[sdiy] Polymoog resonator question

David G Dixon dixon at mail.ubc.ca
Sat Oct 6 10:03:35 CEST 2018

> In this circuit, with the 10% loop gain factor, it's the same 
> as multiplying the gain of each integrator by sqrt(10), or 10dB.

Yes, I see that in my calculations.  I've learned a fair bit about transfer
functions today, so thanks for helping me with that (really).  I had never
derived any that had anything other than unity gain all around, and this has
been an eye-opener.

> That said, I have no idea why they did this, and I see no 
> good reason for it.  

That's what I was thinking as well.  Wouldn't it just make the LP outputs
too loud, and the HP outputs too soft?  It's not going to affect the timbre
of anything, but it might cause some nasty clipping, which (to me) kinda
defeats the purpose of the resonator in the first place (which is to add
richness to the sound, but not distortion).

> And it is somewhat compensated for in those resistor mixdowns 
> before the 4016.

I haven't really looked at that part of the circuit too closely yet, but I
am intrigued by the "E" output, which seems just to add an inverted version
of the middle resonator's BP output to the other BP outputs.  I wonder how
cool that sounds.

It's an interesting circuit, but I suspect that there is scope for
improvement.  I'm curious about adding different modes of the three
resonators together, not just all LP or all BP, etc.  That might be
interesting.  Also, I'll probably add a fourth resonator (and change the
frequency ranges accordingly) because I need two 2164 VCAs for each
resonator, so I'll have two spares.  I've figured out a way to build each
resonator with one quad and half of a 2164, and that includes both manual
and CV control of frequency, so the layout will be very efficient --
074-2164-074-074-2164-074.  I'll probably build one tomorrow.

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