Self on Audio, Second Edition

483 Pages·2006·10.95 MB·English

by  Douglas Self

Self on Audio
This Page is Intentionally Left Blank
Self on Audio
Second Edition
Douglas Self
AMSTERDAM•BOSTON•HEIDELBERG•LONDON
NEWYORK•OXFORD•PARIS•SANDIEGO
SANFRANCISCO•SINGAPORE•SYDNEY•TOKYO
NewnesisanimprintofElsevier
NewnesisanimprintofElsevier
LinacreHouse,JordanHill,OxfordOX28DP
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Firstpublished2006
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Contents
Introduction viii
Preamplifiers and related matters
1 Advanced preamplifier design
(November 1976) 1
2 High-performance preamplifier
(February 1979) 13
3 Precision preamplifier
(October 1983) 24
4 Design of moving-coil head amplifiers
(December 1987) 37
5 Precision preamplifier ’96, Part I
(July/August 1996) 49
6 Precision preamplifier ’96, Part II
(September 1996) 67
7 Overload matters
(February 1997) 86
8 A balanced view, Part I
(April 1997) 90
9 A balanced view, Part II
(May 1997) 103
10 High-quality compressor/limiter
(December 1975) 117
11 Inside mixers
(April 1991) 127
12 Electronic analogue switching, Part I: CMOS gates
(January 2004) 142
13 Electronic analogue switching, Part II: discrete FETs
(February 2002) 155
vi Contents
Power Amplifiers
14 Sound mosfet design
(September 1990) 170
15 FETs versus BJTs: the linearity competition
(May 1995) 181
16 Distortioninpoweramplifiers,PartI:thesourcesofdistortion
(August 1993) 187
17 Distortion in power amplifiers, Part II: the input stage
(September 1993) 201
18 Distortion in power amplifiers, Part III: the voltage-amplifier
stage
(October 1993) 217
19 Distortion in power amplifiers, Part IV: the power amplifier
stages
(November 1993) 232
20 Distortion in power amplifiers, Part V: output stages
(December 1993) 251
21 Distortion in power amplifiers, Part VI: the remaining
distortions
(January 1994) 265
22 Distortion in power amplifiers, Part VII: frequency
compensation and real designs
(February 1994) 279
23 Distortion in power amplifiers, Part VIII: Class A amplifiers
(March 1994) 294
24 Power amplifier input currents and their troubles
(May 2003) 310
25 Diagnosing distortions
(January 1998) 318
26 Trimodal audio power, Part I
(June 1995) 333
27 Trimodal audio power, Part II
(July 1995) 347
28 Load-invariant audio power
(January 1997) 363
Contents vii
29 Common-emitter power amplifiers: a different perception?
(July 1994) 380
30 Few compliments for non-complements
(September 1995) 391
31 Loudspeaker undercurrents
(February 1998) 400
32 Class distinction
(March 1999) 404
33 Muting relays
(July 1999) 421
34 Cool audio power
(August 1999) 434
35 Audio power analysis
(December 1999) 448
Index 463
Introduction
ThisbookisacollectionofthearticlesIwroteforthejournalWirelessWorld
(nowElectronicsWorld)betweentheyears1979and1999.Thevastmajority
of these deal with hi-fi preamplifiers and power amplifiers, and this book
concentrates on this field.
Inthelasttwenty-fiveyearsthescopeoftechnologyavailabletotheaudio
designer has greatly widened. At the beginning of this period the only
choice in preamp design was between discrete transistor stages and the
relatively new op-amps. The latter had dubious characteristics as regards
noise and distortion—particularly crossover distortion. This most unloved
of audio defects was tolerated in power amplifiers because it had to be,
but there was considerable resistance to incorporating it in preamplifiers.
At this time no-one would have considered using valve circuitry in a new
design.
My association with this influential and much-loved magazine actually
beganwithadesignforacompressor/limiterin1975,whichstartedlifeas
my third-year project at university. This design is not reproduced here as
it is unarguable that it has been overtaken by advances in technology.
This debut was followed by what I called ‘An Advanced Preamplifier’
in1976.TheAdvancedPreamplifiercertainlygave(andgives—Istillhave
the prototype) exemplary performance, obtained by making each stage a
discrete-componentoperationalamplifier.Thismadenecessarytheuseof
