Xlr grounding

[EmilFrid]

Pretty straightforward question. If I had two live rooms that I wanted to use simultaneously, say channel 5 from live room A and channel 7 from room B, and instead of using a clunky XLR patch panel built a smaller panel consisting of 24 switches toggling between the rooms, would it be kosher to use a more accessible 2 pole switch (as opposed to 3 pole) and simply tie mic ground A and mic ground B together and then directly to the desk? I tried something similar in a temporary setup long time ago and didn't notice any problems, but there might still be pitfalls I'm not aware of.

 

[RSRecords]

Pretty sure you don’t want to be switching the cable shields (grounds) but I’ve never done this for a bunch of channels. Putting shields on a switch seems like the wrong move especially with a phantom mic.

 

[ccaudle]

Do you mean you want a panel that lets you select room A or room B for each channel of your mixing desk?
I think you should be able to get away with just switching the hot and cold legs from each, but be aware that you risk creating some transients that could be really hard on the front end of your preamps (and possibly the microphones as well) if you switch with phantom power enabled and the two legs to not make and break contact at exactly the same instant.

RF interference is also a possibility depending on how you tie the shields together. I think the best case would be all of the XLR connectors on the same side of an enclosure, with a highly conductive chassis connection for all of the pin 1 connections.

 

[ruffrecords]

A balanced signal only exists between the hot and the cold pins. The cable screen is just a screen. It does not carry signal so it is fine to connect them all together,

Cheers

Ian

 

[EmilFrid]

Thanks for the replies everyone!

@RSRecords Maybe I was unclear, or maybe I misread you, but the thing I  do want to switch is the balanced signal and keep the screens permanently tied together, screens from room A and B joining at one single point in the control room.

@ccaudle yes, RF interference is one thing I worried about as a possible pitfall. As channel count increases this (hypothetical) interference could become a nuisance. Your idea with a very conductive chassis sounds good. Perhaps a copper bar connected to the inside of such a chassis.
And I won't switch with phantom activated. Seems like a really bad thing to do.

@ruffrecords thanks Ian! Yes, I'm aware of the pinout. I was just worrying I might get a noise problem if the screen gets too long. I think we're talking an extra 20 meters. Maybe not a big deal. I'll follow your advice and go for it!

Thanks again everyone for your advice. I like this community a lot!

 

[GeoffDoane]

Why do you need switches at all? Or do you plan to leave microphones plugged in but not used in both rooms?

I've worked in studios that had multiple wall boxes in the main room and iso booths all just wired in parallel, daisy-chained from one panel to the next. As long as you don't have a crazy amount of wire on each channel (100+ meters), capacitance shouldn't be a problem. Needless to say, you shouldn't plug a mic into the same channel at more than one panel simultaneously, but even if you do, it won't damage anything except the engineer's ego.

To my mind, switches are an unnecessary failure point, both human (wrong position) or equipment (poor contact after years of use, or disuse). You're only dealing with mic level signals here, and switching line level in a console (with a mechanical switch) eventually gives problems.

Geoff

 

[EmilFrid]

@GeoffDoane I see. No, I never leave any mics plugged in after sessions are done. I agree with you about avoiding as many failure points as possible. However, you could also see the XLR panels eventually fail as well.
For the switches to be able to compete with a decent XLR panel I guess I'd need some quite sturdy ones though... thanks for your input. Made me think.

 

[pucho812]

Any reason you don’t want them on a patchbay?
Many a studio has been wired with live room feeds showing up on a patch bay with a full normal into the mic pre on the desk. It passes phantom without issue and allows you to patch around as needed.
About the only downside is patch with phantom turned on, can damage ribbon mics and give a nice pop.
Nothing turning phantom off as you patch cannot cure.

 

[GeoffDoane]

Except for the particular XLR input being used, all the others on each channel are merely tie points for the wires, so there shouldn't be any reliability problems. The picture below is of the back of a mic input panel, with 12 of an eventual 24 inputs wired. The shielded pairs come from the mic pre inputs on one 12-pair cable, and go out to the next box in line on the other. IIRC, the main room had 24 inputs, and each iso booth had 12, with a couple more daisy-chained off to some more XLRs in the ceiling of each booth (that was the client's request).



