On Oct 24, 6:57 am, flipper wrote:
On Wed, 24 Oct 2007 13:44:13 +0100, Eeyore



wrote:

Patrick Turner wrote:

Eeyore wrote:
Multi-grid wrote:
maxhifi wrote:

Stating that "the amplifier is Class A until XXX watts", really, is
telling you how hot the tubes are biased relative to the two extremes of
pure class A (full dissapation), and pure class (cut off).- Hide quoted text -

Class A has nothing to do with dissipation either. Just because some
marketing group noticed that class A means something good, does not
make it right either. Just because it seems to make sense is no reason
to *******ize the definition. Find some other way to describe it.

The definition of Class A is very simple. It requires that the output device(s)
never cease conducting under any signal condition.

It means slightly more than this because tubes don't cut off as sharply
as other devices.

I see what you're saying but I do believe that the definition is unchanged. Obviously
avoiding any region of significant non-linearity is preferable but that in its own right
doesn't change the definition.

People are talking semantically past each other with some speaking of
the definition of the 'amplifier' class while others are speaking of
it's behavior under a restricted set of operating conditions; and it
is useful to observe that under an appropriately restricted set of
conditions the output tubes conduct 360 degrees as in 'Class A'
operation.

"Useful", indeed, Flipper. Thanks. But I would go further and say that
the signal and dissipation restriction is part of the definition, as
you go on to imply:

For example, if maximum power, or efficiency, were the primary concern
then one might bias more to the 'B' side of the equation while if
fidelity were the primary concern one might bias more to the 'A'.

Precisely. Both Class A operation and Class B operation are inherent
in the nomenclature and definition and their relative importance is
clearly intended to be in the designer's discretion.

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. 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.

One might also observe that's likely why it's called Class AB and not
Class insert unique letter.

There is in fact a class between Class A and Class AB with a unique
description: "Limiting Class A1", which is set up so that the
crossover happens when one valve just reaches current cutoff and the
other simultaneously reaches zero bias. It makes for an amazingly
smooth sound but it is a bitch to set up and keep tuned if you want to
keep your circuits simple. I was therefore rather interested in what
Patrick said elsewhere in this thread (in more general sense rather
than specifically about LImitiing Class A1) about within 10 per cent
of conduction angle being imperceptible...

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