[sdiy] Synth applications of CMOS inverter as SPDT switch?

Russell McClellan russell.mcclellan at gmail.com
Wed Oct 31 20:28:32 CET 2012

Thanks, Richie, for the historical info!  That 909 tom circuit is
pretty interesting.

Olivier - good point on the dynamic range issue.  At modest
frequencies - 32MHz, 32kHz, for example, you could get 10 bits or so.
This probably isn't enough control for a filter or a oscillator, as
you say, but might not be bad for an amplifier.  I like your
DAC-as-amplifier idea a lot, too, though.

I tried to read the rockit circuit but with all the nets that aren't
visibly connected, it's hard to read.  It seems like it's a sallen-key
filter with the cap that's traditionally connected to ground in the
negative feedback loop of the op-amp.  I'm not sure how they get
around the dynamic range either - it's hard to believe that the filter
doesn't audibly step.  Unless I did the math wrong, the resistor
connected to the wiper doesn't dramatically affect linearity of the

Here's a curve of equivalent resistance of the whole pot with the
wiper resistor to the digital control value:

Thanks so much for your great insights,

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