Patchbays are certainly convenient, but I've had trouble with signal integrity with full size jacks (B-gauge). XLR patchbays were reliable, but take up space, and they do add another connection point to each signal's path. Surprisingly (to me), the Moses+Mitchell bantam patchbay that came as part of the SSL 4000G+ I used to look after was reliable for the life of the console. Other than the jacks working loose in the frame, I had no signal problems with it.

Geoff

 

[EmilFrid]

@pucho812 I'm considering just using a patchbay for mics more and more. Might be the best solution after all, although @GeoffDoane has good points too. I'm not afraid that any of us would hot-patch anything. We always patch before anything else.

 

[Brian Roth]

A mic patching system using all XLRs is a good solution.

Folks complain "Ugh! takes up too much space!" Well yes and no in the real world.

The snapshot attached shows it can be relatively compact. Indeed, 1 RU space would hold a TT bay with two rows of 48 jacks (96 total). Nevertheless, it's not totally crazy in terms of rack space required.

Note: the snapshot was taken during build-out of a new control room and the labels for the jacks were not completed/attached to the panels.

Edit: found another snapshot showing the XLR mic patchbay in "context" of the entire setup.

Bri

 

[sodderboy]

All shields should be independent, so you would HAVE to switch shields as well because of phantom powering. You have to have a pure shield path for P48 use.
The most common mistake that I fix with programmed bays is shield busing. Very bad, don’t do it for mic or tie lines.
I would have no problem multing a second panel to the first, either at a patchbay with a “Y” or at the panel as previously pictured. Don’t be surprised if you get 2 mics plugged!
If a customer HAD to have switches I would either pass on the job or demand sealed relays- DPDTx2 for every channel with whatever cheesy switches they wanted firing the relays.
A quality programmed bay is a better path, and changes with the eqpt.
Mike

 

[ccaudle]

All shields should be independent, so you would HAVE to switch shields as well because of phantom powering.

Can you explain why you think that?

 

[sodderboy]

I can explain why I know that. Two reasons, actually, P48 and noise drainage.

What are the shields there for anyway? The main function is to drain EMF noise away from the signal wires inside. A balanced wiring configuration further rejects noise that makes it past the shield. There is EMF energy everywhere inducing noise into neighboring electronics. Once on the shield, the energy should have a distinct potential, read low impedance, towards which it is drained away from the signals we want, and it gets drained to "ground" or mother earth through the load's chassis. Connecting shields together at one point or multiple points along the path from source to load reduces the "distinct potential" path of noise drain, and the differing noise on the signal lines gets pooled together, to be passed along again on independent shields to the next joining or the load chassis connection, a high gain mic pre in our case.

I liken EMF noise draining in systems to rain draining off a roof. The better the pitch, the better the drain.

Enter condenser microphones are their need for external power, and the development of phantom power, P48. Now the signal shields have a second function as the return/ 0vl/ power ground for P48. You have two energies flowing in opposing directions on a condenser mic line. NOTE: I am speaking from a system design point of view using mainframe console and commercial outboard equipment, where P48 comes from the desk and/or multiple mic pre's in different chassis. Otherwise, P48 could come from local Stewart boxes, or a 8CH DIY P48 box, or no console and only DIY outboard, DAW, and a monitor controller, but the theory is the same. You do not want the P48 signals of different sources sharing their returns. This will induce noise in the mic signals and get amplified in the pre.

The functions of P48 power to one microphone, and the noise draining for that one mic line will be disrupted when you connect the shield together with another microphone signal shield. It will exhibit itself as increased noise floor of hiss and or hum. Scale up to 4, 8, 48 channels connected together and you can have a real mess.

The theoretical perfect studio has individual mic cables connecting the sources, mics, back to the loads, mic pres. A working studio needs more flexibility than that, so we use multipair, patchbays, multiple panels sharing one line, etc. That is all cool as long as the separation of shields is followed.

This also applies to the line level signals, but exceptions are voluminous and application specific.

Mike

 

[JohnRoberts]

While the audio exists between pins 2 and pin 3, phantom powered mics return current through pin 1. Ideally these pin 1s would be kept independent and clean but any corruption of pin 1 would be common mode so "mostly" ignored.

