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Book Review by Nicholas Vinen Small Signal Audio Design Handbook, by Douglas Self. 1st edition, 2010. 558 pages, soft covers, 190 x 235mm. ISBN 978 0 240 52177 0. Price $103.95 I f you are going to design any analog audio signal processing circuitry, such as a preamplifier, mixer or even just a volume control or input switcher, read this book first. After doing so, you will design better circuits than you could have thought possible. It doesn’t take expensive parts to design circuits with low distortion and noise. The Author, Douglas Self, is an audio performance guru. Few people have his extensive experience designing audio gear (such as power amplifiers, preamplifiers and mixers). He combines the resulting experience with a thirst for rigorous analysis, with the aim of achieving impeccable performance. We reviewed another book by Douglas Self, the Audio Power Amplifier Handbook (Fifth Edition), in the March 2010 issue of SILICON CHIP. The two books can be viewed as companion volumes and together they endow the reader with a profound understanding of the performance determining factors in analog audio circuits. You do not need an engineering degree to understand Self’s writing but you will find it much easier if you are familiar with basic operation of op amps, transistors and passive components. He assumes that you can analyse basic circuits but regardless, does a good job of explaining his design techniques and why they are superior to the alternatives. The chapters are logically arranged and the prose is easy to read. Both text and diagrams are large and clear. You can read the book one chapter (or a few pages) at a time but it also makes a good reference volume. There are plenty of circuits within it that will come in handy in future. Rigorous analysis Self’s technique for improving circuits is quite straightforward. First, he takes the basic circuit and measures its performance and Audio Precision frequency response, total harmonic distortion and other graphs are heavily featured. If performance of a proposed circuit is found wanting, he then breaks it down into smaller pieces and uses basic circuit theory to figure out where the problems lie. He then presents the revised circuit and a performance comparison to the original. If the performance is still not up to scratch, this process is repeated as many times as necessary. Often the final circuit isn’t much more complicated than the original but works much better. He tends to shy away from exotic parts or complex designs as they increase the size and cost of the finished product. As he demonstrates quite thoroughly, it’s more about 90  Silicon Chip how you arrange the circuit than what’s in it. Once you understand his approach, you can use this technique yourself. By the end of the book you will understand the sources of noise and distortion (and other non-ideal properties) well enough to figure out for yourself how to avoid them in your own designs. The book features plenty of circuit diagrams, many of which can be built right away. It also contains many distortion graph plots, often comparing the performance of several different versions of a circuit, along with tables showing noise performance against source impedance and so on. No circuit is too simple to be improved using Self’s method. Take the humble voltage divider; a signal can be attenuated by 6dB (reduced in voltage by half) with just two equal value resistors. In isolation, though, this is a lousy way to do it since it either loads the source excessively (with low value resistors) or has poor noise performance (with high value resistors). He shows how the addition of a buffer and carefully chosen resistor values can produce a much better attenuator. Questions answered Throughout the book, a number of questions are answered which have been the source of discussion in the SILICON CHIP offices quite recently. Questions such as: Can electrolytic capacitors be used for signal coupling without introducing siliconchip.com.au distortion? Can polarised electrolytics be operated reliably with very low bias voltages? Is there an advantage in using polypropylene capacitors over polyester types? He answers all of these questions, and more. Some of the answers provided in the book are to questions we had not thought to ask – but in retrospect, we probably should have. For instance, thick film resistors have significant nonlinearities which can introduce distortion. As he puts it, they don’t really “obey Ohm’s law”. This is an important consideration for designing audio circuits using surfacemount components. The book contains some pleasant surprises in the form of simple circuits with surprisingly good performance, some of which are quite unique. Some of the circuits presented are improved versions of those which are so common that in the past, we have not batted an eyelid before using them. Often the performance can be improved without increasing the component cost at all. One of the themes throughout the book is that of low impedance design and minimising noise through low source impedances. Have you ever wondered why high value DC bias resistors do not compromise noise performance but high value feedback resistors can? Once you understand Johnson noise, voltage/current noise, source impedances and related phenomena, it all makes sense. Another surprising revelation: most engineers and technicians know that balanced audio signals result in less hum and interference but did you realise that a typical balanced input circuit is a lot noisier than an average unbalanced input? As usual the Author presents various methods for solving this dilemma but it turns out that to get very good noise performance with a balanced input is not easy. Highlights In the chapter on Discrete Transistor Circuitry, Self presents multiple circuits which use a handful of bipolar transistors to create buffers, inverters and gain stages with surprisingly good performance, given their simplicity. One benefit of these circuits is that they will operate with supply voltages well in excess of a typical op amp. In the chapter on Op Amps and Their Properties, he shows how common and cheap op amps can be used to very good effect. In fact for the most part, the more expensive and exotic op amps are shown to have worse performance than older designs like the NE5532 and LM833. Other useful circuits are presented such as ultra-lownoise microphone preamplifiers with variable gain, active volume controls, signal swing limiter circuits and more. Douglas Self reveals in the book that he is experienced at designing mixers (including at least one award-winning design) and this clearly shows. Only a relatively small portion of the book is dedicated to mixer circuits and configurations but there is a lot of good information in there and he gives several simple but very effective techniques for maximising performance, yet again. Criticisms Obviously, we really like this book. There are however a few sections that could be better. For example there are two chapters that deal with designing circuits to interface with turntables (ie, moving magnet and moving coil cartridges). While they contain interesting information and circuits, it could be argued that the application for which they are designed is essentially obsolete. And while the circuit descriptions are mostly very thorough, in a few places Self presents a circuit and seems to assume that the reader can figure out how it works. This is mainly evident with the circuits which combine discrete transistors with an op amp for improved noise performance. It is possible to use basic theory to analyse the circuits and understand them but a more detailed explanation would be nice. Summary If (like several SILICON CHIP staff members) you are an audio enthusiast, this book will give you inspiration and ideas. It will also help you become a better designer with, among other things, a better understanding of component peculiarities and how to work with them (and in some cases, around them). It is also a pleasant read for the technically inclined and can be leafed through at leisure. Self even has a sense of humour and the book is good for the occasional chuckle. The Small Signal Audio Design Handbook is available SC from the SILICON CHIP book shop. 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