[sdiy] Feedback in filters [was: Re: From a commercial standpoint...]

Lanterman, Aaron lanterma at ece.gatech.edu
Wed May 4 11:08:28 CEST 2016

On May 4, 2016, at 4:28 AM, Mattias Rickardsson <mr at analogue.org<mailto:mr at analogue.org>> wrote:

Den 3 maj 2016 6:46 em skrev "Quincas Moreira" <quincas at gmail.com<mailto:quincas at gmail.com>>:
> You can always patch VC control of resonance.  Just send the filter
> output to an inverter, then to a VCA, then back into another filter
> input.  Voila, control the VCA = control filter resonance.

Nope, not on all filters. The usual state-variable topology has its Q feedback from the bandpass output and with opposite direction:
Less feedback -> higher resonance
More feedback -> lower resonance

So there you need to do some odd tricks *and* have the bandpass.

There’s also a slight quirk in the way that the negative feedback works depending on the number of poles, in the case of a filter where the core is a series of cascaded 1st-order sections.

If you have four sections in your cascade, like a Moog ladder (or Prophet 5, etc.), then inverting the output and feeding it to the input sharpens the curve makes the usual resonance, the poles expand in an X pattern, and at a feedback multiplier of 4 you get self-resonance.

But if you have just two sections, and you apply negative feedback from the output to the input, the poles split apart and spread, but vertically along the imaginary coordinate but. They never actually hit the imaginary axis itself, so it won’t self-resonate. I seem to recall that the Elka Synthex in 2-pole lowpass mode behaves like that, or something like that.

- Aaron
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