Bob Cordell has a long-standing track record of designing audio equipment as well as advanced test equipment. He has published numerous articles and papers on power amplifier design and distortion measurement in the popular press and in the Journal of the Audio Engineering Society. He holds 17 patents and has frequently presented at the International Solid State Circuits Conference (ISSCC). His error correction amplifier design of 1983 achieved unprecedented distortion levels of less than 0.001% at 20kHz. Many of his designs and papers can be found on his website www.cordellaudio.com.When his book Designing Audio Power Amplifiers (DAPA) came out in 2011, it was enthusiastically received by professional designers and serious amateurs alike [1]. The second edition followed in 2019. We knew that Bob was working on another update, expecting an expanded and improved third edition, but that’s not what happened! Instead, Bob decided to write a different type of book, aptly titled Designing Audio Circuits and Systems [2]. I now have this new book in my hands, and it is time for a serious look.
This book includes pretty much everything electronic there is to be found in audio. The Introduction chapter discusses the content of the book, not as a listing of chapter headers and page numbers but by short discussions of the various areas covered. (There is also a traditional Contents section, running some 30 pages). Surprisingly, the first content chapter discusses the design of a simple preamplifier. Though simple in design, it is a full-fledged preamp with volume and tone controls as well as a phono preamp section. His reasoning here is that this would familiarize the reader with audio circuit design before delving into more advanced topics later. A good way to gently get readers onboard who are still developing their understanding of audio design!
Touching on the Various Chapters
Next are the circuit blocks that make up complete audio equipment designs. The usual basic transistor stages, current sources and mirrors, buffers, op-amps, what have you. It concludes with an introduction to Analog Devices’ LTspice circuit simulator for audio. I know that many audio designers are leery of using circuit simulators, but that is totally unwarranted, and I think they miss a great tool toward better and quicker design results. True, simulation results are not 100% accurate with respect to the real thing. That’s not the simulator’s fault, but many part and device models only implement a subset of the device’s properties, and what the simulator isn’t told, it cannot consider. But if your simulation results show your design is sound, your first prototype will come very close to what you want, very quickly, letting you skip several prototype iterations.
Then it is on to the parts and devices that we need to build these circuit blocks—passive components, surface-mount technology, semiconductors, and op-amps. Between the chapters on surface-mount technology and semiconductors is one on Poles and Zeros. At a certain point in your design career, you will start to look at stability, phase margins, gain margins, that sort of stuff. These are more advanced topics, and you need to master them to design increasingly high-performance audio circuits. I just thought it odd to place that chapter where it is. A more logical place would be following the chapter on negative feedback.
Distortion and its measurement is another great chapter. Here, Cordell also discusses often-heard comments such as “we don’t listen to single sine waves” or “a low distortion amplifier can still sound bad, so why bother?”. He agrees that a single number 1kHz distortion is of limited use but makes a strong case about why other distortion measurements can and do correlate much better with perceived sound quality. Measuring the distortion at 20kHz is a much better indicator of amplifier linearity—if only because it always accompanies transient intermodulation distortion.
Other methods such as Intermodulation Distortion in various forms and slew rate limiting distortion are similar indicators of what goes on with high-frequency signals. He notes that there is only one amplifier with one specific set of nonlinearities, which can be measured with different methods and different sensitivities and presentations. This gives the designer different views of what is going on in an amplifier, and where to expend more design effort. Cordell also spends several pages explaining the interpretation, advantages, and limitations of total harmonic distortion (THD) measurements. A very balanced view everyone should read!
There’s much more information that’s extremely valuable such as the connection between negative feedback and phase intermodulation, measuring with multitone signals and very high sensitivity distortion measurements using his Distortion Magnifier [3].
The chapter on switches and relays covers not just mechanical switches and relays but also solid-state versions using junction field-effect transistors (JFETs), complementary metal–oxide–semiconductor (CMOS) and double-diffused metal–oxide–semiconductor (DMOS) switches, offering many insights to eke the best performance out of these devices.
Then we come to Chapter 14 on Power Supplies and Grounding. These subjects are often treated as an afterthought after finalizing a design, but the performance of the power supply can make or break a complete design! The choice of transformer type, rectifiers, and reservoir capacitors are just the start — the whole subject of grounding could be worth a book [4].
Linear power supplies and regulators are covered, as well as switched-mode supplies — becoming increasingly popular, as their performance steadily improves. I was happy to see that my SilentSwitcher, a low power dual-polarity switch-mode supply followed by a pair of low-dropout very low noise linear regulators, was mentioned as an example implementation.
I strongly believe that switch-mode power supplies (SMPSs) are becoming more and more attractive even for sensitive low-level equipment. A very high-frequency, low ripple switcher followed, if you want to go all out, with the latest linear regulators can be so much cleaner than a transformer and rectifiers hanging off the polluted mains!
