The mod that does what you want isn't too hard, but it may be challenging to find the parts you need.
The filter knob is a pot that presents a certain range of resistances across it's rotation from 0 ohms to the rated value of the pot (25K, 250K, etc.). So you need to determine the high and low values of resistance for your useful range, then replace the pot with a circuit that creates that more limited range.
Determining the range is pretty easy if you have some electronics experience and equipment. Using an ohmmeter, you want to read the resistance values off the pot in the high and low positions of your favored range. Set the knob to the lower position, turn off your amp, then read the resistance between the middle lug and the outer leg that has a wire on it. Now, turn your amp back on and set the knob to the high position, turn your amp off and measure the resistance across the pot in this position. The reason you're turning your amp off between tests is to prevent a speaker- blowing transient if you brush the wrong wire while testing.
Now, you need to go find a regular resistor that approximates the value of the lower range reading. It doesn't need to be exact, but in the general range. This is a low-power application, so a small 1/2-watt resistor will be more than enough. Ideally, you would want to find one with tighter tolerance (5% would be better than 20%) just so the value is reasonably close to your desired value.
Next, you need to find the difference between your high and low value and try to find an audio taper pot that has roughly this range. Unlike plain resistors, pots don't come in every arbitrary value, so you may have to improvise here a bit. Finding a pot that the right value, taper, and physical size may not be possible.
Resistances in serial are cumulative, so what you want to do is remove the existing pot, then install the new one. The little resistor goes inline on either leg of the new pot.
Now, when the pot is turned all the way up, the new pot doesn't present any resistance, so the total resistance that onboard preamp sees is the resistor value, which should be like the high-end of your favored range. When you turn the pot down, the preamp sees the sum of the little resistor and the new pot which should be the low-end of your favored range.
The mod itself is very simple, but finding exactly the parts you need will probably be a fairly big challenge unless you're lucky.
David Fung
