[sdiy] Polymoog resonator question

David G Dixon dixon at mail.ubc.ca
Fri Oct 5 06:57:24 CEST 2018

```I did have one other question, though...

What is the significance of "V_CH" which seems to play the role of ground
almost everywhere in the schematic?  Is this some specific voltage which is
not 0V?

_____

From: Donald Tillman [mailto:don at till.com]
Sent: Thursday, October 04, 2018 7:08 PM
To: David G Dixon
Cc: synth-diy at synth-diy.org
Subject: Re: [sdiy] Polymoog resonator question

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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