dual IC regulators to produce +/−15V, and at the time the cost of this
power-supply scheme was significant.
Asareactiontothiscomplexity,Idecidedtotrymyhandatwhatmight
becalled‘traditional’discretecircuitryinapreamplifier,andthisbecame
the ‘High Performance Preamplifier’ published in 1979, though actually
designed nearly two years earlier. It was conceived in an era when op-
amps were still regarded with considerable suspicion by designers seeking
the best possible audio performance. In the search for simplicity a single
supply rail was used, without regulation, but with a simple RC filter after
the reservoir capacitor to reduce ripple to a manageable 50mV or so.
Experiment had proved that this minimal-cost arrangement could give
hum and noise results that were as good as those yielded by the dual-IC-
regulator approach.
In contrast, the Precision Preamplifier of 1983 was designed at a time
whentheremarkable5534/5532op-ampshadbecomeavailableatreason-
able prices. Since they delivered very low noise with almost unmeasurable
Introduction ix
distortion,itwasclearlytimetotrya‘thirdway’asregardspreampdesign.
Havingexploreddiscreteop-amps,andconventionaldiscretecircuitry,an
IC solution was an obvious next step. The return to op-amps meant a
return to dual power supplies, but this was a small price to pay for the
convenience of dual rails. This design later gained a moving-coil head
amplifier. I had designed several of these stages before, at least two of
which made their way into commercial production, but this was the first
version that got both noise and distortion down to what I considered to
be acceptably low levels. The salient features are the discrete transistor
inputdeviceswhichthen,andindeednow,providethebestpossiblenoise
figure.Atthetimemanyheadampswereoutboardunits,oftenrelyingon
battery power, presumably to sidestep intractable ground-loop problems.
However, no difficulties were found in grafting this design onto existing
preamplifiers.
Someyearslater,havingdevotedmuchtimeinbetweentopowerampli-
fier design, I felt the call to take another look at preamplifiers. My last
design was twelve years old, and it seemed likely that some significant
improvements could be made. Much thought and a lot of calculation and
simulation led to the ‘Precision Preamp 96’ articles, including in-depth
mathematical modelling of the noise generated by the RIAA stage. This
allowedeachnoisecontributiontobestudiedindependently,andpermit-
ted comparison between the actual noise and the theoretical minimum.
The latter is rather sensitive to the exact assumptions made. It was also
possible to discover why op-amps that appeared to be quieter than the
5534 in theory, were actually slightly noisier in practice. The answer was
that op-amp bias-cancellation networks maybe great for DC precision, but
the extra common-mode noise they generate in audio circuitry is just an
embarrassment.The96preampisinfactnotsomuchanupdatedversion
as a thorough re-design, with only the moving-coil input amp remaining
essentially unaltered. It demonstrated, amongst other things, that obtain-
ing an interchannel separation of 100dB on a stereo PCB is perfectly
possible with careful component and track layout.
In 1990 I had again turned my attention to power amplifiers. For many
years I had felt that the output stages of power amplifiers presented very
great possibilities for creative design, and so I explored some of them.
OneofthefirstdifficultiesImetwithwastheproblemofdetermininghow
much of the overall distortion was produced in the small-signal sections,
and how much was generated by the output stage. Traditionally the latter
was regarded as the major source of distortion, but there was very little
published research to back this up, and so I attacked the problem myself.
When I began it was not clear if there were two, twenty, or two hundred
significant distortion mechanisms, but after a good deal of study it sud-
denly became clear that seven or eight were sufficient to explain all the
observabledistortion.Thisisnottosaythattherearenototherdistortion