High Performance Audio Power Amplifiers for Music Performance and Reproduction

480 Pages·1996·5.4 MB·English

by  Ben Duncan

High Performance
Audio Power Amplifiers
for music performance and reproduction
Newnes
An imprint of Butterworth-Heinemann Ltd
Linacre House, Jordan Hill, Oxford OX2 8DP
A division of Reed Educational and Professional Publishing Ltd
OXFORD BOSTON JOHANNESBURG
MELBOURNE NEW DELHI SINGAPORE
First published 1996 Reprinted with revisions 1997
© Ben Duncan 1996 © B. D. 1997
All rights reserved. No part of this publication
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TRADEMARKS/REGISTERED TRADEMARKS
Computer hardware and software brand names mentioned in this book are
protected by their respective trademarks and are acknowledged.
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0 7506 2629 1
Typeset by P.K.McBride, Southampton
Printed and bound in Great Britain
High Performance
Audio Power Amplifiers
for music performance and reproduction
Ben Duncan
, A.M.I.O.A., A.M.A.E.S., M.C.C.S
international consultant in live show, recording & domestic
audio electronics and electro-acoustics.
Foreword
Ben Duncan is one of those rare individuals whose love and enthusiasm for a subject
transcends all the usual limits on perception and progress. In fact, without the few people
of true independent spirit, progress in the world would be swamped by the xylocaine of
vested interest, narrow attitude and corporate monoculture.
Amongst my early experiences of Ben Duncan’s thinking, many years ago, were his
contention that electronic components have qualitative audio properties and his recom-
mendation that we listen to the sound of capacitors of various dielectrics. The outcome
was the exclusive use of polypropylene capacitors in all Turbosound’s passive hi-pass
networks. This is not only illustrative of the depth to which the man goes, but also his
extensive seen and unseen influence on the whole audio community. He is an holistic
thinker and I believe there are very few things in the Universe that he has not, at one
time or another, considered having an effect on audio quality. Does he keep his flights of
fancy and strokes of brilliance to himself? Not one bit of it. He communicates compul-
sively and in large quantities as anyone who has followed the general audio press for the
last dozen or so years will tell you.
A memorable early experience of power amplifiers was with the then relatively new
transistor variety powering a P.A. I had built for the Pink Fairies, that was at the original
Glastonbury in 1971. After the sixth failure of an HH TPA100, for no apparent reason,
I was running out of working stock. On sitting down to consider the hopeless situation it
became worse when I found the live soldering iron. My next immediate thoughts were
about a change of career. Anyhow, the point of this sad little tale is that in those days
power amplifiers were absolutely horrible things because despite the fact that they had
somewhat puny voltage swings they were, nevertheless, always blowing up at the slightest
opportunity and particularly in the hour before show time. These days things have pro-
gressed a long way and sound system operators bask in the luxury of equipment that is
almost indestructible and capable of audio quality usually associated with esoteric hi-fi
as well as delivering arc welding levels of power.
I am extremely grateful to Ben that he has undertaken the Herculean task of collating all
the relevant facts on, and to do with, power amplifiers ranging from the in depth assess-
ment of household mains to determinations as to whether it actually sounds any good.
The breadth of the book enables an average human to purchase or design power ampli-
fiers knowing that all relevant information is at their disposal and as such this book
should be considered a positive contribution to the sum total of mankind. I hope it has a
similar effect on his bank balance.
