[sdiy] Good 24db/oct HP-LP-BP-Notch filter Schematics ??

Tom Wiltshire tom at electricdruid.net
Wed Mar 5 20:13:39 CET 2014


On 5 Mar 2014, at 17:13, Neil Johnson <neil.johnson71 at gmail.com> wrote:

> Jean-Pierre Desrochers wrote:
>> I'm looking for schematics of a good 24db/oct HP-LP-BP-Notch VCF.
>> All the filter modes outputs should be available simultaneously..
> 
> The Oberheim matrix 12/Expander multimode VCF might give you some clues.

I've changed my mind. I think Neil's right. It basically *has* to be done by pole-mixing because of the simultaneous outputs and 4-pole response.

I had a read of Olivier's excellent paper on the topic:

http://mutable-instruments.net/static/documentation/pole_mixing.pdf

They must have used some of the input in the mix as well. That should allow you to get a 4-pole HP response too. The Xpander doesn't do this, and reading Olivier's paper made me realise that switching the first stage out is essentially a way of faking a connection to the input.

So, it can done. 

T.





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