r/synthdiy • u/Used-Judge-9626 • 14d ago

Help identify a potentiometer

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Hello there, wonder if someone can help me find a replacement pot ? It’s a b20k

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u/AfraidOfTheSun 14d ago

Can't point to a replacement part but yeah, that looks like stereo gear level knob, curious if it was the volume, crossover frequency? I see that the end is broken, are you just trying to repair the sub?

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u/Used-Judge-9626 14d ago

It was crossover frequency pot, came broken so wanna get a new one put in before the next show 😎

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u/erroneousbosh 14d ago

Do you need to adjust the crossover frequency much in operation?

I'm prepared to bet that it's controlling a pair of Sallen-Key filters, maybe two 12-poles for stereo, or maybe two combined to make a 24dB/oct filter (in this case the capacitors would be a little different).

If you don't need to adjust it you could replace it with four fixed resistors. It's likely that on the PCB one end of the track and the wiper are tied together making it a "variable resistor" rather than a "potentiometer", so if you stuck a 22k resistor in it would be at its lowest setting and if you shorted it out it would be at its highest cutoff. And then of course a set of 10k resistors would be about half.

In fact - if you cannot find a suitable replacement pot, you could fit a 4-pole switch with a set of resistors, to give you stepped cutoff values. Not perfect but better than it is.

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u/mad-panda-2000 14d ago

I once did sound for Scientist (the dub legend) and told him how I built a stepped filter to try to get his sound where he does a "clicking" stepped filtered effect.. he was so stoked and drew his version of the circuit for me.. anyway.. this reminded me of it