Lately I've been playing around with the idea of designing my own booster to get a better understanding of how some of these effects work. Now I've read Cook Your Own Distortion and an article on Geofex and have a pretty good understanding of high and low pass filters and such.

Ok, say I have an input section like this:

C1 & R1 form a high pass filter, R2 & C3 (which I should've labelled C2 ) form a low pass filter and the input impedance is the parallel value of R3 and R1. Right?

Okay, now my question is, when I'm working out the value of say, C1 (1/2*pi*R*Freq), does R3 play a role at all? Is R the parallel value of R1 and R3, or just R1? What about when I'm working out C3?