARLY. cuits attempting to capitalize on this publicity, but few have come anywhere near equaling it in popularity or in quality. The biggest difficulty for many AUDIO ENGINEERING who have built or obtained MAIL this this ampli- 342 Madison Avenue, New York 17, N. Y. fier has been the lack of a good quality preamplifier with adequate controls. Coupon Enclosed is J Check C Money Ordo fer $ Please send a copy of each new issue of Audio Engineering for the next This is a resumé of some of the pop- 12 months; C 24 months le: ular amplifiers in this country. There NOW Please Print of many Name are, course, others available, but most of them lack tone controls suf- Address ficiently flexible for living room opera- City Zone Slate tion.

Subscription Prise: U.S.A., Canada and Pon American Union: 1 year.. $3.00 2 yea.. .55.00. Other countries: 1 year..$4.00 2 years..$7.00. ' David Sarser and Melvin C. Sprinkle, "Musician's amplifier." AUDIO ENGINEER- ING, Nov. 1949.

42 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com WILLIAMSON AMPLIFIER [from page 271 1 ûPE/ transformers *- of the circuit utilize power sc which furnish 400 volts at 200 nia. .ATANEW Since the drain of the circuit does not LOW PRICE! exceed 130 to 140 ma, the voltage ob- filter This superior LABORATORY instru- tained out of a capacitor input and ment possessing many outstanding 5V4 rectifier is about 450 volts. This is features. A vertical amplifier of 5 -mc the correct value for the circuit as con- bandwidth; a sweep oscillator variable verted. Lower voltage will limit the to 150 kc in 6 ranges; full 4" vertical power output capabilities. deflection without overload. Excellent reproduction of 1 -mc square waves. 30-cycle square wave tilt less than Performance 5 %. Slotted bezel for insertion of Figure 2 shows intermodulation dis- graduated scale and light filter. Cali- tortion versus power output. It can be brated continuously variable vertical seen that the power output of the cir- gain control. cuit is effectively doubled over that of Never before could you obtain so many the original circuit for a given distor- outstanding features in a laboratory pre- Model WBO -50 cision scope at this low price. tion. At low levels, around 1 watt, the IM hits such phenomenal values as Write for Bulletin. .06 per cent. It is only 0.3 per cent at The Vertical Amplifier has a sensitivity of 20 millivolts RMS per inch of deflection, a fre- quency response (sine wave) of 20 cycles to 5 megacycles that is down only 3 db at 5 me, 13 watts. This curve is based on equiv- a square -wave response that is an excellent duplication of all square waves between 30 alent sine wave power in order to make cycles and I megacycle with a maximum tilt of 5% for 30 -cycle square wave. a maximum it comparable with all the other pub- input potential of 1000 volts peak -to -peak, and an input attenuator of XI-X10-X100 positions, data on the Wil- frequency compensated. lished and advertised The Horizontal Amplifier has a sensitivity of 0.3 volts RMS per inch liamson circuit.' The values graphed of deflection. and a frequency response (sine wave) flat to 300 ke. in Fig. 2 can be divided by 1.47 for The Recurrent Sweep Oscillat, has a frequency range of 10 cycle to 150 kilocycles in i those who wish to have direct compara- ranges, with excellent linearil, r entire range. Expanded horizontal deflection. bility with the meter readings obtained AtA on the intermodulation test equipment. $24950` Figure 3 shows oscillograms of INC. F O.B. Factory square -wave traces taken through the Pncr subject complete amplifier with repetition rates to chançr of 20 cps and 50 kc. Traces at inter- 2667 N. Howard St., Phila. 33, Pa. mediate frequencies approach theoreti- cal perfection, and even such a rigorous HARRISON AUDIO SHOP REPORTS .. . test as the 50 -kc wave shows up ex- Sensational New ACRO TRANSFORMERS tremely well. The waveform has not Sweeping High Fidelity Field in Sales! "sined off," and the extent of ringing is less than that exhibited by the 5000 - cps wave of many good quality amp- lifiers. These square -wave tests were W5/19 a Niliam5an 77 made at a comparatively low level which makes the test even more rig- Double your power output, and orous. At low excitation levels, the in- reduce distortion by 50%:: ductance of an output transformer de- creases, the phase shift increases and the tops of the square wave tilts. A high - ACRO TO -300 level square wave will appear better The World's Finest Transformer " than a low -level one at low frequencies. Similarly, high powers at high fre- quencies will clip any supersonic peaks Response - 1 db 10cps to 100.000cps in the response and improve the ap- Rated to handle 40 Watts pearance of the square wave. The use Resonance -free response of a high level of power can make a Jnequalled square wave response s2475 relatively poor amplifier appear better Low phase shift net on square wave tests. Most quality per dollar! The frequency response of the con- FREE verted amplifier is flat ± 1 db from less ACRO TO -310. Some ratings but 20 watts. For 12 page ACRO than 5 cps to 200 kc. Its phase shift Wnte for con - converting all popular 10 -watt 6V6 amplifiers to Qs Ampliher and reaches 3 deg. at 20 cps and at 20 kc, data; the sensational Ultra Linear Operation 8.758 indicating symmetry of response with sion schematics respect to the audio band. Convert to Ultra Linear Operation by simply KIT The amplifier puts out 30 watts of changing three resistors and condensers! ... $1.00 power over a range greater than the audio spectrum. However, this type of All units fully potted in attractive ACRO TO -290. For the original Williamson power curve, as measured by response grey ha mmertone case. Camped, Amplifier. Performance equal to the finest, re- at high power levels, is not too meaning- only 3'." x alb" x 41/4" high. gardless of price ...... $15.75 ful. The important consideration is the amount of undistorted power available u C O D J.,...J 'Arm. .,.dow 25x D.po,e ARRISON RADIO CORPORATION a Sarser and Sprinkle, "Musician's 227 GREENWICH ST. amplifier," AUDIO ENGINEERING. November NEW YORK 7, M. Y. BArcl 7 -7777 1949.