JR

 

[ccaudle]

Once on the shield, the energy should have a distinct potential, read low impedance, towards which it is drained away from the signals we want, and it gets drained to "ground" or mother earth through the load's chassis.

Since currents flow in loops, what is the source of electric potential of the EMF that would need to be returned to earth to complete the circuit?

What happens to circuits in a vehicle insulated from earth by rubber tires? Does the EMF build up inside like a sink with a plugged drain?
Is that why aircraft get the frame connected to a ground cable when they pull up to a terminal, to drain off the EMF which has filled up the cargo bay during flight?

 

[moamps]

In my career, I designed audio systems for several large theaters and concert halls, and in doing so, I applied the knowledge and conclusions I gained from working on smaller systems such as radio stations over a longer period of time. I have absolutely never connected pin 1 of XLR connectors together at a remote location (stage box, etc). And especially not to some kind of grounding. I have absolutely never connected shields from pairs of multicores coming from two or more consoles from different spaces together in some remote location.
Absolutely always one source from a remote location was connected with a cable whose shield was connected only to the input of the device for which it was intended (or to the local TT patch bay next to the console).
I never use switches for the microphone signal in such applications. Stage boxes are places with rather unregulated working conditions, from temperature, humidity, smoke, dust, etc. In such positions it is best to use XLR patch bays. Even the cheapest ones will provide better contact in the long run than the most expensive TT patchbays and especially switches. And especially if phantom power is used.
Several stage boxes can have connected XLRs in parallel if they are not very far apart as someone has shown in the picture, but these inputs can only go to one console or preamp at the same time. The metal panel should be grounded, but it should not be connected to the shield of the audio cables.
My 2 euro cents.

Finally, the idea that a shield is just a shield and that the audio signal is in no way affected by what's going on in the shield shows a misunderstanding of the actual operating conditions, IMO.

 

[sodderboy]

EMF is just there, like your neighbor cranking reggaeton. The sound pressure fluctuations from their speakers do not build up like your frustration or a sink with water. It’s just there. AC wiring, scr dimmers, wall warts, those cool white led bulbs, ext. and commercially you might have a 5kA three phaseAC riser behind that wall next to where you might put a console, or the building electrics room on the other side, all sources of EMF. And I am speaking to AC spectrum. RF is another world, especially near any tower, or I’ve had next door hotel wifi put a hurt on microphones in a location.
I always bring an analog AM radio with me on new jobs. It is my first stage EMF detector. A lot of time it is in a raw potential space in NYC where I need RF detection as well, measured differently.
Theaters, what a huge world of EMF noise!
Planes, aviation is really paranoid about anything electric.
Cars, search auto and AM. Manufacturers want to bag AM in vehicles. The electronics in modern cars make AM very difficult to manage reasonably, and the EV’s, AM is impossible in those EMF cookers. I remember the CB days when you had to put filters in-line with the spark plugs to filter the distributor noise! Didn’t really work for the engine OR the radio.
Mike

 

[nick_t]

We use a 19" XLR patch bay as per Brian's comment. Ideally situate it away from your other racks and main patch bay, and away from all power and line level cabling. Run all console and outboard mic pre inputs to that rack only.

I wouldn't parallel anything, I'd keep it all separate. That gives you the most flexibility to run signal back and forth between any of the rooms. You can also run another 8 channels from your main patch bay to the XLR patch bay, for running line level into mic pres during mixing, extra feeds to the live rooms etc.

 

[Matt Syson]

In the 'good old days' the main concerns were powerline frequency (or low harmonics) or (usually) AM transmitters nearly. Balanced wiring eliminiated most problems of powerline pickup although AM transmitters /Taxi radio!?) were occasionally a problem in specific locations. Now everyone is carrying a microwave transmitter in their pocket and switchmode supplies are ever present, it tests the old equipment designs like never before. Any conductor of a few inches (Centimetres) or more is a very handy receiving aerial so now to prevent interference spoiling your day you need to have effective filtering on gear inputs and outputs because balancing alone is not sufficient to prevent HF interference from being demodulated.