Ground lift to cancel humming - how does that work?

[bassbiscuits]

Did a gig last weekend and halfway through my bass amp started making a horrible humming noise (it's a Little Mark III into a couple of MB cabs).

 

Even with the head turned off, there was a massive loud noise thru the PA every time the channel for my DI was unmuted.

 

Eventually we flicked the Ground Lift switch on the back of the LMIII and the buzz stopped.

 

a) What on earth might have been going on there, for it to be generating such a racket even when turned off?

b) How does that ground lift thing even work?

c) Could it have been something to do with the cabs (I was test driving one of them for the first time, but it seems to work ok), the amp head, the wiring at the venue, or something else?

 

Just interested to know what was going on really! Did my head in mid-gig.

 

 

[bertbass]

That's an earth loop.  When two pieces of earthed equipment are connected together with a lead you can form an earth loop.  The ground lift breaks the earth connection between them eliminating the loop and stopping the noise, all the equipment is still earthed through it's own earth so no safety hazzard.

[Bill Fitzmaurice]

https://www.ranecommercial.com/legacy/note110.html

[bassbiscuits]

7 hours ago, bertbass said:

That's an earth loop.  When two pieces of earthed equipment are connected together with a lead you can form an earth loop….

Nice one that’s a nice clear explanation.

 

I do wonder why it has suddenly happened tho and where in the system the problem might lie - plugs, cables, leads, basses, pedals, venue wiring etc?

[tauzero]

It could be that something in the environment became more electrically noisy - fluorescent light, fridge, dishwasher - there's quite a lot of stuff that can create interference.

[Phil Starr]

On 05/10/2023 at 12:09, bassbiscuits said:

 

a) what on earth might have been going on there, for it to be generating such a racket even when turned off?

b) How does that ground lift thing even work?

c) Could it have been something to do with the cabs (I was test driving one of them for the first time, but it seems to work ok), the amp head, the wiring at the venue, or something else?

 

Just interested to know what was going on really! Did my head in mid-gig.

no pun intended

 

So earthing works by assuming that if everything is connected to the ground it is always at the same voltage. Mainly this is for safety reasons. The second thing you need to know is that we are surrounded by electromagnetic radiation coming from just about any electrical wires carrying an alternating current. This varying radiation will induce a current to flow in any wire that forms part of a circuit. If everything is perfectly connected to ground then that current will flow through the earth to ground.

 

The problem is that earthing isn't perfect, there is always a small resistance in the earth circuit and a small voltage difference between different earths. Some of that resistance is in your equipment and some of it is in the house wiring. Sometimes it is enough to even give you an electric shock. So if you have a cable between two earted pieces of equipment the cable completes a circuit and current will flow along the cable down to earth and back up the other earth to complete the circuit. The earth shielding in the cable will pick up any electromagnetic radiation and a current will be induced. Most of the radiation will be from the mains flowing through all the mains wiring and things like electric motors so most of the noise now in your cable will be 50Hz hum you might also pick up radio too in this situation or the crackles from something with a loose connection.

 

So (b) the ground lift thing works by breaking the circuit and (c0 no it wasn't the cab.

 

It could be that you plugged the PA and your amp into a different mains socket and that one or both weren't earthed properly or were at the end of entirely different ring mains within the building, then there was a large motor running intermittently nearby during the gig or some other source of electromagnetic radiation. I try to run all our gear off a single socket, obviously you can only do this in the UK if you draw less than 13A in total that reduces the chance of this happening so long as all your gear is properly earthed and it eliminates the shocks you sometimes get from the mics due to floating earths. The ground lift switch is there just for this reason, to break the ground circuit, no circuit no current no hum.

[bassbiscuits]

Thanks Phil that’s dead helpful. 
I used the same kit for last weekends gig and it was fine so I assume it was a weird characteristic of the venue wiring. 

[Chienmortbb]

In my humble opinion, there is no real need to have the earth/ground connected on a signal feed. It is not a safety issue, and you are far more likely to have no trouble with the ground lifted. Many DI boxes have the ground lifted by default.

[Bill Fitzmaurice]

The problem isn't the power ground, it's the signal ground. With unbalanced 2 wire interconnects the signal ground and power ground are connected, which can lead to ground loops. The usual cure is lifting the signal ground at one end of the connection. You never lift the power ground. Properly configured 3 wire balanced connections don't have ground loops, as the power and signal grounds are not connected.