AUDIO ENGINEERING JUNE, 1952 43

www.americanradiohistory.com at various frequencies. The Ultra Linear Williamson arrangement puts out close to 25 watts at 20 cps and at 30 kc with- CLASSIFIED out clipping, attenuation, or other visual Rates: 10e por word per Insertion fer noneommorslal distortion of the waveform as viewed advertisements; 25e por word for oommorelal adwr- on a 'scope. By observing the transfer tlsements. Ratos ars not, and no dlsseunts VIII M characteristic, it is possible to by allowed. Copy must be accompanied by reminaM. la detect full, and must reach the New York o1Ne by tas Mat eye harmonic distortion of less than 2 of rho month preceding the date N Issue. per cent. The power curve of the amp- lifier thus deviates from flat by less than TILE AUDIO EXCHANGE, INC. buys and 1 db over the range 20 cps to 30 kc. sells quality high -fidelity sound systems and As intimated above, the circuit has components. Guaranteed used and new equip- ment. Catalogue. Dept. .kl, 159 -19 Hillside excellent listening qualities. This is a Ave., Jamaica 32, N. Y. Telephone OL 8 -0445 confirmation of the measurements. The additional power available shows up in MAGNECORD, model PTO -A with PTO -J cleaner and better articulated bass. The Amplifier, $425 ; Presto disc recorder, model K, $250 ; High -fidelity amplifiers -Altec, where precision overall effect is of greater smoothness, American Amplifier Corp., Stromberg Carl- more definition of detail in the sound, son. Goodell. from $70 to $180. Il. Untried, Main Buffalo N. Y. and better transient response. Ultra - 3104 St., 14, Linear circuits seem to have a wider matters... FOR SALE : Stephens 2 -way system transient bandwidth -an audible benefit (103LX 15 -In. woofer, P -15 tweeter, 824 which is not readily susceptible to horn, 800X crossover network). 15 -watt Wil- liamson Amplifier, custom built. RV -10A measurement. The combined effect of Browning FM tuner. Above equipment like the Williamson circuit configuration - new, used less than 20 hours. Price $310. S. a wide -band, low- distortion arrange- Villanti, Phone HAvemeyer 6 -8724 (New York) after 7 : 00 p.m. weekdays. ment -plus an output stage of decreased distortion and higher power capability, FOR SALE: DuMont 185 electronic switch a stage which exceeds the original $65 DuMont 208 5 -in. oscilloscope, $135 RCA W-54a audio oscillator, NEV, $65 specification and operating parameters, RCA WV-73a audio voltmeter, NEW, $65 must be heard to be appreciated. RCA 170 -a Audio Chanalyst $135 ; Weston 769 Electronic tester, like new, $150 ; Shall - cross 638.1 Wheatstone- Kelvin bridge with 619 -B d.c. supply, $140. Adolph Hoefer, 8330 AUDIO PATENTS Kingsbury, Clayton 24, Mo. ro1S , WANTED: RCA 77 -B, 77 -D, 44BX micro- 1 , phones. D. R. Steele, 78 Shearer Dr., Ather- in one phase or the other. Thus, the result- toll, Calif. ant, shown on both sides of the baseline, will come to the phase -insensitive ear FOR SALE : Craftsmen C10 tuner and Transformers for Television C500 amplifier, Pickering 230 -11 preamp, merely as a curve shaped as in Fig. 3. Garrard RC -80 changer with GE heads and An approximate idea of the action of Radio Shack Compensator, Jensen H -510 ... Radar... Aircraft... the circuit may be had by using this same speaker and Klipsch Rebel horn enclosure. Matched for superb reproduction- including method of preliminary plotting for any the first honest bass I've known in ten years Geophysics... Radio kind of filter. Single- frequency and band of buying and building. New in February. emphasis circuits may ako be made by Priced $102 below net at $400. Edward Burks, 7302 Birch Avenue, Washington 12, You will find Thermador ready, willing D. C. SLigo 3900 evenings. +12 and fully qualified to handle your trans- LO PASS FILTER CONVERT your Webster record changer former requirements. Engineering RESULTANT with a 22X084 arm, GE RPX -050 Triple - Play cartridge. Installed in plug -in head. experience and manufacturing know - Your cost $11.75. Send check or money order. FRIEND's WIIOLESALE