Audio-Anthology-3-19
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itplc %.- the 3rd audio anthology COMPILED FROM AUDIO From August 1952 to June 1955 by C. G. McProud Editor RADIO MAGAZINES, INC. MINEOLA, NEW YORK: 1 9 5 5 Copyright 1955. by Radio Magazines, Inc., 204 Front Street, Mineola, New York. Henry A. Schober, Presi- dent. Printed in the United States of America. All rights reserved. This book, or any part thereof,may not be reproduced in any form without permission of the publishers. CONTENTS Article Issue Page Simple High -Quality Phono Amplifier Aug. 1954 5 Quality Amplifier For The Home May 1955 7 Transformerless 25 -Watt Amplifier For Conventional Loudspeakers June 1954 10 The White Powrtron Amplifier Nov. 1953 13 The Maestro-A Power Amplifier Nov. 1952 16 Medium -Power Tetrode Amplifier With Stabilized Screen Supply Feb. 1954 20 Zero -Impedance Output Stage Jan. 1953 22 Dual -Channel Control Amplifier for Stereophonic Music Systems May 1954 24 Improved Phonograph Compensation Circuits Nov. 1954 27 Note on Volume Controls Apr. 1953 28 The Selection of Tone -Control Parameters Mar. 1953 30 Flexible Tone -Control Circuit Sep. 1953 34 Three -Element Bass Control Oct. 1953 39 Hi -Fidelity Phonograph Preamplifier Design Apr. 1953 36 Versatile Control Unit For The Williamson Sep. 1954 42 Preamp with "Presence" Jan. 1954....45 Miniaturized "Preamp with Presence" May 1955 50 New Approach To Miniature Preamplifier Design Feb. 1955 54 Preamplifier Switching, and Equalizing Unit for Critical Listening Sep. 1952 57 Low -Distortion Tone -Control Amplifier Aug. 1954 61 Record Improvement with H -F Cut-off Filters Nov. 1952 63 Cathode -Follower Audio Control System Nov. 1952 66 Simplified Equalizer Design Dec. 1952 69 Pickup Loading and its Effect on Frequency Response July 1954 71 Motor Rumble Reduction in Wide Range Phonographs Nov. 1954 72 Two -Channel Conversion for AM -FM Receivers Jan. 1953 74 Binaural or Stereophonic? Jan. 1953 77 Stereophonic Reproduction Oct. 1952 80 Discussion of Dividing Networks Dec. 1952 84 Transistor Phonograph Preamplifier for Magnetic Pickups Oct. 1954 86 D.C. Pack for Heaters and Bias Sep. 1953 90 Corner Horn For The Small Listening Room Feb. 1954.... 91 Tuned -Pipe Enclosure for Bass Enhancement May 1955 93 Evolution of "The Horn" Dec. 1954 96 Corner -Mounting Infinite Baffle Oct. 1952 99 Coupled Loudspeakers May 1953 101 The Concrete Monster July 1954 105 Real Theater Sound in a Small Package Feb, Mar, Apr. 1953....108 Vibration Reduction in Loudspeaker Enclosures Dec. 1953 115 Feedback and Loudspeaker Damping Apr. 1953 118 Damping of Loudspeaker Cabinet Panels Sep. 1954 119 Speaker Treatment for Improved Bass Dec. 1952 121 Improving Loudspeaker Performance Sep. 1954 123 Foreword Continuing the practice of providing the reader with a single volume that contains most of the material from issues of the magazine of greatest interest to the serious audio experimenter, the 3rd audio anthology is offered in the sincere hope that its pages will con- tinue to provide a wealth of useful reference material without the necessary additional pages that go to make up a monthly magazine. The original audio anthology was created to bring the early articles that appeared in AUDIO ENGINEER- ING into the hands of those who did not learn of the magazine's existence until some time after it was intro- duced. The articles that were in the first thirty issues of AUDIO ENGINEERING were among the most valu- able that had been published in magazines up to that time, and once the early copies of the magazine were exhausted, there was not any way in which the reader could secure them for his own use. Thus the audio anthology was born. As months went on, more and more readers-both of the magazine and of the anthology-came to ex- pect that there would be another compilation of the more popular articles, and so the 2nd audio anthology was introduced. With a precedent thus created, it was natural to expect that a third would be along soon, and here it is. We are grateful to our readers for their continued interest, and it is to them that this 3rd audio anthology is appreciatively dedicated. C. G. McPROUD, Editor AUDIO Mineola, N.Y. September, 1950 A Simple High -Quality Phono Amplifier R. D. MIDDLEBROOK An inexpensive audio amplifier with sufficient gain to drive a loudspeaker from a magnetic pickup and which uses an unusually large amount of negative feedback over the output tube and transformer. THE BEST AUDIO AMPLIFIER IS wasted extended frequency response. The am-clipped. This effect is clearly visible on if the associated equipment can not plifier to be described uses a single 6V6an oscilloscope, not only for a sine wave take advantage of it. Many high -qual- beam tetrode tube, with an ordinary uni-but also for the complex waves of music ity amplifiers are available commercially, versal