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hum question

Discussion in 'Amps and Cabs [BG]' started by frankosaurus, Jul 14, 2002.


  1. frankosaurus

    frankosaurus

    Feb 27, 2002
    San Jose
    A while ago I was at a gig and we heard a hum coming out of the bass channel. I was running an XLR out the back of my eden nemesis 2X10 (great amp btw) to the sound board.

    First we tried flipping the ground / lift switch on the amp. that didn't help so much, so then the sound guy did something to the power plug on my amp. I'm not sure what he did -- I think he might have put a 3 to 2 prong adapter on it. Anyways, whatever he did it worked.

    I had a gig tonight, with a different guy doing sound, and he didn't know what to do, so we had to deal with the hum :( I wanted to tell him what the other guy did but I wasn't sure.

    2 questions:

    1) What is the difference between the ground and lift switch positions (let me know if this one's been asked before)?

    2) What did the first sound guy do to fix the hum? I would really like to know to prevent this from happening again.

    Thanks very much!

    --phrank
     
  2. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
    frank:
    You had a ground loop problem. In simple terms, the chassis ground of the P.A. mixer and your bass amp were at different potentials, meaning there was actually a voltage across the two grounds. The hum was created by that voltage.
    This often happens when the sound guy doesn't do a power tie-in, or the venue doesn't have a good onstage electrical wiring system. Things get plugged into receptacles that may not be grounded to the same location. The ground lift on your amp won't help much in this case, as you found out.
    To try to answer your questions:
    1)It opens the chassis ground going out of the DI in the "lift" position. (It doesn't lift the chassis ground at the receptacle.)
    2)He almost certainly put in a 3 to 2 prong adapter which did lift the chassis ground at the receptacle.
    Hope this helps. It can be confusing.
     
  3. Nick man

    Nick man

    Apr 7, 2002
    Tampa Bay
    Ive been shocked by mics and guitars that are plugged into amps or PAs that have had the ground prong detached. It was only a slight shock every time, but it can make your lip numb after a while.

    Peace
    Nick
     
  4. frankosaurus

    frankosaurus

    Feb 27, 2002
    San Jose
    thanks a lot for the replies. I guess I may need to carry a 3 to 2 prong adapter with me for the crappy venues. Hopefully I won't get shocked-- well I don't sing into a mic so maybe I am safer than the lead singer!

    --Frank
     
  5. Ty McNeely

    Ty McNeely

    Mar 27, 2000
    TX

    Yeah, but then you'll just get a nifty little shock every time you touch a string...kinda like the one you get when you touch the guitarists strings:D

    Not really, but it'd be funny:D
     
  6. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    There's a reason the third prong is called a safety ground. It's there to shunt any major electrical faults within the amp or other gear to ground so that the circuit breaker or fuse opens. Without it, you're the potential route to ground, and you might not conduct enough current to open the circuit, so you'd just sizzle in your own juices for a while until someone catches on and is wise enough to shut off the AC.

    Anyone using a 3 to 2 adaptor and not attaching the ground connector to and actual ground should get a hard kick in the ass, because nearest to his or her brain.

    Instead of lifting a safety ground, which is stupid, take care of the ground loop instead.

    To learn about finding/eliminating/avoiding ground loops, read http://www.rane.com/pdf/note110.pdf
     
  7. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    BTW, the correct--that is, safe--way to correct the ground loop in the situation you described would've been to disconnect the shield at one end of the XLR cable. Probably within the male XLR connector.

    Forget the 3-to-2 prong adapter. Get a short balanced XLR cable, like maybe a foot or two long. Open the male XLR connector and unsolder the connection from the shield to pin 1. Solder a small capacitor, about 0.01 to 0.1 µF, at least 150 V between the shield and pin 1 instead. A ceramic disk capacitor should work well. Close up the XLR connector and clearly label the cable as a GROUND LIFT.

    Then, if you ever get hum from running a DI out of your amp, disconnect the XLR cable where it goes into the board or the snake, insert your GROUND LIFT cable, and then attach the XLR cable to it. This will open the ground loop between your amp and the PA system without compromising the safety grounds of either.
     
  8. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    I had the same problem with an Aguilar DB359 that I posted a few weeks ago. My solutions was slightly different that what Bob suggests, but the intent was similar. The shield on an XLR cable is typically connected to both pin 1 and the shell of the connector. The DB359 has a ground lift switch, but the shell is connected to chassis. I believe it’s done the same in the Nemesis (I also have one, and I agree, it’s a great little amp). I modified a short XLR cable by disconnecting the shield from the shell connection, but left it connected to pin 1. This way, the ground lift switch in the amplifier is now functional.

