Mute Switch

[BigRedX]

I'm making a mute pedal. When it's engaged is it best that it makes an open or a short circuit?

And why? (Just because I like to know the reasons for these things)

Thanks!

[flyfisher]

An open circuit will make noise pick-up more likely. A short circuit will minimise/eliminate this. Although it matters less with a balanced cable.

One consideration is what the amp will make of a shorted cable? I'm sure amp inputs should be protected against being shorted but if you connect to a wide range of devices then it might be safer to include a resistor is the circuit such that, when muted, the guitar is disconnected and a resistor is connected across the amp input.

Keep the resistor value low-ish to minimise noise pick-up within the cable. 1K should do it.

[BigRedX]

It's not actually for a bass, but for my Theremin which doesn't have a standard output volume control, but just a volume sensitivity control which adjusts the distance you need to move your hand from the volume arial to get maximum output. We need a way of silencing this at the more "intimate" gigs we play where audience members are likely to set the thing off. Also the signal path is:
Theremin>Mute Switch>DI Box>Balanced line to PA.

Thinking about it short circuit will probably be the way to go. Why do I need a resistor? Surely connecting the signal live to the ground is the same as turning the volume to zero on a guitar or bass?

[flyfisher]

If you're happy to short-circuit the input of your DI box then you don't need a resistor.

I'm not sure how every guitar pot is wired, but if the wiper goes to the pickup and the 'ends' are connected across the jack socket (i.e. across the amp inputs), then turning the volume to zero is not the same as shorting the amp input.

[flyfisher]

If you're happy to short-circuit the input of your DI box then you don't need a resistor.

I'm not sure how every guitar pot is wired, but if the wiper goes to the pickup and the 'ends' are connected across the jack socket (i.e. across the amp inputs), then turning the volume to zero is not the same as shorting the amp input.


Edit: Just checked the wiring for a P-bass and the signal output is connected to the volume pot wiper so, yes, in this case, when the volume is zero the output is effectively connected to ground and a resistor is not necessary (although it wouldn't do any harm).

Edited August 12, 2010 by flyfisher

[BigRedX]

This is exactly why I need to "talk" this through with someone.

You're quite right. Setting the volume on a guitar or bass to zero connects both sides of the pickup to earth and keeps the maximum resistance of the volume pot between the live and earth on the amp input side.

Off to buy a 1k resistor unless anyone has a better idea.

[flyfisher]

Oops - looks like I was editing my last post as you posted yours. Your description of wiring a guitar was the one I was thinking of when I suggested a resistor, but it seems that's not the way a P-bass is wired. All of which is the reason for suggesting a resistor in the first place.

It seems a pity to have to go out just to buy one resistor though. PM me if you can't find one and I'll send you a few.

[BigRedX]

Thanks!

However, I need it today, for a rehearsal for tomorrow's gig, so nipping out over lunch to Maplins will be the quickest way.

[flyfisher]

No worries. Good luck with it.

Edited August 12, 2010 by flyfisher