All you need to do is take that diagram and add a second treble pot and capacitor, but put an inductor between them. This will give you LPF, HPF and band-stop filters.
Originally Posted by Ross AriaPro
I dont see anyone bleeding off all three to zero...anymore than anyone would do so with amp controls.
Equalizers on amplifiers are generally active filters. They are designed to boost and cut particular bandwidths of frequencies in very predictable ways. The problem here is that you are dealing with rather crude passive circuits. When all three are turned down, you are left with a near zero-Ohm load on mid and high frequencies, and a blockage of low frequencies. The only frequencies that can make it through without blockage by the HPF's capacitor, or shunt to ground by the other two filters would be frequencies that fall outside of all of the filters. Usually, there will be nothing, due to overlapping frequency cutoffs.
You can certainly experiment with capacitance and inductance, however. Just be aware that the band-stop and LPF need to have pots with high resistances at one extreme of the rotation, so that they can be set flat, when desired, but lower resistances throughout the sweep. Choose audio taper pots, preferably 500k, depending on signal impedance. Any higher will cause an on/off switch-like behavior, as the usable range of resistance is squashed into a small area of the sweep. The HPF needs a pot with a rather high resistance range, to allow a useful range of attenuation. 1M Ohms is traditional for high impedance pickups.
Capacitors C2 and C3 in your G&L diagram are optional. C2 is a treble-bleed capacitor, meant to offset the loss of high frequencies when turning down the volume. C3 is a fixed LPF, probably meant to tame the frequency response a tad.