[sdiy] State Variable Filter Practical Maximum Q

[Antti Huovilainen]

On Sat, 17 Oct 2009, Scott Gravenhorst wrote:

> Thanks Jim, that's sorta what I figured, noise generated by active 
> components would be something that would supply the energy to kick it 
> into oscillation given enough gain at high Q settings.  I assume this 
> also applies to voltage controlled state variable filters.

The "problem" with SVF is that the resonance works by decreasing damping 
unlike cascaded RC filters (moog & most other 4 poles) or those 
using bandpass coupled positive feedback (MS-20). Decreasing opamp gain 
results in decreased damping and thus higher Q.
You should be able to compensate for this by increasing the damping at 
high frequencies.

Antti

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