JUNE, 1955 504 ENGINEERING MUSIC SOUND REPRODUCTION...a.' CN -.tar N -Qu\P pNpGßQ e P N 60i, 3Z.pa$ \ c w s.oc TVAL INPUT GAVOL The search for more power, lower di:tor:ior, and better listening quality goes on ad infini_um. Using secordary emissio-z tubes in a cross -coupled phase splitter and four KT456's in the output stage, one author chains great improvement. See page 15. This elaborate installation is capable of giving recording studio quality -it has to be for its owner is a recording technician. Amu) IN THE HOME takes us to see it, describes it thoroughly. See page 18. NOVEL TAPE RECORDING AMPLIFIER VALIDITY OF SOUND TRUCK RESTRICTIONS NEGATIVE FEEDBACK HIGH -POWER LOW-DISTORTION AMPLIFIER www.americanradiohistory.com NOW supreme fidelity costs 1/ as much 9 _.42D 40 to 15,000 cycles response at 71/2 in /sec. Over 55 db. signal -to -noise ratio Flutter and wow under 0.25% rms. Today the Ampex 600 is a tape recorder in a class by itself. Separate record and playback heads to permit At $545 it provides a degree of fidelity that is beyond reproach monitoring while recording - and it is a modest machine that weighs only 28 pounds. Built -in mixing between microphone and line It is a professional recorder priced within reach of thousands Prices - $498 unmounted, $545 in portable case of critical music listeners and tape recording enthusiasts. n 1947 this same class of fidelity could only be had by buying an Ampex 200 for $5200. It was worth its price because it A companion unit that not only matches the 600 in portability and oppearance, but also in quality as was the was only thing of its kind. It the first commercial re- well. Price is $149.50 in portable case. corder that made radio transcriptions sound like live broad- casts. But the Ampex 200 weighed 250 pounds. Few, if any, were bought for home use. The man who has seen it all has this to say: 4 "When I bought the first Ampex Model 200, folks thought I was a goner, springing for $5200.00. But it was a bargain - transcribing my radio program without losing any of the fresh- ness of a live appearance. Some of my records came off the same tape. I could be three places at once. But now here's an Ampex I can carry with one hand. Records and reproduces perfectly, but costs little compared with the first one. So I have a commercial recording studio wherever I roam." For full information, writ* today fo Dept. 8 -1 AMPEX CJfgnainre ofCléFecifon Tn CJound 934 CHARTER STREET REDWOOD CITY, CALIFORNIA CORPORATION Distributors in principal cities (see your local telephone directory under "Recording Equipment "). Canadian distribution by Canadian General Electric Company. www.americanradiohistory.com cuntinurd JUNE, 1955 VOL. 39, No. 6 Successor to RADIO, Est. 1917. AUDIO f. \1.I\Fa.111 , itt :sic sot 'o RF.PRIInI 4770\' C. G. McProud, Editor and Publisher Henry A. Schober, Business Manager Ilarrie K. Richardson, Associate Editor Florence Rowland, Production Manager Edgar E. Newman, Circulation Director Sanford L. Cahn, Advertising Director lu dastry Special Representatives - "Workhorse" H. Thorpe Covington and Dick Knott, 7530 Sheridan Road, Chicago 30, Ill. `, Nid West Representative - San ford R. Cowan, 67 West 44th St., 635 New York 36, N. Y. BROADCAST DYNAMIC West Coast Representatives - MICROPHONE lames C. Galloway and I. W. Harbison, 816 West 5th St., Los Angeles 17, Calif. CONTENTS Audio Patents -Richard H. Dorf 2 USED ON New Literature 5 Letters 6 TV London Letter -Richard Arbib 8 Editor's Report 12 AND A High -Power Amplifier with Minimum Distortion -Bruce DePalma 15 At Home with Audio -Lewis C. Stone 18 Negative Feedback-Edgar M. Villchur 22 BC A Novel Tape Recording Amplifier-A. F. Fischman 24 Phonograph Equipment -A complete 32 -page section 25 Uniform high quality performance day- after -day Record Revue Edward Tatttall Canby 57 in studio and remote - pick -ups has proved Equipment Report the rugged -Crestwood 304 Tape Recorder, "Dual" Record Player - dependability of the "635 ". Pilotrol Preamplifier-F_qualiser 60 Exclusive E -V Acoustalloy New Products 64 diaphragm assures smooth, Audio E T C-Edward Tatnall Canby 68 peak -free response 60 to 13,000 cps. Output is db. 50 -250 Coming Events 69 -55 ohms impedance selector. Tiltable Employment Register 91 head. %"-27 thread. Cannon XL -3 About Music - Harold Lawrence 72 connector. 