On Tue, 22 Dec 2015 10:34:32 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
It does have what I used - a snubber across the first long-tail
pair(s). Also a cap across them *and* some later caps. If anything it
seems to have roll-off applied in multiple places. Which one may
dominate I haven't checked. But it doesn't seem to lack HF roll-off.


This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling.

Yes, it does seem the 'automatic' choice of many designers. To the extent
that I've seen articles on amp design that take it for granted. However
when I experimented I found a suitable snubber across the long-tail pair at
the input of the designs I worked on worked much better. I can't say if
that was peculiar to the designs I worked on, or if I simply didn't pick up
the usual hammer other chose.


Now I know this is easiest when you have open loop gain by the
bucketload, as you have in a modern integrated op amp. But this is still
an op amp and it seems strange to dot bits of this and bits of that
around it.


Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.


Yes. The Rotel does seem rather complicated to me. Maybe the designer got
desperate and fitted caps everywhere and played "hunt the battleship" until
it worked. ...but then forgot to add an output inductor. An omission that
might have ruined a few proud buyer's day. 8-]

Jim

Adding the snubber across the input long tail pair will allow you to
lighten up a bit on the dominant pole on the voltage amp and give you
a bit more open loop gain, hence less distortion. But it is a bit of a
balancing act, and you have to do a pretty careful tolerance analysis
to make sure you've got away with it in all circumstances. A normal
dominant pole is a bit more of a blunt weapon.

d

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