[sdiy] Polymoog resonator question

Richie Burnett 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 
>and cap.

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...

https://damien.douxchamps.net/elec/param_eq/param_eq_sch2-big.png

...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.

-Richie, 



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