speaker noise

[mattt]

The problem is with a speaker thats tied into a smart board set up when we plug the speaker into the 110v outlet there is a loud humming if I disconnect the connector from the computer [ie the speaker input ] the hum stops.
also if we feed the speaker from a different outlet there is no hum at all
and yes I checked all the grounds and replaced all cables the run between the smart board and the computer Thanks Matt

[Dennis Alwon]

I have no idea but the first thing I would do id to make put the outlet that is causing the hum on the same phase as the outlet that did not cause a hum.

[LarryFine]

also if we feed the speaker from a different outlet there is no hum at all
I have no idea but the first thing I would do id to make put the outlet that is causing the hum on the same phase as the outlet that did not cause a hum.
Better yet, from the same receptacle.

[ray cyr]

Probably a 60Hz. hum, quite common in the audio world, check for a loop in the grounded conductor (ground loop).

[BushCreaM]

Try disconnecting the ground prong where the amp is plugged into the outlet.

[George Stolz]

Try disconnecting the ground prong where the amp is plugged into the outlet.
Please tell me you're joking. :-?

[HighWirey]

"Try disconnecting the ground prong where the amp is plugged into the outlet"

"Please tell me you're joking".

George,

We know he meant by using an isolation transformer, don't we? Certaintly not the alternative!

Best Wishes

[LarryFine]

Please tell me you're joking. :-?
As a temporary trouble-shooting method, it's okay to use a "cheater" to see if ground-looping is the problem.

[iwire]

As a temporary trouble-shooting method, it's okay to use a "cheater" to see if ground-looping is the problem.

FWIW I don't think it is unless your wearing proper PPE.

You could be exposed to live parts.

[LarryFine]

FWIW I don't think it is unless your wearing proper PPE.

You could be exposed to live parts. If that were the case, wouldn't the EGC have caused a breaker to trip when it was intact?

[iwire]

If that were the case, wouldn't the EGC have caused a breaker to trip when it was intact?

No, not necessarily at all.

Just like many appliances will run on a 'normal' circuit but will trip a GFCI immediately.

Look let me say that I am not trying to tell anyone how to work and I have never said that I have not broken the live work rules.

But when anyone suggests doing some work like this I feel it is important to point out that there are rules and safety procedures for hot work. And more importantly there is almost always a safer way to do things they just may take longer or more skill.

[ELA]

I would like to add that I would also temporarily remove the EGC as a troubleshooting method only. And with care. As long as it is not left unattended and precautions are taken it is a good and quick tool.

The key is to be aware that the chassis of the equipment may become "live" . The filter capacitors from line to ground in the EMI filter can then lift the case to 60volts and the leakage (in older filters) may be enough to trip a GFCI.

That being said, if there is a ground loop it is possible there is also a secondary grounding path.