Tony Andrews, Hoyle, Surrey
March 1996
Contents
Preface.......................................................................................................................xi
Acknowledgments...................................................................................................xiii
System of presentation............................................................................................. xv
1 Introduction and fundamentals 1
1.0. What are audio power amplifiers for ?...............................................1
1.1 What is the problem ?.........................................................................1
1.2 What is audio ?...................................................................................2
1.3 What’s special about audio ?..............................................................2
1.4 The ramifications of quality on audio................................................3
1.5 Some different aims of sound reproduction.......................................3
1.6 About people and their hearing..........................................................4
1.7 Limits of a ‘objectivity’. Why listen ?...............................................5
1.8 Why are power amplifiers needed for audio ?...................................6
1.9 Music fundamentals ...........................................................................8
1.10 Adjectives that describe sound...........................................................9
1.10.1 Tonal qualities.............................................................................................. 10
1.10.2 Broader tonal descriptors..............................................................................11
1.10.3 General sonic adjectives ...............................................................................11
1.10.4 Dynamics...................................................................................................... 12
1.10.5 Space............................................................................................................ 13
1.10.6 Botheration or Abomination........................................................................ 14
1.11 Nature and range of music (alias programme).................................14
1.12 Bass and subsonic content................................................................15
1.13 HF dynamics and ultrasonic content................................................16
References and Further reading................................................................... 18
2 Overview of Global Requirements 19
2.1 Common formats for power amps ...................................................19
2.2 Loudspeakers....................................................................................21
2.2.1 Loudspeaker drive-unit basics..................................................................... 21
2.2.2 Loudspeaker sensitivity vs. efficiency......................................................... 25
2.2.3 Loudspeaker enclosure types and efficiencies............................................. 26
2.2.4 Loudspeaker configurations: a résumé........................................................ 27
2.3 The interrelation of components ......................................................32
2.3.1 What loudspeakers look like to the amplifier.............................................. 32
2.3.2 What speakers are looking for..................................................................... 35
2.3.3 What passive crossovers look like to amplifiers......................................... 38
2.4 Behaviour of power amps as voltage sources..................................40
2.4.1 Drive-unit power ratings after EIA/AES..................................................... 40
2.4.2 Output power capability requirements......................................................... 41
2.4.3 Loudspeaker vulnerabilities......................................................................... 43
2.4.4 High power, the professional rationale........................................................ 44
2.4.5 Active systems, power delivery requirements............................................. 46
2.5 Current delivery requirements..........................................................46
2.5.1 The low impedance route............................................................................. 47
References and Further reading................................................................... 48
3 The input port – Interfacing and processing 49
3.1 The Input...........................................................................................49
3.1.1 Input sensitivity and gain requirements....................................................... 49
3.1.2 Input impedance (Z )................................................................................... 52
in
3.2 RF filtration......................................................................................58
3.3 The balanced input ...........................................................................59
3.3.1 Balancing requirements ............................................................................... 59
3.3.2 Introducing Common Mode Rejection........................................................ 60
3.4 Sub-sonic protection and high-pass filtering ...................................63
3.4.1 Direct Coupling............................................................................................ 65
3.5 Damage protection ...........................................................................68
3.6 What are process functions?.............................................................71
3.6.1 Common gain control (panel attenuator)..................................................... 71
3.6.2 Remotable gain controls (machine control)................................................. 74
3.6.3 Remote control considerations..................................................................... 77
3.6.4 Compression and limiting............................................................................ 78
3.6.5 Clipping (overload) considerations.............................................................. 79
3.6.6 Clip prevention............................................................................................. 79
3.6.7 Soft-Clip....................................................................................................... 79
3.7 Computer control..............................................................................80
References and Further reading................................................................... 82
4 Topologies, classes and modes 83
4.1 Introduction ......................................................................................83
4.1.1 About topologies.......................................................................................... 84
4.2 Germanium and early junctions .......................................................87
4.2.1 Out of the vacuum-state............................................................................... 87
4.2.2 Push-pull, Transformer-coupled.................................................................. 88
