Odd Ground Loop

[Eboy87]

Ok, so I wanted to get yous guy's opinion on this. The other week, I spent the better part of two hours tracking down a buzz in my system, at least until the SM called places.

Rig: Apogee AE5 mains (1 per side) over Bag End dual 18" subs (model escapes me). Stage fill was another AE5 per side. The mains were driven by an Apogee processor at FOH (A5?) feeding two old JBL amps (Urei 6725 IIRC). The side fills were driven by a Crown XLS402. Yamaha LS9-16 console. The problem was downstream from the console.

Problem: Up until this point (had to have been Thursday of last week, maybe Wednesday), the PA had been running swimmingly. When I powered up before the show, I had a nasty 60Hz hmm coming from the house left stack (subs, mains, and fills), but the house right side was acting normal: no buzz. All our cables run down the same path (signal, speaker, and power). The entire rig is driven off the same circuit at FOH (side fill amp was on-stage, fed from this circuit). I switched cables coming out of the console, going into the amps, even unplugged the power cable running to the XLS. Nothing seemed to get rid of the buzz. Both my boss and myself had never seen, heard rather, a hum quite like this out of only one side of the PA.

THe next day it was fine, but we have our open house coming up here next week that we need the PA for. I want to be ready to tackle the problem from a different angle should this crop up again. Does anyone have any thoughts of what I can try different next time? Has anyone ever heard of a ground loop in half of an amplifier before? Our amps were running in stereo mode. If you need more information, please ask and I'll do my best to answer. We're sort of stumped at this.

 

[TimmyP1955]

I've had this a few times. The cause was dirty XLR contacts causing a balanced line to go one-legged. Break and make all of your connections a few times to rule this out.

 

[NUSound]

I've been lurking around these forums for a little while, here's my .02

I had a really nasty buzz a couple weeks ago tracked it back to the patch snake running from my main snake to the amp rack. It looked like whoever had setup it up let the patch snake hang by just one connector (House Left of course) and the cable was kinked and visibly broken probably causing the exact same problem (broken shield) that's described above. Cut it down and re-soldered, good as new.

If it's a really noticeable issue, it's probably going to end up being a bad cable. Ground loops usually aren't audible over program material like this would have been.

 

[museav]

As I read it, Ian tried disconnecting cables and even unplugged the amp and still experienced the noise. If the noise is present with nothing connected into the amp and with no power to the amp it is probably not caused by a broken shield or dropped leg in one of the audio lines.

You might try disconnecting the outputs of the amp or even swapping output channels. You could also disconnect the outputs and see if you measure voltage between the conductors going to the speakers. Are there any lights, transformers, dimmers, etc. near the speaker runs in question that might only affect in under certain conditions that happened to occur at that time?

 

[Eboy87]

Ok, a bit more info, err... clarification. We've ruled out the cables. I disconnected them from the amps and still have a buzz through house left. The main amps live at FOH with me (40 feet from the stage), but the amp for the side fills was on stage (behind the subwoofers actually). Both the main amp and side fill were exhibiting this increasingly annoying hum. I swapped speaker cables, and the buzz followed to the house right stack. I haven't tried putting a meter on the binding post outputs.

Here's a photo of the house right stack.

<img src="http://farm3.static.flickr.com/2749/4357587157_acaff426ff.jpg" width="375" height="500" alt="The Trough" /ima>

The side fill is on the box boom aimed at the stage, while the main is on the stick in front of it. The house left stack is exactly the same, including the lighting positions. Dimmers live in a loft backstage right, and my understanding is they're on their own power.

 

[museav]

So the buzz is there with no inputs connected and it follows the amp's left channel output. However, it is happening with two separate amps in different physical locations.

This reminds me of a mean trick. Someone caught a live fly, took off the windscreen on a mic, put the fly in it and put the windscreen back on the mic. Imagine the frustration trying to get the buzz out of that system!

 

[TimmyP1955]

Two guesses:

Speaker cable has been damaged and one of the conductors is shorting to something metallic in the building, causing a ground loop.

Speaker cable is picking up EMI or RFI that is sufficiently strong that it is entering the amp's input stage via its negative feedback loop.