Real world testing – Rationale and procedures

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

MELBOURNE



BOSTON

NEW DELHI



First published 1996

© Ben Duncan 1996



JOHANNESBURG

SINGAPORE



Reprinted with revisions 1997

© B. D. 1997



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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 recommendation 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 compulsively 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 progressed 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 assessment 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 amplifiers 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.

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

1.10



What are audio power amplifiers for ? ............................................... 1

What is the problem ? ......................................................................... 1

What is audio ? ................................................................................... 2

What’s special about audio ? .............................................................. 2

The ramifications of quality on audio ................................................ 3

Some different aims of sound reproduction ....................................... 3

About people and their hearing .......................................................... 4

Limits of a ‘objectivity’. Why listen ? ............................................... 5

Why are power amplifiers needed for audio ? ................................... 6

Music fundamentals ........................................................................... 8

Adjectives that describe sound ........................................................... 9



1.10.1

1.10.2

1.10.3

1.10.4

1.10.5

1.10.6



Tonal qualities .............................................................................................. 10

Broader tonal descriptors .............................................................................. 11

General sonic adjectives ............................................................................... 11

Dynamics ...................................................................................................... 12

Space ............................................................................................................ 13

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

2.2



Common formats for power amps ................................................... 19

Loudspeakers .................................................................................... 21



2.2.1

2.2.2

2.2.3

2.2.4



Loudspeaker drive-unit basics .....................................................................

Loudspeaker sensitivity vs. efficiency .........................................................

Loudspeaker enclosure types and efficiencies .............................................

Loudspeaker configurations: a résumé ........................................................



2.3



The interrelation of components ...................................................... 32



21

25

26

27



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

2.4.2

2.4.3

2.4.4

2.4.5



Drive-unit power ratings after EIA/AES .....................................................

Output power capability requirements .........................................................

Loudspeaker vulnerabilities .........................................................................

High power, the professional rationale ........................................................

Active systems, power delivery requirements .............................................



2.5



Current delivery requirements .......................................................... 46



40

41

43

44

46



2.5.1 The low impedance route ............................................................................. 47

References and Further reading ................................................................... 48



3



The input port – Interfacing and processing

3.1



49



The Input........................................................................................... 49



3.1.1 Input sensitivity and gain requirements ....................................................... 49

3.1.2 Input impedance (Zin) ................................................................................... 52



3.2

3.3



RF filtration ...................................................................................... 58

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

3.6



Damage protection ........................................................................... 68

What are process functions? ............................................................. 71



3.6.1

3.6.2

3.6.3

3.6.4

3.6.5

3.6.6

3.6.7



Common gain control (panel attenuator) .....................................................

Remotable gain controls (machine control) .................................................

Remote control considerations .....................................................................

Compression and limiting ............................................................................

Clipping (overload) considerations ..............................................................

Clip prevention .............................................................................................

Soft-Clip .......................................................................................................



3.7



Computer control .............................................................................. 80



71

74

77

78

79

79

79



References and Further reading ................................................................... 82



4



Topologies, classes and modes

4.1



83



Introduction ...................................................................................... 83



4.1.1 About topologies .......................................................................................... 84



4.2



Germanium and early junctions ....................................................... 87



4.2.1

4.2.2

4.2.3

4.2.4

4.2.5

4.2.6

4.1.8

4.1.9



Out of the vacuum-state ...............................................................................

Push-pull, Transformer-coupled ..................................................................

Sub-topology: the Darlington ......................................................................

Transformerless push-pull (transistor OTL) ................................................

Sub-topology: diode biasing ........................................................................

Complementary push-pull OTL ...................................................................

Quasi complementarity: the faked match ....................................................

Sub-topology: paralleling .............................................................................



87

88

89

91

91

92

94

94



4.3



Silicon transistors ............................................................................. 95



4.3.1

4.3.2

4.3.3

4.3.4

4.3.5

4.3.6



The Lin topology .......................................................................................... 96

Sub-topology: the long-tailed pair (LTP) .................................................... 98

Sub-topology: the Vbe multiplier (VbeX] ................................................... 99

Sub-topology: the triple (compound BJT) ................................................. 100

Sub topology: Dual supplies (+/–Vs) ........................................................ 102

Sons of Lin ................................................................................................. 104



4.4



True symmetry: the sequel ............................................................. 105



4.4.1

4.4.2

4.4.3

4.4.4



Later topologies .......................................................................................... 106

IC power ..................................................................................................... 108

The Op-Amp topologies ............................................................................. 110

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

4.6.3

4.6.4

4.6.5

4.6.6

4.6.7

4.6.8



Class A alternatives ....................................................................................

Class A sliding bias and ‘Π-mode’ ............................................................

‘Super Class A’ ...........................................................................................

Dynamic biasing and Stasis .......................................................................

Sustained plateau biasing ...........................................................................

Class B and A-B .........................................................................................

Class A-B, developments and ameliorations .............................................



123

123

126

126

126

127

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

5.1



169



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

5.5.2

5.5.3

5.5.4



V-I output capability .................................................................................. 198

V-I output limiting (adverse load protection) ............................................ 200

Mapping V-I capability .............................................................................. 210

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

6.1.2

6.1.3

6.1.4



50/60Hz EMI considerations .....................................................................

Surge handling ...........................................................................................

Actively-adaptive 50/60Hz PSU ................................................................

Regulated 50/60Hz ‘passive’ supplies .......................................................



6.2



Supply amongst channels ............................................................... 240



235

237

238

239



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

6.3.2

6.3.3

6.3.4



HF power supplies (SMPS, HF switchers) ................................................

Resonant power ..........................................................................................

The higher adaptive PSUs ..........................................................................

HF switching summary ..............................................................................



