Valve and Transistor Audio Amplifiers

262 Pages·2009·7.88 MB·English

by  John Linsley Hood

VALVE AND TRANSISTOR
AUDIO AMPLIFIERS
JOHN LINSLEY HOOD
AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD
PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO
Newnes
An imprint of Elsevier
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First published 1997
Reprinted 2000, 2001, 2003, 2006
Copyright (cid:14)9 1997, John Linsley Hood. All rights reserved
The right of John Linsley Hood to be identified as the author of this work has been
asserted in accordance with the Copyright, Designs and Patents Act 1988
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ISBN 0 7506 3356 5
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CONTENTS
PREFACE ix
CHAPTER 1 ACTIVE COMPONENTS
Valves or Vacuum Tubes I
Solid State Devices 11
CHAPTER 2 PASSIVE COMPONENTS 25
Inductors and Transformers 25
Capacitors 30
Resistors 36
Switches and Electrical Contacts 39
CHAPTER 3 VOLTAGE AMPLIFIER STAGES USING VALVES 41
Circuit Configurations 41
Grid Bias Systems 41
Cathode Bias Resistor Calculations 45
Anode Load Systems 45
Grounded Grid Stages 47
The Cascode Circuit 48
The Long-tailed Pair 48
The Cathode Follower 49
The/.1 Follower 50
Tetrodes, Beam-Tetrodes and Pentodes 50
Load Lines 53
CHAPTER 4 VALVE AUDIO AMPLIFIER LAYOUTS 57
Single-ended vs. Push-pull Operation 59
Phase Splitters 61
Output Stages 64
Output (Load-matching) Transformer 65
Effect of Output Load Impedance 68
Available Output Power 69
vi Valve and Transistor Audio Amplifiers
CHAPTER 5 NEGATIVE FEEDBACK 71
Benefits of Negative Feedback 73
Stability Problems 74
Series and Shunt NFB 75
The Effect of NFB on Input and Output Impedances 77
Influence of NFB on Output Impedance 80
Effect of NFB on the Harmonic Components of the Signal 81
Practical Valve NFB Systems 82
The Baxandall Negative Feedback Tone Tontrol System 83
Frequency Response Equalisation 85
Filter Circuits Based on NFB 86
CHAPTER 6 VALVE OPERATED AUDIO POWER AMPLIFIERS 93
The Mclntosh Amplifier 94
The Williamson Amplifier 95
Distributed Load Systems 98
The Quad System 99
Baxandall's Amplifier 101
The Baxandall 5 Watt Design 104
The Radford Valve Amplifiers 105
The Leak TL/12 106
The Mullard 5/10 and 5/20 Designs 106
The GEC Audio Amplifiers 107
The GEC 912 + Amplifier 109
Other GEC Audio Designs 110
The Brimar 25P1 Design 110
The Sonab Design 112
Increasing Available Output Power 113
Conclusions 115
CHAPTER 7 SOLID STATE VOLTAGE AMPLiFiERS 117
Junction Transistors 117
Control of Operating Bias 119
Stage Gain 121
Basic Junction Transistor Circuit Configurations 122
Emitter-follower Systems 124
Thermal Dissipation Limits 126
Junction Field Effect Transistors (JFETs) 127
Insulated Gate FETs (MOSFETs) 131
Power BJTs vs. Power MOSFETs as Amplifier Output Devices 134
U and D MOSFETs 135
Useful Circuit Components 137
Circuit Oddments 138
Slew Rate Limiting 140
Contents vii
CHAPTER 8 EARLY SOLID STATE AUDIO AMPLIFIER DESIGNS 141
The Lin Circuit 143
Quasi-complementary Output Stage Asymmetry 145
Listener Fatigue 148
Alternative Circuit Choices 150
Class A Operation 153
Fully Complementary Designs 156
Gain Stage Designs 158
HF Compensation Techniques 160
Symmetry in Circuit Layout and Slewing Rates 162
Stability of Output Current 163
CHAPTER 9 CONTEMPORARY POWER AMPLIFIER DESIGNS 167
Remaining Design Problems 167
Power MOSFETs vs. Bipolar Junction Power Ttransistors (BJTs)168
Frequency Response of MOSFET Devices 169
MOSFET Linearity 171
gm Values for BJTs and MOSFETs 174
Quiescent Current Stability in Push-pull Output Systems 175
Other Iq Control Techniques 176
The Quad Current Dumping Amplifier 178
The Sandman Class S System 180
The Technics Class AA system 180
Voltage Amplifying Stage Gain and Overall THD 181
Basic Gain Stage Constructions 182
Output/Input Signal Isolation 186
Effect of NFB on Signal Distortion 187
Symmetry in Gain Block Structure 187
Negative Feedback and Sound Quality 188
Power Amplifier Output Stages 190
Output DC Offset Control 191
Hybrid Systems 193
Listening Trials 193
CHAPTER 10 PREAMPLIFIERS 197
Gramophone Record Replay Equalisation Characteristics 198
Replay Equalisation Circuitry 199
Practical Circuit Layouts 203
Very Low Noise Input Systems 206
Low Noise Devices 209
Headroom 212
Tone Controls and Filters 215
Signal Channel Switching and Remote Controls 218
(cid:12)9 (cid:12)9 (cid:12)9 Valve and Transistor Audio Amplifiers
VIII
Voltage Controlled Gain Systems 219
Digital Inputs 221
CHAPTER 11 POWER SUPPLIES 225
High Power Systems 225
Solid State Rectifiers 226
Music Power 229
Influence of Signal Type on Power Supply Design 229
High Current Power Supply Systems 230
Half-wave and Full-wave Rectification 231
DC Supply Line Ripple Rejection 232
Voltage Regulator Systems 232
Series Regulator Layouts 233
Over-current Protection 234
Integrated Circuit (Three Terminal) Voltage Regulator ICs 236
Typical Contemporary Commercial Practice 239
Battery Supplies 239
Switch-mode Power Supplies 241
INDEX 243
PREFACE
An old friend of mine was in the habit of remarking that an engineer is someone who
can do for five pounds what any damn fool can do for fifty. Needless to say, he was
an engineer. I suppose, in a sense, that this is a fairly good description of the major
aims of engineering practice; that the designer should seek elegant, efficient and cost-
effective ways of achieving a clearly specified objective. Only when satisfactory ways