output transformer. The perform-or speech. As mentioned previously, the all of them using push-pull output stages. ancefiguresshowthattheresults guiding principle in the design of the Their capabilities are impressive-andobtained are definitelyin the "high - amplifieriseconomy inconstruction so are their prices. Of the thousands ofquality" class: while maintaining adequate performance non -technical people who have bought for ordinary home listening: the acid these amplifiers, how many have spent Maximum output:3 watts test for success is therefore a listening Total distortion : 1 per cent at 3 test.Ifan oscilloscopeisconnected a proportionate amount of money on watts (1000 cps) their loudspeaker systems? across the loudspeaker, while reproduc- There is room for a cheaper and sim- Frequency range:3 db down at 20 ing heavily -recorded music,it will be pler audio amplifier which can still be cps and 25,000 cps seen that the gain can be increased until considered high -quality. It is not gener- Output resistance:0.1 ohm the volume is sufficient to drown all con- ally believed that a single -ended output The maximum power output of3 versation before the peaks start to clip. stage is capable of producing results in watts is obtained at middle frequencies; In other words, all activity must cease keeping with present-day standards ; yetthe maximum power obtainable at ex-while the music is at full volume-a con- it can be done. This article will describe treme low and high frequencies dependsdition which must surely warm the heart how excellent listening quality can be on the quality of the output transformer. of any hi-fi enthusiast. Need more be obtained from a circuit which, at first No amount of negative feedback can ef- said about the adequacy of 3 watts? fect any improvement in this respect. The complete amplifier has sufficient glance, doesn't look as though it would gain to produce full output from a mag- sound any better than a small table radio. Using the Thordarson Type T22S58 universal output transformer, the maxi-netic pickup, such as the General Elec- The secret of this transformation ismum available power output falls to 3tric variable reluctance type. Figure 1 negative feedback. It is the writer's be-db below 3 watts at 80 cps and at 8000 is a block diagram of the various stages, lief that most of the unpleasantness of cps. and itis seen that the circuit can be the reproduction from an ordinary sin- Since most present-day amplifiers pro-divided into two distinct parts, each em- gle tetrode or pentode output is due tofesing to be "high fidelity" ace capablebodying heavy overall negative feedback. the low speaker damping, and not to theof at least 10 watts output, a word in de- harmonic distortion. Without enteringfense of 3 watts may not be out of place. Circuit Description into the fierce discussion on the merits 10 or 20 watts certainly give a consid- Figure 2 shows the complete circuit or otherwise of negative output impe- erable margin of safety, so that even thein detail. The grid resistor R, is chosen dance of amplifiers, it can be stated withpeaks of the signal give rise to distor- to be a suitable load for the pickup. The some confidence that an output resistance tion less than the figures quoted for max- first two triode sections form a straight somewhat less than the speaker resist-imum output. What theloudspeaker cascaded amplifier with a gain of 1400 ance is desirable in order to obtain sat-does to the transient is, of course, an-times, which is reduced to an over-all isfactory damping. By using enough neg- other story. In this amplifier, owing togain of 100 times by the negative feed- ative feedback this can be achieved withthe large amount of negative feedback,back from the plate of V, to the cathode the signal contains very small percent-of V,. This link contains the tone control a tetrode output tube, with all the attend-ages of distortion right up to the maxi-circuits. Various tone control arrange- ant advantages of reduced distortion andmum output of 3 watts, and if the signalments are possible, and can be chosen * 1928 Cooley Ave., East Palo Alto, Calif. isincreasedfurther,thepeaksaret(, suit the individual : however, two net- works which have been found adequate are described here. Figure 3 is the bass control, which 2 3db 36db gives a maximum of 12 db bass boost, NEGATIVE FEEDBACK NEGATIVE FEEDBACK the magnitude of the rise being con- VOLUME CONTROL trolled by the potentiometer. The point BEAM PENTODE at which the rise begins is selected by TE TRODE II the switch. These turnover frequencies are 80, 140, 200, 300, 400, and 500 cps. DOUBLE TRIODE The bass boost is sufficient to compensate for the recording characteristic of LP records, and in practise it will be found TREBLE BASS TONE TONE that usually the maximum amount of CONTROL CONTROL boost is unnecessary.