4.2.3 Sub-topology: the Darlington...................................................................... 89
4.2.4 Transformerless push-pull (transistor OTL)................................................ 91
4.2.5 Sub-topology: diode biasing........................................................................ 91
4.2.6 Complementary push-pull OTL................................................................... 92
4.1.8 Quasi complementarity: the faked match.................................................... 94
4.1.9 Sub-topology: paralleling............................................................................. 94
4.3 Silicon transistors .............................................................................95
4.3.1 The Lin topology.......................................................................................... 96
4.3.2 Sub-topology: the long-tailed pair (LTP) .................................................... 98
4.3.3 Sub-topology: the Vbe multiplier (VbeX]................................................... 99
4.3.4 Sub-topology: the triple (compound BJT)................................................. 100
4.3.5 Sub topology: Dual supplies (+/–Vs)........................................................ 102
4.3.6 Sons of Lin................................................................................................. 104
4.4 True symmetry: the sequel.............................................................105
4.4.1 Later topologies.......................................................................................... 106
4.4.2 IC power..................................................................................................... 108
4.4.3 The Op-Amp topologies .............................................................................110
4.4.4 Power cascades and cascodes.....................................................................112
4.5 Introducing bridging....................................................................... 113
4.5.1 Bridging the bridge.....................................................................................117
4.6 Class-ification................................................................................. 118
4.6.1 Class A........................................................................................................118
vi
4.6.2 Class A alternatives.................................................................................... 123
4.6.3 Class A sliding bias and ‘Π-mode’............................................................ 123
4.6.4 ‘Super Class A’........................................................................................... 126
4.6.5 Dynamic biasing and Stasis....................................................................... 126
4.6.6 Sustained plateau biasing........................................................................... 126
4.6.7 Class B and A-B......................................................................................... 127
4.6.8 Class A-B, developments and ameliorations............................................. 131
4.7 Introducing higher classes..............................................................136
4.7.1 Class G....................................................................................................... 138
4.7.2 Class H....................................................................................................... 141
4.7.3 G and H, the comparison ........................................................................... 143
4.8 Beyond analogue............................................................................146
4.8.1 Class D....................................................................................................... 147
4.8.2 ‘Digital’ amplification ............................................................................... 152
4.9 Class summary ...............................................................................153
4.10 Introducing modes of control.........................................................155
4.10.1 Negative feedback modes.......................................................................... 156
4.10.2 Other Error Correction Modes................................................................... 162
4.11 Conclusions ....................................................................................164
References and Further reading................................................................. 168
5 Features of the power stage 169
5.1 Overview ........................................................................................169
5.1.1 Operating with high voltages..................................................................... 169
5.1.2 Operating with high currents..................................................................... 170
5.2 Power devices.................................................................................171
5.2.1 Bipolar Junction Transistors (BJT)............................................................ 171
5.2.2 MOSFETs (enhancement-mode power FETs)........................................... 177
5.2.3 Insulated Gate Bipolar Transistors (IGBT)............................................... 183
5.3 Recognising large signals...............................................................184
5.3.1 The slew limit............................................................................................. 185
5.4 RF stability .....................................................................................191
5.4.1 Power stage, critical layout requirements.................................................. 191
5.4.2 Critical nodes ............................................................................................. 192
5.5 V&I limits on output, the context ..................................................193
5.5.1 V-I output capability.................................................................................. 198
5.5.2 V-I output limiting (adverse load protection)............................................ 200
5.5.3 Mapping V-I capability.............................................................................. 210
5.5.4 Audio protection, by fuse............................................................................211
5.6 Clip indication – external relations ................................................213
5.6.1 Overdrive behaviour – internal relations................................................... 215
5.6.2 Output stability and the output network (OPN) ........................................ 215
5.6.3 RF protection.............................................................................................. 217
5.7 DC offset, at output ........................................................................218
5.7.1 DC (Fault) protection (DCP, DCFP).......................................................... 219
5.8 The output interface........................................................................220
5.8.1 Muting systems.......................................................................................... 221
5.9 Output stage, cooling requirements................................................222