DISTRIBUTORS, how, developed over a period of 35 106 N. 6th St., Philadelphia 0, Pa. years, form the hard core that makes FOR SALE : Electronics 5/48-4/52 (48 Thermador today's largest West Coast issues), Proceedings IRE 12/50 (17), +6 -4/52 manufacturer of electrical appliances and 02. FM -TV 3/50 -3/52 (23). All excellent condi- tion. $25 for lot, shipped express collect. C. transformers. We would like to work Benson, 14 Hedge Lane, Westbury, N. Y. 12 with you on your next project involving 3 5 7 10 20 PRESTO 900R1 tape recording mecha- the design and production of transform- FREOUENCYIRCI nism (less amplifier and power supply) ex- cellent condition RECO ART, 1305 for specific $300. ers requirements...including Fig. 2. Market Street, Philadelphia, Pa. joint Army -Navy specifications. FOR SALE : Browning tuner, RJ-20, $75 ; +42 Altec Lansing A -323 -B amplifier, $65 : Elec- tro -Voice SP -15 speaker, $48 ; University transformers: 4402 tweeter, $14 University 4405 high - pass filter. $3. All in good condition. Ralph Ashworth, Charlton City, Mass. Audio Auto Geophysical THE ULTRA LINEAR Williamson Ampli- Filament High -Fidelity Audio fier Acro world's finest. Driver 3 ,5 7 10 20 with transformer. Custom construction, $124.50. Albert Selig- Input- Output Midget Plug-In FREQUENCY ( RC) son, 512 E. Thompson St., Philadelphia 25, Pa. Phone -NE 4 -6057. Plate Power Television Fig. 3. SPECIAL INPUT EQUIPMENT-new but Tube to Line using a parallel combination of low- and shopworn. W.E. Model 22E consisting of am- high -pass filter both at A and at B, or at plifier control unit and power supply, both with carrying cases and connecting cables. one alone, the slopes being steeper if both Price $300. Minnesota Electronics Corp., 47 ...also Chokes and Reactors positions are used. A few minutes spent W. Water St., St. Paul 1, Minn. in plotting possible results as in Fig. 2 will give an idea of the full possibilities. And STEPHENS P15 driver and horn. $50. I'aul Brown, 3418 NE Liberty. Portland, Ore. THERMADOR ELECTRICAL the patent specification itself gives the formulas which may be used to begin ac- tual design. Like all other U.S. patents KLIPSCH speaker, brand -new, utility MANUFACTURING CO. it model. Reduced. Hugh McGafSn, c/o Fergus 3.320 is available for 250 from the Commissioner Motors, 444 W. 55th St., New York City. XDistrict Boulevard Los Angeles 22, Calif. of Patents, Washington 25. D. C. JU 6 -2155. 5110 44 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com NOW...YOU CAN AFFORD THRILLING Available from stock HIGH -FIDELITY MUSIC THE OTL [from page 23] at TRIAD OWN THIS HI -FI CUSTOM PHONO. graph can always be rearranged to use jobbers! GRAPH SYSTEM shunt feed with a center -tapped choke, ONLY though this would be an unwelcome $12685 compromise at best. 20 watt Feedback Considerations Amplifier AMPLIFIER t t RESPONSE ( +1 db 20- 20,000 cps. In any O'1'1, output stage negative Rated an outstanding value by a lead- ing product research organisation feedback can be taken from the voice coil without the degree of concern about Enjoy the brilliant record repro- low- frequency phase shift that plagues duction you've always wanted the designer of conventional amplifiers .. at a sensible low price. This superb hi -fi phonograph system who likes to pick his feedback voltage gives you the top -value Knight off from a secondary tap on the output 20 -watt amplifier, G.E. S1201D transformer. Where a single loop is em- 12' hi -fi speaker and Webster- Chicago 106 -27 3 -speed record ployed, this can be carried back over at changer with plug-in heads and least one extra stage, assuming careful 2 G.E. cartridges. Complete with cables (no solder connections re- design. quired), hardware and instruc- In cases where d.c. must be kept out tions. Hear your records as they of the feedback path, and if there is al- should be heard Webster. ... order today! ready a blocking capacitor in the circuit Chicago Shpg. wt., 57 lbs. "The World's Smallest 3-Speed Changer 93.320. Only $126.55 to keep d.c. out of the