18 ft. cable. Industry Notes and People 79 List Price $75.00 Advertising Index A0 Normal trade discount applies. See your Authorized E -V Distributor or Write for further information. AUDIO (title registered U. S. Pat. Off.) Is published monthly by Radio Magazines, Inc., Henry A. Schober, President: C. O. hIcProud, Secretary, Execut.re and Editorial Offices, 204 Front St., Mineola, N. Y. Subscription rato -U. 8. NO FINER CHOICE THAN Possessions, Canada and Mexico, $4.00 for one year, $7.00 for two years, all other countries, $5.00 per year. Single copies 50c. Printed in U. S. A. at Lancaster, Pa. All rights reserved. Entire contents copyright 1955 by Radio Mega. vino, Inc. Entered as Second Class Matter February 9, 1950 at the Post Off ce, Lancaster, Pa. under the Act of March 3. 1879. gieeMS,CC RADIO MAGAZINES, INC., P. O. Box 629, MINEOLA, N. Y. BUCHANAN MICHIGAN AUDIO JUNE, 1955 1 www.americanradiohistory.com Ersin Multitore Solders America's most complete line AUDIO PATENTS RICHARD H. DORF* 1 TlIE USUAL AUTOMATIC gain control, point (selected by adjusting the delay bias. limiter, or volume compressor operates no doubt) despite increases of several on the basis that a negative -going d.c. hundred times at the input point. control voltage derived from the signal is Just what happens to the tube's operating applied to the grid of a control tube, usually point when the grid voltage is played a variable -mu tube, which reduces the gain around with in this manner we have not of the stage as the signal input level in- bothered to figure out. Very likely R. creases. Angus A. Macdonald of Catons- creates enough additional voltage drop ville, Md., is the inventor of a volume - when the grid goes negative and plate cur- limiting device which does not affect the rent increases to help keep actual bias fairly ROTARY POWER gain of the amplifier but actually limits the constant. Or perhaps it doesn't matter in IS BEST signal itself at the grid of a voltage- ampli- he "clop- clop" of this application. Anyway, the inventor says Old Bess" gave fier stage by voltage- divider action. Al- it works and in this column we generally Grandma's buggy ride though the inventor states in his preamble take his word if it doesn't look too out- more vibration thon the smooth Rotary Power that his device obviates the disadvantages rageous. He says the system can be used of today's modern au- of the usual types, he does not specify with any voltage -amplifier stage, and that tomobiles. ROTARY just what the gains over the prior art are. parameters should be adjusted initially for POWER is best for mo bile radio, too He has at least come up with something normal operation without control voltage. and for all DC to AC different and interesting, covered by Patent Naturally, higher signal levels initially can r conversion ... smoother No. 2,703,825, which is assigned to West- be handled by low -mu than by high -mu . more dependable. inghouse. tubes. The control voltage can, of course, DC TO AC CONVERTERS One circuit of his invention is shown in he derived from anyplace, the method of Fig. 1. Signal comes in through blocking Fi,q. 1 being only a for -instance. Delay bias capacitor Cs and passes through R, to the is also a matter of choice and need not be For operating tape re- is corders, dictating ma- grid of V,, which the amplifier stage. used at all. chines, amplifiers and Output is taken in the usual manner other 110 -volt radio - across plate resistor R, through blocking audio devices from DC or storage batteries. Used Cut Signalling by broadcast studios, program producers, exec- capacitor C,, R. is an unbypassed cathode - utives, salesmen and other "field workers ". bias resistor. Attell B. Anderson has recently been Output signal from the amplifier tube granted Patent No. 2,703,344, which con- DUO -VOLT GENEMOTORS is also passed through blocking capacitor tains some interesting implications in the The preferred power sup- C, to the grid of V,, which is the signal field of signalling and warning of blind and ply for 2-way mobile radio ¡ rectifier which produces the d.c. installatio ns. , control deaf persons by passing currents through from either 6 or 12 -volt batteries. Carter Gene - voltage. A negative delay bias may also be the skin. The patent, which is assigned to motors are standard equipment in leading makes connected to the grid of V, to prevent any Bell Labs, should be read by anyone much of auto, aircraft, railroad, utility and marine communications. control action until the signal level reaches interested and we will just skim the sur- a certain maximum point. The delay bias face here. CHANGE -A -VOLT DYNAMOTORS keeps V, at or near cutoff under no- signal The general idea is that if electrodes are or low -signal conditions. Operates 6 -volt mobile rodio attached to the skin, the wearer can detect sets from 12 -volt automobile When the signal reaches the level at changes in both the level and the frequency batteries ..