The Alpha to Omega on power amps
Oct 1, 1997 12:00 PM, Bob Bushnell
High Performance Audio Power Amplifiers, by Ben Duncan, published by Newnes, an imprint of Butterworth-Heinemann Ltd., 1996; 1,462 pp., hard cover, $56.95.
In opposition to the many books that end up as remainders or sold by the pound at the local supermarket, here's a tome that will grace your bookshelves and your life for some years. As the title states, the topic is audio power amplifiers -- and in detail. Is the book confined only to "hi-fi" power amplifiers, touring power amplifiers or the esoteric world of high-end power amplifiers? To all three, the answer is a resounding "NO." Duncan delves into power amplifiers from input to output to power supply -- from input connectors to output connectors to power connectors. Duncan dedicates chapters both to real-world testing, as well as his professional opinions.
Throughout the book, Duncan has drawn legible schematics that are easily understood because of his consistent drawing style. They are not just poor reproductions from some manufacturer's service manual. The book has good margins (for notes), and bold-face section headings improve readability over pages of solid copy.
This isn't a book just for the amplifier designer. Even if you're designing basic sound reinforcement systems for normal installations with less than kilowatts of output power, Duncan covers many topics you need, not only for the initial installation, but also for continued, trouble-free operation.
In his opening chapter, the author covers "adjectives that describe sound." He lists the adjectives and defines the various synonyms for each adjective. In cases of negative adjectives, he describes a possible cause for the condition.
Duncan analyzes many amplifier designs, both good and bad. Some designers will appreciate this book; others will realize their masquerade. He uses detailed real-world information, not a cookbook approach. What do you want to know about amplifier topologies, classes and modes? Duncan wrote some 80 pages on this frequently misunderstood subject.
How basic is a gain control? Some people feel that gain control should not be accessible on a power amplifier. Nevertheless, Duncan examines this urbane device, and in a context dear to my heart, considers the source and output resistances presented by a potentiometer in a real circuit.
Gain controls, VCAs, common mode rejection and frequency response of an input port are some of the many topics the author covers in Chapter 3. Chapter 5 studies features of the power stage. Some of the topics are output stability and the output network, mapping V-I capability, output stage cooling requirements, heat exchange and thermal protection.
The chapter on power supplies is not just dry data, but meaningful information concerning the ubiquitous part of any active electronic device. He scrutinizes passive supplies, switching supplies, resonant supplies, and Carver's use of Class G amplifier topology together with PWM (pulse width modulation) power supplies. Before you shrug aside switching supplies as being too noisy because of EMI considerations, Duncan pays more than lip service to the vast subject of EMI as applied to non-passive power supplies used in the real world.
How many test specifications can you perform on audio power amplifiers? Duncan probes into 27 separate specifications, ranging from the test load to peak output current capability. They won't be listed here, but his specification list is thorough, and the examination of each is quite complete, considering that entire books cover only THD measurements.
EMI and EMC (electro-magnetic compatibility) are probed in the chapter on real world testing. Adverse loads (what about loudspeakers?) and acoustic noise are other topics found in that chapter.
In Chapter 9, "Choice, application installation and set-up," the author spells out those topics in detail. He includes a neat section on what he terms "howlers," which aren't a form of feedback but are, to quote his opening paragraph, "some common and 'it really happened' defects to look for in manufactured units based on hundreds of reviews of both domestic and professional power amplifiers costing from 150 pounds ($325) up to 5,000 pounds ($10,850), or even more."
His notations are revealing, even without the manufacturers' names (for obvious reasons). He discusses the use of RCCBs (residual current circuit breakers) - what we Yanks call GFI protectors.
Chapter 10 covers maintenance and surgery. One section of that chapter covers bipolar transistors. The UK refers to NPN or PNP devices as BJTs or bipolar junction transistors. Much more descriptive than simply transistor, as we colonials tend to lump FETs with BJTs.
His examination of BJTs begins with a simple but effective circuit for testing BJTs and then proceeds to effective use of a DMM or VOM. There's a circuit for matching of complementary pairs. He reviewsMOSFETs, then diodes, zeners and LEDs. This is just the chapter on maintenance and surgery, not the entire book! Exclamation points are generally frowned on in book reviews, but this book deserves them.
One appendix lists power amplifier manufacturers throughout the world. Another appendix is a short list of active devices by type and pinout; ready reference in emergencies. Still, another appendix is on power amplifier terminology. As opposed to humans, an appendix in a book is actually useful, and more than one is even better.
Throughout the book, Duncan references various terms, some unique to the UK. (But the author is English, after all.) Fortunately, he provides a thorough glossary, including some terms that were new to me. At the end of each chapter, Duncan provides a further reading list. Granted, many of his references are to British publications. He refers to at least two writings of the late but stellar physicist, Richard Heyser. Among other accomplishments, Heyser was responsible for the field of time domain symmetry (TDS) measurements. TDS doesn't apply to power amplifiers, but then, why not?
Criticisms? Very few. In Appendix 1, he provides a thorough history of audio as relating to power amplifiers. He might have given more credit to AT&T and Bell Laboratories for their many contributions to audio, notably loudspeakers and power amplifiers. He also credits Audio Fidelity for introducing the first stereo LP in the United States without mentioning that the disc was cut by Westrex as a trial for Audio Fidelity. Westrex, in fact, developed the first stereo LP disc recording head.
Not a cheap book ($62.65 from an Internet bookstore), but not casual reading. In short, this is a book for reading, reference and lending (to close friends only, otherwise you may not get it back).
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