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Ground lift

David Jayne

David Jayne

Jan 16, 2006
16,337
8,096
Brookfield, CT
#1
At a recent gig, the engineer plugged his XLR into my amp DI and promptly found a large amount of hum or other noise on my line(I didn't hear it so can't say for sure what it was). I went away and when I came back I found he'd unplugged my power cord and added a ground-lift plug to it and plugged it back in. I protested, citing the potential safety hazard and he countered that I was still safe because in the event of a problem my amp was now grounded to the house system, via the XLR. True?
 
#3
Most likely the problem (hum/noise) was caused from either crummy/faulty PA gear and or the soundman didn’t have the backline equipment (your amp) on the same AC power circuit as the PA like he should have had.

dmusic148 said:
At a recent gig, the engineer plugged his XLR into my amp DI and promptly found a large amount of hum or other noise on my line(I didn't hear it so can't say for sure what it was). I went away and when I came back I found he'd unplugged my power cord and added a ground-lift plug to it and plugged it back in. I protested, citing the potential safety hazard and he countered that I was still safe because in the event of a problem my amp was now grounded to the house system, via the XLR. True?
Click to expand...

FALSE!!!

TIP/WARNING: Don't EVER let anyone do that again - NEVER, PERIOD!!!
 
#5
Hmm...it seems like a reasonable conclusion by the sound "engineer". So I checked a schematic for a bass amp (an older Carvin) with an integrated DI. The ground pin of the XLR jack runs through a 100K resistor on its way to the amp's chassis ground. So your amp was slightly (poorly) grounded to the house system through the XLR. This is not much different than being not grounded at all. Danger, Will Robinson!
 
#7
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#11
I have a ground lift on my DI box and on my amp. That should be tried first.

Don't let the sound guy bully you. I once played a gig where if I didn't have my polarity checker, I would not have been able to discern that one of the plugs was wired with the hot and neutral flipped when the owner tried to rewire the stage on the cheap. That would have caused electrocution. We refused to play until I checked every socket to assure that each socket we used was wired correctly.

OK -- they still could have wired the ground to the neutral, but at least that would not have been the electrocution hazard that some of the other wiring cross-ups can be.

Hmmm...a few months after we played that gig all those years ago, the place closed. Coincidence? I think not....
 
#12
scottbass said:
Hmm...it seems like a reasonable conclusion by the sound "engineer". So I checked a schematic for a bass amp (an older Carvin) with an integrated DI. The ground pin of the XLR jack runs through a 100K resistor on its way to the amp's chassis ground. So your amp was slightly (poorly) grounded to the house system through the XLR. This is not much different than being not grounded at all. Danger, Will Robinson!
Click to expand...
But the shells of the XLR cable are probably connected directly to the shield of the cable and to the chassis of the amp and to a grounded chassis at the board. I agree that using a ground lift is not a good practice, generally, but it's not an automatic death sentence. The hysteria is a little over the top, IMO; amps didn't even have a ground pin until the late 60's. I have used one when nothing else will work.

Direct outs of bass amps are notoriously noisy, no matter what you do. A good DI box is a better way to go than a ground lift adapter if you can't get it to stop humming.
 
#13
Electricians are required provide less than 25 ohms resistance to ground at the service panel, in some jurisdictions 10 ohms. A 100k resistor is, what, 100,000 ohms? What's the resistance of the human body? Depends on how well you're connected to ground. If your resistance is less than the ground connection, current leakage will flow through you instead. 15 milliamperes is all it takes. That's 1/1000 of the current required to trip the breaker, if it's calibrated to spec. And you're trusting your life to a 100k resistor and a pair of sneakers. If you got zapped and killed, that ground lift would disappear in a second and it would be like,"bummer dude, it was just your time to go"
 
#16
ggunn said:
The hysteria is a little over the top, IMO; amps didn't even have a ground pin until the late 60's. I have used one when nothing else will work.
Click to expand...

