This schematic is for a low-fequency oscillator, used in a phaser effect pedal. I know it works because I have built it, but I am not 100% sure how it works and
Guitar Phaser Pedal Types: JFET, Optical, OTA, and Others - Function f(x)
Guitar Phaser-Tremolo with a LM386 and a 555 timer chip - YouTube
Mxr Phase 90 Schematic - filmcelestial
ElectroSmash - MXR Phase 90 Analysis.
Transistor Clustering for DIY Guitar Effects – Slackpropagation