5.9.1 Heat exchange............................................................................................ 224
5.9.2 Thermal protection..................................................................................... 226
vii
5.10 Logical systems..............................................................................228
5.11 Output transformers........................................................................228
References and Further reading......................................................230
6 The power supply 231
6.1 Mains frequency (50/60Hz) supplies.............................................231
6.1.1 50/60Hz EMI considerations..................................................................... 235
6.1.2 Surge handling ........................................................................................... 237
6.1.3 Actively-adaptive 50/60Hz PSU................................................................ 238
6.1.4 Regulated 50/60Hz ‘passive’ supplies....................................................... 239
6.2 Supply amongst channels...............................................................240
6.2.1 Bridge benefits........................................................................................... 242
6.2.2 Operation with 3 Phase AC ....................................................................... 242
6.3 Pulse-width power (PWM PSU)....................................................243
6.3.1 HF power supplies (SMPS, HF switchers)................................................ 244
6.3.2 Resonant power.......................................................................................... 247
6.3.3 The higher adaptive PSUs.......................................................................... 250
6.3.4 HF switching summary.............................................................................. 250
6.4 Power supply (PSU) efficiency round-up......................................251
6.4.1 Amplifier efficiency summary................................................................... 252
6.5 Power supply fusing.......................................................................253
References and Further reading................................................................. 256
7 Specifications and testing 257
7.1 Why specifications? .......................................................................257
7.1.1 Types of spec.............................................................................................. 257
7.1.2 Standards for audio power amps................................................................ 259
7.2 Why test things ?............................................................................259
7.2.1 Test tools and orientation........................................................................... 260
7.2.2 Realtime test signals .................................................................................. 261
7.2.3 The test equipment revolution................................................................... 264
7.3 Physical environment.....................................................................265
7.3.1 Mains measurement and conditioning....................................................... 265
7.3.2 Power amplifier preconditioning............................................................... 266
7.3.3 The test load............................................................................................... 268
7.4 Frequency response (Bandwidth, BW)..........................................272
7.4.1 Gain and balance........................................................................................ 274
7.4.2 Output impedance (Z)............................................................................... 276
o
7.4.3 Damping factor .......................................................................................... 278
7.4.4 Phase response ........................................................................................... 278
7.5 Introducing noise............................................................................281
7.5.1 Noise spectra.............................................................................................. 282
7.5.2 Breakthrough and crosstalk (channel separation)...................................... 283
7.5.3 Understanding CMR measurements.......................................................... 284
7.5.4 Measuring CMR......................................................................................... 285
7.6 Input impedance (Z ) .....................................................................286
in
7.7 Introducing harmonic distortion.....................................................286
7.7.1 Harmonics: the musical context................................................................. 289
7.7.2 Harmonic distortion (THD, %THD+N).................................................... 293
7.7.3 Individual harmonic analysis (IHA).......................................................... 297
viii
7.7.4 Intermodulation (% IMD, Intermod)......................................................... 298
7.7.5 Dynamic intermodulation (% DIM 30/100).............................................. 302
7.7.6 Sundry intermodulation checks ................................................................. 303
7.7.7 Other distortion tests.................................................................................. 303
7.8 Power output (P)............................................................................304
o
7.8.1 Output voltage capability (V rms, MOL)................................................. 305
o
7.8.2 Dynamic output capability......................................................................... 306
7.8.3 Clipping symmetry..................................................................................... 307
7.8.4 Dynamic range........................................................................................... 308
7.9 Dynamic tests .................................................................................308
7.9.1 Rise time (small signal attack)................................................................... 308
7.9.2 Slew limit (slew rate, large signal attack).................................................. 309
7.9.3 Transient response (impulse response)...................................................... 310
7.9.4 Peak output current capability................................................................... 312
References and Further reading................................................................. 314
8 Real world testing – rationale and procedures 315
8.1 Scope and why essential.................................................................315
8.2 Listening.........................................................................................315
8.3 Operable mains range.....................................................................317
8.3.1 Inrush current............................................................................................. 318
8.3.2 Soft start..................................................................................................... 319
8.3.3 Mains current draw.................................................................................... 320
