high- fidelity circuit design norman h.crowhurst and george fletcher cooper published by gernsback library, inc. First Printing — April, 1956. Second Printing — April, 1957. C © 1956 Gernsback Library, Inc. All rights reserved under Universal, International, and Pan-American Copyright Conventions. Library of Congress Catalog Card No. 55-11190 jacket design by muneef a/wan 1 chapter page Part 1 — Feedback Feedback effects Feedback improves response. Basic mathematics. Effects of phase shift. Feedback design. Sample problems. Gain without feedback. Where to apply feedback. Safety margins. Calculated response. Interpreting the phase-shift curve. -j Analysis and design Response curves. High-frequency response. Increasing stability. The feedback circuit. Amplifier design. The output stage. The phase splitter. Low-frequency response. The feedback resistor. The preamplifier stage. 3 Response and stability Circuit refinements. Cathode and screen circuits. Coupling net- works. The feedback network. Nyquist testing. Polar response curves. Stability margin. Phase margin. Degree of stability. Com- ponent tolerances. A-r Distortion Using positive feedback. Reducing intermodulation and harmonic distortion. Limitations of positive feedback. Input and output im- pedance control. Passive networks. Optimum load. Load lines. Sensitivity. Positive grid swing boundary. f.c Extending design factors Alpha-beta vs. mu-beta effect. The alpha-beta evaluator and its use. Stability margin. Design comparisons and procedures. Narrow-pass band-pass amplifiers. Feedback considerations. Series and shunt feedback injection. Internal noise. 07 Part 2 — Design and Testing Techniques Drivers and inverters Two good phase splitters. Some circuits from life. Methods of pre- serving balance. Practical applications. Some important problems. The seesaw circuit. Noncritical circuits. Choosing the best circuit. I The "long-tailed pair." Ill Power supplies The full-wave rectifier. Regulation. Capacitor-input filters. Choke input filters. Swinging-choke filters. Filter ripple. Choke current saturation. Regions of constant voltage output. Swinging-choke design chart. Eliminating filter hum. 1 ao chapter page Attenuators Calibrated attenuators. Basic configurations. Balanced and single- ended networks. Constant impedance. Attenuator design. Attenua- tion losses resulting from impedance mismatching. Mismatch reflec- tions. Correcting for impedance changes. 151 Filters and equalizers Derivation from transmission line. Characteristic impedance. Arti- ficial lines. Significance of image impedance. M-derived niters. Constant resistance networks. Equalizers. Terminal impedance. Bass-boost circuits. 1 yc Scope test procedures Detecting amplifier distortion. Phase-shift pattern. Determining the phase angle. Sine of phase angle. Cosine of phase angle. Common distortion patterns. Clipping. Transformer-core magnetizing current distortion. Ringing. 1 05 Speaker systems Source of sound. Realism, Constant resistance networks. Imped- ance variations. Dissociation effect. Directional sensitivity. Speaker phasing in sound reinforcement work. Phase difference. Crossover network design chart. 205 Advanced techniques Remote control amplifier. Bass control circuit. Vari-slope technique. The grounded-grid amplifier. The inverted amplifier. Grounded- cathode amplifier. Interstage transformers. Negative resistance. The follower. cathode 1 O High-power amplifiers Hookup problems. 500-ohm bus system. Ground potentials. Line impedance. Improvising outputs. Multiple signal sources. Isolating grounds. Remote microphone control. The story of an amplifier. High-frequency problems. 241 Test equipment design Single-frequency generator. Oscillator circuits. Modulators. Detec- tors. Circuit problems. Tuning modulator plate. Self-balancing push-pull oscilloscope amplifier. Basic rules for calibrating audio oscillators. 269 Improving old amplifiers Cost limitations. Modification procedures. Gain problems. Output transformer saturation. Phase characteristics. Amplitude response. Feedback applications. Interstage coupling. Low-frequency consid- erations. Compromise solutions. 