audio power amplifier design handbook [electronic resource]

Audio Power Amplifi er Design
Handbook
This book is dedicated to Julie,
without whom it would not have happened.

Audio Power Amplifi er Design
Handbook
Fifth Edition
Douglas Self
AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD
PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
Focal Press is an imprint of Elsevier
Focal Press is an imprint of Elsevier
30 Corporate Drive, Suite 400, Burlington, MA 01803, USA
Linacre House, Jordan Hill, Oxford OX2 8DP, UK
First published 2009
Copyright © 2009, Douglas Self. Published by Elsevier Ltd. All rights reserved
The right of Douglas Self to be identifi ed as the author of this work has been asserted in
accordance with the Copyright, Designs and Patents Act 1988
P e r missions may be sought directly from Elsevier’s Science & Technology Rights
Department in Oxford, UK: phone ( ϩ 44) (0) 1865 843830; fax ( ϩ 44) (0) 1865 853333;
e mail: . Alternatively visit the Science and Technology Books
w e b site at www.elsevierdirect.com/rights for further information
Notice
No responsibility is assumed by the publisher for any injury and/or damage to persons or
property as a matter of products liability, negligence or otherwise, or from any use or operation
of any methods, products, instructions or ideas contained in the material herein
British Library Cataloguing-in-Publication Data
Self, Douglas.
Audio power amplifi er design handbook. – 5th ed.

1. Audio amplifi ers—Design. 2. Power amplifi ers—Design.
I. Title
621.3’81535—dc22
Library of Congress Control Number: 2009920721
ISBN: 978-0-240-52162-6
For information on all Focal Press publications
visit our website at www.focalpress.com
Printed and bound in the United States of America
09 10 11 12 13 12 11 10 9 8 7 6 5 4 3 2 1
v
Contents
Acknowledgements xviii
Preface to fi fth edition xix
Abbreviations xxi
Chapter 1 Introduction and general survey 1
The economic importance of power amplifi ers 1
Assumptions 1
Origins and aims 1
The study of amplifi er design 3
Misinformation in audio 5
Science and subjectivism 6
The subjectivist position 6
A short history of subjectivism 7
The limits of hearing 8
Articles of faith: the tenets of subjectivism 11
The length of the audio chain 15
The implications 16
The reasons why 16
The outlook 17
Technical errors 18

The performance requirements for amplifi ers 18
Safety 19
Reliability 19
Power output 19
Frequency response 20
Noise 20
Distortion 21
Damping factor 21
Absolute phase 23
Amplifi er formats 24
vi Contents
Chapter 2 Power amplifi er architecture and negative feedback 26
Amplifi er architectures 26
The three-stage amplifi er architecture 26
The two-stage amplifi er architecture 27
The four-stage amplifi er architecture 28
Power amplifi er classes 31
Class-A 31
Class-AB 31
Class-B 32
Class-C 32
Class-D 32
Class-E 32
Class-F 33
Class-G 33
Class-H 35
Class-S 35
Variations on Class-B 35
Error-correcting amplifi ers 35
Non-switching amplifi ers 36

Current-drive amplifi ers 36
The Blomley principle 36
Geometric mean Class-AB 36
Nested differentiating feedback loops 37
Amplifi er bridging 38
Fractional bridging 39
AC- and DC-coupled amplifi ers 41
The advantages of AC-coupling 41
The advantages of DC-coupling 42
Negative feedback in power amplifi ers 44
Some common misconceptions about negative feedback 48
Amplifi er stability and NFB 50
Maximizing the NFB 57
Overall feedback versus local feedback 58
Maximizing linearity before feedback 60
Chapter 3 The general principles of power amplifi ers 62
How a generic amplifi er works 62
The advantages of the conventional 64
The distortion mechanisms 65
Distortion 1: Input stage distortion 65
Distortion 2: VAS distortion 66
Contents vii
Distortion 3: Output stage distortion 66
Distortion 4: V
AS-loading distortion 67
Distortion 5: Rail-decoupling distortion 67
Distortion 6: Induction distortion 67
Distortion 7: NFB take-off distortion 67
Distortion 8: Capacitor distortion 67
Distortion 9: Magnetic distortion 68

