[sdiy] Polymoog resonator question

Michael E Caloroso mec.forumreader at gmail.com
Fri Oct 5 05:19:53 CEST 2018

I own a Polymoog and the resonator is overrated.  It is 6dB/oct which
results in buzzy sound.  I prefer 12dB/oct resonators which are much
more flexible.


On 10/4/18, Donald Tillman <don at till.com> wrote:
> Hey Dave,
> The inverter stage in those filters attenuates, reducing the loop gain by a
> factor of 10, lowering the tuned frequency by a factor of the square root of
> 10.
> On a more philosophical note, this is because a state variable filter isn't
> R's and C's; it's two integrators and an inversion in a loop.  It doesn't
> matter how those parts are implemented, the overall equation is the same.
> In this case, the attenuation in the inverter stage is equivalent to each of
> the integrators tuned down by sqrt(10).
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> http://www.till.com
>> On Oct 4, 2018, at 1:00 AM, David G Dixon <dixon at mail.ubc.ca> wrote:
>> 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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