[sdiy] Digisound SVF no resonance

[Andrew Simper]

The standard trick to get more resonance out of an SVF (thanks to
Osamu Hoshuyama for pointing this out clearly in his pdfs describing
the ARP4023 and OSCar filters!) is to have a large resistor in a
positive feedback loop around the bandpass output back to its input
(if the buffers of the 2040 are inverting then you can just add this
resistor from the bandpass buffer output to the negating bandpass
input, start at around 300k and work you way down to taste). You will
also probably need to add the regular parallel but in opposite
direction diodes from the bandpass output to the high pass op-amp
input, which increase damping to maintain resonance stability.

Cheers,

Andy

On 31 August 2016 at 05:45, ian <ioddity2001 at hotmail.com> wrote:
>
> I have 2 Digisound state variable filters (CEM type) and neither has any
> resonance to speak of just a very mild effect. I've read of others having
> the same issue so I am wondering if there is a common fault, a design issue
> or whether this module is supposed to have any resonance at all. Can anyone
> shed any light on this?
>
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