Bypass box for balanced signal

[ckeene]

Hello,

I'd like to make a bypass box for an XLR balanced signal. It would be like a stompbox switch where, when engaged, it would route through a loop (also balanced) and when disengaged, would pass straight through.

As I know, you need a DPDT switch to do this with an unbalanced signal, and a 3PDT if you want to put an LED in, too. SO, for a balanced signal, I want a 4PDT (5PDT for LED), correct?

To me, this seems like a job for a relay. Has anyone had to do anything like this, and if so, are there any particular components you'd recommend? I don't have experience working with relays.

Thanks,

Chris Keene

[nclayton]

What's the nature of the incoming signal? Most balanced sources can drive two normal loads without too much trouble. Depending on the setup, potentially you could have the box always sending the signal out to your loop and then just switch the box's output back and forth from the return to the through. You'd only need a DPDT switch for that.

Ned

[ckeene]

It's for a mic signal. It's for a live switcher box that a singer can kick in a little blue tube or whatever mic pre, and then feed the signal back into the stage XLR box. It's to get some crunch on the vox without having to make the soundman have to do anything different.

[rodabod]

You can probably leave the grounds permanantly connected as long as no ground loops arise.

A DPDT will allow you to switch the hot and cold either to your loop, or straight to your output socket. However, I'm not too sure about having the return from the loop permanently hanging on the output jack too - it would mean that your mic signal would be driving into it, even when the loop was "bypassed". Therefore, it would be best if the loop was completely switched out when in bypass mode - ie. the switch would disconnect the loop output from the output socket.

Driving the two mic inputs constantly in parallel is not a good idea. It would cause loading of the mic.

[nclayton]

Driving the two mic inputs constantly in parallel is not a good idea. It would cause loading of the mic.

If it's for a live setup, chances are the mixer input will be transformerless and at least a few K ohms, so parallelling it with a different mic preamp input probably won't actually be that detrimental to the sound...although technically of course it will load the mic harder than it otherwise might be....still, most mics have pretty low source impedance and sound acceptable into a pretty wide variety of loads. I've parallelled mics into two pre's in studio quite often and only rarely noticed an unacceptable difference in sound as a result.

I'm thinking the main nice thing about what I proposed compared to switching one leg only is that the pair going out to the permanently attached auxilliary pre would still be humbucking. If you just switch a single wire on the in and out using a DPDT then you've got two long hanging wires attached to one leg of the balanced signal just sitting there like antennas. The wires will be shielded of course, but you'll lose a lot of the advantage of common mode rejection.

Probably the very ideal best way to do this is to use a 4PDT switch, but if you want to use a DPDT I still think driving both preamp inputs and just switching the output is the way to go. I honestly think you'd be perfectly happy with the results, but, you know...I'm kind of dumb.



BTW, you probably already thought of this, but unless your external mic pre has a transformer balanced output you'll want to be SURE the sound man has phantom turned off on the channel you're using.


Wait....so are you thinking you want to have two XLR's coming out of your box, one for the regular mic channel and one for the switched in preamp? That is, is the output of the preamp going to go into the mic channel or do you want it to go into its own special line level channel? Cause if you have two outputs you can just leave both of them always attached and just switch the input between them with a DPDT.

Ned

[ckeene]

Ned,

Thank you for the advice. The idea on the box is to have
1 XLR in (from stage mic)
1 XLR out (to stage box)
Balanced Send/Return to outboard pre (used onstage, sort of like a stompbox)

The single out would be going to one mic input on the board, to make things minimally disruptive for the live engineer, who'd probably me mixing 3-4 bands each night.

The outboard pre would likely need to be padded back down before returning to the splitter box.

So what you're saying is put a DPDT on the back end of the box? This will result in the mic "seeing" the load from the effects loop at all times, even when disengaged, but it shouldn't really be an issue - am I picking up what you're saying?

-Chris

[nclayton]

Ned,


So what you're saying is put a DPDT on the back end of the box? This will result in the mic "seeing" the load from the effects loop at all times, even when disengaged, but it shouldn't really be an issue - am I picking up what you're saying?

-Chris

That's what I was saying, and I think that it's even true. If you've got a normal dynamic vocal mic spec'ed like 150 or 200 ohms and you're driving a transformer input tube pre on the loop and a non-inductive 2-5K ohm load in the mixer I don't see this as being a serious kind of problem as far as mic loading. As long as you don't run into an "exotic" live mixer with unusually low impedance mic pre's I think you'll be fine. Even if the mixer has lower impedance inputs the worst that will happen is the mic will get a little pinched in the highs and lows, maybe noticeably, maybe not. Maybe Brian or somebody might chime in and either confirm or deny what I'm saying here.