We dive into designing complete audio equipment in Chapter 15. The chapter addresses analog source preamps such as moving magnet and moving coil preamps for record players, tape and NAB/IEC preamps and equalization for tape equipment and microphone preamps.
Chapter 19 discusses balanced inputs and outputs, and of course, Cordell isn’t one to make the common mistake of mixing up balanced lines with symmetrical signals. I admit, it’s a pet peeve of mine. The signal level on a balanced line is the signal amplitude measured between the lines, period. The signal level on an individual line to some reference (e.g., ground) is irrelevant, if the signal levels remain within the common mode range of the sending and receiving side. The unique noise suppressing capability of a balanced line stems from the fact that both at the sending and receiving end impedances to ground are equal. That means that electromagnetic interference (EMI), mains, and other noise radiated ingress will be of equal amplitude in each line, so that it is cancelled at the differential receiving end.
Now, if you want to maximize signal levels in a particular system, maybe to maximize signal-to-noise ratios, you can use a symmetrical connection: two signal lines that carry signals of equal amplitude but opposite phase with respect to ground. As an example, with a ±15V supply system you can use two lines of 12V amplitude and opposite phase, providing a peak-to-peak differential signal amplitude of 48V, which normally would not be possible with 15V supplies. But such a connection would not have the noise cancelling properties of a balanced line, unless the line impedances are equal, which normally would not be the case.
Equalizers and Tone Controls are next. Even if these capabilities have receded from most home hifi equipment, they are still important for studio and live music installations. Several equalizer bandpass-based topologies are discussed and the implications for boost, cut and Q control are shown. Multiple bandpass equalizers can be combined in graphic equalizers of course, extending the granularity of the frequency-response shaping options.
With headphones and headphone amplifiers increasingly taking over listening spaces, a whole chapter (Chapter 21) is dedicated to this field. It starts with a discussion on headphone types, then goes into power requirements for their use. Many design options and designs, both integrated circuit (IC)-based and discrete are proposed, and accompanied by matching power supplies, volume and balance controls, DC coupling and DC servos, muting, protection, crossfeed circuits, low-frequency equalization, and more. If you are into headphones, this chapter alone is worth the book.
A separate chapter (Chapter 22) is dedicated to the important subjects of Volume and Balance control, Faders and Panning circuits. This chapter is especially valuable for professional recording and production facility designers. You’d think that there’s not too much to say about the lowly pot meter volume control, but you’d be wrong. Think of subjects such as wiper-loading distortion, tracking/matching, thermal noise versus rotation, and such. The chapter also discusses many alternate level-control methods such as (relay-based) ladder attenuators and IC-based digitally controlled analog attenuators.
Then it becomes time to delve into the world of digital-to-analog converters (DACs). Characteristically, Cordell begins by discussing how DACs work and their analog interface. Next are power supplies, references and (again) grounding. Many pages are dedicated to clock recovery, jitter and digital interfaces. The chapter finishes with a discussion of DAC types and topologies and reconstruction filters.
After a chapter on active crossovers and speaker equalization we move into the world of studios and music production facilities with chapters on voltage-controlled amplifiers, compressors/dynamic limiters, level displays and metering circuits, and mixers and recording consoles. Somewhere in that lineup is also a chapter on microcontrollers and computers.
I realize that this very cursory discussion of chapters and subjects cannot do justice to the wealth of topics and details discussed. As an example, take Chapter 11 on filters. Everything you ever want to know about filters is there! Not just filter types, familiar and unfamiliar (e.g., the Hall notch filter), as well as pink noise and other weighting filters and anti-alias and reconstruction filters as used in AD and DA converters. The section on Twin-T notch filters also explains things like notch filter depth related to parts tolerances, fine tuning and trimming the notch. It’s all there!
Conclusion
Bridging the audiophile and studio world, the analog and digital world, Designing Audio Circuits and Systems is essential reading for those in the professional audio engineering community, as well as students and enthusiasts who wish to design audio circuits and functions for pro audio or audiophile applications, and live sound or studio mixing consoles.
It is very courageous for an author to not take the easy road with another edition of a book that already does very well. Bob Cordell has invested several years in this book, and he can be justifiably very proud of the result. Highly recommended — if you are reading this, then you need this book.
References
[1] J. Didden, “Designing Audio Power Amplifiers,” audioXpress, October 2019.
[2] J. Didden, “Designing Audio Circuits and Systems,” audioXpress, January 2024.
[3] B. Cordell, “The Distortion Magnifier,” Linear Audio, Volume 0, September 2010.
[4] B. Putzeys, “The G-word, or how to get your audio off the ground,” Linear Audio, Volume 5, April 2023, www.hypex.nl/media/fa/d8/a3/1682342122/The%20G%20word.pdf.
This article was originally published in audioXpress, October 2024