    Also, NEVER defeat the ground lead on plug. It’s there for your protection. Any sound guy that does this to fix a ground loop has no idea what he is doing.
     
  9. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It shouldn't be! Reputable cable manufacturers should know better than to do that. Cable shields should be connected only to pin 1. The connector shells should be floating, unless there is a specific reason to tie them to the shield; otherwise they could cause ground loops or other problems when used on metal stages, in equipment racks, etc.

    I agree with everything else you said, though! :)
     
  10. frankosaurus

    frankosaurus

    Feb 27, 2002
    San Jose
    thanks for the info. I'm getting kind of paranoid since I have a show coming up this weekend at the venue where the guy was using the 2 prong adapter...

    The modified XLR sounds good to me but my soldering isn't so great and I don't know if I could figure it out by this weekend (and I'd hate to screw it up and end up causing some damage to me or my equipment). Are modified XLR cables like this sold in stores? It seems like maybe they should be since they can solve the problems that me and 44me are having.

    As an alternative, would it be okay to just ensure that the PA and bass amp are plugged into the same socket (through a power strip or whatever)?

    ...I remember in college when I played shows the school electrician would come in and put in a special patch for the sound people to hook us into... those were the days....

    -phrank
     
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    If your soldering skills are lacking, then forget about soldering the capacitor in the XLR connector for now, and just cut the shield-to-pin 1 connection.
     
  12. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    Bob, funny that you should mention this. When I posted my solution to the hum problem I was having, I was told that it’s convention to connect the shield to pin 1 and the shell. The cables I had were mostly from Carvin (a few were unknown), and were all done this way. My feeling at the time was that it may be convention, but it is electrically incorrect. Interesting to see that in may not be convention after all.
     
  13. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    Good quick fix! I think Phrank should also be sure to cut the shield from the shell, which is the likely cause of the problem. The Nemesis has a ground lift switch that should open pin 1, but I think the shell is still connected to ground. I've got the same amp at home so I'll check tonight.

    Phrank, ensuring that the PA and your amp are on the same circuit may solve the problem, but it may not. I've run my Nemesis through a PA dozens of times without a problem, so I'm inclined to think it will, but it is possible that something else is causing the problem.
     
  14. 44me

    44me

    Jun 17, 2002
    Bedford, NH USA
    I checked the function of the ground lift switch on my Nemesis last night. The connector shell is not connected to ground, so when the ground lift switch is opened, you should be breaking the ground loop. This leads me to think that the cable modifications that Bob and I wrote about aren’t going to help. The problem I had was a little different. My Aguilar has the XLR jack directly connected to the chassis, so if the cable modification is not made, the ground lift switch is defeated. Phrank, are you sure the output level was set high? Low gain can be compensated for at the board, but you will end up with more hum. Bob, any other ideas?
     
  15. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    It's okay if the shell of the XLR connector on the amp or preamp is connected to the chassis because that's a terminus of the cable; you're not likely to accidentally ground the amp or pre chassis to something it's touching because it should already be grounded. You just don't want the XLR shell connected to pin 1 in the cable ends because that would definitely defeat the ground lift switch.

    The ground lift switch on the amp or preamp should only interrupt the connection from the chassis to pin 1. Aside from the switch, the electrical connection from pin 1 to the chassis should be as short and direct and conductive as possible. If it's not--if pin 1 is connected to signal ground, or the power supply ground, or whatever--the piece of equipment has what's often called a "pin 1 problem" and should be modified. This goes for both inputs and outputs.

    Some design engineers--especially in MI, but also in pro audio--never learned the real gist of balanced connections, so they designed products that work well most of the time but sometimes caused hum problems. If all audio gear had pin 1 directly connected to the chassis, the chassis well grounded, and signal circuitry that was independent of the chassis, there'd be no such thing as ground loops causing hum problems and we wouldn't have to lift shields or grounds anywhere.

    Well-balanced signal runs have an inherent immunity to noise and hum pickup, even without shielding. Phone companies run tens, hundreds, and even thousands of miles of unshielded twisted pair balanced lines with absolutely minimal pickup of hum and noise.
     
  16. Amen to that!

    :D

    There are several instances of musicians getting KILLED on stage because of wiring issues. Radio Shack stocks and sells a $6 line checker. Mine goes into every power receptacle, at every gig. No exceptions.

    Bad Wiring = No Play (and middle finger extended)