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Ground Lift

agreatheight

agreatheight

Dec 23, 2005
4,269
33
55
Portland Area, ME
#1
A funny conversation last night leads me to ask this simple / dumb question to the electronics guys/gals out there: Is there any different between engaging the ground lift switch on a piece of gear and hooking up its power cord to an 3 prong to 2 prong adapter? And while we are at it, what is the specific danger in running something ground lifted or 3 prong to 2 prong adapted?

Thanks in advance!
 
#2
agreatheight said:
A funny conversation last night leads me to ask this simple / dumb question to the electronics guys/gals out there: Is there any different between engaging the ground lift switch on a piece of gear and hooking up its power cord to an 3 prong to 2 prong adapter?
Click to expand...

Yes, they are not the same thing at all. Don't do that.

The ground lift disconnects pin 1 of the XLR connector, breaking continuity between ground on one end of the connection and ground on the other. Both pieces of equipment are still grounded.
 
#3
thumbknuckle said:
Yes, they are not the same thing at all. Don't do that.

The ground lift disconnects pin 1 of the XLR connector, breaking continuity between ground on one end of the connection and ground on the other. Both pieces of equipment are still grounded.
Click to expand...

Thank you. This supports my argument. Next question is a bit trickier. If a piece of gear doesn't have a ground lift on it and has electrical based hum, how can you get rid of said hum without the 3 to 2 adapter?
 
#4
^ what thumb said!

Say your mixer is at one end of the room, and your powered speaker or amp at the other end. both plugged into grounded outlets on different circuits.
When connected via XLR cables, now both components are grounded to each other via pin 1 of XLR cables. So each has 2 paths to ground which can sometimes create a ground loop issue.

If you don't have a ground lift switch, taking a short XLR cable and opening one end and cutting the wire to pin 1, and inserting the cable between the existing path and you've got a ground lift. Both components still grounded just not to each other. Of course you should mark/tag the cable so you know it's got the ground removed. I keep a few 1 foot cables like this in my toolbox.

If this doesn't help then one of the components in the signal path my not have a good ground.
 
#5
uhdinator said:
If you don't have a ground lift switch, taking a short XLR cable and opening one end and cutting the wire to pin 1, and inserting the cable between the existing path and you've got a ground lift. Both components still grounded just not to each other. Of course you should mark/tag the cable so you know it's got the ground removed. I keep a few 1 foot cables like this in my toolbox.
Click to expand...

Thank you very much! That was exactly what I am looking for!
 
#6
agreatheight said:
Thank you. This supports my argument. Next question is a bit trickier. If a piece of gear doesn't have a ground lift on it and has electrical based hum, how can you get rid of said hum without the 3 to 2 adapter?
Click to expand...

You probably have at least one piece of gear with a "pin 1 problem." That's where the ground connections on the inputs and outputs (pin 1 on XLR connectors, or sleeve on TRS) don't connect directly to the chassis via the shortest possible path, but instead connect to the ground on the circuit boards and (maybe) eventually to the chassis through a circuitous route.

The long-term solution would be find the offending piece(s) of gear and solder short wire jumpers to the chassis on all the incorrectly wired inputs and outputs, and also snip their connections to circuit board ground. I've done this myself with some of my old analog processing gear.
 
#7
thumbknuckle said:
Yes, they are not the same thing at all. Don't do that.

The ground lift disconnects pin 1 of the XLR connector, breaking continuity between ground on one end of the connection and ground on the other. Both pieces of equipment are still grounded.
Click to expand...

I agree... mostly. Ground lifts should be used exceedingly sparingly, only when nothing else will work (it happens), and only by someone with a good grasp of how electricity works.

I agree that anyone who asks the question in post #1 should not do that.
 
#10
agreatheight said:
Since this is still getting looks, someone I know (different person from the first post) suggest simply snip the #1 pin on the male end of a short cable to use as a jumper (as opposed to ope wire surgery). Any reason not to do that?
Click to expand...
Even simpler is to buy an adapter that connects the signal pins but does not pass ground. About $7, IIRC. You cannot do either for a high Z unbalanced connection, though. Two passive DI's with a ground lifted XLR between them would work.
 
#11
agreatheight said:
Since this is still getting looks, someone I know (different person from the first post) suggest simply snip the #1 pin on the male end of a short cable to use as a jumper (as opposed to ope wire surgery). Any reason not to do that?
Click to expand...

Yes, you can certainly do that if you happens to have a few extra mic cables laying around. Make sure you use a sharpie or a p-touch labeled to ID the Cale from your other cables. That has been my solution many a time. And mic cables are cheap to replace, so in a pinch, why not my friend.
 
#12
People who lift the ground on the mains feed to their kit stand a good chance of electrocution and there was a video on youtube only a few days ago.

If people do this in the UK now, they stand the chance (if an employee) of being sacked for gross misconduct, and possible prosecution by our Health and Safety Executive if somebody got hurt. The ground is there for your protection, so the old fashioned short-cut of lifting the ground is frowned on nowadays - especially as many bits of kit now have ground lifts, and a few cables with no ground connection one end usually work fine.

In the video, you can see the guy go rigid and just keel over. I've had quite a few shocks on stage through old, dangerous and lifted ground gear, and it's just stupid!
 

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