voice coil, this same capacitor can also serve the feed - 212 Page Hi -Fi,TV back circuit. In a Sinclair -Peterson cir- Hermetically Sealed FREE & Radio Catalog cuit, feedback would be taken from the The complete Buying Guide point marked F in Fig. 7, and depending Transformer" to everything in Hi -Fi, Radio, on how and where the feedback voltage TV and Electronic equipment. is applied it might be possible to omit a Miniaturization has become increas- Unsurpassed for quality ap- blocking capacitor. In a push -pull OTL ingly important in the design of all paratus and fine values. Ask stage, such as the one at (B) in Fig. 6, types of electronic equipment. Use for your FREE copy today. feedback would be taken off at the plates !raterials and bet- ALLIED of improved core RADIO in two balanced loops and carried back ter winding techniques in our JAF $33 W. Jackson Blvd., Dept. 17 -F -2. Chicago 7, 111. push -pull stage, and prob- to an earlier series permits these great reduc- ably in a cross -coupled configuration. The positive current feedback ap- tions in size and weight of low level proach suggested by Clements' and de- audio transformers. TRIAD JAF Trans- veloped further by Childs' does not seem formers, as lister below. are "the readily applicable to OTL design but, world's smallest hermetically sealed it might be possible to obtain the same transformers." All have 45 db. shield- with a method sug- or better results ing and are availat le in MIL standard gested by Tanner'. Analyzing the re- quirements of speaker design and gen- AF case as shown above. Carried as erator impedance for obtaining flat re- stock items at all Triad jobbers. sponse-i.e., in actual sound output - I mpedc nca Reg the feedback Type List Tanner concluded that Resp Price voltage should be a motional voltage, No. Primary Seealdary and he obtained such a voltage "by the JAF-1 600/250/50 50000 100-10000914.50 simple expedient of winding a separate JAF-2 600/250/50 25o0m 300.3000 15.30 feedback coil of very fine wire over the JAF-3 600/250/50 60000 C.T. 110.1000 15.30 existing voice coil." How simple an ex- JAF-11 15000 50000 100 -10010 13.60 pedient this alteration and addition JAF-12 15000 60000 C.T. 100-10010 14.50 would be for most audio experimenters JAF-13 1530 95000 C.T. 350-500e 15.30 is but it should be fairly questionable, 15300 600¡250/50 100 -10000 14,50 easy for such coils to be included in JAF-21 original speaker designs, for both low - 1AF-22 15300 600,250/50 350 -5000 14.50 and high- impedance types. (Bifilar JAF-23 20300 C.T. 600,250/50 100.10000 15.30 windings ?) JAF-31 600/250/SO 630;250/50 100 -10000 14.50 In addition to the advantages claimed JAF-101 50 h.-1 Ma. 11.60 Directional characteristics of the for this method of feedback by its author. +10 dim. 'DC in primary. acoustical lens at 5000 cycles. Maximum level two other favorable considerations come ' except JAF-1, 2 ind 3 (0 dbm.) The new Jim Lansing acoustical lens gives the to mind. It eliminates the need for a highs a smoothness your ear will appreciate in- blocking capacitor where one might be Write for Catalog TR -52C stantly! The new lens, a natural development brought about by the inadequacies of the multi - otherwise required, and secondly, for cellular horn, distributes sound constantly and conventional push -pull output circuits, uniformly over the entire audio spectrum...giv- ing the highs a smoothness impossible to match á by any other method. 5 Clenletlts, "A new approach to loud- Drop into your high fidelity dealer and ask for speaker damping," AUDIO ENGINEERING, the full story today. .Aug. 1951. 6 Chills, "Loudspeaker damping with TRANSFORMER MFG. C3. dynamic negative feedback," AUDIO EN- first in fine sound GINEERING, Feb. 1952. n , enera i -¢es:.. .ewoo. ve. ' Tanner, "Improving loudspeaker re- Venice Area, Los Angeles, Calif. JAMES B. LANSING SOUND, INC. sponse with motional feedback," Elec- Address correspondence to: Box 17813 2439 Fletcher Dr., Los Angeles 39, California tronics, Mar. 1951 (p. 142). Los Angeles 34, Calif.

