Ground Loop with multiple amps

Discussion in 'Amps and Cabs' started by stevel, Oct 7, 2009.

  1. stevel

    stevel Member

    Joined:
    Apr 6, 2008
    Messages:
    8,375
    Location:
    Hampton Roads, Virginia
    Why?

    What to do?

    I have heard that if you plug two amps into the same power strip, they will not create a ground loop (in all cases here, running one guitar into two amps via A/B switch, either, not both).

    I've also heard, at various times, if you plug them into the same circuit, and even, if you plug them into different circuits, you can get rid of the ground loop.

    TIA,
    Steve
     
  2. Billy Penn

    Billy Penn Member

    Joined:
    Dec 4, 2007
    Messages:
    1,057
    Location:
    NJ
    When using multiple amps you need to only ground one of them. Use a ground lift on the others. Unless you use a splitter like the Lehle P-Split or similar that has transformers and ground lifting features. I like that splitter alot. They all can't be grounded or you get a loop.
     
  3. riffmeister

    riffmeister Gold Supporting Member

    Joined:
    Jan 4, 2003
    Messages:
    15,360
    Location:
    near Philly
    Lehle, Tonebone, Switchazel, Framptone, are examples of ABY boxes with transformer isolation of the two signal paths and........no GLH.
     
  4. bigeasy

    bigeasy Member

    Joined:
    May 10, 2008
    Messages:
    123
    If it is the AC line going to the amps, yes, usually if they are on the same circuit it's better. But it still doesn’t always solve the problem. What should solve the problem is a 3-prong to 2-prong adapter on lower powered amp. This will lift the earth ground connection to the second amp, and use the earth ground for the first one for both.
    Give that a try. It may also be something else in your set up I don’t know about?
     
  5. OrangeAD30TC

    OrangeAD30TC Member

    Joined:
    Oct 30, 2007
    Messages:
    1,716
    Location:
    San Francisco
    Try it and address it if you have a problem. Many times you don't need to lift one. You won't always get a ground hum.
     
  6. stevel

    stevel Member

    Joined:
    Apr 6, 2008
    Messages:
    8,375
    Location:
    Hampton Roads, Virginia
    So the other thing I've heard is - using one of those 2 prong adapter (or snipping the ground pin off an extension cord) is a good way to get electrocuted.


    So it looks I'm reading that, the problem is, you have two amps grounded.

    Why does it happen though? What actually causes the loop?

    Steve
     
  7. riffmeister

    riffmeister Gold Supporting Member

    Joined:
    Jan 4, 2003
    Messages:
    15,360
    Location:
    near Philly
    See my post above and keep both amps grounded and be safe and you will never have GLH. And there are other benefits to using these ABY boxes as well, for example no additivity of cable capacitance when running in Y mode, phase switching if the two amps are not in phase, etc. A good investment that will provide a lifetime of music making joy. :)
     
  8. Billy Penn

    Billy Penn Member

    Joined:
    Dec 4, 2007
    Messages:
    1,057
    Location:
    NJ
    That little gray three to two prong adapter is called a ground lift. If you have multiple amps just ground one. You only need one point of grounding.
     
  9. smolder

    smolder Silver Supporting Member

    Joined:
    Jun 16, 2008
    Messages:
    10,796
    Location:
    up in the rockies
    was reading this site today...

    http://www.paulrubyamps.com/info.html#Ground

    "OK, that optimizes our individual piece of equipment. But, it leaves open the possibility of ground loops when stringing multiple things together. Each thing has it's own mains ground with a reference connection to signal ground. So, the mains power line and the line cords between items form loops. Some amps have a "ground lift" allowing the quick elimination of ground loops by removing the mains ground connection. This SUCKS. Throwing that switch eliminates the safety of mains ground and is NOT needed to solve ground loop problems. The argument is that "if there was a ground loop, then there must be another path to earth, so there's no safety issue lifting the ground." That's crap. Do you trust the cheesy patch cords between your foot pedals to protect you against a B+ short to chassis?? So what's the solution? Think about your mains as extensions of the star ground principle. Keep your equipment physically localized and keep all the mains cords running parallel and tight to each other (there's no surface area to any ground loops and thus no noise pickup). Run them all back to one group of sockets on a wall rather than plugging things in willy nilly around the room. For pub gigs, this is an easy policy to follow. For the big guys, use isolation transformers and run balanced XLR to break the ground loops."
     
  10. ShavenYak

    ShavenYak Member

    Joined:
    May 23, 2009
    Messages:
    1,999
    Location:
    Birmingham, AL
    Smolder's quote is right on. If you use a cheater plug on one amp, and are relying on the signal path to keep its chassis grounded, what happens if the other amp gets accidentally unplugged or the guitar cable gets yanked loose? You have no safety ground, and are now at risk of electrocution.

    Listen to riffmeister. Spend the extra money for an A/B switch with isolation transformers.
     
  11. B_of_H

    B_of_H Member

    Joined:
    Jan 3, 2007
    Messages:
    4,506
    Location:
    Jayhawk Country
    I always carry a ground lift 'cheater' adapter or 3 in my cable/utility bag for gigs. It really helps sometimes.
     
  12. Third Stone

    Third Stone Supporting Member

    Joined:
    Sep 20, 2004
    Messages:
    867
    Location:
    santa clarita, CA
  13. jimbo13

    jimbo13 Supporting Member

    Joined:
    Apr 28, 2009
    Messages:
    244
    Why all the concern about getting shocked? They played without ground plugs back in the 50's like real men. Now shut up and have a smoke you pu$$ies.
     
  14. bigeasy

    bigeasy Member

    Joined:
    May 10, 2008
    Messages:
    123
    The problem with those 2-prong amps then (like the original Fender Tweeds) was that the "ground" switch they had in there actually AC coupled the chassis to one or the other hot sides of the AC comming in. If you had it the wrong way with respect to you, your strings, you would get skocked. Never do this or run one of those amps this way. Cut that cap out out there and add a 3-prong cord. Again, that ground switch was not a ground lift, it actually AC coupled your chasiss to one or the other the HOT AC lines.
     

Share This Page