From bruce@ashbysolutions.com Sun Nov 11 15:48:59 2001
Subject:Re: Tube amp designs and such

All,

There is a lot of discussion on this list -- and many other places -- about
Class A, Class AB, push-pull, transformer-coupling, solid-state, tube,
etc., etc., etc. The purpose of all amplifiers is to make a sound louder,
period. So, why do they sound so different? Let me add my $0.02. Bear
with me a bit, because it's hard to talk about this stuff without getting
technical:

All amplifiers have two modes of operation: 1) where the amp faithfully
reproduces the input, with a minimum of by-products, and 2) where the amp
alters the sound of the original. Musicians refer to Mode 1 as the "clean"
response of the amp, while tech folks call it the "linear region." I'm
going to go out on a limb here and say that the response of ANY
well-designed amplifier -- Class A, AB, tube, (bipolar) solid-state, MOSFET
-- when running in Mode 1 is pretty much indistinguishable from the output
of any other amplifier of the same size, when also running in Mode 1,
neglecting any difference in the EQ of the two amps.

Mode 2, however, is a completely different story. Electronic components
can't continue in the linear region indefinitely as the input is
increased. When an amp is asked to provide more than it's capabilities, it
can do one of two things. Solid-state (bipolar) amps keep trying until
they sort of hit their head against a wall -- the maximum output of the amp
-- at which point they just stay at that level until the input drops back
to where they can work properly again. This is often referred to as
"clipping," and it's an unpleasant sound. The more thoroughly the amp
clips, the harsher the sound, and as Steve mentioned, solid-state amps are
almost too accurate in this respect.

Tube amps, OTOH, begin to give out when the get NEAR the "wall," and
continue to sag more as the demand increases, resulting in a more rounded
waveform during overload. This effect is considered a deviation from
"perfect" tube design, but it sounds more pleasing to the human ear. "Why"
is a great answer to the question, "What should I base my Ph.D. in
acoustics on?" Take the statement at face value for now. An amp that has
a bit of "give" to its response sounds better than one that doesn't. Well,
guess what? Transformers have a bit of give to their operation, too, so
transformer-coupled tube amps have a second area where they can even out
the response when pushed too hard. Other components used in earlier amp
design like optocouplers and tube rectifiers can also add to this effect.

So, what about Class A vs. Class AB/push-pull? That's a little tougher to
quantify. Class A sounds great because it's the closest to the initial
design goal: make the signal bigger, and don't do anything else. Class A
is popular for high-quality preamps, but unpopular in power amp designs,
because it's fairly inefficient. Class AB was considered to be a
reasonable compromise between sound quality and efficiency. While Class A
should always sound better, especially at low output levels, many Class AB
amps sound great in practice. Both the Fender Twin and the Leslie 122 amp
are clearly a Class AB designs, yet both are often described as "warm" and
"sweet." So, I guess the answer is that amplifier class alone does not
make or break the sound quality of the amp.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ www.ashbysolutions.com
CloneWheel Support Group and HiNote moderator
978.386.7389 voice 978.964.0547 fax
bruce@ashbysolutions.com