| How long have you got??
This is from memory dredged up after about twenty years. Perhaps
someone would like to correct me if I go wrong.
There are two basic modes of operation for audio power amplifiers,
known as Class A and Class B. In Class B, one device conducts
during the positive-going half of the signal waveform while
another does so during the negative-going half. Since there is a
need to switch from one to the other when passing through zero,
and since all amplifying devices are non-linear around zero, this
leads to a kink in the output waveform which is audible as
"crossover distortion". In Class A, a bias current is used to
ensure that the output devices never switch off, but conduct in
the most linear part of their characteristic curves throughout
all excursions of the input signal. Distortion is thereby
reduced to an absolute minimum.
Why bother with Class B, you may ask? The big problem with
semiconductor amplifiers is getting rid of enough heat to stop
the devices going into meltdown. With Class A, the necessarily
high level of quiescent current means lots of heat to dissipate
even when it's doing nothing. With Class B, you only draw
current, and therefore generate heat, when you drive it - the
louder, the hotter.
Most modern hifi amplifiers compromise by allowing a small bias
current to eliminate almost all of the crossover distortion.
This is sometimes referred to as Class AB operation and gives
(almost) the best of both worlds - reasonable linearity with
minimal quiescent current and heat buildup.
The upshot of all this is that Class A amplifiers:
- are generally recognised as sounding "better" than the
rest;
- run very, very hot;
- tend to be very large, very heavy and very expensive;
- tend to have (relatively) very low rated power outputs.
So, if you are a *real* purist, or you need your amp to double as
a room heater/griddle, get yourself a Class A amp. Otherwise,
don't worry about it and save on the electricity bills.
BTW, Technics have a habit of inscribing "Class AA" (along with
about another 500 words of advertising copy) on their amplifiers
- ignore it. I don't know what it means but it sure ain't *true*
Class A!
Dave M.
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