gain control problem

[Joecool85]

I tried having a gain control that controlled how much ground (and consequently power) was going to the tranny...but didn't like the outcome, it worked ok and I might go back to it.  But I was thinking about doing a gain control by having a 100k audio taper pot before the in cap on my pedal.  I tried it, and it works pretty well, with the except of two things, it seems to cut the highs when turning down the gain, and also the bottom 25% or so of the pot doesn't do anything, no sound comes out at all.  Any ideas?

[d95err]

It would be easier to help you if you mentioned what it is you're trying to build...

[Joecool85]

Its a custom OD I'm working on...I guess I'll need to get that schematic drawn out so you guys can help more...sorry 

[Joecool85]

Well, here is the big thing, when the gain pot is at 0 resistance, its full on gain, around 85k ohms is as low as I want, it starts shutting down the volume.  I know 85k audio pots don't exist, is there a way to make a 50k an 85k?  like have one of the lugs have a 35k resistor on it or something?  This is basically a volume pot before the effect btw.  So the in is on lug 3, the out going to the effect is lug 2, and ground is lug 1.

[d95err]

Well, here is the big thing, when the gain pot is at 0 resistance, its full on gain, around 85k ohms is as low as I want, it starts shutting down the volume.  I know 85k audio pots don't exist, is there a way to make a 50k an 85k?  like have one of the lugs have a 35k resistor on it or something?  This is basically a volume pot before the effect btw.  So the in is on lug 3, the out going to the effect is lug 2, and ground is lug 1.

A series resistor with a 50k pot would mean you can't get full gain, which is probably not what you want.

Use a 100k pot. The difference to 85k should be very small, it's up to the user to avoid those last 15k. If you really want to get less than 100k, put a resistor in paralell with the pot. A 500k resistor should get you a range of about 0 - 83k, although the taper will be sligtly different.

[Joecool85]

If you put a 35k resistor off lug one before ground, wouldn't that mean that at all the way to the left it would still have 0 resistance and would throw all of the signal through lug 2 and at full to the right it would be 85k resistance?

[d95err]

If you put a 35k resistor off lug one before ground, wouldn't that mean that at all the way to the left it would still have 0 resistance and would throw all of the signal through lug 2 and at full to the right it would be 85k resistance?

Hmm, I forgot it's a voltage divider, I was thinking series (feedback loop) type gain. Sorry!

Since it is a voltage divider, the absolute value of the pot has very little importance. A 100k or 50k pot should make very little difference. What is important is the ratio between the series resistance on the signal (lug 3 to 2) and the resistance to ground (lug 2 to 1).

I assume that the 85k value you mention is the resistance between lugs 3 and 2 when the volume starts to cut off. In this case you have a 15k/85k ratio or about 0.18. So, in order to get that ratio when the gain control is turned fully down, insert a resistors between lug 1 and ground that gives you the ratio you are looking for.

The pot is 100k, so the series resistor would be about 0.18 x 100k = 18k.