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Connecting to two amps -- shock hazard?

Discussion in 'Amps and Cabs [BG]' started by BFunk, Jun 7, 2002.


  1. BFunk

    BFunk Supporting Member

    I have read in several places that it is generally a bad idea to connect an instrument directly to two amps. The argument is that the amps may be at different ground levels. This could cause a potentially lethal voltage.

    Lately, I heard another view on this. About a week ago, I was in a store run by a very knowledgable and respected owner/player. While I was trying out some basses, he was talking to an amatuer player who wanted to hook his bass up to two amps. The owner sold him an standard A/B/Y box. I was a little concerned that he gave the buyer information that could be potentially hazardous. I did not want to upset the sale, so I waited until the customer left. At that point I mentioned to the owner that the solution could be potentially lethal. He said that if one of the amps has the ground lifted, there is no chance of shock. It doesn't make sense to me, but I am not expert on the subject. Is what he said true?
     
  2. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
  3. BFunk

    BFunk Supporting Member

    So what do you have to do to lift the ground?
     
  4. Connecting two amps: shock hazard? How does this happen, and at what points (where do you touch for zaps)?
     
  5. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    I actually deal with this quite a bit. A lot of the guitars players I work with have an affinity for POS old amps that have only a 2 prong plug.

    To prevent them from getting a jolt everytime they walk up to a mic, or touch me, I run their guitar into an AB box, tuner with multiple outs or even a fx pedal with multi outs. Then one output goes to the working amp and the other to a grounded (3 prong plug equipped) dummy amp.

    The theory, I believe, is that the ground wire on the plug is the path of least resistance especially compared to the human body. By hooking up the 2 amps' chassis (though the ground on the cable, through the pedal) you are effectively using the gound on the 3 prong amp as the ground for the groundless one. I wish I could explain it better.....

    FWIW, I haven't lost a guitar player yet. I don't know if that is such a good thing.
     
  6. So you're saying that by me connecting my 2 Ampegs with a stereo chorus pedal, I can die? The grounds on both my amps are intact and plugged into the same power strip, as is the pedal.
     
  7. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    NO. What I'm saying is that electricity has an easier time going through metal than people, so as long as at least one amp has a good ground connection, all the exrta juice is going to go through that.

    I mean, let's suppose you've got 2 amps, and you're going into both of them through a stereo chorus. If the ground on one fails (however that happens) the electrical jolt will leave that amp, go through the pedal and then onto and out of the other amp as it is the path of lower resistance versus your fingers.
     
  8. BFunk

    BFunk Supporting Member

    Wait, I thought the problem was because the grounds could be at different levels causing a voltage potential over the ground wires. The one ground that is guaranteed to have a lower voltage is your bass, which you are holding in your hands. Isn't that the problem?

    Isn't that at least part of the reason why they star-ground studios? (All electrical equipment going to the same ground.) If you have one ground lifted, I suppose that means that the two amps would end up sharing the same ground of the grounded amp. I think that is what he is saying.
     
  9. Ohhh ok. Good to know I won't die now.
     
  10. BFunk

    BFunk Supporting Member

    I think you are alright since you are going through the chorus pedal. The electronics will shield you. (Please don't bet your life on my advice! Check with a qualified technician.)

    Here is a quote from Roger Mayer's site. He is talking about his crossroads pedal. It's an A/B/Y box with a transformer on one leg.

    Source: http://www.roger-mayer.co.uk/crossroads.htm
     
  11. geshel

    geshel

    Oct 2, 2001
    Seattle
    I was just about to say what your Roger Mayer quote says. Don't lift the grounds on anything with AC power inside of it - if a hot lead were to touch the chassis, you would become the new ground. Ouch.
     
  12. ebozzz

    ebozzz Supporting Member

    May 17, 2001
    Denver, Colorado
    Knowing that the danger is that much of a factor, why would the option of lifting the ground even be available on power amps?
     
  13. BFunk

    BFunk Supporting Member

    Because in permanent installations they ground all the amps to the same ground point. It's called start grounding. Disconnect the ground. Attach a ground wire to the chassis and connect it to the ground point. It prevents ground hum and is very safe because everything is at the same ground level.
     
  14. ebozzz

    ebozzz Supporting Member

    May 17, 2001
    Denver, Colorado
    That makes sense. I guess I was just looking at it from the perspective of how we would use a power amp. I never thought about permanent installation.
     
  15. geshel

    geshel

    Oct 2, 2001
    Seattle
    Well - now that I think about it, I bet the "ground lift" switch on the back only lifts the signal ground, but keeps the chassis grounded to the third prong. If that's the case, it's not as dangerous.
     
  16. This is correct.
     
  17. BFunk

    BFunk Supporting Member

    You're talking about the ground lift on the DI. That's a whole different story. Some power amps, like my Aguialr DB 728, have a mains ground lift for the power. You actually have to unscrew the ground connection on the outside.
     
  18. geshel

    geshel

    Oct 2, 2001
    Seattle
    I hope they have a *big* damn warning in the manual about doing that. . .that you'd better ground it some other way if you do.
     
  19. BFunk

    BFunk Supporting Member

    They do have a big warning!