[sdiy] VC Sallen-Key with a 2164

Andrew Simper andy at cytomic.com
Wed Apr 27 04:48:37 CEST 2016

Hi Pru,

There is no way to build an MS20 type structure with virtual grounds,
but there is another structure I came up with independently and have
called SMP in my software. The equations are:

0 = g (v0 - v1 + k (v1 - v2)) - s C (v1 - 0)
0 = g (v1 - v2) - s C (v2 - 0)

but because each stage has an inverting op-amp virtual ground buffer
this turns into:

0 = g (v0 - v1 + k (v1 + v2)) - s C (v1 - 0)
0 = g (-v1 - v2) - s C (v2 - 0)

where g is the cutoff gain, k is the resonance gain (from 0 to 2), and
C is the value of the capacitor, which in turn alters how much g needs
to be.

Olivier has done a 4 pole cascade using 2164, so the setup is similar
but you just need the first two one pole low pass stages, but the
resonance path becomes the sum of the first and second stage outputs
(which is actually the difference because of the inverting buffers),
gained up by the resonance amount, limited in some way, and fed back
to the first stage input:


All the best,

Andrew Simper

On 27 April 2016 at 08:53, prudence prendergast
<prudence.prendergast at gmail.com> wrote:
> long time, first time.
> i was hoping for some advice. is there a way to build a VC sallen-key
> low pass filter with a 2164? since 2164s require a virtual ground at
> the output, the matter seems complex.
> i think dave dixon might have written an article on feeding true
> buffers with the 2164, if he's here to weight in.
> -pru
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