[sdiy] Weird Serge Resonant Filter topology in upper bands

Guy McCusker guy.mccusker at gmail.com
Tue Dec 13 09:47:02 CET 2022

This is intriguing. Don's re-draw looks much more likely to be the intended

I had a look at some photos of original ResEq PCBs and as far as I can tell
they match the schematic that Aaron posted, so if there was a transcription
error somewhere it happened before the boards were laid out, or in that
process I guess.

I am very tempted to perform some modifications to my CGS ResEQ to see what

On Tue, Dec 13, 2022 at 6:57 AM Donald Tillman <don at till.com> wrote:

> Hey Aaron,
> I don't recognize it either.
> And the filter circuit, as presented, doesn't make any sense.  If you work
> out the transfer function, it's not a bandpass filter; it's some highpass
> bandpass combo with a low Q.  And like you said, the opamp buffer doesn't
> make any sense either.
> Hmmm, y'know...
> If the schematic was in error...
> And the filter's output signal was not the opamp output as shown, but
> instead tapping off of the RC network that leads to the opamp's + input,
> that would make more sense.
> Something like this:
> That's a proper bandpass filter.  And it would require a buffer.
>   -- Don
> --
> Donald Tillman, Palo Alto, California
> https://www.till.com
> On Dec 11, 2022, at 5:11 PM, Lanterman, Aaron D via Synth-diy <
> synth-diy at synth-diy.org> wrote:
> I’ve been looking at the Serge Resonant Filter:
> https://electro-music.com/wiki/pmwiki.php?n=Schematics.SergeResonantEqualizer
> The lower 7th bands are standard bandpass Multiple Feedback filters. But
> the 3 top band (see rightmost two on the bottom row, rightmost one on the
> top row) are something else weird I haven’t seen. They vaguely remember a
> Wein-bridge-style filter in the Boss SP-1 Spectrum pedal, but instead of
> having a capacitor in series in the positive feedback loop, Serge has it in
> series with the input.
> Does anyone recognize this filter? I have an urge to give things names.
> I tried to work out the transfer filter under the assumption that the
> negative feedback is stronger than the positive feedback so “golden op amp”
> rules hold. But when I plot the frequency response of my transfer function,
> I get peaks that are way too broad — they’re more curves than peaks. So I’m
> doing something wrong somewhere.
> My transfer function has a numerator of
> c*s*(r+rp*(1+r*c*s))*rn
> And a denominator of
> rp*r^2*c^2*s^2 + (rp*2+rn)*r*c*s + rp
> Where rn is the resistor in the negative feedback look, rp is the resistor
> in the positive feedback loop, and the input resistor and resistor to
> ground are the name and denoted by r.
> Serge doesn’t remember where he got the filter from. He said he went to
> this topology for the higher bands because limitations in the open loop
> gain bandwidth product prevented the MFB topology from working well at the
> higher frequencies.
> One interesting thing is he felt the need to add an op amp buffer at the
> output of the filter, which suggests to me that non ideal op amp effects
> are in play.
> Aaron Lanterman, Prof. of ECE, Georgia Tech
> My blog on Education and Innovation: https://edupocalypsenow.wordpress.com
> My blog on Electronics and Programming: https://lantertronics.blogspot.com
> My YouTube channel on Electronics and Programming:
> https://www.youtube.com/c/lantertronics
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