Adding expression jack for Tremolo speed

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Adding expression jack for Tremolo speed

Postby Faldoe » Wed Jul 19, 2017 1:43 pm

I have a Demeter Tremulator clone that I built. Speed pot is 500K Lin.

How would I wire an expression jack so that when the exp pedal isn't plugged it, the speed control works as normal?

I can't just wire the expression jack parallel with the stock pot, right? Cause if I do the the exp pedal won't do anything to alter the values cause the stock pot will still be connected

Are there plus switching jacks that also have a make/break connection lug?

I was thinking of wiring the stock pot's wiper to the jack so when the exp pedal is connected, the stock pot wiper is disconnected​ and when the exp pedal is unplugged the stock pot wiper is reconnected.

Make sense?

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Re: Adding expression jack for Tremolo speed

Postby crochambeau » Wed Jul 19, 2017 2:15 pm

Faldoe wrote:Are there plus switching jacks that also have a make/break connection lug?

Yes. It so happens I've been looking into installing an expression jack that will take priority over the panel mount frequency control on the RMA Monobius (how's that for back door spamming? Sorry!). The criteria for this working is that one leg of the potentiometer being replaced MUST connect to ground, as the shaft connection of the jack is not switched, and is generally mechanically fastened to the chassis.

Switchcraft 114BX is a TRS female 1/4" jack that has normalling (switch closed until plug is inserted) contacts on both the tip and ring. You simply wire your "high" and wiper legs of the pot through the switched contacts so the pot is taken out of circuit when an external pot is plugged in.

If you have a circuit node at "low" instead of ground I've seen plastic case switching jacks that will interrupt the shaft as well, but I have no part number handy.
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Re: Adding expression jack for Tremolo speed

Postby Dr Satan » Fri Jul 21, 2017 1:39 pm This and what Crochambeau said. If your EXP pedal doesn't have a switch to reverse the outer lugs of the pot, you may want to mod it too as it is common for LFO speed controls to have a reverse log taper.
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Re: Adding expression jack for Tremolo speed

Postby ianmarks » Sun Jul 23, 2017 12:27 pm

Here is a drawing for reference. Jsut be sure to check the datasheet for whatever jack you buy so you know what lugs switch when plugged or unplugged. ... xterne.gif

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