You can find 4PDT toggles and rotary switches pretty easily. I think you're right that you're not going to find a footswitch pushbutton with that many poles. I don't remember ever seeing one. If the only alternative is to wire a relay and then deal with powering it and everything else, I think the DPDT on output only is a good alternative.

A third alternative would be to use a 3PDT where two poles switch the output and one pole breaks just one of the two connections to the unused input. That would eliminate loading, but leave you stranded as far as an LED goes. It also would have potential hum rejection problems and might do bad things with phantom power if present. So never mind this whole last paragraph.

Ned

[ckeene]

Awesome. Thanks for the great replies. I think I'll go ahead with the 3PDT (with LED) and just switch outputs.


As an academic question, would a 4-pole relay work reasonably well for this? I'm still sort of interested in trying out a relay, just for sheer curiosity.

-ck

[0-it-hz]

Not to be an ass, but... This sounds like it could be a nightmare for your FOH mixer as far as feed-back and levels are concerned.

A box with seperate outputs for each signal makes me much less nervous as an engineer because I know I can EQ an problems out of a crunchy mic pre effect without carving up the dry vocal signal.

Just my two cents.

[ckeene]

Not to be an ass, but... This sounds like it could be a nightmare for your FOH mixer as far as feed-back and levels are concerned.

A box with seperate outputs for each signal makes me much less nervous as an engineer because I know I can EQ an problems out of a crunchy mic pre effect without carving up the dry vocal signal.

Just my two cents.

This is a good point, and this project is actually for a friend's band. This guy is a professional FoH engineer, so he definitely knows both sides of the fence and I think wants overwhelmingly to be able to not have to give engineers at other clubs a headache managing 2 separate vox feeds.

I guess this is where having a 4PDT switch would be helpful, because you could either return back into the box, OR just give the club a second feed, directly off the "crunchy" outboard pre.

Hmm

[nclayton]

I've got basically nothing to do all day today other than sit around and wait for the jazz to play.

How about this?



Now the DPDT is on the input, but it's not a huge advantage loading-wise since in the "through" setting it has to drive pad's shunt resistor. Still...shouldn't be a problem. If you're worried about that, you can scale the pad's values up, but that will make it noisier.

In the through mode, the pad protects the mic from the external pre's output and vice versa.

In the loop mode you'll have signal at both line and mic level outputs. The mic output will be down about 40dB with these values.

Ned

[brianroth]

I basically see no problem with "multing" the mic into the "through" pathway as well as the input of the "auxiliary" mic preamp.

That is the situation you have when using a passive or 1:1:1 transfomer splitter to simultaneously feed a FOH board and an onstage monitor board...a jillion PA systems have been doing that for years.

In theory, multing a mic into two preamps will alter the mic's response in some form or fashion, but it is reasonably minimal in *this* application. The PA guys really have had no other logical alternative when needing to feed a mic signal into two boards.

After you "mult" the mic into the "aux" mic preamp, then a DPDT switch can select between the "through" signal (which is 1/2 of the mic mult) or the padded output signal from the "aux" preamp.

I can see some possible problems if the mic requires phantom power...to make this sensible, stick with a dynamic or self-powered condensor mic.

A relay is a possibility...used a sealed relay module with contacts rated for low level/low current ...typically bicurcated contacts. These sorts of relays are used in telecom gear for switching low level, "dry" signals. But the relay will require a power source....but so will the indicator LED! <g>

Bri

[ckeene]

Thanks, Ned, for the diagram. I like this idea and will probably run with it. Having the pad built in is really helpful, too.

[nclayton]

Whoops, forgot something. In addition to the DPDT footswitch you probably want a little DPDT toggle to disconnect the loop return from the mic output when you're use a separate line channel. Probably doesn't help to use a separate line channel if the "looped" signal is still coming through the mic channel.

[ckeene]

Whoops, forgot something. In addition to the DPDT footswitch you probably want a little DPDT toggle to disconnect the loop return from the mic output when you're use a separate line channel. Probably doesn't help to use a separate line channel if the "looped" signal is still coming through the mic channel.

imagine how sweet it would be to have a relay kick in and do this when you plugged in a 1/4" line level cable?