Guide Evaluate: Designing Audio Power Amplifiers
Designing Audio Power Amplifiers, 2nd Edition, by Bob Cordell, Routledge / Focal Press, 2019. 776 pages. $ 160.00 hardcover; $ 97.95 Paperback.
I first met Bob Cordell at clinics he gave at the last audio show organized by Stereophile Home Entertainment in Manhattan in 2007. In these clinics, Bob shared his views on why amplifier measurements are not always good at predicting differences in sound quality. When I did a seminar “Amplifiers: Are Measurements Important?” At this year’s AXPONA, Cordell was on the short list of designers I thought would provide valuable insight.
Bob describes himself as “both an engineer and a handyman, and I love to mix the two.” He has been extensively involved in audio since developing vacuum tube amplifiers in his youth, and has been involved in bipolar transistor and MOSFET designs and audio test equipment development in recent years. Bob has worked at Bell Laboratories and other telecommunications companies and also advises in the audio industry. Probably only Nelson Pass and John Curl know as much about amplifier circuits as Bob Cordell.
The first edition of Designing Audio Power Amplifiers was published by McGraw-Hill in 2011. For the long-awaited second edition, Cordell has rewritten many of the original chapters and added five new ones, on topics such as low noise design and professional performance amplifiers and switching power supplies (footnote 1).
This massive book? 36 chapters, each with a comprehensive list of references? Is divided into six parts. In the first part, Bob covers the basics of power amplifier design, including discussions of noise, damping factor, negative feedback, and slew rate. Each of these preliminary sections is brief and gives a clear picture of what an amplifier should do and how it can do it. The specifics of the circuit design and operating parameters are introduced by guiding the reader through the design of an actual amplifier.
Modeling a typical 50W solid state amplifier circuit follows in the third chapter with examples of how to improve the performance of each stage, illustrated by SPICE calculations of total harmonic distortion. (For a full description of the SPICE circuit and component simulations, see Part 4.) The next chapter is new in this edition: A case study of actually building a practical 125W amplifier, from the first principles to prototyping , Build, and test. It ends with ideas for “upgrades” like using a DC servo to eliminate the need for series coupling capacitors.
These ideas are explored in depth in the second part of the book “Advanced Power Amplifier Design Techniques”, which also examines the main issues of feedback compensation and how to deal with crossover distortion, as well as how to use error correction to get very low values THD over the audio tape.
Part three, “Real Design Considerations”, examines how an amplifier deals with real world life and discusses things like power supply design, grounding, output stage thermal design and stability, and short circuit protection, control of clipping and “Civilized Amplifier Behavior”. In addition to the SPICE simulation, part 4 contains chapters on measurements and instrumentation.
Part 5, “Amp Design Topics,” begins by examining the pros and cons of negative feedback, a controversial topic. You will appreciate it when you look at stereophile measurements from amplifiers using high, low, or no feedback (footnote 2). . It’s clear from the start where Bob stands. “Negative feedback has gotten a largely undeserved bad rap. Much of it is because poorly designed solid-state amplifiers of the 1970s used large amounts of negative feedback … Some designers carelessly believed that negative feedback could be used to linearize a design that was from the start not very linear; … the negative feedback allowed the designers to make bad decisions or to compromise. “Amen about that ?? and in addition: “[T]The open loop bandwidth of a feedback amplifier should extend to the highest audio frequencies. ”
Bob continues his analysis of the feedback by examining how it can cause problems. I wrote one of them: A speaker cable can act as an antenna that feeds RF noise into the amplifier via the feedback network (footnote 3). Bob is skeptical of this potential problem, but admits that it is a good reason to use an input stage with good signal processing capabilities up to high frequencies.
The book concludes with four chapters on class D amplifiers, including how best to measure such designs. “The stumbling block for the introduction of Class D amplifiers in the past has been sound quality,” Bob summarizes this section. “This has changed dramatically in the past few years, but there is still a lot to be done for high-end audio.”
I know some engineers who will make an exception to this statement!
Designing Audio Power Amps is not a book to be gobbled up in one session. On the contrary: every chapter requires days of reflection. Is it dense and expensive? But it’s also important read for anyone who is intrigued by the superficially simple idea of making a small electrical signal powerful enough to drive a speaker without affecting that signal.
Footnote 1: For more information on designing audio power amplifiers, see here. A one-minute video interview with the author can be found here.
Footnote 2: See Martin Colloms’ primer on this subject.
Footnote 3: See my discussions of this mechanism here and here.