"N_Cook" wrote in message
...
As I cannot find anything wrong as far as broken wrong, I will assume this
is as designed. I cannot see why there is a low 4K7 dropper from 27V to
the
4.7V zener that basically causes this 5V DC pulse, when only supplying
80uA
at most into the base of a TO92 transistor, Q13.


That equates to a zener current of around 5mA, which seems an altogether
reasonable amount ?

Unless some defined minimum current flows through a zener, then it will not
produce a stable and accurate voltage across itself. The current into the
base of the transistor is supplementary to the current through the zener,
and of itself is generally an insignificant amount, depending on the current
demand from the transistor, and the DC gain of that transistor. If you
increase the value of the R, then the standing current through the zener
will drop, and it will likely not produce the correct voltage across itself.
Whilst 5mA through the zener is a 'reasonable' value (assuming it's a little
400mW job), it is never-the-less on the 'low' side of reasonable, and if I
was incorporating such a zener into any design of mine, I wouldn't want it
to be any lower than that to guarantee correct zener action.

I will try upping the
dropper and adding a C to give a time constant of 2 seconds and change the
main hold off time constant from 4 sec to 2 sec also but keeping 13V bias
point, both channels. A few strange noises in the first couple of seconds
,
if that and only if the phones are connected, is surely preferable to 5V
DC
pulses fed , delayed, into the PA.
So both will rise approximately together and so no 5V surge is possible.



I hear what you're trying to do, but for myself, I'm reluctant to start
modifying circuits that I don't fully understand the purpose or functioning
of, just because I feel that they may not be working as designed, or even as
desired. Sometimes I find that elements of a design turn out to be a 'best
effort' compromise by the designer and, even if that circuit element doesn't
work quite as he had hoped, it's as good as it can be. Be very careful that
any mods you dream up don't make the situation worse. There's no second
chances if the wrong amount of DC arrives at the input of a DC coupled amp,
as I'm sure you well know ...

Didn't Graham used to be involved with Audiolab gear at some time in the
past ? Could be wrong, but it seemed to ring some distant bells in the back
of my mind. Where is Graham these days, anyway ? Don't seem to see him on
here like we used ...

Arfa