limit range of potentiometer

[bthm3288]

I'm wondering if there's a way to limit the "range" or a potentiometer.

Here's my scenario - I use a Diamond Memory Lane Jr delay pedal, and I like to adjust the mix with my foot while playing. However the mix control is very sensitive to movements.. ideally I would like the mix, when set to maximum, to sound more like where a stock unit sounds at noon (I never ever use more mix than that).

After talking to Diamond, they suggested I replace the stock Rev Log 100k pot with a Linear taper 100k pot... which I did, and it does allow a little more precise control over the lower mix settings.. but the taper is kinda all messed up (pot movements from 1/3 to 2/3 make very little change in mix as you might expect).

What I really want is to go back to using the stock C100k mix pot (for the ideal taper), but somehow limit the pot to only getting to max 50%. I other words, the entire range of the pot from minimum to maximum, would allow 0-50% of a stock pedal.

The mix pot appears to be wired as a voltage divider FYI. I had wondered if using a 50k pot and placing a 50k resistor of the pot "output" might work, but can't get my head around it. I've read the secret life of pots but can't find anything on limiting the range of a pot. Thanks all!

[Walto]

I had wondered if using a 50k pot and placing a 50k resistor of the pot "output" might work, but can't get my head around it.

Put the 50K resistor in series on the high side of the pot, not on the wiper. That way your sweep behaves correctly, but can only sweep the first half of the full voltage divider.

[BluffChill]

It's a while since I read it but doesn't the secret life of pots recommend putting a resistor across lugs 1 and 3 to decrease the range of a resistor? So if you put a 100k across lugs 1 and 3, it turns it into a 50k pot? Not sure how this would work if it's in voltage divider setup, though.

[bthm3288]

@BluffChill, yeah that was the first thing I thought of, didn't seem to change anything.. I guess because the ratio of the voltage divider stayed the same??

@Walto thanks I'll try that and see how it works.. kinda hard with the surface mount components but should be doable.. out of curiosity, if I put a 25k resistor on the hot side, and a second 25k resistor off the wiper side, would that allow the pot to sweep between ~25% and ~75% of a stock unit??

[MRiddickW]

You've got the right idea. You want the resistor to go from CW to GND.



The only issue is that you'd have the same taper, just over a smaller area. This might be fine for your purposes, or you might want to swap out for a B taper.



The modified taper I graphed may not be exactly right, but it should be close enough for demonstration purposes.

[Walto]

if I put a 25k resistor on the hot side, and a second 25k resistor off the wiper side, would that allow the pot to sweep between ~25% and ~75% of a stock unit??

Since this is a blend wet mix control, I'm assuming the high/low ends are the dry/wet inputs and the wiper (middle) is the output. In that case, putting the 25K resistors in series on the dry and wet inputs will do that.

Edit:
Sorry, that's High = Wet and Low = Gnd like the @MRiddickW diagram. Still, the 25K resistors would be in series on top and bottom ends (wet and ground.)

[bthm3288]

thanks everyone for the info, this is starting to make sense now I've read it all a couple times. I'll give it a go asap.

While we're talking about pot ranges - I have a deep blue delay pedal with delay range between ~ 25ms - 450ms, and since I never use delays under about 120ms, I'm thinking of applying this to the delay time knob to limit the minimum setting to around 120ms. The mix pot in question is 50kB, wired as a voltage divider.. wondering if I can attach a 25k trim pot in series into the pot lug 3 (hot) so I can gradually increase the series resistance to tweak the minimum delay time.. obviously as I increase the value of the trim pot I'm also increasing the overall value of the 50kB pot too.. but would this make a difference assuming that the ratio of the voltage divider is what's setting the delay time...?

Thanks!

EDIT: actually now that I look at the schematic, the deep blue delay does not use voltage divider for the delay time control, so that's a bad example. But I could apply the Pot + trim pot to other places that a voltage divider pot was used, correct?