289 Index 297 introduction Many books have been published on the various aspects of audio that can broadly be divided into two groups: the "theoretical" group, which undoubtedly give all the theory any- one needs, provided he has sufficient knowledge of advanced mathematics to be able to apply it; and the "practical" group, which tell the reader, step by step, how to make some particular piece of equipment. What has been lacking in the literature is some information in a form that will enable the man with only the most elementary knowledge of mathematics to produce his own design and make it work. With the objective of filling in this gap, we have written various articles that have appeared in Radio-Electronics magazine. And the correspondence we have received assures us that we have achieved our objective in this. In fact, many people find the only complaint is the inconvenience of having numerous copies of Radio-Electronics strewn about the place for easy reference! For this reason the Gernsback Library undertook the assembly of this material into book form. But a number of separate maga- zine articles do not too readily go together to make the best book, just like that. The result is apt to be rather a mass of bits and pieces. So the authors' help was called upon, and the original articles have been considerably edited and rewritten. In several places additional material has been added, in the interest of over- all clarity, and to fill in some gaps that naturally result from pre- paring a book in this way. But now that we have finished the work we feel confident in offering it to the reader as a real primer on designing the best in audio. Often people ask where we get the ideas for articles. We feel this introduction is a good opportunity to give the answer to this ques- tion, as credit to whom credit is due. Practically all the subjects for our articles arise from questions various people have asked, and the discussions that have followed. To be able to explain a subject successfully, it is necessary not only to understand it prop- erly one's self, but also to understand the obstacles that make it difficult for others to grasp. It is odd how obstacles in the attain- ment of knowledge seem insurmountable as we approach them, but having passed them, they seem to vanish, and we find it diffi- cult to realize the obstacle ever existed. This is why questions from people seeking knowledge, and the discussions that ensue, are invaluable in providing material for this kind of presentation. Knowing that many look in the introduction of a book to find out for whom it is written, we should answer that. While we have avoided using expressions that would put it over the heads of the many enthusiasts who do not possess very much theoretical knowl- edge, we are also confident that much of its contents will prove helpful to many who have more advanced training, but who have failed to visualize adequately some of the problems they en- counter, largely due to the vagueness of the "classical" approach. As a "primer" to read, it will give a sound basic knowledge of the subject, after which it will serve for years as an invaluable refer- ence book. We make no apology for such a claim — we use our own writings for reference. It's so much easier than trying to memorize it all! Norman H. Crowhurst George Fletcher Cooper feedback effects Boil water and butter together. Add flavor. Cook till it forms a ball. Season, and beat in an egg. This is not a book on cookery, nor is it one of those with a cook- book approach on how to build the perfect amplifier. "Take a ripe output transformer, about four pounds," he says; "two large well- matched tubes and an assortment of smaller tubes, capacitors and resistors. Connect as shown. Add feedback to taste." The trouble is, that the amplifier described this way is never just what you want, and nothing in the book tells how you can alter it without ruining the performance completely. Consequently, when you or your employer needs an amplifier, it has to be designed from scratch. Amplifiers without feedback are no problem, but the ad- dition of a reasonable amount of feedback to an amplifier with more than two stages usually leads to instability unless the circuit is carefully designed. To begin with, why do we want to use negative feedback in am- plifiers at all? There are four reasons which assume different orders of importance, depending upon the function of the amplifier. If the amplifier is part of an a.c. voltmeter, the gain must be con- stant in spite of changes in supply voltages and aging of tubes. A voltmeter with a drift of 10% would be a thorough nuisance in any laboratory. After all, unless you are a magician, you must trust something. By using negative feedback, the overall gain can be made almost independent of the internal gain of the amplifier; once it is adjusted, the gain will be the same even if the tubes , are changed or the line voltage drops 5 % . If the gain is independ- ent of the plate supply voltage, ripple caused by inadequate filter- ing will not modulate the signal. This aspect is especially impor- tant for ordinary program amplifiers. The reasons for using feedback are also important in audio. By using negative feedback we can flatten the frequency response, and reduce the harmonic and intermodulation distortion. It can also be used to modify effective input or output impedances. Feedback