Distortion 10: Input current distortion 68
Distortion 11: Premature overload protection 68
Nonexistent or negligible distortions 69
The performance of a standard amplifi er 70
Open-loop linearity and how to determine it 70
Direct open-loop gain measurement 71
Using model amplifi ers 72
The concept of the Blameless amplifi er 73
Chapter 4 The input stage 75
The role of the input stage 75
Distortion from the input stage 75
BJTs versus FETs for the input stage 77
Advantages of the FET input stage 77
Disadvantages of FET input stage 78
Singleton input stage versus differential pair 78
The input stage distortion in isolation 79
Input stage balance 80
The joy of current-mirrors 82
Better current-mirrors 83
Improving input stage linearity 85
Further improving input linearity 87
Increasing the output capability 90
Input stage cascode confi gurations 91
Double input stages 92
Input stage common-mode distortion 92
Input current distortion 96
Input stage noise and how to reduce it 104
Noise sources in power amplifi ers 107
Noise in bipolar transistors 108
Reducing input transistor noise 112

Offset and match: the DC precision issue 114
The input stage and the slew rate 115
Input stage conclusions 116
viii Contents
Chapter 5 The voltage-amplifi er stage 117
Measuring VAS distortion in isolation 118
VAS operation 118
VAS distortion 120
Linearizing the VAS: active-load techniques 121
VAS enhancements 122
Some more VAS variations 124
VAS operating conditions 125
The importance of voltage drive 126
The push – pull VAS 127
The high-current capability VAS 128
Single input stages 128
Double input stages 130
Manipulating open-loop bandwidth 134
Conclusions 137
Chapter 6 The output stage 138
Classes and devices 138
The distortions of the output 139
Harmonic generation by crossover distortion 141
Comparing output stages 142
The emitter-follower (EF) output 143
The complementary feedback pair (CFP) output 147
Output stages with gain 149
Quasi-complementary outputs 151
Triple-based output confi gurations 154
Triple-EF output stages 156

Quadruple output stages 158
Output stage distortions and their mechanisms 159
Large-signal distortion (Distortion 3a) 159
The Load-Invariant concept 162
The LSN mechanism 163
Doubled output devices 164
Better output devices 164
Feedforward diodes 166
Trouble with triples 167
Loads below 4 Ω 168
Better 8 Ω performance 168
A practical Load-Invariant design 168
More on multiple output devices 170
Load invariance: summary 172
Contents ix
Crossover distortion (Distortion 3b) 173
Output stage quiescent
conditions 180
An experiment on crossover distortion 181
V
q
as the critical quiescent parameter 184
Switching distortion (Distortion 3c) 185
Thermal distortion 186
Thermal distortion in a power amp IC 188
Selecting an output stage 189
Closing the loop: distortion in complete amplifi ers 190
Conclusions 193
Chapter 7 More distortion mechanisms 194
Distortion 4: VAS-loading distortion 194

Distortion 5: Rail-decoupling distortion 195
Distortion 6: Induction distortion 198
Distortion 7: NFB take-off point distortion 201
Distortion 8: Capacitor distortion 202
Distortion 9: Magnetic distortion 206
Distortion 10: Input current distortion 208
Distortion 11: Premature overload protection 209
Design example – a 50 W Class-B amplifi er 209
Chapter 8 Compensation, slew rate, and stability 215
Frequency compensation in general 215
Dominant-pole compensation 216
Lag compensation 217
Including the output stage: output - inclusive Miller compensation 217
Other forms of inclusive compensation 218
Two-pole compensation 218
Stability and VAS-collector-to-ground capacitance 222
Nested feedback loops 223
Output networks 224
Amplifi er output impedance 224
Minimizing amplifi er output impedance 227
Zobel networks 227
Output inductors 228
The output inductor value 234
Cable effects 235
Crosstalk in amplifi er output inductors 235
Coil crosstalk conclusions 241
Reactive loads and speaker simulation 241
Resistive loads 241
x Contents
Modeling real loudspeaker loading 242

Loudspeaker loads and output stages 246
Single-speaker load 246
Two-way speaker loads 250
Enhanced loudspeaker currents 252
Amplifi er instability 254
HF instability 254
LF instability 255
Speed and slew rate in audio amplifi ers 255
The basics of amplifi er slew-limiting 257
Slew-rate measurement techniques 257
Improving the slew rate 259
Simulating slew-limiting 259
Slewing limitations in real life 261
Some additional complications 262
Further improvements and other confi gurations 264
Chapter 9 Power supplies and PSRR 266
Power-supply technologies 266
Simple unregulated power supplies 266
Advantages 266
Disadvantages 266
Linear regulated power supplies 267
Advantages 267
Disadvantages 267
Switch-mode power supplies 268
Advantages 268
Disadvantages 269
A devious alternative to regulated power supplies 270
Design considerations for power supplies 271
Mains transformers 272
Transformer mounting 274

