Had an interesting thing happen the other day...

[tenor_singer]

Hi everyone...

I was setting up our stage for the last choir concert of the year. When they built our stage, they put in hanging microphone jacks on the ceiling.... three down and three up. The sound package that we got included four hanging microphones. They hung two down and two up. The problem... the four microphones were hung literally 1' in from each wing, leaving the entire center of the stage uncovered by the mics' patterns. I decided to move them in more to better mic the choir. Our TD was AOL, so I had our stage manager, who is more of a "jack of all trades" type of individual, go up in the lift and uncoil the zip-tied extra of each microphone and move them towards the center. He did so to one and we had an awful ground hum. Here is what he did...

He wrapped the microphone around one of our teaser batons (which are just schedule 40 pipe). I tried everything and still the hum was terrible (you couldn't hear anything other than it). Thinking... "GREAT... the concert is in a half an hour... what can I do?"... I remembered comments made in earlier posts about possible ground loops causing this hum. I told him to take the microphone off of the baton and use the conduit that was run to the mic jack to hang it because it was probably grounded whilst the schedule 40 wasn't. He did and the hum was gone.

Thanks again to the brilliant minds at CB.

[audioslavematt]

The fact that having a cable in close proximity of a pipe that had no current flowing through it caused a ground hum is kind of disturbing to me. What kind of mic cables do you have? Are they balanced? Do they have a good shield in them? You may want to buy/make come cables with good shielding and pins matched appropriately. Hanging a cable from a pipe should not cause a hum like that.

[jbeutt]

This isn't a grounding problem. What most likely happened, as matt suggested, was that when you moved the mic/cable, you put it in proximity to some interference like a power line. What you probably have is a pin 1 problem and an unbalanced cable. Cables don't ground themselves through their rubber casing.
Again, as matt said, check that you're actually using balanced microphone cable with proper shielding.

If for some reason it were a grounding issue, the way to check would be to recreate the situation where you got humming, then put an iso-transformer in line with the mic.

[mbenonis]

In addition to what jbuett has said, I would check the mic line's XLR connector to make sure it is wired correctly - it is possible that perhaps the shield has come loose from it's solder joint and thus is acting as an antenna (and thereby inducing current into the conductors).

[tenor_singer]

The microphone in question has an incredibly thin wire (maybe 2 mm in diameter). Picture a metal junction box drilled into the ceiling at stage right with a 3-pin jack wired in the center connected by about 15' of electric conduit to the center box of similar design which is in turn connected to another box at stage left by 15' of conduit. Then there is conduit run from there to the wall where the wire is then pulled through the wall and laid on a cable try with about 300 other wires which is run to our booth (our school has "cable trays" where all internet, phone, speaker, etc. wires are laid. The wires are branched off at certain intervals and run on other trays to the differing parts of the building. Since our stage is in line with our vocational rooms, all wiring from our tech center, in addition to our stage wiring, gymnasium wiring, etc... run past our stage... about 100,000 feet of different wires). The microphones at SR and SL came with a 15 - 20' wire. The extra wire was coiled and zip tied in a 4" circle to the conduit allowing the microphone to dangle above the stage. The baton I am refering to is only a half foot away at best, but is chained to the metal girders that run the entire length of the auditeria.

I am confused as to why it would stop when I switched it from the baton to the conduit which was only 6" away? Is the sheathing of the mic an issue? I know that rubber insulates, but have also been in lab situations where electricity of a strong enough current passed through rubber... ie... an electrical line worker doesn't use rubber kitchen gloves when handling the 13 kV lines. But then again... there is absolutely no power source near that area. The closest is the transformer that is in our construction shop a good 20 feet away on the other side of a brick wall. I even went back and duplicated the problem last night. I hung the mic from the baton again and I got the same hum. The led on our board lit up instantly with very little gain (my gain knob was at about 8:00... analog time simile).

I don't know enough about wiring XLR cables and such, so when you mention a pin 1 problem I am not sure what you are meaning. I am assuming that pins 1 - 3 are your hot, neutral and ground, but I don't know which is which. Is this an issue with the mic itself or with the wiring that has been run from the booth? I am shipping some microphones that were damaged by our choir back to their respective companies for summer maintenance (school is over is six days here) and will send that one along too if it is an issue with it.

I am a bit embarassed at being so wrong with my ground theory. I brought up ground because the hum sounded exactly the same as the times when I had to lift the ground on other devices.

thanks for the help

[cutlunch]

First off. Here is a link to a Rane application note on the proper wiring of cables. At the moment we are interested the balanced to balanced connection digrams which is number 1 in the list of connectors. NB they show that the shield is only connected at one end. This is a common practice to help prevent ground loops.
But your cable may have it connected at both ends.

