Coping with a Ground Loop

[thelatinist]

When my theater was built, our amplifier cabinet was wired on a completely different panel from our sound booth. Indeed, it's on a completely different leg of the building's 600A service. I know this is not an ideal situation, but it's not something that I can change for the foreseeable future.

As you can probably foresee, I'm suffering from fairly consistent speaker hum which persists even when I've taken out all peripherals. It varies somewhat depending on what else is going on in the building (we're in the same building as a gymnasium). I'm wondering if there is anything I can do to mitigate this issue short of rewiring to try to fix the ground loop. In particular, I wonder if some sort of isolation transformer might help and, if so, where such a transformer should be placed for best effect.

Thanks in advance for your help!

 

[themuzicman]

Place an Iso on the service causing the issues -- if it's the console, place one there. If it's the amps, place one there. Can you not just run power from your amp rack to FOH?

 

[JD]

So, the stage and amp rack are grounded off one panel, and the board/booth are grounded off a different one. Running a dedicated power line to the booth from the amp rack panel is the best option, but.... First, make sure that is your problem. The power cord is not the only injection port for ground loops. I once tracked one down at a theater that had the stage audio feeds via a jack plate in the booth. Tried ground lifting everything in the world but could not get rid of the hum. Finally unscrewed the jack plate and discovered that all of the XLRs had pin 1 jumped to the shell tab. So what was happening was the loop was injected by the building frame / conduit ground, not an electrical ground at all. Snipped the jumpers and no more hum!

 

[thelatinist]

I know that the best solution would be to put FOH and the amps on the same circuit, but, as I said in my first post, correcting the wiring issue is not feasible at this time. Believe me, I'm looking into it in the longer term, but I'm looking for a solution that will help me cope with the issue in the shorter term.

The real issue is that whoever installed the system subbed the panel that serves the amplifiers (and house lights) off of the feed for the dimmer rack. Ultimately, it needs to get off that panel and onto the panel that feeds FOH. But I'm sure the reason it was done that way was to avoid having to drill through the concrete floor and fish wires extensively -- and, as I say, there's not money in our budget to redo it right now.

 

[rwhealey]

There are other problems that cause hum unrelated to ground loops (especially with dimmers), but a ground loop is a good place to start.

Option #1 is to lift the shield at the amplifier/DSP inputs, if they aren't lifted already - it is fairly common installation practice to do this anyway, even if there isn't a problem. Make sure you do this for all the audio lines headed from FOH to the remote location.

Option #2 is to put an audio isolation transformer at the output of the sound board: http://www.jensen-transformers.com/product/po-2xx/ Again, you will need one channel of isolation for each audio line going from FOH to the amp room.