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I’m building a full analog chorus-effect pedal. My schematic is based on that of a Rockman X100 headphone amp, designed by Tom Sholz. In this schematic, there is a 2nd order low pass filter that looks like a variant of a Sallen-Key low pass filter, but I have never come across a topology like this. Can someone help me to derive the formulas for calculating the frequency, gain and quality factor? Thanks in advance!

Regards,

Linus

enter image description here

link to the original schematic: https://www.dropbox.com/s/3kii0qlqiwtoosj/rockman-x100-and-soloist-schems.pdf?dl=0 It starts on page 2 underneath the ‘Stereo Chorus’ title.