Get full potentiometer range from 45 or 90 deg rotation?

Thread Starter

nanok66

Joined Jul 14, 2016
75
Hi,

I am modifying a synth keyboard with new pitch bend (PB) and mod wheel (MW) potentiometers. The existing pots are 10k and they must be special because they turn about 90 degrees and my ohmmeter shows they reach full range of a 10k pot. The new pots cannot rotate the full range either due to the mechanics of the device. The PB pot rotates about 90 degrees and the MW pot rotates about 45 degrees. Is there any way to modify the potentiometer circuit to get a wiper voltage output closer to full voltage range when I am only able to rotate the pots a small degree? Appreciate any ideas on this!
 

Papabravo

Joined Feb 24, 2006
16,188
Hi,

I am modifying a synth keyboard with new pitch bend (PB) and mod wheel (MW) potentiometers. The existing pots are 10k and they must be special because they turn about 90 degrees and my ohmmeter shows they reach full range of a 10k pot. The new pots cannot rotate the full range either due to the mechanics of the device. The PB pot rotates about 90 degrees and the MW pot rotates about 45 degrees. Is there any way to modify the potentiometer circuit to get a wiper voltage output closer to full voltage range when I am only able to rotate the pots a small degree? Appreciate any ideas on this!
Sure. Follow the wiper with a gain stage. As a bonus you can also add or subtract an offset.
A pot is a pot after all and the modifiers you are using are meaningless to the electronics. I don't care what the ohmeter tells you I want to know the voltage levels at each end of the available travel for both the old and the new pots. Then I can tell you how to fix it. It of course may be way more sensitive than before especially if it is a non-linear (audio taper pot). It for sure won't have the same feel as the old board.
 
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Thread Starter

nanok66

Joined Jul 14, 2016
75
@Papabravo Thanks! Yeah I assume the output is being fed into some microcontroller ADC, aka the pot functions as resistor divider. But I have only tested the pots unconnected to the circuit. I guess I should test with the keyboard on, I do need to find out what voltage is being used on these pots after all...
 

Thread Starter

nanok66

Joined Jul 14, 2016
75
@JohnInTX Thanks, I knew about parts express but not the other supplier. I will check them out. I think a pot that only turns 90 or 45 degrees is gonna be pretty exotic and quite possibly expensive but it would be cool to do this without an opamp. I'll search those websites and see...
 

Papabravo

Joined Feb 24, 2006
16,188
Those are specials for musical instruments. Try the manufacturer or outlets like parts express and amplified parts and go from there?
https://www.parts-express.com/
https://www.amplifiedparts.com/
So what makes them special from an electrical point of view. Is either one different from a linear taper or an audio taper? What would the model of such a thing be like? Would an opamp circuit even be appropriate to change the voltage range? I'd feel better about helping if I knew what I was dealing with.
 

JohnInTX

Joined Jun 26, 2012
4,451
A lot of times a musical instrument manufacturer will do a special from a parts manufacturer.. like a pot for a wahwah pedal that gets way more wear than a conventional part and uses a compressed range. I pointed out those suppliers because they specialize in those weird parts that you can’t buy at Digikey.
When I was taking music classes after retirement I made myself useful by maintaining and repairing the amplifiers (and playing horn parts on guitar) and found many special parts in the process.
I also found most of the manufacturers very helpful. Sometimes they will have parts left over from an old product and can be sweet-talked out of what you need
 
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Papabravo

Joined Feb 24, 2006
16,188
A lot of times a musical instrument manufacturer will do a special from a parts manufacturer.. like a pot for a wahwah pedal that gets way more wear than a conventional part and uses a compressed range. I pointed out those suppliers because they specialize in those weird parts that you can’t buy at Digikey.
When I was taking music classes after retirement I made myself useful by maintaining and repairing the amplifiers (and playing horn parts on guitar) and found many special parts in the process.
I also found most of the manufacturers very helpful. Sometimes they will have parts left over from an old product and can be sweet-talked out of what you need
Well and good, but the TS has a problem about what to do with some apparent limitations of available replacement parts and it would help if we could understand the differences between the original parts and the replacement parts. For example I think a linear gain stage following and audio taper (logarithmic) pot would be a disaster in the case of limited travel on the low end, and equally ugly with limited travel on the high end.
 
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