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Complaints of shocking mic

Discussion in 'Sound, Music, and Intercom' started by zac850, Feb 5, 2006.

  1. zac850

    zac850 Well-Known Member

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    This isn't really related to the other active discussion right now, so I'm creating a new thread.

    My school has a new guitar teacher who set-up a small sound system in one of the classrooms. He has complained that when he is playing his guitar, which is connected to his guitar amp, and he kisses the microphone, he gets a shock.

    Now, besides telling him he should step back from the mic, I would love to figure out why. I remember a discussion of this came up a year or two back, but I couldn't find it in the search.

    I doubt Phantom Power is on, but I will check that when I get to school and look the system over. I'm also not quite sure what kind of mic he is using, its an SM-58 type, but a cheeper knock-off of it I believe.

    I know somehow an electrical connection is being made between his guitar and the mic, but I have no idea how to stop it. Would putting both the amp and the sound system on the same circuit help?

    And the system was grounded, because he told me he lifted the grounds, which didn't help. Not that I would expect it to, but he gave it a shot anyway. And yes, I will re-ground the system when I go to help him.

    Thanks guys,
    Zac
     
  2. soundman1024

    soundman1024 Active Member

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    Sounds ground related to me. See that the console the mic is hooked up to has a good ground.
     
  3. zac850

    zac850 Well-Known Member

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    I'm almost sure it is. In passing the system, I saw there was a ground lift on the power supply, so I removed it, and unless he put it back, I am assuming it was grounded.

    Besides the grounding issue, is there anything else that could cause this?
     
  4. NickJacques

    NickJacques Member

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    I had almost this exact same issue.

    Mic: Shure 58
    Board: Soundcraft Live! (24ch) with Phantom on
    Instrument: Bass guitar

    During variety show rehearsals, the bassist kept complaining the mic was shocking him. The next day, the bassist actually got a welt on his lips from the shock. I suspect that his bass wasn't grounded properly, as he dropped the bass and cracked the entire body of it. I turned off Phantom power for his act and the shocking never happened again.
     
  5. mbenonis

    mbenonis Wireless Guy Administrator

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    Improper grounding is almost always the cause of problems like this. You are right to remove the ground lift - there is no reason to use such an unsafe device. Also, make sure the guitar amp is plugged into the same power strip or leg of the circuit as the sound system. If it's a large auditorium, you might even consider running an extension cord from the sound area to the stage for the amp to ensure there is no potential between the sound system's ground and the amp's ground. Finally, make sure your cables are all made correctly: all three wires should be attached to their respective pins, and not attached to anything else (i.e., the shell of the connector).

    Phantom Power should not cause any kind of shock, unless the microphone is poorly designed/broken or the cable has a wire cut in it. However, it's worth trying both ways - if it causes a problem, you know you need to get a new mic or cable.
     
    Last edited: Feb 6, 2006
  6. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    The cause is some stray voltage that is getting to into the signal. Either this is coming from the desk down into the mic or from the guitar amp into the guitar. Either way it is getting from the teacher into the power ground when he completes the circuit between the two circuits. I have never heard of phantom power causing it but I would turn it off just to be sure.

    If you have a DMM you can meter the instrument to ground and the mic to ground and check for voltage. If there is a detectable voltage, you will probably need to have it looked at by a service tech.

    However, I could be wrong so in addition to the above – check the following:

    * Is all of the sound equipment that is being used in this scenario grounded?

    I have seen people replace power cables with non-grounded ones because they thought that it would clean up the sound. Essentially it is a ground lift device (which should not only be removed, but also hit with a hammer). If the connection to ground is not there, then the person kissing the mic will act as the ground.

    * May sound like a stupid one – but ALL the gear is plugged into a grounded circuit isn’t it?

    * Does the amp have a ‘ground liftswitch on it?

    Ideally, all power ground and signal ground should be joined but whilst making things safe, it can introduce noise into the sound if the power is dirty. Some amps have a switch that lift/break this connection.

    * Does the guitar hum badly if the strings/bridge/pic ups are not being touched?

    If so, you have an earthing problem with the guitar and whilst this may not have anything to do with the problem here, it is annoying and should be rectified.

    If possible try different guitars and different amps and perhaps check with a DMM between the guitar strings and the mic grill rather than making him kiss the mic grill whilst playing. As tempting as it is to watch the facial contortions, he could get a serious shock and one that could kill him. Take this seriously.
     
  7. kingfisher1

    kingfisher1 Active Member

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    The venue is about the size of my living room. We've used this system before without a problem, but stuff may not have been on teh same circut. neither zac nor i were there to witness the shocks but my sister said that the teacher was bering shocked with his guitar, and without it
     
  8. zac850

    zac850 Well-Known Member

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    Ian's sister lied. :)

    The teacher gave me a call. He went out and got a circuit tester, and it turns out the circuit he plugged his amp into has a faulty ground. So for now we're pulling power from another circuit for his amp, and we'll eventually get an electrician in to fix the ground so we can use that circuit again.

    So, his amp was plugged into a ground, that ground just didn't work. Oh well.
     
  9. mbenonis

    mbenonis Wireless Guy Administrator

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    In the mean time, this receptacle should be clearly marked as functioning improperly so that nobody else plugs something in and expects the ground to work. Better yet, just tape it over with some Gaff tape so that it can't be used until it's fixed.
     
  10. Mayhem

    Mayhem Senior Team Emeritus Premium Member

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    Good point - if only people wouldn't peel off the gaff and use the receptacle after you have taped it up. Best bet would be to throw the breaker and await the services of a qualified electrician. Whilst the problem might be isolated to the receptacle, it could actually be at the grounding bus, meaning that every receptacle on that circuit could potentially be faulty.

    I would also get the electrician to check all ground connections whilst they are there. The problem with a ground is that you never know it is faulty until you either do a routine check or something goes wrong and someone gets a shock. Should the Hot or neutral fail then whatever is plugged in will not work and so the problem will be rectified.

    Just because a circuit has hot and neutral, it does not guarantee that the ground is good or even connected.
     

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