The circuit is composed of four phase shift opamp stages each with a JFET used as a variable resistor. Part of the circuit I do not understand is the DC biasing arrangement. I can see that it uses a 4.5V Zener diode to provide the 1/2 VCC supply as this circuit runs off single 9VDC supply. I do not understand how the 4.5VDC gets to the stages. Do R6/R9/R12/R15 provide the DC component to the stages? Or does R2/R3 simply bring the DC to the first stage and it gets passed through each stage?
Finally, I can't figure out the purpose of R18, the 1k resistor at the output of the last stage.