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Church and H.O.W. Forums for HOW Sound and AV - Your Displayed Name Must Be Your Real Full Name To Post In The Church and HOW Forums => Church Sound Archive => Topic started by: ekemini on September 13, 2004, 09:44:57 am

Title: Ground Loops
Post by: ekemini on September 13, 2004, 09:44:57 am
Hi everybody.
I've got a new sound system in a new buiding that has been working ok for about 3 months. Now all of a sudden, there a noticable ground hum on both main outputs and all monitor outputs. Its even there when all the channels on the desk are muted. Turning the levels down on the outputs gets rid of it, so I know its from the desk and not the amps. I believe its some kind of ground issue, but have no idea what to look for or why its only just come to light. Any ideas anyone?
Title: Re: Ground Loops
Post by: AVsupport on September 13, 2004, 11:31:31 am

There are many variables that could contribute to this problem.  If the system was fine for 3 months and the "hum" just appeared, I start by analyzing what has been introduced into the system recently.  Just bringing your faders down is not a completely accurate test.  If you can, disconnect F.O.H. from the system (make sure you mute and power down the P.A. and Monitor rig. If the system is turned on and clean with the F.O.H. disconnected, your problem probably is at F.O.H.  Also check and see what "non" A/V gear has recently been plugged into the A/V electrical system.

Disconnect all input and outputs from the console.  Start by introducing the main outputs one by one, then the channel inputs, then your EFX, then your outboard processing.  By using a systematic approach like this you should find the "area" fairly quickly that needs further looking into.  Good Luck!

Dave Miranda
Title: Re: Ground Loops
Post by: John Roberts {JR} on September 13, 2004, 12:55:16 pm
Ah troubleshooting 101...

You state that you can stop the hum by "turning down the outputs". Does this mean you are turning down the console master faders or the power amp inputs?

This is an important difference because if you can kill the hum by turning down the console masters the hum is localized inside your console. If the hum is only killed by turning down the power amp inputs there is probably an interface issue between the desk and power amps.

We need more data....

Title: Re: Ground Loops
Post by: ekemini on September 13, 2004, 07:47:47 pm
In response JR's question, I meant when I turn down the master faders on the desk, or the monitor faders for that matter. I have also tried what David suggested and I still have a hum with just the master outputs and nothing else plugged into the desk. Its clean if I unplug the master outputs. The entire sound system is run off a single one-phase 32A circuit breaker, and nothing else plugs into it so I'd really not know where to start if I was to go looking for the guilty electrical appliance elsewhere in the building. Actually, now I think about it, we recently had a 100V background music system installed. does that sound like a probable culprit?
Title: Re: Ground Loops
Post by: John Roberts {JR} on September 13, 2004, 08:21:51 pm
OK, it appears that it's in the console and apparently not coming from external gear.

Most obvious vector from your previous description that channel mutes don't kill it is the console power supply (or perhaps low mains voltage).

For multiple console outputs to be dirty doesn't sound like a single circuit fault (you haven't taken any lightning or static hits recently?).

Title: Re: Ground Loops
Post by: AVsupport on September 13, 2004, 08:37:09 pm

you mentioned a 100v system that was installed.  Is it tied into the main P.A system?  If so where and how.  It is possible it could be "inline" between your console outputs and your amplifier inputs.  I would imagine you have some sort of eq and processor between the two.  Sometimes people will "split" off the eq or processor for the 100v system when they should be using an output or DA.  

Dave Miranda
Title: Re: Ground Loops
Post by: ekemini on September 14, 2004, 07:37:25 am
The 100v system is not tied to the main system at all. It actually not even physically near it. I mentioned it 'cos it was the only recent installation I could think of and I was thinking along the lines of the audio transformers possibly messing up the AC supply?