On Oct 24, 10:48 am, 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?

Well, yes, but who would want a strictly Class B tube amp? It'll be
harsh and nasty and the THD will be grim. On the other hand, there was
an urgent demand (possibly only from the marketing department,
possibly from speaker manufacturers -- these things are very rarely
consumer-led) for more output than available from SE or even Class A
triodes. Class AB, a natural for the new multi-grid tubes, was for the
time a perfect compromise between the "waste" of Class A and the (at
the time) incredible power available from push-pull operation of beam
tubes and pentodes, *and* Class AB had a lower NFB requirement than
Class B, all others things being equal of course, and thus better
stability margins. All of this happened at the same time ever-lower
THD numbers became the chief marketing tool; it followed in turn that
the THD should be attacked where it was most vulnerable and where it
would give the biggest fix in the least time for the lowest cost, and
that was at the second harmonic. So, you don't want Class B because it
is crude, you can't have Class A because it is too expensive for the
power you want, you must have a lot of stable power with low THD,
bingo, Class AB saves your butt. You have to look at the entire
package of elements that drove the general swing towards Class AB.

Having looked, from a closer vantage point than ours, at the package
of elements, Langford-Smith himself tells us in the RDH4 (Newnes
1997) on p 545 that:
"Class AB operation indicates overbiased conditions, and is used only
in push-pull to balance out the even harmonics."

In Langford-Smith's eyes, therefore, it seems that what drove the
choice of Class AB was the ability to retain most of the power
available in Class B while reducing THD a very big chunk, without the
instability that would follow on the heels of the amount of NFB to
achieve the same task in Class B.

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.

Sure, Class A1 amplifiers, as in SE 300B amps, are big since say the
mid-80s. But I think if you go into the history of how much of the
total power of typical Class AB amps at every period was available in
Class A, I think you will find that in the days of sensitive speakers,
when the first watt truly was everything that mattered, the Class A
benefice was quite low, a handful of watts perhaps. It is only in fact
since the 1950s that it was known to specialists that third and higher
odd harmonics are disproportionately more disturbing than the even
harmonics; you can still see the willful resistance, arising from
ignorance, to my practice of designing amps to shape the residual
harmonic artifacts so that the odd residuals are miles below the
fractional remaining second harmonic. Again, those tubes like 807s
when operated in triode were naturals for Class AB, with a naturally
beneficial harmonic spectrum; these things fell out naturally without
the obsessive thought we put into the tiniest detail these days,
bedevilling retrospective analysis.

Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

Of course I could be wrong. I wasn't there, I don't have twenty-twenty
hindsight, and the few amps whose histories I know about are not
exactly in the mainstream. One has to read very carefully between the
lines to understand what someone like Langford-Smith tell you when he
speaks of motives driving commercial choices rather mere engineering
facts: his milieu and assumptions were very far from ours.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
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