AUDIO ENGINEERING JUNE, 1552 45

www.americanradiohistory.com whether plate or cathode loaded, it per- GOOD REASONS WHY mits the use of a single feedback path df balanced loops. and one instead of two 74 72eW MAGNEMITES* which can be carried back beyond the phase inverter if desired. Are The Perfect Tape Recorders for Use 7ir Portable One drawback which became apparent World's Smallest, Lightest Tape Recorders is that at the very high frequencies the O Letterhead size, weighing 10 lbs., the mutual inductance between driving voice Magnemites* The Magnenrite* increas- offer every desirable fea- consuls ,series coil and the pickup coil has an ture for field recording, compactly assem- of 3 dual - ing effect, resulting in attenuation of bled in beautiful, precision recorders. track models or (174,3%, the highs. Tanner's solution to this diffi- 7% in. per sec.) on the Record Without AC Power ad single-track culty involved an extra winding applicable to O The Magnemites* are powered by inex- mnd 1 output transformer -not sec,) design. On the other hand, in a pensive dry -cell flashlight -type batteries designed to NAB OTL that last 100 operating hours. No wet standards. two- or three -way speaker system, loss " r cells that require daily recharging. of highs in a woofer or mid- frequency driver would be of no concern and Playback and Monitoring O Facilities Built -In Many exclusive fea- might even be advantageous by elim- An output connection which feeds ear- tures distinguish need for shunt capacitance. phones or an Magnemites* inating the auxiliary amplifier and as the Studies by different investigators speaker during playback, is also used outstanding portable gi for monitoring while recording. recorders. have shown that critical or optimum lf'rite today related to speaker for complete literature damping is closely Exceptional Recording Quality and direct factory efficiency and to acoustic loading of the O 4 Magnemite* models are available for prices. speaker. In a talk before the AES during any recording need. Designed to record sessions held in *Trade Mark one of the technical music, interviews, or any type of sound Reg. connection with the 1951 Audio Fair. effects with lifelike reproduction. 0 Mr. D. J. Plach of the Jensen Mfg. Co. and AMPLIFIER CORP. of AMERICA 398 -4 Broadway New York 13, N.Y. showed the effects of under, over, optimum clamping on the actual sound output (free -field measurements) of a high quality 15 -in. woofer mounted in a properly matched hass -reflex cabinet. NOW AVAILABLE With insufficient damping the character- istic twin peaks were present. With The GRAMOIOt Feedback optimum damping they smoothed out and the response was practically flat Disk Recording Cutterhead Unit 40 cps. With too much Engineers and musicians now realize a perfect transient response down to about of any recording system is of paramount importance-the Grampian damping, the response fell off badly at cutterhead with its precision -made true balanced armature and flux correcting feedback loop obtains this ideal. the low end. Used by the BBC and other international broadcast networks where In the light of the above, it seems to fidelity is of primary importance, you too, can be assured of the the writers that present practices in highest fidelity disk recording. may be revised con- Many outstanding recordings, such as the LP Mercury rating speakers Olympian Series, were recently produced with the Grampian. siderably in the future. Some of the For more information, write to Wide freq range specifications which a manufacturer 30 c/s to 20 kc for a woofer, for example, R E E V E S E Q U I P M E N T C O R P. might provide Low Distortion 10 EAST 52nd STREET NEW YORK 22, N. Y., U. S. A. might include: resonant frequency (free Superior transient re- NE CABLE: REEVESOUIP, N. Y. air), impedances at several key fre- sponse quencies within the normal operating range of the unit (including resonant frequency), cl.c. resistance and induc- IF YOU ARE MOVING tance of the voice coil. efficiency, power Please notify our Circulation Department at least five weeks in advance. The handling capacity (again, at a low key lost Office does not forward magazines sent to a wrong destination unless you frequency), and the optimum output im- pay additional postage, and we can NOT duplicate copies sent to you once. pedances which the speaker should "see" When notifying us, please give your old address and your new address. when mounted in various types of en- Circulation Department semi -horn RADIO MAGAZINES, INC. closures-such as true horn, 342 Madison Avenue New York 17, N. Y. (corner), bass reflex, infinite baffle, and R -J enclosure. Some of the aforemen- tioned enclosures might require the same degree of damping. If such information were macle avail- able by the speaker manufacturers, then 9205 DEGAUSSER perhaps more amplifier manufacturers would add to their present ratings the magnetic tape and film Demagnetizes internal impedances or damping factors to erase recording and residuals. of their amplifiers, so that an ideal Accommodates 5400 foot reels of V4" match between speaker, encolsure and tape; 1000 foot, 35mm. Write for amplifier would be assured. It is even Catalogue. Net Price $69.50 conceivable that the amplifier of the future would have a selector switch on the rear panel which will permit the choice of any one of several "standard" CINEMA ENGINEERING COMPANY damping factors. Such a selector would 1510 WEST VERDUGO AVENUE, BURBANK, CALIFORNIA in all likelihood also compensate auto- matically for changes in overall gain as C Manson. Export Ayants: Froror t Ud. 301 Clay Lead San Frond-ce. Calif., U. S. A. the feedback is varied.

