[sdiy] Polymoog resonator question

David G Dixon dixon at mail.ubc.ca
Thu Oct 4 10:00:13 CEST 2018


Hey Team,

I have been looking at the Polymoog resonator schematic (the one from the
synthfool site), and I'm a little confused.  It's just three state-variable
filters, and the RC networks have Rs which range from 2.4k to 12.4k (a 2.4k
resistor and a 10k pot), and the Cs are 68nF, 12nF, and 2.7nF.

The stated frequency ranges are 60-300Hz, 300-1500Hz, and 1500-7500Hz.
However, when I calculate the frequencies one should get from these
component values (assuming the formula f = 1/(2*pi*RC), I get frequencies
which are about 3.3 times higher than the targets.  By my calculations, for
the same resistance ranges, the correct capacitors would be roughly 220nF,
47nF, and 8.2nF.

Is the design wrong, or am I stupid, or am I missing something?  If the
stated component values are the actual ones used in the Polymoog, then one
cannot argue with the fact that it sounds fantastic, even if the stated
frequencies are all wrong.  However, I'd just like to know that I'm not
smoking crack here.

Cheers,
Dave Dixon



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