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PostPosted: Mon Jun 03, 2013 5:44 pm 
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Marker Magician
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I am helping a colleague here at my school. This is a brand new school, so the wiring is just about one year old now. In the auditorium there are floor access pots, with power and DIN plugs for the lighting board. Ther is no audio access in the thes floor pots (????).

For stage audio, the school owns a very good Allan-Heath board.

The young guys in the drama/tech class asked me to help them sort out a hum. They have the board running to a wall mic input (XLR). This goes to the amplifier rack in the tech closet on stage. It seems as if every time they plug the board into the floor power receptacle, the stage amp emits a rather load hum. The board need not be powered up. the hum starts as soon as you plug its powere cord into the floor receptacle. Power up the unit and the hum changes a bit, but is still very evident.

To trouble shoot, I plugged the board into a portable amp/mixer we have for use away from the auditorium. No hum. (amp and mixer both using the same power circuit).

I then used an extension cord to draw power from the backstage receptacle, into which is plugged the amplifier rack. The hum does not appear in this circumstance either. So, I assume I am looking at a ground loop issue. Before I call in the electrician, any suggestions as to what might need be done?

The board does not have a ground lift switch upon it that i have found.

Any electrician types out there who may be able to advise me, or ask some questions to help me problem solve?

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PostPosted: Mon Jun 03, 2013 6:20 pm 
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HamelnStock Survivor and Midi Guru
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Hey Byron,

50Hz hum ? Are there any audio leads towards the amp running in parallel to or close by the power leads that feed power to the board ? I would disconnect all audio cables from the amp, switch on the circumstances that usually create hum. Then plug audio leads back in one by one untill the hum re-apears. I suspect one audio lead picks up a power cords electric field. i supose you know all this anyway ... just trying to be helpfull.

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PostPosted: Mon Jun 03, 2013 9:02 pm 
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Kind of what i have done so far, but I have not completly exhausted all the connections, as the unit is installed in an auditorium.

I will post again when I discover the source. Any other suggestion welcomed, as I may need a memory jog to help me along the way.

Thanks Dirk.

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PostPosted: Tue Jun 04, 2013 3:55 pm 
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Im not sure of the configuration exactly..but maybe a di box with a ground lift?

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PostPosted: Tue Jun 04, 2013 7:02 pm 
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That make sense Beer. I'll try that.

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PostPosted: Wed Jun 05, 2013 12:56 am 
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Byron wrote:
I then used an extension cord to draw power from the backstage receptacle, into which is plugged the amplifier rack. The hum does not appear in this circumstance either. So, I assume I am looking at a ground loop issue. Before I call in the electrician, any suggestions as to what might need be done?


Hi,
You already tried powering everything to the same power line and problem is gone if I understand correctly?
First I'd look into power-safety issues, because if there's any possibility of different ground potentials (not sure if this is correct eng term, sorry..), it could be dangerous too: Don't remember who it was, but had guitar plugged into amp (1st ground) and touched mike with lips (2nd ground) = killed him.
At the end it might still be a job for electrician...

Other - I'm sure you know alot about xlr cables and ground on/off... sometimes it's just a cannon connector chassis ground problem, not cable shield. This one is time consuming, I know... good luck :wink:


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PostPosted: Wed Jun 05, 2013 7:56 am 
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HamelnStock Survivor and Midi Guru
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You could be on to soething Dejan. I remember when i was out recording i had to be connected to the same power source as the pa providing the audio signals. It is true that ground differences can cause dangerous situations.

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PostPosted: Thu Jun 06, 2013 12:48 am 
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Yeah, ground hum occurs when too different circuits grounds "see" each other. For example. Mixer plugged into outlet a. Powered speakers are connected to outlet b, but then with a speaker cable to the mixer. Ground a and ground b have suddenly met. HUUUUUUUMMMMMMMMMMMM!

Atleast that's how I understand it. I've yet to solve a real hum problem though. LOL

-= Beer

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