[sdiy] Filter problems

David G Dixon dixon at mail.ubc.ca
Sun Jan 28 22:22:52 CET 2018

We put zeners across the first integrator cap in our SVF design, and it
works great.  Gives very stable oscillation at the critical damping point,
and doesn't color the filter's sound very much in filtering mode.  The only
issue is that the self-oscillatory sine waves do not have a constant
amplitude with changing frequency, but this is a pretty minor problem.
Cascaded-stage filters oscillate in a much simpler fashion in my experience.

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Neil Johnson
> Sent: Sunday, January 28, 2018 12:00 PM
> To: Tom Wiltshire
> Subject: Re: [sdiy] Filter problems
> Hi Tom,
> > You'd recommend putting them in the feedback path instead, Neil?
> Yes.
> > It does make more sense in many ways (since what you're 
> trying to do is limit the resonance). But the problem is that 
> VC-resonance control is really a "damping" control, so it's 
> not as simple as just moving them to the Res VCA's op-amp. 
> Would be nice if it was that simple.
> Umm, yes it is:
> http://www.milton.arachsys.com/nj71/pdf/cesyg/vcf-sch.pdf
> You certainly don't want them anywhere near the integrator 
> capacitors otherwise you'll end up with the poles not aligned 
> due to the diode capacitance slugging one of the integrators.
> Neil
> --
> http://www.njohnson.co.uk
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