of doing this cannot be found is there a need to seek more elaborate or costly ways to
get this result.
In the field of audio amplifiers there has been a great interest in techniques for making
small electrical voltages larger ever since mankind first attempted to transmit the
human voice along lengthy telephone cables. This quest received an enormous boost
with the introduction of radio broadcasts, and the resulting mass-production of
domestic radio receivers intended to operate a loudspeaker output. However, the final
result, in the ear of the listener, though continually improved over the passage of the
years, is still a relatively imperfect imitation of the real-life sounds which the engineer
has attempted to copy. Although most of the shortcomings in this attempt at sonic
imitation are not due to the electronic circuitry and the amplifiers which have been
used, there are still some differences between them, and there is still some room for
improvement.
In this book I have looked at the audio amplifier designs which have been developed
over the past fifty years, in the hope that the information may be of interest to the user
or would-be designer, and I have tried to explore both the residual problems in this
field, and the ways by which these may be lessened.
I believe, very strongly, that the only way by which improvements in these things can
be obtained is by making, and analysing, and recording for future use, the results of
instrumental tests of as many relevant aspects of the amplifier electrical performance
as can be devised. Obviously, one must not forget that the final result will be judged
in the ear of the listener, so that, when all the purely instrumental tests have been
completed, and the results judged to be satisfactory, the equipment should also be
assessed for sound quality, and the opinions in this context of as many interested
parties as possible should be canvassed.
Listening trials are difficult to set up, and hard to purge of any inadvertent bias in the
way equipment is chosen or the tests are carried out. Human beings are also
notoriously prone to believe that their preconceived views will prove to be correct.
The tests must therefore be carried out on a double blind basis, when neither the
listening panel, nor the persons selecting one or other of the items under test, know
what piece of hardware is being tested.
Valve and Transistor Audio Amplifiers
X
If there is judged to be any significant difference in the perceived sound quality, as
between different pieces of hardware which are apparently identical in their measured
performance, the type and scope of the electrical tests which have been made must be
considered carefully to see if any likely performance factor has been left unmeasured,
or not given adequate weight in the balance of residual imperfections which exist in
all real-life designs.
A further complicating factor arises because some people have been shown to be
surprisingly sensitive to apparently insignificant differences in performance, or to the
presence of apparently trifling electrical defects- not always the same ones - so, since
there are bound to be some residual defects in the performance of any piece of
hardware, each listener is likely to have his or her own opinion of which of these
sounds best, or which gives the most accurate reproduction of the original sound - if
this comparison is possible.
The most that the engineer can do, in this respect, is to try to discover where these
performance differences arise, or to help decide the best ways of getting the most
generally acceptable performance.
It is simple to specify the electrical performance which should be sought. This is that,
for a signal waveform which does not contain any frequency components which fall
outside the audio frequency spectrum - which may be defined as 10Hz-20kHz - there
should be no measurable differences, except in amplitude, between the waveform
present at the input to the amplifier or other circuit layout (which must be identical to
the waveform from the signal source before the amplifier or other circuit is connected
to it) and that present across the circuit output to the load when the load is connected
to it.
In order to achieve this objective, the following requirements must be met.
(cid:12)9 The constant amplitude (+0.5dB) bandwidth of the circuit, under load, and at all
required gain and output amplitude levels, should be at least 20Hz-20kHz.
(cid:12)9 The gain and signal to noise ratio of the circuit must be adequate to provide an
output signal of adequate amplitude, and that the noise or other non-signal related
components must be inaudible under all conditions of use.
(cid:12)9 Both the harmonic and intermodulation distortion components present in the
output waveform, when the input signal consists of one or more pure sinusoidal
waveforms within the audio frequency spectrum, should not exceed some agreed.
level. (In practice, this is very difficult to define because the tolerable magnitudes
of such waveform distortion components depend on their frequency, and also, in
the case of harmonic distortion, on the order (i.e. whether they are 2nd, 3rd, 4th or
5th as the case may be). Contemporary thinking is that all such distortion
components should not exceed 0.02%, though, in the particular case of the 2nd
harmonic, it is probably undetectable below 0.05%.)