What are some effects that use Reverse Audio Pots?

[Arn C.]

I am placing an order to small bear and was wondering what are some effects that use Reverse Audio Pots.

Thanks!
Arn C.

[MikeH]

Mosfet boost, SHO(?), micro amp, dist+, ea tremolo, and certain pots in some fender tonestacks.  To name a few.

[Mark Hammer]

Reverse audio is typically used when you have to get through a lot of resistance initially, and then small variations in resistance matter.

One context is the use of a pot on the ground leg of an op-amp in some distortion circuits where you need to skip past the first 90% of resistance quickly.  A second would appear to be in setting LFO speed in some circuits, where it requires large resistance changes to go from, say, .1hz to 1hz (once every 10 seconds to once per second), but only small resistance changes to go from 1hz to 2hz.

[Joe Hart]

DOD 250 uses them.
-Joe Hart

[Mark Hammer]

Quick tutorial that can help flag contexts where reverse-audio makes sense.

In a non-inverting op-amp configuration, you can increase gain by increasing the feedback resistance (1M in both the DOD250 and Dist+), or by decreasing the ground leg resistance (pot plus a 4k7 fixed resistor in both of the aforementioned cases).

The gain is calculated by (feedback+groundleg)/(groundleg).  If the ground leg is anything less than infinity (open-circuit), the gain will be greater than 1.  If you work out the math, you'll see that it takes very big resistance changes to move gain in the direction of 1 if the feedback resistance is high. 

In contrast, when the gain is set for a non-inverting op-amp by the feedback resistance - as it is in the Rat and Tube Screamer - it takes comparatively little change in feedback resistance to change the gain from 1, or to move it in that direction.

So, whenever you see a circuit that uses the ground leg of a non-inverting op-amp to adjust gain, there may be a need to use a reverse-audio taper pot there, depending on what sort of gain range one is aiming for.  If the pathway used to control gain is the feedback path, then the odds are slimmer that a reverse-audio is needed to do the job.

[caress]

fuzz face type circuits?

[Arn C.]

Thank you for your inputs everyone and Mark for the explanations!

Arn C.

[kurtlives]

Most fuzz designs use a B pot on the emitter of some transistors gains stage. I find a C pot works way better in those positions than B pots.

[Mark Hammer]

As I keep parroting, often the pot-taper needs are a function of what it is you need to be able to dial in most.  If you could give a rat's ass about slow speeds on a phaser, and are really only interested in dialing in modulation speeds between 1hz and 5hz, then a 50K linear pot may do the job YOU need doing every bit as well (if not better) than a 500 reverse-audio pot that has to cover the range from .1hz to 5hz with roughly equal dialability.  A 50k linear pot on a Dist+ does an admirable job of finding different shades of grind If you don't really have a need to also dial in the cleanest possible tones.

[cpm]

most "speed" controls on LFOs, i think