[sdiy] SVF question

[Tim Ressel]

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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