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

[Dan Snazelle]

Thanks for posting this andrew 

And Don thanks for all those links 



Sent from my iPhone

> On Dec 21, 2014, at 2:56 AM, "Andrew Simper" <andy at cytomic.com> wrote:
> 
> 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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