46 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com RESCO HAS THE FAMOUS ACRO JftierI/loldlduifrI// MODEL TO -300 ULTRA LINEAR Meie. REPOR TRANSFORMER The most versatile and convenient test instrum >nt AMONTHLY SUMMARY of product de- for audio measurements... velopments and price changes of radio electronic- television parts and equipment, supplied by United Catalog Pub- lishers, Inc. 110 Lafayette Street, New York City, publishers of Radio's Master. These REPORTS will keep you up -to -date in this ever -changing industry. They will also help you to buy and specify to best advan- tage. A complete description of most prod- ucts will be found in the Official Buying Guide, Radio's Master - MODEL INS -available through \ local radio parts wholesalers. MEASURES distortion, hum, noise, power Ss output, frequency response, pickup Miscellaneous Radio, TV and frocking .. . COMBINES signal source, disto -tin Electronic Parts meter, and VTVM in one instrument. The World's Finest AMERICAN PHENOLIC (AMPHENOLI. Added 4 type BNC SIMPLIFIED >, OPERATION for fast connector. . . . 34 RP I series 921 connectors . 12

measurements thon possible with fray UHF (series 931 connectors . 16 replacement pings Audio Output Transformer other method. BURGESS BATTERY CO. Advised a price differential no longer exists between east and west roast. Brochures eyed /cote. The east coast price now covers the entire C. S. also advised that elec- READ IN THIS ISSUE engrrs tronic labeled battteries are replacing the hearing aid and fabricator: os line now being sold to the radio and lndnslrial trade. ULTRA LINEAR OPERATION OF magnetic and electro- Burgess advises, however. that they have not discontinued l mechanical products. the manufacture of hearing aid batteries. ii A WILLIAMSON AMPLIFIER f INTERNATIONAL RESISTANCE (I.R.C.). 295 TV coneen- Irie dual controls for replacement in ever 5000 TV models by David Hafler added . . also added 41' otter shafts In types Pl, P22. P3, and 14 . . . 44 inner shafts in types RI. R2, WI. ON PAGE 26 and R4 . . . added 20 base elements for panel or rear carbon sections and 13 base elements for wire wound DISTRIBUTED BY panel sections . . . 2 Concentrikits 3 Cnneentrik- park for l'hilco, RCA and Admiral ... 2 Universal rams shaft kits for t'oncentrikit. as PASADENA 1, CALIF. EAST ORANGE, N. 1. .. MALLORY B CO. Increased prices on their VA series of 7

power supplies . . . added 7 new 2 watt wire -wound front section controls . . . 8 new WP dry electrolytic DOUBLE YOUR POWER OUTPUT capacitors . decrease prices of 3 WP electrol111rs ' WP 605 to $1.20 net, WP 510 to $1.32 net and WP CUT DISTORTION IN 540 to $2 40 net - HALF! VAN CLEEF. Decreased prices on 60 -yard rolls of Dutch Brand m:nking tale available In "bulk case packing." Response - PROFESSIONAL 1 db 10 cps to 100.000 cps Recording Equipment, Speakers, Amplifiers, Unequalled DIRECTORY Needles, Tape, Etc. . . . square wave response ASTATIC CORP. Phonograph cartridges models 511 -2 -J and MI -2.133 . pickups models 510 -MI -2.33. 400 -MI- Low phase shift 2.33. 510 -311 -2 and 400.311 -2 are discontinued. ELECTRO- VOICE. Added "Baronet" folded horn corner Rated to handle 40 watts loudspeaker enclosure for %" speakers at $35.70 net In mahogany and $37.80 net In blonde. Resonance -free response Custom -Built Equipment GOLDRING GRAMAPNONE. Reduced price of . T17, head- master transcription pick -up to $16.77 net. KENT PRODUCTS. Added CB -400. steel buse to fit O. I. ACRO TO -300 $24'S record changer and CB -950, steel base to fit VM record net changer at $4.0% net each. U. S. Recording Co. LANSING SOUND. Added f, comer enclosures for D -1050 ACRO TO -310 $1875eí system and 6 corner enclosures for D -1001 system. 1 /21 Ver>sont Ave., Warhigtoe 5. D. C. LOWELL MFG. CO. Added 10 items to their line of Ceiling Lincoln 3 -2705 Baffles and l'arts. 20 watt (same rating) used to convert IO MILLER MFG. CO. Added 3 new replacement cartridges for watt amplifiers to ultra linear operation. Aatatic . 1 for Electro -Voire . 1 for Magnovox . 3 for Shure Bros. Withdrew Diamond Stylll 3121:1 and M513 for Magnavox cartridges and PI1413 ID) and Write for ACRO Catalogue P11 413 (Psi for Phileo cartridges. showing schematics PRECISION ELECTRONICS, INC. Added 6 new amplifier amplifier models . . . redesignated amplifier model W to LJ2 at FREE and conversion information. $36.50 net and 50PG to 501'02 at $55.00 net. PRESTO RECORDING. Added Y -5 recorder (for low im- pedence mike) at $771.00 list . . T -99 -N dynamic microphone at $32.50 list . A-15 -S floor stand at DIAMOND $10.50 $10.00 list and L -2 transcription player at $290.00 list. SERVICE CO. OF PENNA., INC. NEEDLES p,, n,,t.Ga Tubes -Receiving, Television, 701 ARCH STREET Special Purpose, Etc. . . flu, main Cosine.. I, tilt- n 11,1inctnr- ( dia- PHILADELPHIA 6, PA. mond styli for brgadcasting stations and high G.E. Increased prices on 51 radio receiving tubes. 9 trans- fidelity sound equipment. As a service to the mitting and 7 industrial type odes. Decreased prices on Please ship ACRO Transformer general public we offer to retip your present 52 radio receiving type tubes. 11 TV picture tubes, and L] No. 300 [] No. 310 replaceable needle with a genuine, uncondi- 23 transmitting & Industrial type tubes. tionally guaranteed broadcast quality dia- HYTRON RADIO & ELECTRONICS. Increased prices [] Check enclosed for on 183 mond tip either LP or Standard records. radio receiving type tubes and decreased prices on 34 Money Order enclosed Send us your replaceable needle (or complete others. cartridge except Pickering) together with 11 NATIONAL UNION. Added radio receiving tubes . . . NAME check or money order for $10.50 plus 250 Increased price of 203 and decreased price of 27. for shipping. R.C.A. Decreased prices on six 17-In. Kinescopes and two ADDRESS 21 -in. Kinescopes . Increased prices on 30 receiving THE TRANSCRIBER CO. tubes . . . added receiving tubes 6BL7GT, 12B117 and CITY 25BQC,l1T. ZONE ... STATE 172 -4 Green St. Dept. R Boston 30, Mass. RAULAND CORP. Decreased prices on 15 TV picture tubes . . . added picture tubes 21AW and 2131P4.

