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

Russell McClellan russell.mcclellan at gmail.com
Wed Oct 31 00:40:37 CET 2012

I recently read the Jim Williams chapter on his famous "zoo circuit"
in which he recounts the design process of a high-precision,
battery-powered VCO.  The critical insight was apparently using a CMOS
inverter as an SPDT switch - signal A is connected to the Vcc pin,
signal B is connected to the ground pin, then the inverter input
selects whether the output is connected to signal A or signal B.

That got me thinking - if you connect the ground pin to ground, and
vcc pin to a signal, and the input to a high-frequency PWM from a
microcontroller, and follow it with a low pass filter that passes
audio but stops the PWM carrier frequency, you'd have in effect a very
cheap computer controlled voltage gain stage.  This could be used as a
control element in many synth circuits. One could perhaps very cheaply
make a full computer-controlled analog synth, using analog
oscillators, filters, and amplifiers controlled in this way.

Is this a crazy idea?  Would it work?


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