# [sdiy] Weird Serge Resonant Filter topology in upper bands

Lanterman, Aaron D lanterma at ece.gatech.edu
Mon Dec 12 02:11:23 CET 2022

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