[sdiy] SVF question

[Magnus Danielson]

Hi Tim,

On 04/10/11 00:13, Tim Ressel wrote:
> Hi,
>
> 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.

Cheers,
Magnus