[sdiy] SSM2164 state variable filter

[Harry Bissell]

some opamps would phase reverse when they are driven into clipping,
never a good thing. A diode clipper would be much more polite.

You might want to limit at an amplitude lower than where the opamp
clips, another good reason for the diode solution...

H^) harry


----- Original Message -----
From: Tom Wiltshire <tom at electricdruid.net>
To: sdiy DIY <synth-diy at dropmix.xs4all.nl>
Sent: Wed, 14 Jul 2010 11:34:29 -0400 (EDT)
Subject: [sdiy] SSM2164 state variable filter

Hi All,

I'm playing with a SSM2164 SVF circuit again. I'm still not happy with it, or perhaps still not happy with my understanding of it.

There are several things that I've been told/learned which I have no particular reason to doubt:

1) The circuit is essentially an oscillator, given the feedback from the LP output.
2) The "resonance" path from the BP output actually controls the damping by cancelling the oscillation. This is why the control works "back to front".
3) Some type of limiting or clipping is required for the oscillation to be stable and low distortion.
4) The best place for such a network would be in the resonance/damping BP feedback path.

Now, it seems to me that (4) doesn't fit. If the circuit is oscillating, there's no signal (or very little) going back along this path, so I can't see why a clipping network would do anything at all. On this basis, I'd be better off putting the clipper in the LP feedback path.

I also don't understand why having no clipping network doesn't achieve the same thing but at a higher voltage level. If left without any clipping, the circuit will oscillate and the output starts to crunch into the rails, producing a clipped sinewave at the level set by the limits of the op-amps. Why does having a clipper with a lower threshold make any difference at all? Why aren't the two clippers (overdriven opamp / clipping diodes) equivalent?

Is one of my "facts" wrong? If so, which one? Am I right that putting clipping in the LP feedback path is a better idea?

I've tried many variations on the breadboard, but at the moment I'm not getting reality and theory to match up at all. That and there's too many options.

Thanks,
Tom

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-- 
Harry Bissell & Nora Abdullah 4eva