[sdiy] Polymoog resonator question

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sat Oct 6 20:31:46 CEST 2018

Thanks for publishing your findings, David.

I was about to ask whether it might be better to design a single module with an SVF, so that folks could chain a triple, quadruple, or whatever they like.

It’s somewhat limiting that the Polymoog resonator has bands that don’t overlap, forcing users to work within the three, pre-determined bands. In addition, the LP/BP/HP switch (and hidden BP with inverted phase) forces all three resonators to the same mode, rather than allowing independent selection.

I suppose that the limited range allows more precision through finer control of frequency with the same slider travel. I assume that the shared LP/BP/HP switch is more likely due to limited panel space and the cost of additional switches that might rarely be used.

In light of what you’re describing, three independent SVF modules would perform vastly differently than the Moog version.

I released an AudioUnit plugin that implements a triple resonator, but it does not mimic the gain disparity between bands (and it certainly doesn’t model clipping or other distortion). It might be interesting to add those features and see what sorts of different sounds it leads to.

Brian Willoughby

On Oct 5, 2018, at 10:15 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> So, I've done the Bode plot analysis of the Polymoog Resonator.  The results
> were a bit surprising to me (and I confirmed them with Multisim).
> The surprise is that the LP output has a pretty high gain (about 13dB) at
> low frequencies, while the HP output has pretty low gain (about -7dB) at
> high frequencies.  The BP output looks normal, with a corner frequency gain
> of about 0.85dB at the minimum resonance.  Of course, all three amplitude
> responses have equal magnitude at a normalized frequency of 1, whereas the
> normalized corner frequency is at 0.316.  This means that the resonating
> frequency has dramatically different gains from the three filter outputs at
> maximum resonance (HP 12.8dB, BP 22.8dB, LP 32.8dB).  This means that the LP
> output is going to suffer from crazy clipping with low-frequency signals at
> high resonance.
> All of this weird behaviour is the direct consequence of that 10% loop gain
> factor.  If the input and loop gains are set to unity (by making the two
> feedback resistors equal to the input resistor, 22k), then the three
> responses all give equal peak gains at all resonances.  It remains to be
> seen how this will affect the sound, but I fear that the resonator as
> designed by Moog will simply be too loud on the LP setting.  I guess I'll
> just have to build it with sockets for those resistors (and the capacitors,
> since the corner frequencies will all change by a factor of 3.16) and hear
> for myself which way is superior.
> Does anyone else have any opinions on this matter?  Does anyone have any
> insights as to why the Polymoog resonator design may or may not be superior
> to three bog-standard unity-gain SVF circuits at the same corner
> frequencies?

More information about the Synth-diy mailing list