8.4 Signal present indication and metering..........................................321
8.4.1 Clip indication............................................................................................ 322
8.5 DC at the input................................................................................322
8.5.1 RF at input.................................................................................................. 322
8.5.2 Large signals at input................................................................................. 323
8.6 Output DC offset (output offset, V )...........................................323
oos
8.6.1 RF at output................................................................................................ 324
8.6.2 Adverse loads............................................................................................. 325
8.6.3 Adverse load proving................................................................................. 325
8.6.4 Adverse loads, low loads and shorting...................................................... 325
8.6.5 Adverse loads, reactive.............................................................................. 327
8.6.6 Hard drive testing....................................................................................... 327
8.7 Thermal protection and monitoring ...............................................327
8.8 Muting behaviour...........................................................................328
8.8.1 Acoustic noise............................................................................................ 328
8.9 EMI and EMC ................................................................................329
References and Further reading................................................................. 330
9 Choice, application installation and set-up 331
9.1 Manufactured goods, a résumé ......................................................331
9.1.1 Choosing the right power amp, domestic.................................................. 332
9.1.2 Choosing the right power amp, for pro users............................................ 335
9.2 Howlers...........................................................................................340
9.3 AC mains voltage...........................................................................342
9.3.1 Safety earthing ........................................................................................... 344
9.3.2 Mains cabling............................................................................................. 346
9.3.3 Power factor correction.............................................................................. 348
ix
9.3.4 Mains fuses and breakers........................................................................... 349
9.3.5 AC mains connectors, amplifier end ......................................................... 350
9.3.6 AC mains connectors, the power-end........................................................ 352
9.3.7 Mains wiring practice, domestic and studio.............................................. 352
9.4 Input connections............................................................................356
9.4.1 Balanced polarity and shielding................................................................. 359
9.4.2 Quasi balanced (unbal-to-bal).................................................................... 360
9.4.3 Input cabling............................................................................................... 361
9.5 Output connections.........................................................................362
9.5.1 Speaker cabling.......................................................................................... 367
9.5.2 Impedance setting ...................................................................................... 370
9.5.3 Output polarity........................................................................................... 370
9.6 Placement and fitment....................................................................371
9.6.1 Cooling airflow conventions...................................................................... 371
9.6.2 Cooling and air pollution........................................................................... 373
9.7 The 1 to 5 of prudent amplifier use................................................374
9.7.1 System back-end troubleshooting.............................................................. 374
References and Further reading................................................................. 376
10 Maintenance and surgery 377
10.1 Classifying failures.........................................................................377
10.2 Problem solving procedures...........................................................379
10.3 Universal repair procedures ...........................................................379
10.4 Repair tools and equipment............................................................389
10.4.1 Useful tools................................................................................................ 389
10.4.2 Test powering tools.................................................................................... 390
10.4.3 Audio test tools.......................................................................................... 392
10.5 Testing components........................................................................393
10.5.1 Testing BJTs (bipolar transistors).............................................................. 393
10.5.2 Testing MOSFETs...................................................................................... 395
10.5.3 Testing diodes, zeners and LEDs............................................................... 397
10.6 Scope traces....................................................................................398
References and Further reading................................................................. 400
Useful addresses for maintenance.............................................................. 400
Appendices
1 History............................................................................................401
2 Makers’ listings..............................................................................409
A-Z, under principal use............................................................................ 409
A-Z of integrated loudspeaker-amplifier makers...................................... 413
Principal output device, by number of makers.......................................... 414
3 Active devices ................................................................................415
A Bipolar transistors, silicon ..................................................................... 416
B Lateral power MOSFETs ....................................................................... 420
C Power D–MOSFETs............................................................................... 421
D Thermionic valves (electron tubes)........................................................ 422
4 Power amplifier terminology .........................................................423
Index 457
x