Transformer specifi cations 275
Electrical specifi cations 276
Mechanical matters 276
Transformer evaluation 277
Transformers and hum 278
External power supplies 279
Advantages 279
Disadvantages 280
Inrush currents 281
Inrush suppression by thermistor 282
Contents xi
Inrush suppression by relay 282
Fusing and rectifi
cation 284
RF emissions from bridge rectifi ers 284
Relay supplies 285
Power-supply rail rejection in amplifi ers 286
A design philosophy for supply-rail rejection 288
Positive supply-rail rejection 2 89
Negative supply-rail rejection 2 90
Negative sub-rails 2 97
Chapter 10 Class-A power amplifi ers 299
An introduction to Class-A 299
Class-A confi gurations and effi ciency 300
Output stages in Class-A 302
Quiescent current control systems 306
A novel quiescent current controller 307
A Class-A design 308
The Trimodal amplifi er 310
Load impedance and operating mode 312

E f fi ciency 313
On Trimodal biasing 318
Class-A/AB mode 318
Class-B mode 320
The mode-switching system 321
Thermal design 321
A complete Trimodal amplifi er circuit 323
The power supply 325
The performance 325
Further possibilities 325
Chapter 11 Class-XD ™ : crossover displacement technology 328
The crossover displacement principle 330
Crossover displacement realization 332
Circuit techniques for crossover displacement 334
A complete crossover displacement power amplifi er circuit 336
The measured performance 337
The effect of loading changes 340
The effi ciency of crossover displacement 341
Other methods of push – pull displacement control 342
Summary 343
Advantages 343
Disadvantages 343
xii Contents
Chapter 12 Class-G power amplifi ers 344
The principles of Class-G 344
Introducing series Class-G 345
E f fi ciency of Class-G 346
Practicalities 349
The biasing requirements 350
The linearity issues of series Class-G 350

The static linearity 353
Practical Class-G design 354
Controlling small-signal distortion 355
The performance 359
Deriving a new kind of amplifi er: Class-A ϩ C 361
Adding two-pole compensation 362
Further variations on Class-G 365
Chapter 13 Class-D amplifi ers 366
History 367
Basic principles 367
Technology 369
Protection 370
Output fi lters 371
E f fi ciency 371
Chapter 14 FET output stages 373
The characteristics of power FETs 373
FET versus BJT output stages 373
Advantages of FETs 374
Disadvantages of FETs 374
IGBTs 375
Power FET output stages 375
Power FETs and bipolars: the linearity competition 378
FETs in Class-A stages 379
Chapter 15 Thermal compensation and thermal dynamics 383
Why quiescent conditions are critical 383
Accuracy required of thermal compensation 384
Basic thermal compensation 388
Assessing the bias errors 388
Thermal simulation 389
Modeling the EF output stage 390

Modeling the CFP output stage 398
Contents xiii
The Integrated Absolute Error Criterion 400

Improved thermal compensation for the EF stage 400
Improved compensation for the CFP output stage 403
A better sensor position 405
A junction-temperature estimator 406
A junction estimator with dynamics 408
Conclusions about the simulations 409
Power transistors with integral temperature sensors 410
Variable-tempco bias generators 412
Creating a higher tempco 413
Ambient temperature changes 414
Creating a lower tempco 415
Current compensation 416
Early effect in output stages 418
Thermal dynamics by experiment 420
Crossover distortion against time – some results 420
More measurements – conventional and ThermalTrak 423
Chapter 16 The design of DC servos 429
DC offset trimming 429
DC offset control by servo-loop 430
The advantages of DC servos 431
Basic servo confi gurations 431
Noise, component values, and the roll-off 432
Non-inverting integrators 433
The 2C integrator 434
The 1C integrator 435
Choice of integrator type 436

