AW: AW: J.H.'s WASP-filter-clone

[Haible Juergen]

	>My version doesn't oscillate at all ! Well, the caps have 10%
tolerance,
	>and I took the original feedback network from your drawing.  Maybe
I
	>have to change the network, you have two diodes in there, I guess
for
	>amplitude limiting, so by that I could have seen, that this filter
was
	>made for oscillation.
	>
	>Very likely: "amplifier" gain insn't enough, these are just
inverters.

Well, I have found this oscillation by chance, and I haven't investigated
the exact reason. It's just my guess that the high output resistance
of the CMOS inverters and the capacitance of the cable form a 3rd pole
which enables oscillation.
The diodes are taken from the original Wasp filter (which doesn't 
self oscillate, as far as I know), which in turn is most probably taken
from the Oberheim SEM, which doesn't self oscillate either. They
are just in to get smooth clipping when you hit a strong harmonic of
the input signal with the resonant frequency of the filter at high Q.
(Maybe they aren't even necessary in the Wasp, because the
CMOS inverters don't get stuck to the rails at overload like
opamps. But this is just another guess, not for sure.)
But they do limit oscillation amplitude, too, when self oscillation
happens.
BTW, for a nice SVF that was *designed* to do self oscillation,
you might look at Joachim's ARP page. Very nice filter, with
the damping loop (and an extra undamping path) around the
second, not the first, integrator stage.

Amplifier gain: CMOS inverters have their largest gain at low
supply voltage. Thus the 5V supply. Self heating shouldn't
be a problem here, either. But I've heard good reports about
a 15V version of the filter, too, from several persons.

JH.