Originally Posted by MMski
It's a gyrator circuit, a simulated inductor, pretty common in EQ pedals. Inductance = R1*(R2+R3)*C
R1 being 470, R2 being the series resistance of the 51k resistor and whatever setting the pot is at, C being C2.
Whoops, I should have looked more closely! http://www.analogzone.com/avt08063.pdf
Should have what you need, unless you've figured it out already