[sdiy] Sallen Key with input mixing to generate all shapes

David G Dixon dixon at mail.ubc.ca
Mon Dec 22 01:10:44 CET 2014

This very concept is the basis behind the Intellijel Korgasmatron II, which
(with the expander) allows one to derive different responses from the same
filter simultaneously.  The only difference is, in that case, the topology
is that of the MS-20 2-pole filter.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Andrew Simper
> Sent: Saturday, December 20, 2014 11:54 PM
> To: synthdiy diy
> Subject: [sdiy] Sallen Key with input mixing to generate all shapes
> Hey guys,
> I've noticed many Sallen Key synth filters can generate 
> different responses by sticking the input into various points 
> of the circuit, but they only ever deliver a 1 pole high 
> pass, and a very weak notch.
> I've worked out the theory for generating all the same 
> responses as you can get by mixing the outputs of an State 
> Variable, but instead mixing the inputs to a Sallen Key. I've 
> put together a technical paper on it here:
> https://cytomic.com/files/dsp/SkfLinearTrapezoidalSin.pdf
> and I've sketched the basic structure of what is going on 
> here (taking an MS20 v2 type OTA Sallen Key as the basis):
> www.cytomic.com/files/dsp/SkfInputMixing.jpg
> the important voltages are:
> vamp1 = (m0*v0) - (m1*v0 + v1)
> vamp2 = (m1*v0 + v1) - (v2)
> vcap1 = (v1) - (res*v2 + m2*(-v0))
> vcap2 = (v2) - (m2*v0)
> where m0 = low mix, m1 = band mix and m2 = high mix
> Also for those interested in a digital implementation I've 
> also included in the paper is also a trapezoidal integrated 
> linear circuit model use state space increments and high 
> precision coefficient calculation.
> Andy
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