6.4



Power supply (PSU) efficiency round-up ...................................... 251



244

247

250

250



6.4.1 Amplifier efficiency summary ................................................................... 252



6.5



Power supply fusing ....................................................................... 253

References and Further reading ................................................................. 256



7



Specifications and testing

7.1



257



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

7.4.2

7.4.3

7.4.4



Gain and balance ........................................................................................

Output impedance (Zo) ...............................................................................

Damping factor ..........................................................................................

Phase response ...........................................................................................



7.5



Introducing noise ............................................................................ 281



7.5.1

7.5.2

7.5.3

7.5.4



Noise spectra ..............................................................................................

Breakthrough and crosstalk (channel separation) ......................................

Understanding CMR measurements ..........................................................

Measuring CMR .........................................................................................



7.6

7.7



Input impedance (Zin) ..................................................................... 286

Introducing harmonic distortion ..................................................... 286



274

276

278

278

282

283

284

285



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

7.7.5

7.7.6

7.7.7



7.8



Intermodulation (% IMD, Intermod) .........................................................

Dynamic intermodulation (% DIM 30/100) ..............................................

Sundry intermodulation checks .................................................................

Other distortion tests ..................................................................................



298

302

303

303



Power output (Po)............................................................................ 304



7.8.1

7.8.2

7.8.3

7.8.4



7.9



Dynamic tests ................................................................................. 308



7.9.1

7.9.2

7.9.3

7.9.4



8



Output voltage capability (Vo rms, MOL) .................................................

Dynamic output capability .........................................................................

Clipping symmetry .....................................................................................

Dynamic range ...........................................................................................



Rise time (small signal attack) ...................................................................

Slew limit (slew rate, large signal attack) ..................................................

Transient response (impulse response) ......................................................

Peak output current capability ...................................................................

References and Further reading .................................................................



Real world testing – rationale and procedures

8.1

8.2

8.3



305

306

307

308



308

309

310

312

314



315



Scope and why essential ................................................................. 315

Listening ......................................................................................... 315

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, Voos) ........................................... 323



8.6.1

8.6.2

8.6.3

8.6.4

8.6.5

8.6.6



RF at output ................................................................................................

Adverse loads .............................................................................................

Adverse load proving .................................................................................

Adverse loads, low loads and shorting ......................................................

Adverse loads, reactive ..............................................................................

Hard drive testing .......................................................................................



324

325

325

325

327

327



8.7

8.8



Thermal protection and monitoring ............................................... 327

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

9.1



331



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

9.3



Howlers ........................................................................................... 340

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

9.3.5

9.3.6

9.3.7



Mains fuses and breakers ...........................................................................

AC mains connectors, amplifier end .........................................................

AC mains connectors, the power-end ........................................................

Mains wiring practice, domestic and studio ..............................................



349

350

352

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

10.1

10.2

10.3

10.4



377



Classifying failures ......................................................................... 377

Problem solving procedures ........................................................... 379

Universal repair procedures ........................................................... 379

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

2



History ............................................................................................ 401

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 .....................................................................

B Lateral power MOSFETs .......................................................................

C Power D–MOSFETs ...............................................................................

D Thermionic valves (electron tubes) ........................................................



4



Index



x



416

420

421

422



Power amplifier terminology ......................................................... 423



457



Preface

There has never been a book like this one, in its interleaving of electronics and audio,

engineering ideality, and musical and practical reality. There haven’t actually been

many books dedicated to audio amplifiers, period. On any level.

Beginning with the electronics, amplifiers for driving loudspeakers are actually rather

hard. So hard, that after 75 years, there is not a lot of convergence – not compared to

say washing machines, which are similarly old. In spite of a century of consumerism, Music remains on a higher, primal level that interfaces with levels of human

perception that precede and can outstep the logical. There have been many gifted

minds at work in amplifierland, but they haven’t had even half the answers. Many

have come unstuck, or lost the plot completely, confused by mathematical catastrophes in audio’s higher dimensions.

Audio power amplifiers are unsung key tools in the immense growth of human massattuned consciousness during the 20th century. Imagine amplifiers were dis-invented.

Without speaker-driving-devices, there wouldn’t be hi-fi systems, radios or (wild

applause) TVs. There wouldn’t be PA systems, and there wouldn’t be any festivals

bigger than village garden fetes, or at best 2000 seater amphitheatres. There wouldn’t

be cinemas and recording studios, and no solid-bodied or electronic instruments.

The huge emotional and psychic amplification, through music (its own capabilities

also vastly expanded by electronic amplification, recording and processing) and the

sound-tracked cinema (and video offspring) and their mass broadcast and

affordability, that has made the 20th century vibrant like none before it, would be

naught. The human world without good amplifiers – or any audio amplifiers – would

be far less linked to spiritual and emotional heights – and probably not a lot quieter.

‘High Performance’ means that the book does not cover equipment where makers

knowingly make significant ‘corner cuttings’ that degrade audio quality, reliability

and utility, particularly so called ‘consumer’ and much so called ‘M.I’ equipment.

This seems a more natural dividing line than any of the more common ones, like

pro.vs.domestic. Everyone who is serious about music, wants much the same things,

however much it needs adapting to suit their particular environment. What ‘High

Performance’ does not mean is any particular price or other label of exclusivity.

The amplifiers covered in this book could cost (at 1996 prices) £135 or £13,500.

They could deliver 25w or 2500w, be used in the home, in the studio, a stadium, or

in a field, so long as their aim or suitability is to permit the faithful reproduction of

some kind of music and all its nuance.

Across this book, you will discover that the contents’ focus purposefully veers from

a wide pan across the most global, broad-minded considerations including amplifiers employing valves (tubes) and/or ‘zero’-feedback, wherein generalities are enough,

through to a narrow concentration on the majority of modern transistor (‘solid state’)



xi