Preface

“Another damned thick book! Always scribble, scribble, scribble! Eh, Mr. Gibbon?”

Attributed to Prince William Henry, Duke of Gloucester, in 1781 upon receiving the second volume of The History of the Decline and Fall of the Roman Empire from its author.

This book deals with small-signal audio design: the amplification and control of audio in the analog domain, where the clever stuff is done with op-amps or discrete transistors, usually working at a nominal level of a volt or less. ‘Small-signal design’ is the opposite term to the large-signal technology used in power amplifiers, welding gear, and the electricity distribution grid.

Audio signals frequently need to be manipulated, by altering their level, frequency response, routing and so on. It is obviously much more economical to perform these functions on a small signal, rather than a large signal that has already been amplified up to loudspeaker level, and so has tens of volts and several amps to its name.

The total flexibility of digital signal processing may allow a greater scope of control – you might think how you would go about implementing a one-second delay in the analog domain, for example – but there are many times when greater quality or greater economy can be obtained by keeping the signal in the analog domain. Sometimes, of course, analog circuitry connects to the digital world, and so a complete chapter of this book deals with the subtleties of analog/digital interfacing.

I have devoted the first few chapters to the principles of high-quality small-signal design, moving on to look closely at, first, hi-fi preamplifiers, and then mixing consoles. These two genres were chosen partly because they are of wide interest in themselves, but mainly because they use a large number of different functional blocks, with very little overlap between them. They cover a wide range of circuit functions that will be useful for all kinds of audio systems. You will find out how to adapt or design these building blocks for audio, and how to put them together to form a system without bad things happening due to loading or interaction. You should then be able to design pretty much anything in this field.

It is my aim to provide information not available elsewhere. As an example, take the near-universal Baxandall bass and treble tone control. Many books give you the basic circuit, with a few conventional remarks, and that’s it, but there are actually a huge number of useful variations on the basic circuit. To begin with, this book reveals that there are actually two different versions of the basic Baxandall control, with a quite different action at the low end of the audio spectrum. It goes on to show how to make a middle control using the Baxandall approach, and even how to fit three bands of tone control into one stage. From there we move on to a whole range of more sophisticated controls with variable frequency, Q, and so on.

You will also find material on how to make amplifiers with apparently impossibly low noise, how to design discrete transistor circuitry that can handle enormous signals with vanishingly low distortion, how to use humble low-gain transistors to make an amplifier with a startlingly high input impedance of more than 50 MΩ, how to transform the performance of low-cost op-amps, how to make filters with very low noise and distortion, how to make incredibly accurate volume controls, how to make magnetic cartridge preamplifiers that have noise so low it is limited by basic physics, and generally how to sum, switch, clip, compress, and route audio signals. Finally, the chapter on power supplies, in which I give some very practical ways to keep both the ripple and the cost down, shows how to power everything.

This book is mainly aimed at the design of audio equipment of the highest possible quality. This does not, fortunately, mean spending money like water and making everything out of unbendium or unobtainium to achieve this end. The limitations on performance are usually set by the laws of physics – which are proverbially hard to break – rather than the cost of components. This is not always the case and sometimes you have to grapple with a price/performance trade-off. I have kept the lupine from the lobby for many years by designing things to sell at a profit, so I have been able to include quite a few non-obvious ways to save money. Only at the so-called ‘high end’ of hi-fi is cost almost wholly irrelevant.

An eye for economy is not merely a negative influence. Possibly the man who said ‘cost is an interesting extra constraint’ was just trying to convince himself he hadn’t sold out to Big Business, but to my mind there is a lot of truth in the saying. If you could always get better performance just by laying out more cash – well, where’s the fun in that?

I have tried to ensure that all the essential points of theory that bear on practical performance are explained, with the mathematics kept to an essential minimum, but I have no intention of trying to create a basic electronics textbook – there are plenty of good ones about already. I do advise, however, that you get the basic theory straight before you try to make use of the circuitry and concepts here.

