[sdiy] Polymoog resonator question

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sun Oct 7 02:56:16 CEST 2018


On Oct 6, 2018, at 12:01 PM, Donald Tillman <don at till.com> wrote:
> On Oct 6, 2018, at 11:43 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> 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.
> 
> The pot taper issue gets interesting.
> 
> An audio taper pot isn't really an exponential curve; it's two linear curves spliced together at the center.
> 
> And the Polymoog resonator uses linear pots in series with the input resistor, and the gain is proportional to the inverse of the resistance:
> 
> TFintegrator = 1 / (2400 + (1-x)10000) sC
> 
> Where x is the pot travel, 0.0 to 1.0.
> 
> And you can plot that, and it's log, on the Grapher app on a Mac, or any plotting facility, and check it out.

I tried this is Grapher, but need some hints. Is C in farads or nanofarads? What about s? Also, there aren’t enough parentheses for me to tell whether sC is in the numerator or denominator.

Sorry for the basic questions. Grapher is very useful.

Brian




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