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

houshu at muj.biglobe.ne.jp houshu at muj.biglobe.ne.jp
Tue Dec 23 02:34:59 CET 2014

Hello list,

Input mixing structure is possible with state-variable-like filter.


HPF response is 12dB/oct not 6dB/oct. Multiple functions with 
beautiful frequency response can be obtained from this very 
simple circuit. 

Basic structure is like ARP4023, which can be derived from
ordinary state variable filter:

I built a module with satisfactory results:
Power consumption is as small as 5mA.

I designed it for single supply operation.  
Bipolar supply operation is possible by changing the 
exponential converter (Untested):

Saitama, Japan

----- Original Message -----
From: "Andrew Simper" <andy at cytomic.com>
To: "synthdiy diy" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, December 21, 2014 16:54:23
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:


and I've sketched the basic structure of what is going on here (taking
an MS20 v2 type OTA Sallen Key as the basis):


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

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