AUDIO ENGINEERING JUNE, 1952 47

www.americanradiohistory.com P.%ItTRIDGE ADVERTISING WI1.1.IAMSION INDEX OUTPUT TRANSFORMER Built to the original specification

Acro Products Co. 30 De -luxe model now available from stock from all Allied Radio Corp. 45 important Radio Stores throughout the U.S.A. paid) IS TRUTH RELATIVE Altec Lansing Corp. 37 (Price $26.00 duty as the most Amperite Co., Inc. 40 This transformer is now accepted OR ABSOLUTE? efficient in the world. According to "Audio En- Ampex Electric Corp. 35 gineering" (Nov. 1949), there is no U.S. equiva- A letter has reached us from Denver. "I have Amplifier Corp. of America 42, 46 lent. Thousands already sold in the U.S.A. been reading your Æ ads for some time now, Arnold Engineering Co., The 9 Partridge CFB Output Transformer, accepted as and recently had an opportunity to listen to Audak Co. 33 without rival. Series leakage induct .10 mh., one of your speakers. Your unit was as good as core Audio Devices. Inc. Cover 2 primary shunt induct, 130 H., with "C" any in its price class. It did not, as your ads construction and hermetically sealed. (Price Audio & Video Products Corp. 34 imply, give anything remotely approaching the $40.00 duty paid) quality of the big multi -channel units. . I The Following Stores are among those now really would advise toning down your ads a Stocking Partridge Transformers. bit." Belden Manufacturing Co. 5 Harvey Radio Co. Inc. Terminal Radio Corpo- A few days before we got another letter. "I Bell Telephone Labs 14 103 West 43rd Street. ration, have New York 18. 85, Cortlandt Street. now lived with the 215 for a fortnight. New York 7. It is mounted in a Boffle made to your specifi- Electronic Wholesalers Gates Radio Company. cations. The reproduction Inc. surpasses everything Cannon Electric Development Co. . 10 2700 Polk Avenue, previously encountered. Why anyone should 2345 Sherman Ave., Houston Texas. Carter Motor Co. 37 Washington. D. C. waste money on special reflex cabinets, cross- Wholesale Radio Parts Transformer Co. Inc. over networks, and so- called treble speakers, Chicago Co. 8 Radio Electric Service Co. 311 W. Baltimore St., which are cnly, at Cinema Engineering Co. 46 Baltimore Maryland. their very best, makeshifts, 5 North Howard Street, I. is incomprehensible. Classified Ads 44 Baltimore 1. Maryland. Sole Agents in Canada: Atlas Radio Corporation, "Your claims for this reproducer are too con- Gates Radio Company, 560 King Street West, Quincy. Illinois. Toronto 2 -B. servative. It is the best that I have heard, and, Dauen Co., The Cover 3 after trying out no less than five of the so- If you are unable to purchase Partridge trans- D. & R. Ltd. 47 formers in your city, write to us and mention the called 'hi -fi' speakers, all costing much more name of your dealer. than the 215, and being very hard to please, Fullest data, including square -wave tests, distor- I can only say how much I regret the money and tion curves, etc., together with list of U. S. El- time wasted on such inferior instruments, and Tronics, Inc. 43 stockists rushed Air Mail to you. not having tried out your reproducer five years PARTRIDGE TRANSFORMERS LTD. ago." TOLWORTH, SURREY, ENGLAND G. Turner, Lancing, Sussex, Eng. Gates Radio Co. 11

