[sdiy] Digisound SVF no resonance

[Nicholas Keller]

Andy, 

  Ian stated in his initial post that he has the CEM version, 80-7A, not the SSM version 80-7.  Documents for both are included on the page I linked to.  The 7A uses a CEM3335 for the resonance circuit, but it wasn't clear to me where the feedback source was coming from.  The 3320 used for the filter poles does not have VCQ on-chip.

Nick 


 

> On Aug 31, 2016, at 8:30 AM, Andrew Simper <andy at cytomic.com> wrote:
> 
> 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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