[sdiy] Polymoog resonator question

David G Dixon dixon at mail.ubc.ca
Sat Oct 6 20:07:20 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.

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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