Choice of op-amps 438
Servo authority 438
Design of LF roll-off point 439
Servo overload 439
Servo testing 439
Performance issues 440
Multi-pole servos 440
Chapter 17 Amplifi er and loudspeaker protection 441
Categories of amplifi er protection 441
Semiconductor failure modes 441
Overload protection 443
Overload protection by fuses 443
xiv Contents
Electronic overload protection 444
Plotting the protection locus 445
Simple current limiting 447
Single-slope VI limiting 449
Dual-slope VI limiting 450
VI limiting and temperature effects 452
Simulating overload protection systems 453
Testing the overload protection 454
Speaker short-circuit detection 455
Catching diodes 455
DC offset protection 456
DC protection by fuses 456
Relay protection and muting control 458
Filtering for DC protection 459
The single RC fi lter 459
The dual RC fi lter 460
The second-order active fi lter 461

Bidirectional DC detection 462
The conventional two-transistor circuit 462
The one-transistor version 462
The differential detector 463
The Self detector 464
Distortion in output relays 466
Output crowbar DC protection 469
Protection by power-supply shutdown 470
Thermal protection 471
Mains-fail detection 475
Powering auxiliary circuitry 477
Chapter 18 Grounding, cooling, and layout 479
Audio amplifi er PCB design 479
Crosstalk 479
Rail induction distortion 480
Mounting output devices on the main PCB 481
Advantages 481
Disadvantages 481
Single- and double-sided PCBs 482
Power-supply PCB layout 482
Power amplifi er PCB layout details 483
The audio PCB layout sequence 485
Miscellaneous points 486
Contents xv
Amplifi er grounding 487

Ground loops: how they work and how to deal with them 488
Hum injection by mains grounding currents 488
Hum injection by transformer stray magnetic fi elds 490
Hum injection by transformer stray capacitance 491

Ground currents inside equipment 492
Balanced mains power 493
Class-I and Class-II 494
Warning 495
Cooling 495
Convection cooling 496
Heat-sink materials 497
Heat-sink compounds 499
Thermal washers 499
Fan cooling 500
Fan control systems 501
Fan failure safety measures 504
Heat pipes 504
Mechanical layout and design considerations 505
Wiring layout 505
Semiconductor installation 505
Chapter 19 Testing and safety 509
Testing and fault-fi nding 509
Powering up for the fi rst time 511
Safety when working on equipment 512
Warning 513
Safety regulations 513
Electrical safety 513
Shocks from the mains plug 516
Touch current 517
Case openings 517
Equipment temperature and safety 517
Touching hot parts 52 0
Instruction manuals 52 0
Chapter 20 Power amplifi er input systems 521

External signal levels 522
Internal signal levels 523
The choice of op-amps 523
Unbalanced inputs 524
xvi Contents
Balanced interconnections 526
Advantages 527
Disadvantages 528
Common-mode rejection ratio 53 0
Balanced connectors 532
Balanced signal levels 532
Balanced inputs: electronic versus transformer 533
The basic balanced input 533
Common-mode rejection in the basic balanced input 535
The practical balanced input 539
Combined unbalanced and balanced inputs 54 0
Superbal input 541
Switched-gain balanced inputs 542
Variable-gain balanced inputs 544
High-impedance balanced inputs 545
The inverting two-op-amp input 546
The instrumentation amplifi er 546
Transformer balanced inputs 548
Input overvoltage protection 549
Noise and the input system 55 0
Low-noise balanced inputs 552
… And quieter yet 556
Noise reduction in real life 556
Unbalanced and balanced outputs 557
Unbalanced outputs 558

Ground-canceling outputs 559
Balanced outputs 56 0
Quasi-fl oating outputs 56 0
Transformer balanced outputs 562
Using a balanced power amplifi er interface 562
Chapter 21 Input processing and auxiliary subsystems 565
Ground-lift switches 565
Phase reversal facility 565
Gain control 565
Subsonic fi ltering: high-pass 566
Ultrasonic fi ltering: low-pass 568
Combined fi lters 569
Electronic crossovers 57 0
Digital signal processing 57 0
Signal-present indication 57 0
Contents xvii
Output level indication 571
Signal activ
ation 573
Twelve-Volt trigger activation 577
Infrared remote control 578
Other amplifi er facilities 578
Index 579
Acknowledgments
Heartfelt thanks to Gareth Connor of The Signal Transfer Company for practical help, never-failing
encouragement, and for providing the facilities with which some of the experiments in this book
were done.
I wish to thank Averil Donohoe for her help with some of the harder sums.
xix
You will have noted from the increased weight of this book that it has been signifi cantly expanded.

