John Byrns wrote:
In article .com,
Andre Jute wrote:
Historically, the original purpose of Class AB was to annihilate the
second harmonic which before made up such a very large part of the
THD, while still allowing beam tubes and pentodes to give much larger
power than available before.
Andre, why was Class AB necessary to annihilate the second harmonic,
didn't Push Pull operation already annihilate the second harmonic
irrespective of the class of operation?
Class AB was found to be more efficient and cheaper per watt to produce,
and give better sonic performance than the SE amps of 1935.
Once anyone needed more than 3 watts, PP was considered,
and the bonus was to banish high THD/IMD of the SE amps of the day.
The SE radio amps using a lone 6V6 in beam tetrode mode with no NFB were
often used,
and had mainly 2H, but many OTHER H, and sounded lousy over a whisperish
level.
HOWEVER, in the misplaced zeal to banish 2H and settle for the clean PP
sound, especially
when you had a pair of output 2A3, designers would labour away to force
an input triode to feed and IST to
drive the PP outputs with a two phase secondary.
SUN amps are a classic example.
Typical primary voltage needed at the 1:2 IST = 50Vrms.
So the bloomin input driver tube was making lots of 2H and there was NO
net betterment in the sonics except
that because the power ceiling was slightly higher.
More thoughtful PP amps were designed in the late 40s by leak, Quad,
Radford etc, where
ALL distortion was considered bad, and where the driver stages were
designed to produce far less THD/IMD
than the output stage.
The Williamson is a classic example.
It can be used with 300B in the output, and NFB needn't be used, and
THD/IMD will remain low enough,
and the 28 watts AB1 will be enough for most folks even now with
insensitive speakers.
With sensitive speakers of the 50s, the 28 watts of AB triode power
was the equivalent of having 112 watts today on average.
But very very fine SE amps can be built, and the 2H is low,
along with other H, and not much NFB need be used.
Patrick Turner.
The "invention" of Class AB as a hi-
fidelity amp is what spurred part of a Olsen's work on perception;
before it wasn't known that odd harmonics are proportionately much
more disturbing than even harmonics. It seems to me that AB amps with
largish parts of their output in Class A is a relatively modern trend,
possibly related to ever less-sensitive speakers.
I'm not sure I would agree with that, class AB amps were common even in
the days of efficient speakers, I don't see it as "a relatively modern
trend", if anything is a modern trend, I would think it is the return to
"pure" class A amplifiers on the part of many audiophiles.
Regards,
John Byrns
--
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