"Ah," we hear you say, "that is only an Eng- lishman who doesn't know our big stuff." O.K. Harrison Radio Corp. 43 He doesn't. So what about the gentleman from Hartley, H. A., Co. Ltd. 48 Washington State who writes: "The more big Harvey Radio Co., Inc. 31 claim I speakers hear, the better I like my 215. High Fidelity Recently I heard a demonstration of four Heath Co. 41 famous speakers: a . corner cabinet con- Hollywood Electronics 48 C E N T E R taining two 15" woofers and a cellular tweeter; a . 2 -way system; a . . . in a On the West Coast bass reflex cabinet; and a . in a corner Lansing Sound, Inc., James B. 45 for the cabinet. I was surprised at the rough and Leonard Radio, Inc. 39 Audio Specialist generally poor quality of sound given by all of these speakers. None of them can approach the 215 in faithfulness and ,AA..A, smoothness of repro- Maynard Electronics, Inc. 1, 29 duction. The 215 can reproduce subtle tran- Minnesota Mining & Manufacturing Co. 4 sients as no other speaker -not even the finest movie theater system -can." Modecraft Pre -Built Cabinets ...... 40

Let's get this straight. It strikes us as futile to say things in ads which we don't be- Partridge Transformers Ltd. 43 lieve or which our readers couldn't believe. Pickering & Co., Inc. 13 Our copy is based on what the user says, be- cause our personal opinion has nothing to do Professional Directory 47 with the case. Actually, we believe that under- statement is a good business investment, and from your point of view, you will find the 215 Radio Electric Service Co. 47 speaker is also a good investment. The cost is Reeves Equipment Corp. 46 only $48.00 (plus $6.00 import duty/. Send Reeves Soundcraft Corp. 7 for your free catalogue today. Rek -O -Kut Co. . 2 R -1 Audio Products, Inc. 3 Come in and See Subscribers to our data service please note Our New that at last we have completed our spring mail- ing. If you haven't had it by the time you read Tech - Master Products Co. 41 Audio Listening Room this ad, please let us know as it may have gone Terminal Radio Corp. 40 For a Friendly Demonstration astray. This mailing contains information of very great importance to all audio enthusiasts. Thermador Electrical Manufacturing Co. 44 Transcriber Co. 47 Those not on this list can become members at the all -time subscription rate of one dollar Triad Transformer Manufacturing Co. 45 HOLLYWOOD bill. Tung -Sol Electric, Inc. G H. A. HARTLEY CO. LTD. ELECTRONICS U. S. Recording Co. 47 152, Hammersmith Road 7460 MELROSE AVENUE United Transformer Corp. Cover 4 London W.6, England LOS ANGELES 46, CALIF.

48 AUDIO ENGINEERING JUNE, 1952

www.americanradiohistory.com Are all bronc Is of Resistors similar in Quality, Q Specifications aid Performance?

Naturally, no ! That's why DAVEN has earned

the right to c dd a superlative in naming its line

Brands of Resistors vary as widely in SUPER DAVOHM 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 RESISTORS 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 wiy 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.

JUST OFF THE bMpORTAN7 PRESS /. NEW BULLETIN D Av OIV S your RESISTORS.óR' niormation 'oPY rois, swpk,d, prove ro ;P'city lhet when only You n the order ResistorsR esistors Ku should AVENco Sis PER DAVOHM Precision 185 CENTRAL AVENUE Wire Wound RESISTORS NEWARK 4, NEW JERSEY

www.americanradiohistory.com Humidity ... temperature ... DeveloFment tests ... 1/2 cycle filter altitude testing Tracing filter curves electronically

Engineering Leadership UTC has the reputation for exceptional quality and the ability to produce units previously considered impos- sible. This position Filter development of engi- Audio development neering leadership has been effected through a continu- ous program of research and development at the UTC laboratories. A few views of these laboratories are shown on this page.

Testing modulation transformer under Pulse transformer development operating conditions

O meter measurements on low frequency coils

I // ` l fjr itfrfr.l.tiPr!I (_-r"`/ ISO VARIER STREET - NEW TORN IE. M. T.

E1PORl DIVISION 13 EAST 4025 STREET. NEW YORK 16. N Y.. CARI?S "R1A6"

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