Tuesday, 3 January 2012

Pots that drive me up the wall.

I get annoyed at pots in guitar amps (potentiometers), usually the volume pots. They have two attributes I hate. Noisy over time and the inability to get from zero to low volume in an orderly fashion. 

Firstly let’s look at issue of a noisy pot, you move the volume and get great crunches out of the speaker. I got to thinking about this the other day and came up with the following plan. I’ll replace the 1Meg volume pot (log scale for volume) with a 2.2Meg stereo log pot.  I’ll then wire the two pots together in parallel, so wiper to wiper, top of pot to top, ground to ground. This will give me a 1.1Meg Log pot.  This way I have two wipers to go wrong, but I should have less chance of a disconnection occurring as one of the wipers should have a good connection at any one time. Something to give a go.

This then set off a train of thought. What about wiring just the wipers and ground connection together. This should give a modified Log response with hopefully a slightly more gentle lift from zero and a bit more granularity in this area. So a pot and a parallel sub-pot.

If I plot out the response from a theoretical Log pot and then as a “sub-pot” I get the following change plotted as the green curve. This assumes zero source impedance and an infinite load.  I do get a slight easing in the very low volume setting area, followed by a slower curve into the 60% rotation which then quickens up to meet at 100% rotation.

Now anyone familiar with a Marshall JCM800 amplifier will tell you that 0-4 is zero to full power, after this you are soon overdriving the power section. This method will give slightly more control outside of the power amp overdrive section, which is where I want it.

You can also play with the same idea with a linear pot, say the Treble tone control in a passive TMB (treble, mid, bass) tone stack. Here the parallel pot is marked as X1.

We will get a degree of log law faking but we also modify the end to end resistance which will play with the impedance sensitivity of the other controls. To be fair it isn't too far from a fixed resistance at the same point, but different enough.

A common treble value for the pot is a 250K linear.  If we substitute in a 250K lin stereo pot wired as a sub pot then the following changes take place. Note that the mid scoop is reduced, and at maximum treble boost the scoop centre frequency changes from ~250Hz to ~350Hz. There is also a more linear change with pot rotation.  You can also get a scoop reduction by just replacing the parallel pot with a fixed 50K resistor but that can’t be as much fun.

Firstly the notmal TMB circuit moving the treble control from 0 to 100% in 10% steps.  6dB / division.

And now with a 250k stereo pot wired as a sub-pot.

Feel free to play and adapt.


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