The text has increased in size by more than 50%, and there are a hundred new illustrations.
There is a completely new chapter on the Class-XD system that I recently introduced at Cambridge
Audio; an amplifi er utilizing this system won an Innovation award at Chicago CES, January 2008.
There is also a big new chapter on balanced line inputs and balanced interconnections in general.
These are becoming more and more common in the hi-fi fi eld and have always been of prime
importance in professional amplifi er systems. This is a vital topic as without good interconnection
technology the signal quality is irrevocably compromised before it gets anywhere near the actual
power amplifi er stage. This chapter also includes a lot of new material on ultra-low-noise design.
There is also a wholly new chapter on amplifi er subsystems such as signal activation, 12 V trigger,
level indication, and more. Amplifi er input stages and voltage-amplifi er stages now have separate
chapters of their own.
I have added lots of new material on four-stage amplifi er architectures, current-mirrors, power
transistors with internal sensing diodes, amplifi er bridging, distortion mechanisms, input stage
common-mode distortion, double input stages, amplifi er stability, output stages with gain,
transformers and their hum fi elds, inrush current suppression, DC servo design, thermal protection,
the subtleties of cooling fan control, line input stages, low-noise design, high- and low-pass
fi ltering, testing and safety, infrared control, and much more. There is signifi cantly more material
on professional power amplifi ers as used in sound reinforcement and PA applications.
I a m aware there is still very little material on power MOSFETs in this book, as I still hold to the
view that they are inevitably more nonlinear and harder to work with than bipolar transistors. I
know that some people – including some I have much respect for – do not agree, but I fi nd the
evidence in both theory and practice to be convincing.
There has been some rearrangement to get a more logical layout of the subject matter. Your favorite
topic has not been removed, but it might well have been moved.
A s you will have gathered, I am still fascinated by the apparently simple but actually fi endishly
complex business of making small signals bigger and applying them to a loudspeaker. An amplifi er
performs one of the simplest possible mathematical operations on a signal – multiplication by a
constant. It is fascinating to see how much more complicated things get after that.
Preface to Fifth Edition
xx Preface to Fifth Edition

Part of the lure of electronics as a pursuit is the speed with which ideas can be turned into physical
reality. In audio amplifi er design, you very often need just a handful of components, a piece of
prototype board, and a few minutes to see if the latest notion really is correct. If you come up with
a brilliant new way of designing large concrete dams then it is going to take more than an afternoon
to prove that it works.
You will also see, in Chapter 1, that in the last few years I have found no reason to alter my views
on the pernicious irrationality of subjectivism. In that period I have repeatedly been involved
in double-blind listening tests using e
xperienc
ed subjects and proper statistical analysis, which
confi rmed every time that if you cannot measure it you cannot hear it. Nevertheless the controversy
rumbles on, although in a more logical world it would have been regarded as settled in the 1970s.
I get a steady fl ow of emails supporting my position on this issue, but I fear I am still regarded in
some quarters as the Gregor Eisenhorn of amplifi er design.
There is in this book a certain emphasis on commercial manuf
ac
ture, which I hope does not
offend those purely interested in amateur construction or intellectual enquiry. In a commercial
environment, if you want to sell something (for more than a very short time) it has to work – and
keep working. This is still a valuable discipline if you are making a one-off design to test some new
ideas; if the design is not reliable then it must be unsound in some way that may ha
ve more impact
on what is
going on than you think.
In a changing world, one of the many things that has changed is the nature of discussion on audio
technologies. For many years Wireless World – later Electronics World – was a major forum for
this, and I contributed many articles to it over 30 years; it has, however, now changed its emphasis.
Elektor since its beginning has hosted serious audio articles and still does. The biggest change is of
course the arri
val of the Internet, whi

ch allows debate to proceed at a lightning pace compared with
the old method of writing a letter and waiting for a month or two to see it published. Currently the
only bulletin-board I frequent is DIYaudio.com; I personally think it is one of the best.
In producing this edition of the book it struck me frequently and forcibly how much has had to
be omitted for reasons of space, despite the generous increase in its size. Audio power amplifi er
design, even if confi ned to solid-state amplifi ers, and even if further confi
ned to those with bipolar
output s
tages, is already too big a fi eld for one person to know everything. I certainly don’t think
I do.
The journey continues.
Douglas Self
xxi
Abbreviations
I have kept the number of abbreviations used to a minimum. However, those few are used
extensively, so a list is given in case they are not all blindingly obvious:
BJT Bipolar junction transistor
C FP C o mplementary feedback pair
C/L Closed loop
C M C o mmon mode
CMOS Complementary metal oxide semiconductor
CMRR Common-mode rejection ratio
CTF Current timing factor
D F D a mping factor
DSP Digital signal processing
E F E mitter-follower
E FA E mitter-follower added
EIN Equivalent input noise
ESR Equivalent series resistance
FEA Finite element analysis

