In article , Don Pearce
wrote:
On Sat, 21 Nov 2009 13:54:53 +0000 (GMT), Jim Lesurf
wrote:
There may be swings and roundabouts. Many years ago I decided to avoid
using rail-ground-rail caps on the amp board as they tend to inject
distortion from half wave ripple into the amp via any ground
imperfections. But you then need to ensure the amplifier has an
inherently high ability to ignore rail variations.
That is where really well designed grounding is vital. Stars rule!
Ideally. The snag is ensuring that the stages well clear of the chosen
point are still happy with ground or rail impedances at ultrasonic/RF that
aren't zero. I used to deal with that by making as many stages as I could
either common mode differential, or hang via constant current or current
mirrors. Then the rails can flap about all over the show, but provided they
have voltages that are 'enough' the amp can ignore all the flapping. :-)
And I wonder how many died through oscillation. Warrantee repairs are
horribly expensive things to deal with.
Given the above was the cause of the problem I'm curious as to why the
oscillations waited until after warranty. :-) Gives me the feeling
that something else has degraded.
If oscillation started because of the inevitable degradation of a
component (a cap losing value somewhere, maybe?) then they didn't go
through the design centering stage of development. A "what happens
if..." scenario chase is really quite important.
Well, to be fair, you can always find failure modes once you accept that
eventually a component will fail or change.
Here's a little stability thought I've been pondering - and modelling in
Spice. Cdom goes around the voltage amplifier, then the output stage
follows. Now, if the output stage is simply a voltage follower with no
gain or inversion, why would I not connect Cdom to the output of that
stage, rather than just the one transistor's collector? Would that not
give an advantage for HF distortion suppression, since it would not
matter that the overall feedback was degraded by lost HF gain in the V
amp...
Not sure ATM. TBH I never used to approach design of power amps that way.
:-)
If you look back at the 700 amp design there isn't an explicit Cdom cap
shoved in at the end of the voltage gain sections. Instead I put a snubber
on the front long-tailed pair and got it to be stable with that.
That said, I also linked the drivers to the output so they could shunt past
the output devices if they weren't able to keep up with any HF. I also
though this would help to 'fill in' as a quasi-class-A stage driving the
output. Bit like current dumping.
I know Doug Self did a lot of models, etc, of various o/p arrangements. But
I found the above simply worked better even if his results indicated
otherwise. Presumably because this all depends on the specific details of
the devices, etc, etc. I also found that tiny movements of the wiring loom
altered the performance. Can't recall anyone in a book dealing with that.
IIRC I tended to dislike having large caps in the output area as it seemed
to just give problems with slew limiting. But I am trying to recall decades
ago, so I'm sure I've forgotten most of this! :-)
Slainte,
Jim
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