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Had an interesting thing happen the other day...

Discussion in 'Sound, Music, and Intercom' started by tenor_singer, May 23, 2006.

  1. tenor_singer

    tenor_singer Active Member

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    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.
     
  2. audioslavematt

    audioslavematt Active Member

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    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.
     
    Last edited: May 23, 2006
  3. jbeutt

    jbeutt Active Member

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    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.
     
  4. mbenonis

    mbenonis Wireless Guy Administrator

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    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).
     
  5. tenor_singer

    tenor_singer Active Member

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    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 :)
     
  6. cutlunch

    cutlunch Active Member

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    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
     
    Last edited: May 24, 2006
  7. kingfisher1

    kingfisher1 Active Member

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    Your Td was AOl? or AWOL

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

    MHSTech Active Member

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    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.
     
  9. Chris15

    Chris15 CBMod CB Mods Premium Member

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    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.
     
  10. jbeutt

    jbeutt Active Member

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    "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.
     
  11. Chris15

    Chris15 CBMod CB Mods Premium Member

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    I just read my post and it does appear to be somewhat confusing. The unbalanced cable is between the mic element and the preamp and in my experience is no more than about 3 metres (10'). After the preamp, you would be running balanced so that is unlikely to pick up the hum. Butt since it appears as though it was this thin cable from mic element to preamp that got moved, it is this unbalanced cable that could be the source of the problems.
     
  12. jbeutt

    jbeutt Active Member

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    gotcha.

    I don't work too much with choir mics. Why? Because they suck and in my experience there are always better options.

    Nonetheless, if Chris is right and this is the setup, then there's little question that it's that unbalanced cable picking up noise.

    The OP now just needs to confirm that this is the case, that he has an unbalanced cable, and either fix the situation or not put the cable near the interference. I like having things work proper, but in the end as long as the noise is gone, who really cares how it gets fixed?
     
  13. tenor_singer

    tenor_singer Active Member

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    I am not sure what type of cable was put in the auditeria. Everything is inside of conduit. I will try to answer any questions as best I can:

    1. The noise happens when the mic is attached to the pipe. The way we attached it was to gaff tape it to the pipe and let the element dangle about 3-m above the stage floor.

    2. I am not using any seperate cord for the microphone. It came with a 25' long wire which was long enough to plug it into the jack.

    Maybe this will help... there are times when different channels in our mixer randomly show signal but yet have no devices run through them. The installers keep telling me that the random noises like that are because the line cables for the sound booth were laid in the same trays as the internet cables, tv cables, phone cables, etc and are picking up some "noise" from them.

    My physics coursework has very little in the way of specific audio training. Most of my courses were mechanics in nature with very little electricity.

    I am going to start my masters next fall and am thinking hard about adding in some technical theater courses, because I have been sitting here in my own little world thinking I knew a lot about sound and lighting, and then I came here and realized that my knowledge is truly lacking. Sure I can run sound, design sound and such, but I have almost zero knowledge of the theory behind the technology (except for what I garnered from my entry level general physics courses taken over 16 years ago). For example... I have never heard of balanced versus unbalanced cables before this thread. It is truly humbling and boardering on depressing.
     
  14. jbeutt

    jbeutt Active Member

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    well, we've come to the point in this discussion where we need to know what specific equipment you're using. Mains power for a mixer should not be affected by internet lines. In fact, if anything, it's the other way around, although cat5 is another type of balanaced cable.

    About these channels with random noise, is there cable or a snake plugged into them? Gain all the way down? Now you're getting into an entirely different problem which could or could not be related to the current issue we've been discussing.

    I think your next step ought to be to meter the pole you've been attatching the mic to. I would haul up a regular, grounded and plugged in power cable to the pole, then meter across the pole and the cable's ground.
     
  15. cutlunch

    cutlunch Active Member

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    jbeutt I may have missed something but I don't remember there being any comment about the mains power for the mixer having interference on it. I thought we were talking about noise on channels that feed into the mixer.

    Tenor Singer about the noise being introduced into unused cable runs. On these channels what gain setting do you have them set at? Ie do you leave the gain set at the same level you use for microphones or is the gain set lower. When you mean show do you only see this noise on the Led's or can you hear it when you bring that channel up? I am wondering if this because on the unused cables the two balanced lines are not connected. So any noise generated by interference is not cancelled out so well at the mixer. Just a thought.
     
  16. jbeutt

    jbeutt Active Member

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    you're right, I just misinterpreted what he said:
    "line cables for the sound booth were laid in the same trays as the internet cables, tv cables, phone cables, etc and are picking up some "noise" from them."
     
  17. Chris15

    Chris15 CBMod CB Mods Premium Member

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    So, I'm assuming that the cable to the mic element is only a few mm thick (that'd be less than 1/8" for you non metric folk). I would then assume that there would be a small box, or even just an oversized XLR that contains the preamp, which will take the 48V phantom and convert it to a couple of volts bias voltage across the mic. It will also have a few bits to convert the signal from unbalanced to balanced and keep that nasty DC out of the signal.

    That is what my comments have been based on. It may be that you have something which is entirely different. You say that the sockets for these are on the ceiling. Are these a standard 3 pin XLR. I am thinking that since these were obviously installed for hanging mics, that the preamp might in fact be in the junction box. Otherwise, I'm not sure.

    As far as noise, it is entirely likely that interference is creeping in. Things like telephone cables would be worst when the phone is ringing. There may also be grounding issues with the cable trays themselves.

    And it is nice to know that some of the more experienced members here are able to get something out of this, not just us younger members.
     
  18. jbeutt

    jbeutt Active Member

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    it's definitely a doozy.
     
  19. tenor_singer

    tenor_singer Active Member

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    I have noticed that the the signal terminates when I back the gain back down. Usually the knob it set about 12:00. The signal terminates around 10:00 (I am using an analog clock analogy).

    As far as the pre-amp goes... the microphone has an element at the end of a long, thin wire. At one end is the microphone and at the other is a 3-pin XLR plug. I did not find any form of a pre-amp.

    My mixer is a Venice which is sending mono-feeds to three QSC amps of varying wattages, which in turn are run to one of three areas... House right fill speakers, house left fill speakers and our main wedge. The system is run through a processor that I cannot find information on (sorry). The installers are used to high schools where people aren't savy with sound equipment and have put some form of plate over it that I cannot remove. The company did not leave any manuals for things, so I cannot even look at the book for the name.

    Here's something else that may be of interest...

    The hanging mic ports (there are 6 of them of which we are using only 4) have to be patched via a 24 channel patch bay through our mixer. I tried different patches and the sound was still there when we were frantically trying to figure out our problem.

    Thanks for all of the help. I don't have a multi-meter and have ordered one through our school's supplier. I will post results when I get the equipment... which is hopefully soon as school is done in two days.
     
  20. Chris15

    Chris15 CBMod CB Mods Premium Member

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    Then obviously you are using a different type of hanging mic to what I have come across.

    But then again, you are a good distance away from me, and given the other thing which differ between here and the states, it doesn't particularly surprise me.
     

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