Audio Power Amplifier Design

Douglas Self

• Publisher: Routledge
• Year: 2013
• Print ISBN: 9781138406605
• eBook ISBN: 9781136123818
• Edition: 6
• Topic: Technology & Engineering
• Subtopic: Acoustical Engineering
• Index: Technology & Engineering

This book is essential for audio power amplifier designers and engineers for one simple reason...it enables you as a professional to develop reliable, high-performance circuits. The Author Douglas Self covers the major issues of distortion and linearity, power supplies, overload, DC-protection and reactive loading. He also tackles unusual forms of compensation and distortion produced by capacitors and fuses.

This completely updated fifth edition includes four NEW chapters including one on The XD Principle, invented by the author, and used by Cambridge Audio. Crosstalk, power amplifier input systems, and microcontrollers in amplifiers are also now discussed in this fifth edition, making this book a must-have for audio power amplifier professionals and audiophiles.

Chapter

1

Amplifiers and The Audio Signal

The Economic Importance of Power Amplifiers

Assumptions

Origins and Aims

The Study of Amplifier Design

The Characteristics of the Audio Signal

Amplitude Distribution with Time

Amplitude Distribution with Frequency

The Performance Requirements for Amplifiers

Safety

Reliability

Power Output

Frequency Response

Noise

Distortion

Harmonic Distortion

Intermodulation Distortion

How Much Distortion is Perceptible?

“Damping Factor”

Absolute Phase

Amplifier Formats

Misinformation in Audio

Science and Subjectivism

The Subjectivist Position

A Short History of Subjectivism

The Limits of Hearing

The Limits of Hearing: Phase Perception

Articles of Faith: The Tenets of Subjectivism

The Length of the Audio Chain

The Implications

The Reasons Why

The Outlook

Technical Errors

References

The microphone-amplifier-loudspeaker combination is having an enormous effect on our civilization. Not all of it is good!

Lee De Forest, inventor of the triode valve

The Economic Importance of Power Amplifiers

Audio power amplifiers are of considerable economic importance. They are built in their hundreds of thousands every year, and have a history extending back to the 1920s. It is therefore surprising there have been so few books dealing in any depth with solid-state power amplifier design.

The first aim of this text is to fill that need, by providing a detailed guide to the many design decisions that must be taken when a power amplifier is designed.

The second aim is to disseminate the original work I have done on amplifier design in the last few years. The result of these investigations was to show that power amplifiers of extraordinarily low distortion could be designed as a matter of routine, without any unwelcome side-effects, so long as a relatively simple design methodology was followed. I have called these Blameless amplifiers, to emphasise that their excellent performance is obtained more by avoiding mistakes which are fairly obvious when pointed out, rather than by using radically new circuitry. The Blameless methodology is explained in detail in Chapters 4 and 5 in this book. My latest studies on compensation techniques have moved things on a step beyond Blameless.

I hope that the techniques explained in this book have a relevance beyond power amplifiers. Applications obviously include discrete opamp-based pre-amplifiers,¹ and extend to any amplifier aiming at static or dynamic precision.

Assumptions

To keep its length reasonable, a book such as this must assume a basic knowledge of audio electronics. I do not propose to plough through the definitions of frequency response, THD and signal-to-noise ratio; this can be found anywhere. Commonplace facts have been ruthlessly omitted where their absence makes room for something new or unusual, so this is not the place to start learning electronics from scratch. Mathematics has been confined to a few simple equations determining vital parameters such as open-loop gain; anything more complex is best left to a circuit simulator you trust. Your assumptions, and hence the output, may be wrong, but at least the calculations in-between will be correct…

The principles of negative feedback as applied to power amplifiers are explained in detail, as there is still widespread confusion as to exactly how it works.

Origins and Aims

The original core of this book was a series of eight articles originally published in Electronics World as ‘Distortion in Power Amplifiers’. This series was primarily concerned with distortion as the most variable feature of power amplifier performance. You may have two units placed side by side, one giving 2% THD and the other 0.0005% at full power, and both claiming to provide the ultimate audio experience. The ratio between the two figures is a staggering 4000:1, and this is clearly a remarkable state of affairs. One might be forgiven for concluding that distortion was not a very important parameter. What is even more surprising to those who have not followed the evolution of audio over the past two decades is that the more distortive amplifier will almost certainly be the more expensive. I shall deal in detail with the reasons for this astonishing range of variation.

The original series was inspired by the desire to invent a new output stage that would be as linear as Class-A, without the daunting heat problems. In the course of this work it emerged that output stage distortion was completely obscured by non-linearities in the small-signal stages, and it was clear that these distortions would need to be eliminated before any progress could be made. The small-signal stages were therefore studied in isolation, using model amplifiers with low-power and very linear Class-A output stages, until the various overlapping distortion mechanisms had been separated out. It has to be said this was not an easy process. In each case there proved to be a simple, and sometimes well-known cure, and perhaps the most novel part of my approach is that all these mechanisms are dealt with, rather than one or two, and the final result is an amplifier with unusually low distortion, using only modest and safe amounts of global negative feedback.

Much of this book concentrates on the distortion performance of amplifiers. One reason is that this varies more than any other parameter — by up to a factor of a thousand. Amplifier distortion was until recently an enigmatic field — it was clear that there were several overlapping distortion mechanisms in the typical amplifier, but it is the work reported here that shows how to disentangle them, so they may be separately studied and then with the knowledge thus gained, minimised.

