Pultec LF frequency selector question

[chris p]

I'm knocking up a design based on the Gyraf Pultec EQ1 schematic, but have lost my way with the LF cut and boost. These share a common frequency selector which is a 2 pole, n-way switch where n is the number of frequency options. There is a set of cap vales selected on one pole for the cut control, and another set of caps for the boost control. The Gyraf version has 6 possible values, I want to up this to 8 or 10 (as I have done with the HF cut and HF boost frequency selectors).

I assume that each position reflects one frequency value (eg 30Hz of whatever), but what is the formula for working out the right cap values (in both poles) for a given target frequency? I assume its the old C = 1/(2*pi*f*R), but working backwards from the Gyraf values (ie knowing the C and f values, working out the constant R for the circuit) produces wildly differing answers. Any help greatly appreciated.

[Milo]

Chris;

Not being an expert, but you also have to incluse the load impedance of the circuit in the formula, which affects the value of R and C. I can't remember where I put the calcs, but I remember C=0.2 x impedance at lowest pass frequency. I'll try and dig it up.
May be a furphy, but...

[chris p]

Thanks Milo - I think the load impedance affects the value you use for R because it forms a parallel to ground, something like that. What I have found is that the value for R working backwards is actually pretty good if you drop the first two values - I suspect those that have labelled their Gyrafs LF selector starting at 20Hz may be a little off the mark.