Small-signal design is very often based on op-amp circuitry, the great advantage being that all the tricky details of low-noise and almost distortion-free amplification are confined within the small black carapace of a 5532. This op-amp is, for very good reasons, employed more than any other in quality audio design, and I make great use of it here. In the course of compiling this work, it has been brought home to me just what a bargain the 5532 is.

There are, however, times when circuitry built with discrete transistors gives better results. You may need more voltage swing than is available between op-amp supply rails, or it may be inconvenient to provide such rails, or you may want to do it purely for marketing purposes. I have therefore included a chapter on the design of discrete transistor stages. This contains a wealth of little-known techniques, ranging from some of the lesser-known quirks of the simple emitter-follower to the design of complete op-amps from discrete devices.

It is sometimes highly advantageous to combine discrete devices and IC op-amps in the same feedback loop, to exploit the best qualities of each. When you need very low noise, say for a moving-coil phono amplifier, you have transistors followed by an op-amp. When you need high output current, say for driving headphones, you have an op-amp followed by transistors. Many examples of this approach are scattered through the book.

An important strand in this book is low-impedance design. By minimizing circuit resistances the contribution of Johnson noise is reduced, and conditions set for the best semiconductor noise performance. Op-amp common-mode distortion and crosstalk are also reduced. The notion is not exactly new (as some people would have you believe) but has been used explicitly in audio circuitry for at least 20 years. This approach relies on amplifiers that can drive heavy loads at very low distortion, and here again the 5532 is extremely useful. In this book you will find many examples of low-impedance design.

What you will not find here is any truck with the religious dogma of audio Subjectivism: the directional cables, the oxygen-free copper, the World War One vintage triodes and all the other depressing paraphernalia of pseudo- and anti-science. I have spent more time than I care to contemplate in double-blind listening tests – properly conducted ones, with rigorous statistical analysis – and every time the answer was that if you couldn’t measure it you couldn’t hear it. If you want to know more about my experiences and reasoning in this area, there is a full discussion in my earlier book, the Audio Power Amplifier Design Handbook.

I have included a few historical vignettes both where they are of interest in themselves, and where they also shed light on some general principles of design.

I have tried to avoid FGAs (Frequent Gratuitous Acronyms) but a few are used extensively so more useful text can be squeezed in. They are listed after this Preface. None of them, I think, will cause any great confusion. I also use the word ‘offness’ – which is not found in any spell-checker but is widely used in the pro audio sector to refer to the ratio (in dB) by which a controlled signal can be suppressed. I try to give value for money, so introductory material has been cut to a minimum, and many a rolling elegiac sentence has been ruthlessly culled to fit in another hard fact.

In many places I have given measured performance figures or test-gear plots. These are inevitably the result of relatively few measurements and are intended to be illustrative rather than defining a figure for all time. When op-amp properties were involved I have been careful to use high-quality parts from the well-known manufacturers rather than me-too parts of doubtful provenance. Your mileage may vary, but I think not by much.

The data here has been gathered over several years. Some of the plots are from the now obsolete Audio Precision System 1, while others are from the newer and quite remarkable AP SYS-2702. It would have been a Sisyphean task to repeat all the measurements, so some graphs look a little agricultural graphically, but they have at least a certain gritty realism.

I have a website (www.dself.dsl.pipex.com) where I will be adding supplementary material to this book. I really mean this; I have already written another thirty pages or so on both discrete and opamp design significantly extending the material in this book. From there you can contact me, so please let me know if you think something is wrong, or an important topic is missing, or if you have any other suggestions as to how to improve this book that do not involve combustion.

Further information, and PCBs, kits, and built circuit boards of some of the designs described here, such as phono input stages and complete preamplifiers, can be found at:

www.signaltransfer.freeuk.com

It is my hope you will both enjoy this book and find it useful.

Douglas Self

London, September 2009

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