Review of Bob Cordell’s Book ‘Designing Audio Power Amplifiers’

‘Designing Audio Power Amplifiers’ by Bob Cordell  – Reviewed by Andrew C. Russell in 2011

Bob Cordell (on the right) pictured with Jan Didden, publisher of ‘Linear Audio’

After a 15 year hiatus from electronics, I returned in 2005 to linear design not as a professional but as a hobbyist. Earlier in my career, I spent six years designing industrial instrumentation – things like thermocouple amplifiers and linearizers, A-D’s, power supplies, isolation amplifiers and so forth. I’ve always been passionate about music and audio equipment, so it was only a matter of time, with children grown up and out of the home, that these interests would be rekindled.

Audio amplifier design is a specialized branch of electronics in which the  engineer seeks to amplify and reproduce very accurately through a speaker system,  low level source signals from CD’s, tuners, turntables and the like.  As such, it is a fairly unique speciality since it combines precision (very high linearity requirements) over a wide bandwidths (200 to 300 kHz) along with high power requirements (amps of output current with voltage swings of up to +-70 V in  a high power unit). Further, with the application of feedback, there is a dash of control theory thrown into the mix as well, and this is especially challenging when one considers that the loudspeaker load is highly non-linear wrt frequency.

Cordell is a professional engineer who has been blessed with the ability to explain complex technical concepts in a concise, understandable manner. This book starts off with the fundamentals of amplification and then goes on to show how to take a basic design, and with a few well-honed circuit approaches, evolve it to create very high performance, low distortion amplifiers. Chapter 5 is an in-depth discussion on feedback and compensation techniques which is traditionally one of the more challenging areas of amplifier design, but he covers this in a practical and succinct manner, exposing even seasoned, professional designers, to an array of advanced compensation techniques.

Voltage amplifier (VAS) and output stage design are also covered, along with the various trade-offs between the circuit approaches and output device technologies (bipolar and mosfet), and associated protection schemes. There is an extensive section on output stage topologies, covering both bipolar and mosfet technologies, along with a very interesting chapter on Hawksford error correction as applied to mosfet output stages, which Cordell helped popularize back in the early eighties with a ground-breaking design at the time.

The book delves into the subtleties of zero global feedback design amplifiers and Cordell diplomatically deals with the debate raging in audiophile circles about feedback (some for, and others against). What is special about this book is that it is grounded in very solid engineering theory and practice, and, rather than express opinions on why a certain design approach or philosophy is best (a temptation most writers and practitioners in the field are unfortunately unable to avoid), Cordell actually covers both sides. The reader thus comes away with an appreciation of the design challenges required in both feedback amplifiers and zero global feedback amplifiers. The same can be said of his discussion around bipolars and mosfets along with output stage protection.

This book is enormously important for the high end audio design community, and makes the state of the art accessible to a whole new generation of practitioners. If you are a professional audio engineer, then this book is an invaluable reference, and will almost certainly help you raise your game. On the other hand, engineering students will find the practical, down to earth explanations a useful resource in helping get from classroom theory into practical designs. And, for a DIY’er, I’d say it is an absolute necessity if one is to gain a solid, clear and unbiased introduction to the art. If Douglas Self is to be credited with removing the misunderstanding around power amplifier design, then Bob Cordell must be credited for bringing state of the art design techniques to within the reach of everyone with any interest in high performance audio in a thoroughly practical and highly readable book.

Highly recommended