[sdiy] Polymoog resonator question

Donald Tillman don at till.com
Sat Oct 6 18:49:44 CEST 2018

> On Oct 6, 2018, at 1:47 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> I’m curious to know what you’re going to do about the resonance/emphasis controls.
> The frequency control is straightforward and you can directly replace the resistor with a VCA.

This is why I brought up the philosophical aspect before.  It's not a "resistor" so much as it is a tunable integrator built from an opamp, resistor, and cap.  So he implements the integrator with a OTA/2164, opamp and cap.

> The emphasis controls runs a variable portion of the signal to ground. I suppose this too could be done with a VCA, but it’s a bit weird, or at least an unusual arrangement.

And here, too: it's not "running a variable portion of the signal to ground", it's a simple attenuator from the bandpass output to the input, inverting.

Voltage controlled Q is an optional feature in VCF's, so it's completely reasonable to leave it as a pot.  Or it's easy enough to voltage-control it.  'Depends on how complex you want to make the thing.

> Or the circuit could be tweaked to allow some more typical arrangement. But then the purists will start having kittens.

'No purists involved here.  He's not duplicating the Polymoog resonator circuit, which is plenty easy to duplicate as is.  He's riffing off of it.  The very first thing was to make it voltage controllable.

  -- Don
Donald Tillman, Palo Alto, California

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