[sdiy] filter resonance

[Gene Stopp]

Good question - don't know of any reference doc that covers VC resonance for
different filter types. I do know that a cascaded-integrator 4-pole (like
Roland 700 or Electronotes ENS-74 4-pole) has no resonance until you add
some output back to the input. On the other hand, a 2-pole state variable
like the Oberhiem SEM is inherently "unstable" until you "cool it down" with
some feedback, the more feedback the less resonance. Just the other day I
was messing with an ASM-1 and noticed that the VCF was pretty tame when set
to max resonance. When I designed that PCB, I copied the Oberheim Q feedback
circuit, which bypasses the Q pot so that there is always some feedback even
if the Q pot is turned up all the way (which is really turned down all the
way, wired backwards at the panel). On a whim I clipped out that circuit to
disable it, so the filter would squeal like the proverbial pig when set to
max Q. That was much better, more out of control at times, but definitely
more fun.

I also remember that the Moog tranny ladder misbehaved if the Resonance pot
was a voltage divider to ground, but rather needs to be a series rheostat
instead per the orignal schemo. Not sure why, just made it work and moved
on...

- Gene



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of jays at aracnet.com
Sent: Thursday, November 11, 2004 9:00 AM
To: john mahoney; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] filter resonance


John,

Yes and no.

It really depends on the type of filter. Some filters decrease res with more
feedback and some increase the res with more. But in general yes putting a
VCA in the feedback loop can control the res. Another issue is what type of
scale do you want log or linear? Some filters give better control with log
scaling on the res some linear.

Has anyone put together anything saying what type of feedback and scaling
works best with different filters?

Jay S.

John Mahoney <jmahoney at gate.net> wrote :

> Is increasing a VCF's resonance the same as feeding more of its output
> back to its input? If so, then any filter can have voltage controlled
> resonance by adding a feedback loop with a VCA in it.
> 
> Correct?
> --
> john