DIY Quad-gang Pot

[dan_thompson87]

Hi everyone!

I know that 4-gang pots is a subject that comes up quite a lot on this forum and I've been reading through various previous posts before stumbling across this website.

This is highly relevant to me as I am in fact building a B-Format compressor. For the final make-up gain stage I have a single Texas TL074, with all four channels going through individual non-inverting op-amps. As each channel requires identical gain, a quad-gang pot is needed for the feedback resistor on each op-amp. The following paragraph is the part from that website which caught my eye.

In a non-inverting op-amp feedback circuit it is easy to use a variable resistor to GND to create increasing gain with a reducing resistance. And in a slightly different op-amp circuit it is easy to have a resistor to GND create reducing gain with reducing resistance. The cunning part is to use each end of one single gang potentiometer connected in both of these circuits, with the wiper grounded. So, as the wiper is grounded there is no crosstalk between the circuits and as you move the wiper from one end of the track to the other, the gain increases or decreases in both circuits. It's clever, and as far as my very sketchy thought about this circuit goes, should still be fine with log pots, so it will be fine for audio weighted controls. Using this system, a cheap double gang pot from Farnell becomes a quad gang pot; Ideal for B format volume control.

This sounds exactly like something that I could make use of, however I'm struggling to understand the configuration of the pot and exactly how it would work. Could anybody shed some light on this solution for me?

 

[Don Hills]

Most pots don't have perfect isolation between the "ends" when the wiper is grounded. There's a small amount of crosstalk because the wiper doesn't span the full width of the track. Also. many "log" pots aren't true log - they have two linear sections with different resistances.

I havent's seen the design you referred to. When you find it, I suggest simulating it using a SPICE program and seeing if you get the control laws you are looking for. You need to check that the gains track well over the range of the pot adjustment.

Another option is "electronic pots". See this discussion:
http://www.diyaudio.com/forums/solid-state/154209-reverse-old-loudness-control-2.html#post1970522