I assume here that distortion is a bad thing, and should be minimised; I make no apology for putting it as plainly as that. Alternative philosophies hold that some forms of non-linearity are considered harmless or even euphonic, and thus should be encouraged, or at any rate not positively discouraged. I state plainly that I have no sympathy with the latter view; to my mind, the goal is to make the audio path as transparent as possible. If some sort of distortion is considered desirable, then surely the logical way to introduce it is by an outboard processor, working at line level. This is not only more cost-effective than generating distortion with directly heated triodes, but has the important attribute that it can be switched off. Those who have brought into being our current signal-delivery chain, i.e., mixing consoles, multi-track recorders, CDs, etc., have done us proud in the matter of low distortion, and to wilfully throw away this achievement at the very last stage strikes me as curious at best.

In this book I hope to provide information that is useful to all those interested in power amplifiers. Britain has a long tradition of small and very small audio companies, whose technical and production resources may not differ very greatly from those available to the committed amateur. I have tried to make this volume of service to both. I also hope that the techniques explained in this book have a relevance beyond power amplifiers. Applications obviously include discrete opamp-based pre-amplifiers,² and extend to any amplifier aiming at static or dynamic precision.

I have endeavoured to address both the quest for technical perfection — which is certainly not over, as far as I am concerned — and also the commercial necessity of achieving good specifications at minimum cost.

The field of audio is full of statements that appear plausible but in fact have never been tested and often turn out to be quite untrue. For this reason, I have confined myself as closely as possible to facts that I have verified myself. This volume may therefore appear somewhat idiosyncratic in places; for example, FET output stages receive much less coverage than bipolar ones because the conclusion appears to be inescapable that FETs are both more expensive and less linear; I have therefore not pursued the FET route very far. Similarly, most of my practical design experience has been on amplifiers of less than 300 Watts power output, and so heavy-duty designs for large-scale PA work are also underrepresented. I think this is preferable to setting down untested speculation.

The Study of Amplifier Design

Although solid-state amplifiers have been around for some 40 years, it would be a great mistake to assume that everything possible is known about them. In the course of my investigations, I discovered several matters which, not appearing in the technical literature, appear to be novel, at least in their combined application:

•The need to precisely balance the input pair to prevent second-harmonic generation.

•The demonstration of how a beta-enhancement transistor increases the linearity and reduces the collector impedance of the Voltage-Amplifier Stage (VAS).

An explanation of why BJT output stages always distort more into 4 Ω than 8 Ω.

•In a conventional BJT output stage, quiescent current as such is of little importance. What is crucial is the voltage between the transistor emitters.

•Power FETs, though for many years touted as superior in linearity, are actually far less linear than bipolar output devices.

•In most amplifiers, the major source of distortion is not inherent in the amplifying stages, but results from avoidable problems such as induction of supply-rail currents and poor power-supply rejection.

•Any number of oscillograms of square-waves with ringing have been published that claim to be the transient response of an amplifier into a capacitive load. In actual fact this ringing is due to the output inductor resonating with the load, and tells you precisely nothing about amplifier stability.

The above list is by no means complete.

As in any developing field, this book cannot claim to be the last word on the subject; rather it hopes to be a snapshot of the state of understanding at this time. Similarly, I certainly do not claim that this book is fully comprehensive; a work that covered every possible aspect of every conceivable power amplifier would run to thousands of pages. On many occasions I have found myself about to write: ‘It would take a whole book to deal properly with…’ Within a limited compass I have tri...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Bound to Create
• Dedication
• Table of Contents
• Preface to the Sixth Edition
• Acknowledgements
• List of Abbreviations
• Chapter 1 Amplifiers and The Audio Signal
• Chapter 2 The Basics of Distortion
• Chapter 3 Negative Feedback
• Chapter 4 Amplifier Architecture, Classes, and Variations
• Chapter 5 General Principles and Distortion Mechanisms
• Chapter 6 The Input Stage
• Chapter 7 The Voltage-Amplifier Stage
• Chapter 8 The Push-pull Voltage-Amplifier Stage
• Chapter 9 The Output Stage
• Chapter 10 Output Stage Distortions
• Chapter 11 More Distortion Mechanisms
• Chapter 12 Closely Observed Amplifiers: Design Examples
• Chapter 13 Compensation and Stability
• Chapter 14 Output Networks and Load Effects
• Chapter 15 Speed and Slew-rate
• Chapter 16 Power Dissipation in Amplifiers
• Chapter 17 Class-A Power Amplifiers
• Chapter 18 Class XD: Crossover Displacement.
• Chapter 19 Class-G Power Amplifiers
• Chapter 20 Class-D Power Amplifiers
• Chapter 21 FET Output Stages
• Chapter 22 Thermal Compensation and Thermal Dynamics
• Chapter 23 The Design of DC Servos
• Chapter 24 Amplifier and Loudspeaker Protection
• Chapter 25 Layout, Grounding, and Cooling
• Chapter 26 Power Supplies and PSRR
• Chapter 27 Power Amplifier Input Systems
• Chapter 28 Input Processing and Auxiliary Systems
• Chapter 29 Testing and Safety.
• Chapter 30 A Brief History of Solid-state Power Amplifiers
• Index
• The Signal Transfer Company