FET Field-effect transistor
H F A mplifi er behavior above the dominant pole frequency, where the open-loop gain
is usually falling at 6 dB/octave
IAE Integrated absolute error
IC Integrated circuit
IGBT Insulated-gate bipolar transistor
I/P Input
ISE Integrated square error
LED Light-emitting diode
L F Relating to amplifi er action below the dominant pole, where the open-loop gain is
assumed to be ess
entially fl at with frequency
LSN Large-signal nonlinearity
MOSFET Metal oxide semiconductor fi eld-effect transistor
N F N o i se fi gure
xxii Abbreviations
N FB Negative feedback
O/L Open loop
O/P Output
P 1 The fi rst O/L response pole, and its frequency in Hz (i.e. the Ϫ 3 dB point of a
6 dB/octave roll-off)
P 2 The second response pole, at a higher frequency
PA Public address
PCB Printed-circuit board
PDF Probability density function
PPD Power partition diagram
PSRR Power-supply rejection ratio
PSU Power-supply unit
PWM Pulse width modulation
R F Radio frequency

SID Slew-induced distortion
SOA, SOAR Safe operating area
SPL Sound pressure level
Tempco Temperature coeffi cient
THD Total harmonic di
stortion
TID
Tran
sient intermodulation distortion
TIM Transient intermodulation
VAS Voltage-amplifi er stage
VCIS Voltage-controlled current source
VCVS Voltage-controlled voltage source
VI Voltage/current
1
© 20XX Elsevier Ltd.2009
Introduction and General Survey
The Economic Importance of Power Amplifi ers
Audio power amplifi ers 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 amplifi er design.
The fi rst aim of this text is to fi ll that need, by providing a detailed guide to the many design
decisions that must be taken when a power amplifi er is designed.
The second aim is to disseminate the results of the original work done on amplifi er design in the
last few years. The unexpected result of these investigations was to show that power amplifi ers 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. This methodology
will be explained in detail.
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 defi nitions of frequency response, total
harmonic distortion (THD) and signal-to-noise ratio; these 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 confi ned
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 amplifi ers are explained in detail, as there
is still widespread confusion as to exactly how it works.
Origins and Aims
The core of this book is based on a series of eight articles originally published in Electronics World
as ‘ Distortion in Power Amplifi ers ’ . This series was primarily concerned with distortion as the
most variable feature of power amplifi er performance. You may have two units placed side by side,
CHAPTER 1
2 Chapter 1
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 fi gures 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 last two decades is that the more distortive amplifi er 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 in
vent a ne
w 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 nonlinearities 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 amplifi ers 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 pro

ved 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 fi nal result is an amplifi er with
unusually low distortion, using only modest and safe amounts of global negative feedback.
M u ch of this book concentrates on the distortion performance of amplifi ers. One reason is that this
v
aries
more than any other parameter – by up to a factor of 1000. Amplifi er distortion was until
recently an enigmatic fi eld – it was clear that there were several overlapping distortion mechanisms
in the typical amplifi er, 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, minimized.
I a ssume here that distortion is a bad thing, and should be minimized; I make no apology for
putting it as plainly as that. Alternative philosophies hold that as some forms of nonlinearity are
considered harmless or e
ven euphoni
c, they 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 attrib
ute that it can be switched of
f . Those who have brought into being our current
signal-delivery chain, i.e. mixing consoles, multitrack recorders, CDs, etc., have done us proud
in the matter of low distortion, and to willfully 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 amplifi ers.
Britain has a long tradition of small and very small audio companies, whose technical and
production resources may not dif

fer very greatly fro
m those available to the committed amateur.
I hope this volume will be of service to both.
I have endeavored 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 specifi cations at
minimum cost.