[sdiy] Polymoog resonator question

Tom Wiltshire tom at electricdruid.net
Sat Oct 6 10:47:21 CEST 2018

I’m curious to know what you’re going to do about the resonance/emphasis controls.

The frequency control is straightforward and you can directly replace the resistor with a VCA. The emphasis controls runs a variable portion of the signal to ground. I suppose this too could be done with a VCA, but it’s a bit weird, or at least an unusual arrangement. Or the circuit could be tweaked to allow some more typical arrangement. But then the purists will start having kittens.

+1 agree with Quincas. Resonanators and body-modelling filters are under-used and under-represented.

> On 5 Oct 2018, at 19:54, David G Dixon <dixon at mail.ubc.ca> wrote:
> Thanks, Don.  That's what I figured.  I have removed CD4000 switches from
> many designs and replaced them with DG400 switches for that very reason.
> I'm going to redesign this Polymoog Resonator, using 2164s for the R's,
> putting the stages under voltage control, and changing the output switching
> situation a fair bit.  I'm still not sure what I'm going to do about the
> loop gain situation.  I'm deriving the transfer function right now to get a
> better idea of what it actually does.
>> -----Original Message-----
>> From: Donald Tillman [mailto:don at till.com] 
>> Sent: Thursday, October 04, 2018 11:04 PM
>> To: David G Dixon
>> Cc: synth-diy at synth-diy.org
>> Subject: Re: [sdiy] Polymoog resonator question
>>> On Oct 4, 2018, at 9:57 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
>>> 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?
>> Indeed, V_CH is 4.85 V.
>> That audio section uses CMOS CD4000 series chips for audio 
>> switching.  Those max at 18V.  So the Polymoog runs the CMOS 
>> at 0 to +15V, and biases the audio signals up a little to fit 
>> in there.
>> The alternative would be to leave the audio centered around 
>> 0V, and run the CMOS at something like +/- 5V.  Then all the 
>> logic lines would have to be shifted.  Which is a drag, but 
>> would have been better for audio quality.
>>  -- Don
>> --
>> Donald Tillman, Palo Alto, California
>> http://www.till.com
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