In article ,
Rich Grise wrote:

Opinions on this, especially the 2.5W amp schem at the bottom.
http://www.intio.or.jp/jf10zl/EF.htm

When the amp is idle, and there's no(?) current flowing in either output
transistor because there's no forward bias, then the V drop 'resistance'
of the E-B junction adds to the 100k feedback resistor, so the loop gain
increases to the open loop gain. So it would seem that the amp would
attempt to hunt in this region, possibly oscillating?

Maybe a 1k resistor E to B on the output transistors would 'bypass'
this. Or should the amp be biased to work class AB.

Hey, Watson. :-)

I'm going to level with you, I'm not an expert, I just play one on the
internet. But it looks to me like the gain of the output stage - you're
talking about the one with an opamp per each output tranny, right? - is
strapped such that the opamp's loop gain predominates, and I would not be
a bit surprised to see the circuit behave just as you describe (emitter
followers are notoriously fast), with two caveats: The slew rate of the
opamps, and something about a pole or a zero in the complex impedance at
that horrendous huge output cap.

I'm not an expert either, but I have a Bad Feeling about this design.
It seems to me that it assumes the existence of theoretically-perfect
components with ideal matching (between IC2 and IC2, and between the
various Tr1 and Tr2 parallel transistors).

I'd be very concerned about the effect of any input offset voltage
difference which might exist between IC2 and IC3. It looks to me as
if the two op amps could end up "fighting" one another pretty badly.
If the input offset voltages are offset from one another in one
direction, the bias in the output transistors would probably tend down
towards zero, and distortion might result. If the offsets are in the
opposite direction, (e.g. if IC2 wanted to see a slightly more
positive voltage on its inverting input than IC1 did, for a given
noninverting input voltage) then the op amp output voltages would
diverge in opposite directions, turning both Tr1 and Tr2 quite hard,
and quite possibly driving them out of their safe operating areas.

Add to this the fact that the design doesn't include base resistors
for the transistors, or emitter ballast resistors for the paralleled
Tr1 and Tr2 transistor clusters, and I think you've got a recipe for
serious instability (oscillatory and thermal) and for the emission of
copious quantities of Magic Blue Smoke.

--
Dave Platt AE6EO
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