[sdiy] Simultaneous Lin and Exp control for 2164-based VCF

[Rutger Vlek]

Hi list,

It's been forever since I posted here, but hope I can still tap into our
collective natural intelligence.

I'm close to finishing my first breadboard proto of a new filter and
preparing PCB layout. I'd like to put the VCF under digital control via the
exponential CV inputs on the 2164. However, at the same time I'd like to
cater for the possibility of doing audio-rate linear FM (from oscillators).
What's the most efficient way to combine them?

Can I take the output of the Irwin linearization circuit and run it into an
opamp mixer circuit, together with the DAC-output for expo control and feed
the sum to the 2164? Or can I tap into the Irwin circuit directly? I think
I once heard someone mention using the REF input for exp control, but I'm
not sure if that worked at all or even part of the range. Ideally I need
full range control.

Best,

Rutger
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