What I would do to test the cable is this:

Disconnect the microphone from the XLR it plugs into in the ceiling and disconnect the cable where it plugs into the mixing desk. Then take a multimeter set to the ohms range or continuity (beeps when probes touch together). Then see if pin 1 is connected to either pin 2 or pin 3. (If it is then the meter will beep or show less than 20 ohms on the meter).

If the meter shows that pin 1 - pin 2 or pin 1 - pin 3 are connected then this means the cables have been wired as unbalanced which would allow for the hum to get in.

Next do this test on the microphone itself if pin 1 - pin 2 or pin 1 - pin 3 are connected then the microphone itself is unbalanced and susceptible to hum.

Could you please post the make and model of microphone so we can look at the specs to see if it is a balanced or unbalanced.

From what you have said in your latest post I can even tell where I think the inteference is coming from. There is almost definetly an unbalanced cable in the microphone run. I would probably say the cable that is attached to the microphone itself is unbalanced or damaged.

You said the baton is schedule 40 so I am taking that to be a metal pipe. Next you talk about the baton being chained to steel girders. I am taking it that the chain is metal not plastic coated metal chain. If this is the case my theory is that there is a small current flowing through the pipe from one beam to the other beam. Power cables placed on or close to a beam may be inducing a current in the beam and if it is not earthed correctly it may have a slightly higher potenial then the beam at the other end causing a path for the current to flow.

You could prove my theory by temporarily disconnecting the baton from the chain by using bits of rope then set the microphone up exactly the same as now when you get the hum. If the hum is gone then my theory is correct .

If you haven't already I would get a copy of the sound installation manual from the school files. This should have copies of all the manuals for all your equipment. But more importantly it should have all the wiring digrams for the system which will help you fault find in the future. But also may show you things about your system you don't know such as hidden connectors etc..

Hope this helps

[kingfisher1]

Your Td was AOl? or AWOL

(sorry to be a typo snob, just that was gave it a whole different sound to your post)

[MHSTech]

Absent with leave or without leave, the TD was gone. Why does this matter?

To the question at hand... I'm assuming this is just one microphone in question, but just in case, is it one mic or all 4 that were doing this? If it was just doing it in one location, move the mic to a different location and tie it to the same batton in question. If it follows the microphone, try a different mic in the same location where you got the hum under the same conditions. If you still get the hum, then you know that it's not a mic issue, but an issue with possibly how it's wired.

[Chris15]

It is my understanding that most hanging mics have a preamp and the actual mic. In most cases, the run between the mic and the preamp is unbalanced and this does not usually cause problems since the cable length is quite short. After the preamp, it would normally run balanced, so that would probably be the unshielded cable that others referred to as causing the hum.

[jbeutt]

"After the preamp, it would normally run balanced, so that would probably be the unshielded cable that others referred to as causing the hum."

That doesn't make any sense. If it's balanced (truly balanced, not just a 2-conductor cable), it's rejecting noise. Did he say it's unshielded? Balanced cables aren't universally unshielded.

Tenor - microphone cables don't work exactly like power cables. The audio circuit in a balanced system is comprised of a + signal and a - signal. These are the same, but with opposite polarities. The shield is the ground, but that doesn't really matter. These two, inverted signals are compared at the end, usually the mixer and whatever is the Same on both is rejected. This would be the noise that's picked up on the cable from the mic to the mixer.

Now you should see why this isn't a grounding issue. The shield is not part of the audio circuit.

Of course, grounding and ground loops are big issues in audio, but for a different reason. If your system is not all powered from the same source, or at one point there's a leak to ground, you most likely have different potentials to ground at these different points in your system.

Now, if your wiring is bad or you just aren't using a balanced cable, then you'd pick up interference. This is where electrical signals are leaking onto your mic cable and because it isn't truly balanced, it can't reject these.

Let's also keep in mind that a balanced cable is not a cure all and a shield is still required to keep noise from getting in. This is because the interference out there probably isn't going to hit both conductors (those + and - i talked about) equally. Therefor, the mixer doesn't reject all the noise. To deal with THIS problem, conductors are twisted in an attempt to make sure they see any interference equally. This still didn't work to somebody's liking, so they came up with 4-conductor or star-quad cable. This basically incorporates two + and two - cables twisted together so that they'll see any noise even MORE equally than the 2 conductor cable.

This is probably wayyyy more info than you needed, but maybe it'll give you a better idea of the situation. Maybe you mentioned this already, but does it only make the noise when the cable and pipe actually touch?

Also, please somebody correct or add to what i've said. It's late and i'm not all here.