Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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ground loop with a two pronger..

Posted by jneutron (A) on August 18, 2006 at 07:57:22

The system:

A pair of eminence delta 12's with selenium d205ti's on stage.
One QSC RMX 1450 on stage, fed unbalanced from:
Aaron gavin 11 band eq, built in 1978 and modded in '95, in the same rack box as the amp, fed unbalanced via an xlr which has left and right in a 125 foot long mike cable (neglects left-right bleed), if ground fails it will get loud).

I have a snake made of a 100 foot 14/2 extension cord, one 125 foot mike cable for a stage mike, one 125 foot mike cable used for the left-right feed, and a 140 foot rg-6 as a video feed to an on-stage tv.

The tv, amp, eq are all plugged into a 6 outlet box, just some cheapie extension cord thingy. Also, the snake cord is plugged into this.

In the balcony, I have a denon DN-1800f dual cd player and a numark GM100 mixer in a rack box. They are fed AC from the 100 foot cord, and pull 20 to 40 watts power max. None of the equip in the balcony is electrically connected to anything else in the balcony.

Part of the show requires playing a Spongebob video clip (yummie), while playing the audio through my system. Last year, I used my own DVD player, and all was well.

This year, they purchased a new DVD player, some small, inexpensive thing. I plugged it into the system (it's ac feed is also from the 100 foot cord, via an outlet pair on the back of the rack), connected the rca audio feed to my mixer, and the rg-6 feed to the stage. And, lo and behold, lots of hum/noise...all 60 cycle derived.

The kid I have running the sound for the show despaired...it was not tolerable..

So what did I tell him to do?...

I told him to wrap the two rca audio feeds from the DVD around the line cord of the DVD...tightly, all the way to the rack box.

Hum and noise went away...(so did his "deer in the headlights" stare).

The DVD was a two pronger, polarized to prevent reversal..

I reduced the loop between the line cord and the audio cords. Even though there is no hard ground there, the loop matters.

I love it when a plan comes together..course, he still thinks I'm nuts.

Cheers, John

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