ВУЗ: Казахская Национальная Академия Искусств им. Т. Жургенова
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Audio Power Amplifier Design Handbook
deviations are not likely to be very significant. This implies a quiescent
current of approx. 50 mA.
It may simplify faultfinding if D7, D8 are not installed until the basic
amplifier is working correctly, as errors in the SOAR protection cannot then
confuse the issue. This demands some care in testing, as there is then no
short-circuit protection.
Safety
The overall safety record of audio equipment is very good, but no cause for
complacency. The price of safety, like that of liberty, is eternal vigilance.
Safety regulations are not in general hard to meet so long as they are taken
into account at the start of the mechanical design phase. This section
considers not only the safety of the user, but also of the service
technician.
Many low-powered amplifier designs are inherently safe because all the
DC voltages are too low to present any kind of electric-shock hazard.
However, high-powered models will have correspondingly high supply-
rails which are a hazard in themselves, as a DC shock is normally
considered more dangerous than the equivalent AC voltage.
Unless the equipment is double-insulated, an essential safety requirement
is a solid connection between mains ground and chassis, to ensure that the
mains fuse blows if Live contacts the metalwork. British Standards on safety
require the mains earth to chassis connection to be a Protected Earth,
clearly labelled and with its own separate fixing. A typical implementation
has a welded ground stud onto which the mains-earth ring-terminal is held
by a nut and locking washer; all other internal grounds are installed on top
of this and secured with a second nut/washer combination. This dis-
courages service personnel from removing the chassis ground in the
unlikely event of other grounds requiring disconnection for servicing. A
label warning against lifting the ground should be clearly displayed.
There are some specific points that should be considered:
1 An amplifier may have supply-rails of relatively low voltage, but the
reservoir capacitors will still store a significant amount of energy. If they
are shorted out by a metal finger-ring then a nasty burn is likely. If your
bodily adornment is metallic then it should be removed before diving
into an amplifier.
2 Any amplifier containing a mains power supply is potentially lethal. The
risks involved in working for some time on the powered-up chassis must
be considered. The metal chassis must be securely earthed to prevent it
becoming live if a mains connection falls off, but this presents the snag
that if one of your hands touches live, there is a good chance that the
other is leaning on chassis ground, so your well-insulated training shoes
420
Testing and safety
will not save you. All mains connections (neutral as well as live, in case
of mis-wired mains) must therefore be properly insulated so they cannot
be accidentally touched by finger or screwdriver. My own preference is
for double insulation; for example, the mains inlet connector not only
has its terminals sleeved, but there is also an overall plastic boot fitted
over the rear of the connector, and secured with a tie-wrap.
Note that this is a more severe requirement than BS415 which only requires
that mains should be inaccessible until you remove the cover. This assumes
a tool is required to remove the cover, rather than it being instantly
removable. In this context a coin counts as a tool if it is used to undo giant
screwheads.
3 A Class-A amplifier runs hot and the heatsinks may well rise above 70°C.
This is not likely to cause serious burns, but it is painful to touch. You
might consider this point when arranging the mechanical design. Safety
standards on permissible temperature rise of external parts will be the
dominant factor.
4 Note the comments on slots and louvres in the section on Mechanical
Design above.
5 Readers of hi-fi magazines are frequently advised to leave amplifiers
permanently powered for optimal performance. Unless your equipment
is afflicted with truly doubtful control over its own internal workings, this
is quite unnecessary. (And if it is so afflicted, personally I’d turn it off
now.) While there should be no real safety risk in leaving a soundly-
constructed power amplifier powered permanently, I see no point and
some potential risk in leaving unattended equipment powered; in Class-
A mode there may of course be an impact on your electricity bill.
