[sdiy] Polymoog resonator question
David G Dixon
dixon at mail.ubc.ca
Thu Oct 11 19:50:51 CEST 2018
Thanks, Rutger! I have built the PCB to reproduce the original three
resonators fairly closely -- same gains, same frequency ranges, but with the
option of voltage control. I have a fourth resonator that is a copy of the
high-frequency resonator, but which will cover the entire frequency range
(60 to 7500 Hz) given the same input voltage range. With this I can see how
overlapping peaks sound.
To test uniform gains, all I have to do is swap out a few resistors (an easy
and pleasant task with my Hakko FR-300 desoldering gun). If it sounds good
as it is, then I may not bother.
I have installed everything except the capacitors and the connector
hardware. I got a fairly ill on Monday night (headcold plus pancreatitis --
not a nice combo) and didn't feel like finishing it. I also searched (in
vain) for 5.6nF caps for the low resonator, and have since changed the
design to use (the much more common) 6.8nF caps. I will finish it tonight,
however, so I expect to be back on SDIY with a vengeance tonight and/or
tomorrow. I don't have a Jupiter 8 to put through it, so I won't be able to
recreate Ken Elhardt's setup, but I'll make do with my Doc Sketchy synth.
Or, maybe I'll try my Hammond digital organ through it.
> -----Original Message-----
> From: Rutger Vlek [mailto:rutgervlek at gmail.com]
> Sent: Thursday, October 11, 2018 10:28 AM
> To: David G Dixon
> Cc: 'Donald Tillman'; synth-diy at synth-diy.org
> Subject: Re: [sdiy] Polymoog resonator question
> Hi David,
> Great that you're working on this! I also had plans do build
> a resonator like along these lines, though probably would
> have changed things w.r.t. the original Moog version, most
> notably the option to have 12dB instead of 6dB slopes.
> I read your email (below) a while ago and haven't found
> anyone directly replying to it. I may have missed it, but
> your findings and questions are interesting!
> I have a hypothesis on why Moog did it this way, with
> different gains for the LP/BP/HP outputs. It has to do with
> the expected power spectrum of the sounds typically used as
> an input. Most synth sounds (saw/tri/pulse) have a spectrum
> where most of the power resides in the fundamental and
> harmonics close to that. Towards the higher frequencies the
> power rolls of (e.g. isn't flat like the white noise
> spectrum). Given the resonator will receive such sounds at
> it's input, I can see it makes sense to balance the amplitude
> of the resonating frequency against that of the pass-band of
> the filter. So if you are in LP mode the filter passes all
> frequencies below the cutoff and resonates around the cutoff
> frequency. The resonance peak will have to be balanced
> against the typically rather bass-heavy remained of the input
> signal. With the HP mode, this is totally different, as in
> the pass-band only very little power remains, meaning that
> the resonance peak becomes masked less easily and risks being
> too dominant over the original high-end remainder of the
> spectrum. The BP being the neutral one also makes sense, as
> here there's little low nor high end to balance the peak against.
> My suspicion is that if you prototype this idea, it will
> indeed turn out to be more musically sounding than making all
> outputs have the same gain. If I can find some time I'll
> rapid-prototype it for you (and others) on my Nord G2. It's a
> great platform for finding out such things.
> What I still don't understand is why they did it inside the
> loop, rather than at the output. The only reason for being in
> the loop is if you wanted to affect frequency as well....
> which isn't switching to a different range along with the
> LP/BP/HP is it?
> On 6 okt 2018, at 07:15, David G Dixon 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?
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
More information about the Synth-diy