Double ground causing loud feedback from bass?

[awilkie84]

So, my local bar & gig spot has a setup that I find odd and makes me not want to use my amp.

They run a grounded power snake into a non-grounded extension cord & then to the wall.
When I asked the sound tech why this was done, he said it's because the DI is grounded through the PA, so I don't need to ground my amp & that grounding the amp would cause my bass to hum and feedback.

This is concerning to me, as I don't see how the DI could possibly ground my amp properly. Am I right to be concerned?

Here's how it's wired.

Bass -> Pedals -> DI -> Amp & PA

 

[ggunn]

So, my local bar & gig spot has a setup that I find odd and makes me not want to use my amp.

They run a grounded power snake into a non-grounded extension cord & then to the wall.
When I asked the sound tech why this was done, he said it's because the DI is grounded through the PA, so I don't need to ground my amp & that grounding the amp would cause my bass to hum and feedback.

This is concerning to me, as I don't see how the DI could possibly ground my amp properly. Am I right to be concerned?

Here's how it's wired.

Bass -> Pedals -> DI -> Amp & PA

Feedback? No.

Ground hum? Maybe; If your amp is grounded at power and also through the shield in the cable from the DI to the board, but the safe way is to lift the ground on the DI and ground your amp. Lifting ground to amps is almost never the best way to open a ground loop.

 

[seamonkey]

This is nuts. The ground is for safety. Keep your amp grounded

Check this thread
http://www.talkbass.com/forum/f203/ground-lift-986652/

 

[Zooberwerx]

So, my local bar & gig spot has a setup that I find odd and makes me not want to use my amp.

They run a grounded power snake into a non-grounded extension cord & then to the wall.
When I asked the sound tech why this was done, he said it's because the DI is grounded through the PA, so I don't need to ground my amp & that grounding the amp would cause my bass to hum and feedback.
This is concerning to me, as I don't see how the DI could possibly ground my amp properly. Am I right to be concerned?

Here's how it's wired.

Bass -> Pedals -> DI -> Amp & PA

Wrong! This is equivalent to snapping off a 3 prong plug's ground blade to create a ground lift. You power line should be an intact 3 prong (both ends) cord and plugged into a verified outlet. Ground hum / loops are a conflict amongst the components within the system and usually dealt with by utilizing available onboard ground lifts and/or signal cable modification (again, a method of ground lift). Gross over-simplification; others will embellish hopefully. All I know is that you don't want to compromise your electrical source or defeat safety measures.

Riis

 

[TimmyP]

Always ground everything, and get a DI with a proper ground lift.

 

[walterw]

the issue is the chance that the ground your amp's AC cable gets is at a different "potential" (essentially at a different voltage) than the ground the PA and its direct box are getting, so plugging in that box creates current flowing from one ground to the other, and the noise, and sometimes also that lovely sensation of getting your lips shocked by a vocal mic!

+1 to avoiding that by interrupting the ground path between your amp and the PA's ground, not from between your amp and its own ground!

that's what the ground lift switches on the direct box are for. also, it's nice in bars and such where the whole system is not all that big to have the PA and the stage amps on the same circuit, to avoid just this kind of weirdness.

 

[ggunn]

...it's nice in bars and such where the whole system is not all that big to have the PA and the stage amps on the same circuit, to avoid just this kind of weirdness.

Not really. The grounding and grounded (neutral) conductors for all the circuits on a service are all bussed together at the main distribution panel. Being on a different circuit just means that the ungrounded conductor is energized from a different breaker.

Plugging into two outlets that are on the same breaker that are some distance from each other can be worse for ground loops than plugging into two outlets on different circuits that are the same distance (wiring-wise) from the service.

But of course I agree (and as I said) that lifting the ground on the DI is a far safer way of eliminating a ground loop than is lifting ground on the amp.

 

[walterw]

Not really. The grounding and grounded (neutral) conductors for all the circuits on a service are all bussed together at the main distribution panel.

when you say "are", i just hear "should be"!

Plugging into two outlets that are on the same breaker that are some distance from each other can be worse for ground loops than plugging into two outlets on different circuits that are the same distance (wiring-wise) from the service.

interesting, i didn't know that.

so you're saying that two different circuits on adjacent outlets likely have closer to the same ground (if everything was done right) than a different outlet across the room on the same circuit?

 

[seamonkey]

Ground lifts can cause a safety issue.
As per wikipedia:
"Ground lifts do not conform to Audio Engineering Society standard AES48"

The lack of good grounds would be the cause of a ground loop. If you get a ground loop hum then Fix the ground(s).

Read the links at the previous TB thread on this.