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

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.