421
Index
Absolute phase, 26–7
AC coupling, 41–2
Acronyms, listing, 27–8
Active load techniques, 95
Adaptive trimodal amplifier, 288
Ambient temperature changes,
accommodating, 360
Architecture:
three-stage, 31–2
two-stage, 32–3
Audio chain, effects of length, 18
Auxiliary circuitry, powering, 394
Baxandall cancellation technique, 17,
18, 111
Belcher intermodulation test, 10–11,
16
Beta-droop, 127
Bias errors, assessing, 332
Bias generator, 177
Bipolar junction transistors (BJTs):
failure modes, 371
in output stages, 123 and following
overheating, 372
Blameless amplifiers, 71
Blomley principle, 39–40
Blondlot, Rene, 8
Bode’s Second Law, 12
Bootstrapping, 96
Boucherot cell see Zobel network
Bridge rectifiers, 240
RF emissions, 241
Cable selection, loudspeaker, 202
Capacitor distortion, 13, 57, 177
Cascode compensation, 248
Catching diodes, for overload
protection, 383
Clamp diodes, see Catching diodes
Class-A amplifiers, 33–4, 107
A/AB mode, 271
Class B mode, 281
configurations, 257
constant-current, 256
design example, 279
disadvantages, 256
efficiency, 256, 272
load impedance, 272
mode-switching system, 281
operating mode, 272
output stages, 257
performance, 286
power supply, 286
quiescent current control, 263, 280
thermal design, 283
trimodal, 267, 283
Class-AB amplifiers, 34–5, 143
geometric mean, 40–41
Class-B amplifiers, 33, 35, 106, 176
50W design example, 176
efficiency, 256
variations, 38–9
Class-C amplifiers, 35, 291
Class-D amplifiers, 35
Class-E amplifiers, 35
Class-G amplifiers, 36–7
shunt, 37–8
Index
Class-H amplifiers, 38
Class-S amplifiers, 38
Collector-load bootstrapping, 96
Common-mode distortion, 57–8
Common-mode rejection ratio, 61
Compensation, 184
dominant pole, 184
lag, 185
two-pole, 188, 312
Complementary feedback pair (CFP)
output, 114
large signal non-linearity, 115
thermal modelling, 342
Complementary output stages, 30–31
Contact degradation, 14
Cross-quad configuration, 76
Crossover distortion, 107
experiment, 145
harmonic generation, 109
Crosstalk, 397
interchannel, 10
Crowbar, protection system, 391
Current compensation, 362
Current limiting, for overload
protection, 374
Current timing factor, 221
Current-driven amplifiers, 39
Current-mirrors, 81
Damping factor, 25–6, 190
Darlington configuration, 104, 291
DC output offset, 89
DC-coupled amplifiers, 41, 42–4
DC-offset protection, 322, 336
by fuses, 385
by output crowbar, 391
relays, 386
Degradation effects, 7
Distortion, 24
capacitor see Capacitor distortion
in complete amplifiers, 158
induction see Induction distortion
mechanism types, 63
NFB takeoff point, 170
output stages, 56–7, 123
rail decoupling, 167
rail induction see Rail induction
distortion, 168
switching, 153
thermal see Thermal distortion
Type, 3a see Large signal non-
linearity, 123
Type, 3b see Crossover distortion
VAS loading, 163
Dominant pole compensation, 184
Dominant pole frequency, 62
Doubled output devices, 128
Dual-slope VI limiting, for overload
protection, 381
Early Effect, 367
Economic importance, 1–5
Emitter resister value, 135–8
Emitter-follower (EF) output, 113
large signal non-linearity, 123
modelling, 327
thermal compensation, 326
Error criterion, 344
Error-correcting amplifiers, 39
Failure modes, semiconductor, 371
Fault-finding, 419
Feedforward diodes, 131
Field effect transistor (FET) output
stages:
advantages, 314
amplifier failure modes, 203
characteristics, 318
in Class-A stages, 321
disadvantages, 316
hybrid, 318
hybrid full-complementary, 319
linearity comparison, 321
simple source-follower configuration,
318
Frequency compensation, 184
Frequency response capability, 23–4
Fuses:
for DC protection, 385
as overload protection, 373
sizing, 240
thermal, 394
Gain margin, 49
Generic principles, 52–4
424