groud lift??

[audiofreqs]

why would one employ a ground lift?

 

[Track Rat]

It's one way of dealing with a ground loop between two pices of equipment.

 

[bleyrad]

It's more of a quick fix and can actually be unsafe if your ground problems are really bad (I heard about a guy who got shocked from a DI's ground lift because he also had an ungrounded tube amp on the stage).

 

[boingoman]

Bleyrad, sounds like that guy had more problems than just an ungrounded amp. The lack of a ground won't cause a shock unless a fault occurs in the gear.


A/C and signal ground are seperate things. It's legit for signal ground, like track rat said, but not A/C, like snapping off the third prong on an extension cord to get rid of a hum.

 

[bleyrad]

I think the thing was that the DI's audio ground was providing a path to earth through the console and was draining the excess crap spewing from the (faulty and ungrounded) tube amp, and when it was lifted it went through the guitar strings into the player's body and to ground.
Obviously it was an incredibly complex setup with more than a few very faulty pieces of gear. The guy had to be carted away to the hospital.
This isn't a remotely common occurance, but in the right situation it's possible. If you're doing FOH for a really complex show you should make sure your grounding is under control and track down the root cause of ground loops or other ground problems rather than just lifting it.

 

[boingoman]

Hmm...still sounds fishy to me. I know current can pass through audio grounds, but it seems like someone would have noticed 120V passing through the FOH console, or the amp's fuse would have blown, if the DI ground wasn't lifted. Or the mic cable would have failed with that much juice running through it.

Eh, I could be wrong.

In any case.....

Part of proper gear set-up and use is figuring out where and how to lift signal ground, if you have ground loop problems. Sometimes two pieces of gear hooked together by signal ground will make a ground loop, no matter how well your system is set up. Different gear can have different resistance to ground. It's just a fact of life.
Lifting signal ground is the right way to deal with the problem, either with a modified cable, or a passive DI with a ground lift, or an isolation transformer, or whatever other solutions there are.

 

[bleyrad]

OK, I stand corrected then. I did think the technicalities of it seemed a little weird, too, but I didn't question it much seeing as how this information and story came from an engineer who recorded Metallica and got a Juno for Best World Music last year.

And as the story went, it wasn't 120V, it was more like 250 or so because it was the tube amp's problem. Hence the hospitalization and all.

 

[boingoman]

OK, I stand corrected then. I did think the technicalities of it seemed a little weird, too, but I didn't question it much seeing as how this information and story came from an engineer who recorded Metallica and got a Juno for Best World Music last year.

Like I said, I could be wrong. Is the info online anywhere?

And as the story went, it wasn't 120V, it was more like 250 or so because it was the tube amp's problem. Hence the hospitalization and all.

That'll set you on your ass.

 

[MOFO Pro]

It's not the voltage that kills you... It's the current...