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

Richie Burnett rburnett at richieburnett.co.uk
Wed May 4 11:37:06 CEST 2016

Yes, you can make some filters self-oscillate by feeding one of their 
outputs back to the input with phase-inversion if necessary.  But it is not 
a sure fire method that works with all filters.  It depends on the maths of 
the filter.  It works for a moog 4 pole cascade because there is 180 degrees 
of phase shift at the cutoff frequency, and that's how the resonance is 
implemented inside the filter normally anyway.  Conversely if you feed back 
the BP output of a 2-pole state-variable filter it decreases the resonance 
and acts like a damping signal, and if you feed back some of the LP output 
it alters the cutoff frequency!  So the effect of feeding filter output back 
to it's input through a VCA can be a bit un-predictable.


-----Original Message----- 
From: Lanterman, Aaron
Sent: Wednesday, May 04, 2016 10:08 AM
To: synth-diy List
Subject: [sdiy] Feedback in filters [was: Re: From a 

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

Den 3 maj 2016 6:46 em skrev "Quincas Moreira" <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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