[sdiy] MS 20 FILTER

[harrybissell]

Aaron Lanterman wrote:


> I'd be very interested in your results on that. I took a Sallen-Key,
> replaced the resistors with OTAs, and then couldn't quite make that match
> up with the schematic... in particular, to do a one-for-one replacement of
> the transistor that hooks to the voltage buffer/amp, you'd need two OTAs
> in parallel facing opposite directions, which isn't what's happening there
> - the equivalent resistor winds up working "one way" and you get a
> different transfer function.

The two resistors that tune the S-K are replaced by OTA.  If you assume that the
driving impedance is much lower than the first (input) resistor... it cannot work

both ways, its essentially grounded.  The second resistor is terminated in a cap
and a high impedance, so that one is essentially 'one way' also.  The MS-20
feedback starts at a low impedance point, so that sould be pretty much 'one way'
as well.

I'm not following this (or I'm thinking of the wrong points ?)

H^) harry

>
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> - Aaron
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