How to reverse log pot in series (trimmer)?

[simon111]

Need your help guys and gals.

I have a capacitor inline with a log 10k pot to ground on my Fezter Valve stompbox.



This works well to control the gain.

Problem is, I cannot get the log taper reverse of the rotation.

I.e. When I turn the pot fully clockwise the gain goes down and fully anti clockwise the gain goes up. As the lower the resistance the more the cap comes into play and thus more gain.

When I change the leads round on the pot E.g.  Using pins 2&3 instead of 1&2 I get it working correctly (i.e. clockwise the gain goes up) but now the log taper is the wrong way round. E.g. Most of the rotation is around the upper range of the 10k pot (7-10K)

What I need is the log taper concentrated over the 0-3K range rather than the 7-9K range, that reaches 0K fully clockwise and 10K full anti-clockwise

Hope that makes sense.

Any help would be much appreciated of how to achieve a finer control over the lower resistance but can still go up to 10K.

[italianguy63]

Just sounds like you need a reverse log trimmer, if there is such a thing...

[nocentelli]

I think the problem is not the taper, but more to do with the fact that a large proportion of the pot's rotation makes no difference to the gain. If you have a 1k5 resistor to ground on the source, any resistance between the source and a source resistor bypass cap greater than 1k5 will not audibly increase the gain: As the pot is turned (e.g. using lugs 2+3 on a 10k pot) the gain will only increase once the pot's resistance falls to below 1k5 - This is in the last 15% of a 10k pot's rotation, so all the gain boost happens in the final few degrees of rotation. Try a 2k pot instead (linear or log), using lugs 2+3 - the first quarter of the turn won't do much, but after that the gain should increase more smoothly over the rest of the pot's travel.

[Mark Hammer]

Leo's right.  Think in terms of value, not taper.

The 22uf cap really only impacts on stage gain when the resistance in series with it drops below the value of the 1k5 resistor.  After all, if the path through the 22uf cap is no more efficient than that through the resistor, why would it make a difference?

So, you can probably do just fine with a 1k or 2k trimmer.  Worst case scenario, wire up a 3-position toggle to insert 3 different resistances in series with the cap - zero ohms, 408R, and 1k5 - to get 3 different gains.  So, stick a 1k5 resistor in series with cap.  Use the toggle to either switch a 560R resistor in parallel with it (408R parallel value), or a straight wire jumper (0 ohms), to get three different gain settings: stock, highest gain, and medium gain.

Note that where the gain is boosted depends on the value of the cap to ground.  If the cap is a much smaller value - like 330nf - then it affects gain of upper mids and highs.  So, technically, you could have a "bright" function by sticking some resistance in series with the 22uf, but connecting a smaller value directly between source and ground, creating a zero-ohm path for the high end, and a somewhat higher impedance path for the mids and lows.

[petemoore]

I.e. When I turn the pot fully clockwise the gain goes down and fully anti clockwise the gain goes up.

To reverse the rotation / effect, swap the outside lugs of the pot.
You might want to try 100k or bigger there, 10k to ground might be small, depending on what is driving the input.

[peterg]

If it's a trimmer are you using it as a set and forget pot? If so does it matter which way the gain increases?

[simon111]

Thanks for all of your replies so far.

No I want this as a control panel pot.

The below is a 1KHz sine wave being inputted into the circuit. The Vac is the AC voltage at the output, and the Resistance is the pot resistance in KOhms
0-10 Knob Position             Vac   Resistance KOhms
0                                    0.73         0
1                                    0.609        0.11
2                                   0.469                0.32
3                                   0.394                 0.63
4                                   0.364                 0.88
5                                    0.351        1.14
6                                   0.316             2.66
7                                   0.301            4.81
8                                   0.295            7.47
9                                   0.292           9.7
10                                   0.292           9.74


I do like the taper and the resistance range to my ears as starting at 2K as others have already stated, but is just too much gain for my liking. I do really need to go from around 10K to 0K.
I just need the above but reversed.

E.g. Knob position on 10 =  0 ohms and Knob Position 0 = 10K , with Knob position 5 (half way) still at around the 1.14K

....sounds simple....!

[samhay]

You can probably use a 1k instead of 1k5 source resistor. If so, you can try replacing it with a 1k pot (I have never seen a 1k5 pot) and now wire the 22u cap to lug 2 - like this: