[sdiy] Polymoog resonator question

p. hendricks ph at comcast.net
Sat Oct 6 08:37:42 CEST 2018

On 10/5/18 11:23 PM, David G Dixon wrote:
>> Seems even many that tried to help you with this, you didn't
>> even say "thanks" after they took the time to try and help you.
> Um, I guess you didn't see my previous message in response to Don Tillman
> (who is the only one who offered any concrete answers to my actual
> questions):
no, this one:
Yes, I realized the next morning that I had overlooked the loop gain 
factor, but couldn't get on email until now.

*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
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