Double ground causing loud feedback from bass?
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
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.
This is nuts. The ground is for safety. Keep your amp grounded
Check this thread
Always ground everything, and get a DI with a proper ground lift.
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.
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.
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?
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.
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