[sdiy] Polymoog resonator question

Stephan Vladimir Bugaj stephan at bugaj.com
Sat Oct 6 21:16:56 CEST 2018


If you made this as a hand built euro module (as hypothesized in another post), I’d buy one especially if it had VC-Resonance. Freq control wouldn’t need to be 1V/o imho.

- One of the throng 


On Oct 6, 2018, at 11:07 AM, David G Dixon <dixon at mail.ubc.ca> wrote:

>> 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.
> 
> I was tempted to just leave the resonance control as is.  I have done
> VC-Resonance on my SVF design using 2164 (my own unit, not the Intellijel
> one, which just has a Q knob), but the output sums with the input and loop
> signals at the inverting input of the inverter, and the 2164 feeds its own
> dedicated inverter in the resonance feedback loop.  With an input resistor
> of 30k and an output resistor of 15k, when the VCA is fed 0V to the VC pin,
> and therefore passes the BP signal at unity gain, it actually gives a gain
> of 2 at the summer, which is zero resonance for an SVF.  Of course, this
> works just fine.  The only thing is that I don't believe that I have
> actually patched VC-Resonance on any of my filters more than twice in 10
> years.  It's just not a feature that I ever use.  I tend to add it to
> designs for the sake of completeness, or to use up a spare 2164 VCA (in the
> case of the SVF, if you also temperature-compensate the CV, then all four
> VCAs are used).
> 
> I would be very happy to hear from the assembled throng here whether
> VC-Resonance on a resonator would be a desirable feature.  Of course, the
> beauty of the 2164 is that one can very easily limit the voltage range into
> the VC pin to give whatever control response is desired.  For example, I
> have calculated that the damping-factor range of the Polymoog Resonator is
> from 0.052 to 0.793, so this would just be a finite range of positive
> voltages to the 2164.
> 
> Another thing I'd like to know from people is: Do you think it would be
> important or desirable for the voltage control of frequency to this beast to
> be temperature-compensated 1V/octave?  In the thing I designed lastnight, I
> just convert a nominal -5V to +5V CV signal into a 500mV to 0V signal for
> the 2164s, and this (along with a 30k input resistor and a 5.6nF capacitor)
> gives the desired frequency range of about 60Hz to about 300Hz for the first
> resonator.  It didn't seem necessary to do 1V/octave for a frequency range
> that is just a bit more than two octaves, and this thing is not going to be
> an oscillator.  As it stands, it's about 4V/octave.
> 
> Again, I don't ask these questions because I want to sell these and make a
> million dollars.  I just want to get it right the first time in case someone
> sends me a PM on Muffs and says "can you make me a board?"  My inclination
> is to put all available features on and let the end user decide whether he
> wants to use them or not.  Having a 2164 in the resonance feedback loop and
> controlling it with a pot is just as easy as wiring a pot from the
> non-inverting input to ground.  It does add 2164s to the design, though.
> 
> As far as the purists having kittens, my official response is "who cares?"
> 
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