[sdiy] Polymoog resonator question
rburnett at richieburnett.co.uk
Sat Oct 6 20:43:45 CEST 2018
>> 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
Yes, It is just a change of gain that changes the cut-off frequency of the
state-variable filter. I realised this a few years ago when I saw a
parametric equaliser schematic drawn like this...
...where the R and C of each integrator are *fixed* and there are a couple
of pots wired like "volume controls" before each integrator stage.
To state it again, it's the *gain* that matters, and implementing it like
shown in the schematic means that you can use a common "log taper" pot to
get an approximately exponential frequency scaling for the cutoff frequency
control. If you had instead chosen to make the R of the integrators
variable (like in the polymoog resonator schematic,) it would require an
"anti-log" taper pot to get the right feel for the frequency control which
is harder to get.
More information about the Synth-diy