[sdiy] SVF question

[Andrew Simper]

Yes, the diodes prevent self oscillation, and for low resonance (high
damping) they are bypassed almost completely. Note that if you allow
signal to flow through the feedback route from the bandpass output to
the input of an SVF you lower the resonance, so the diodes are placed
so that they limit resonance for high signal values.

Andy
--
cytomic - sound music software



On 4 October 2011 08:28, Tim Ressel <madhun2001 at yahoo.com> wrote:
>
> Ah, I looked at that one a while ago. Will the diodes prevent self-oscillation? The filter I'm doing needs to.
>
>
>
> ----- Original Message -----
> From: KA4HJH <ka4hjh at gmail.com>
> To: synth-diy at dropmix.xs4all.nl
> Cc:
> Sent: Monday, October 3, 2011 4:14 PM
> Subject: Re: [sdiy] SVF question
>
> >> I need to make a bunch of bandpass filters. I will be using the standard
> >>State Variable Filter topology. Many moons ago Paia produced an SVF
> >>design that used a dual pot for resonance. As the resonance went up the
> >>input level got turned down. This kept the resonance peak from clipping.
> >>Is this necessary? Do modern designs do it? I'd be using a VCA to control
> >>the input level if it is needed.
> >>
> >> Thanks!
> >
> >You need to do this to avoid clipping. Recall that the Moog VCF for
> >instance achieve the same thing just by it's feedback-path as it reduces
> >pass-gain automaticly.
> >
> >It's trivial to show and the double pot is an easy way to compensate it.
> >You should be able to mix the signals together before you damp them in
> >the common VCA since they go to the same mix-opamp. If you do OTA it can
> >look straight into the mix-opamp virtual ground.
>
> Also of interest is the Oberheim SVF--It uses a diode clipper, IIRC:
>
> http://www.crowncity.net/ratcave/Electro/Oberheim/SEM-1A.pdf
>
> --
>
> Terry Bowman, KA4HJH
> "The Mac Doctor"
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