Stereo XLR Killswitch Pedal

[CamH132]

My live guitar rig has the the guitar being split into a Line 6 HD500 and a Sansamp VT Bass DI, from there the unbalanced outputs from each of these pedals go into an interface / laptop for additional processing before being sent via DI to FOH.

I'd like to have a backup sorted so that if the computer crashes I can bypass the whole computer setup and send the output of the pedals straight to FOH. Since each of the pedals also have their own balanced DI outputs, I'm able to plug them straight into the stage box and run them directly to FOH. I'd like to have a mute / killswitch pedal between the DIs and the stage box so that I can keep these two direct DI signals muted and have the ability to easily unmute them from my pedal board if the computer crashes and i lose signal through the main signal path.

I've read that the best way to mute an XLR signal is to just short out pins 2 and 3 together. I quickly made up this little diagram but to be honest I have no idea if it would actually work so I'm wondering if you guys were just able to check it and possibly lead me in the right direction if it is wrong...

[CamH132]

anyone?

[ashcat_lt]

Assuming that the contacts on that switch go from side-to-side, it should work okay.  I kind of don't love it, though, because you're shorting the low-z outputs of whatever is driving it.  If those are not somehow current limited, there could be damage.  I'd rather have you break the connection and just short the receiver (what you've got labeled as out - which goes to the input of the other thing) side, but to do it properly would need another pole.  I think you could get away with switching the receiver side of one of the signal wires (from each) between the transmitter side of the same wire and the middle of the other wire because then there's infinite resistance between the two low-z outs, but the inputs (which shouldn't be sourcing any current at all) are still shorted for silence.

(Sometimes XLR inputs actually do source current - as phantom power - but usually the voltage on 2 and 3 are the same, so no current flows from shorting 2 to 3.  If you short either of them to 1, though... )

[thermionix]

What is FOH?

Why do you plug a guitar into a computer?

I don't know how your XLRs are wired, but assuming pin 3 is ground, shorting to ground is a good way to mute a signal.  It might be better to short 1 and 2 to ground simultaneously, but that may be unnecessary.  To do it as you have illustrated you would only need a DPST switch.

[ashcat_lt]

What is FOH?

Front of House.  You might call it the PA.  Technically, FOH is the speakers that point at the audience - not to be confused with the monitor mix - but unless you're playing stadiums, it's usually all the same thing, and you'd probably want this stuff going both places anyway.

Why do you plug a guitar into a computer?

Because it's 2016!

I don't know how your XLRs are wired, but assuming pin 3 is ground, shorting to ground is a good way to mute a signal.  It might be better to short 1 and 2 to ground simultaneously, but that may be unnecessary.  To do it as you have illustrated you would only need a DPST switch.

Most XLRs are pin 1 ground, with pin 2 and 3 carrying the signal (one is inverted compared to the other).  Shorting either 2 or 3 to ground only kills half the signal (unless it blows up whatever is amplifier stage is actually driving it) and worse yet, it could connect the 48V phantom power riding on both 2 and 3 to the 0V on 1, and that ain't gonna be cool! 

I answered the question.

[PRR]

XLR can be "balanced", two hots.

Shorting is fine for guitar and weaker pedals, dubious on stronger outputs *such as* good balanced line drivers.

You want to dis-connect the source (so it idles unloaded) and short the load (to cut the buzz you'll get on a long open wire).

This needs a DPDT switch.

If the feed is "stereo" (or dual mono), then it needs to be 4PDT.

You will want an indicator, to reduce the go-nuts when signal is not getting through. Toggle and rotary switches can be self-indicating (look at the lever/knob). The stomp switch implied in the sketch looks the same either way. This leads to an LED and a  5PDT(!) and some power.

Pulling the plugs is often sufficient, less wire-work, and reasonably self-indicating (unless both plugs and sockets are hidden deep in a rack).

[thermionix]

Because it's 2016!

And we have the internet!  To order tubes, transformers and Alnico speakers with!  Ha!

(Just messing around)

Yeah, like I said I didn't know how OP's XLRs were wired, and wouldn't expect phantom power on a guitar signal.  Unless it's a "cue stick" with a capacitor microphone in it.

[ashcat_lt]

Yeah, like I said I didn't know how OP's XLRs were wired, and wouldn't expect phantom power on a guitar signal.  Unless it's a "cue stick" with a capacitor microphone in it.

Well, no the phantom power isn't coming from the guitar.  It comes from the FOH, and who knows WTF that dude behind the mixer is trying to do?  Some mixers have global phantom, or phantom for some subset of the inputs, and if there's a single condenser mic in the mix, then all the XLRs are going to be hot.

PRR - I still think we only need to disconnect one of the two signal wires from the source, so it should be doable on a 3PDT.  Do you disagree?  What am I missing there?

[PRR]

> only need to disconnect one of the two signal wires

"Balanced" is un-defined.

It can be transformer floating. (One cut suffices.)

It can be center-tapped transformer. (Two cuts needed.)

It can be "impedance balanced" (unbalanced signal with dummy side.) One cut would suffice; but we can't know which is the "live" side.

There are huge differences in how the receivers handle a one-side-cut "balanced". And how they handle a one-cut one-short situation. (Not all will reject one-side signal when shorted.)

In a *specific* situation, you can experiment to find the minimal answer for that situation.

Guys who say "FOH" will probably face a different FOH rig every week, thus different interface details. Even if you hire one company for a 6-month tour, they may switcharoo the consoles because of failure, upgrade, or a bigger customer needs the board you were using.

[ashcat_lt]

Ok I dig that.  Better to be safe.

I personally think that if the OP is that worried about his computer failing, he should upgrade or clean it up or whatever is necessary to be more confident.  My entire band goes through my computer - just hand the FOH a single balanced and tell them "turn it up and blame me" - for the last couple years and have never had a problem.  It's at least as reliable as a tube amp or DIY pedal.

Leads to the question though of what might happen if the interface output somehow ends up seeing 48V.  I haven't had an issue yet, but I kind of think I should get myself a 1:1 transformer....

[thermionix]

I think a lot of microphones that don't want to see phantom power use a blocking cap.

[CamH132]

I personally think that if the OP is that worried about his computer failing, he should upgrade or clean it up or whatever is necessary to be more confident.  My entire band goes through my computer - just hand the FOH a single balanced and tell them "turn it up and blame me" - for the last couple years and have never had a problem.  It's at least as reliable as a tube amp or DIY pedal.

I'm fairly confident in the durability of my computer - it's a fairly recent macbook pro with pretty close to the highest specs you can get. The only reason i'm looking for a backup plan is because we're a two piece band running all synths / guitar / bass through the computer and if some freak crash was to occur we don't just automatically lose everything except for drums.

At this stage I'm thinking maybe PRR's suggestion of just pulling the plugs if a crash happens should do the job well enough and then maybe later down the line I might just wire up individual 3pdt switches for each signal path if I feel a better backup plan is necessary.

[ashcat_lt]

I think a lot of microphones that don't want to see phantom power use a blocking cap.

I think a lot of dynamic mics are transformer coupled, so it's a non-issue.  I'm also pretty sure my interface has blocking caps on the outputs, but are they rated for 48V?  I suppose they have to be able to pass 26V p2p audio signals...

If my computer crashes during a show and won't come back up, the show is over.  Most of the time the crowd would appreciate that anyway, so...