This is NOT my experience. Any time my amp has been on a 2-wire circuit (correct polarity but no ground) and the PA is on a different 2-wire circuit (again, correct polarity but no ground) I have gotten the notorious blue flame of death when my lips get near my SM58. I immediately stop singing backing vocals until someone does something about it. I have also begun carrying my own circuit tester as others have mentioned in previous posts. I don't consider this "over the top hysteria." :rollno:

ggunn said:
Direct outs of bass amps are notoriously noisy, no matter what you do. A good DI box is a better way to go than a ground lift adapter if you can't get it to stop humming.
Click to expand...

This is very true. My old Carvin amp's DI was essentially useless. Even my Ampeg SVT-7 Pro's DI and my Ampeg PF-800's DI do not play well with the PA in a few of the venues we play. (Sorry JimmyM, you know I love them as much as you do!)
 
#17
dmusic148 said:
Thanks fellas. Just as I suspected. It ticked me off, and I argued with him about it. He was starting to get 'hot' with me about it and it was nearly downbeat so I let it go. Not next time.
Click to expand...

+1.

If that ever happens again - pour some drinking water on the stage floor and tell him to stand in the water barefoot while holding a live mic and lick the back of your amp chassis to prove its safe. :D And even if he does it and lives - still DON’T let him or anyone ever unplug the AC ground plug on your amp.

TIP: After plugging your bass in and turning your amp on - hold your bass so that your not touching any metal parts while holding the bass up close to a live mic so that one of the bass guitar tuner pegs is about an 1/8" away fron the mic grill - and listen for corona (a cracking or popping like sound) while watching for a blue arch/spark between the mic grill and tuner key - if you hear or see either of these, there is a grounding problem, so DON’T touch any mic's (or anything) while touching/playing your bass.

I also highly recommend investing in an AC outlet checker and ALWAYS check outlets before use. http://www.amazon.com/Electrical-Re...=1384358255&sr=8-4&keywords=AC+outlet+checker
 
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#18
dmusic148 said:
Yep, my amp has one but he INSISTED on lifting the AC ground instead. Never again. I'll simply walk away-it's a sub gig that isn't very important to me, at least not as important as my life.
Click to expand...

He could've "insisted" all he wants - I'd tell him to get bent, anyways... I always play and sing - what you're talking about is a sure way to get "bit" by the mic... I'm not subjecting myself or my gear to that, regardless of what some ****** running sound wants...


- georgestrings
 
#19
ggunn said:
But the shells of the XLR cable are probably connected directly to the shield of the cable and to the chassis of the amp and to a grounded chassis at the board. I agree that using a ground lift is not a good practice, generally, but it's not an automatic death sentence. The hysteria is a little over the top, IMO; amps didn't even have a ground pin until the late 60's. I have used one when nothing else will work.

Direct outs of bass amps are notoriously noisy, no matter what you do. A good DI box is a better way to go than a ground lift adapter if you can't get it to stop humming.
Click to expand...

The reason why the 2 prong amp power setup was discontinued was because musicians have been electrocuted that way - to deny that is being dishonest... Furthermore, a hum and "fix" as described in the OP is almost guaranteed to nail you while playing and singing - NO THANKS...

Lastly - the DIs of quality amps are not "notoriously noisy", IME...


-georgestrings
 
#20
scottbass said:
This is NOT my experience. Any time my amp has been on a 2-wire circuit (correct polarity but no ground) and the PA is on a different 2-wire circuit (again, correct polarity but no ground) I have gotten the notorious blue flame of death when my lips get near my SM58. I immediately stop singing backing vocals until someone does something about it. I have also begun carrying my own circuit tester as others have mentioned in previous posts. I don't consider this "over the top hysteria." :rollno:



This is very true. My old Carvin amp's DI was essentially useless. Even my Ampeg SVT-7 Pro's DI and my Ampeg PF-800's DI do not play well with the PA in a few of the venues we play. (Sorry JimmyM, you know I love them as much as you do!)
Click to expand...

Agreed 100% with the 1st paragraph...

IME, the DIs in amps by Mesa, Genz Benz, G-K, and Peavey have all been excellent - DIs in Ampeg amps, not so much... FWIW, I ALWAYS use a DI send to the FOH, and currently own a Countryman 85 - and before that a Radial JDI - so, I know a good DI when I hear/use one...


- georgestrings
 

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