Global Hummmmm

[skorper]

I am working on a church that was built in 1968. Everything that has a speaker in the building has a 60 cycle hum in it. The building ground is one ground rod in the main electrical room that is connected to the cold water pipe. (There are no prints for the building.) It looks like the cold water pipe is used for the ground for all three breaker panels. (Zinsco). A basic outlet tester shows that the ground is intact. I loosened and retightened the visible ground connections in the main electrical room. I also shut off ALL of the circuits that were not connected to a 'humming' device. NOTHING seemed to help. My question is-Is there some definitive way to diagnose this problem?

 

[hey_poolboy]

Start with checking to see if any of their audio equipment has ground lift switches in it. Go through one at a time and turn each one on and off again until you find the source. Especially equipment located remotely, such as amps, Mic pre-amps, etc.

These buttons do not remove or interrupt your equipment ground, rather they lift the ground connection to from the shield in the mic/instrument cable.
It is possible there is RF noise on one of the lines, and this should eliminate the noise.

 

[skorper]

All of the equipment was just moved from their previous location. It all worked just fine there. If these devices DID have 'ground lift' switches, could just ONE being in the wrong position corrupt the ENTIRE building?

 

[hey_poolboy]

It's possible. This may get nasty, but bear with me. I am an audio geek.

Depending on what mixer they have you should be able to get a pre-fader listen. This allows you to listen to a channel before it goes through the fader in the mixer. See if the hum is present there, and on which chanels. If the hum is present check to see if it is plugged into a direct input box. (usually a small black box on the stage for an instrument to plug into) Often a DI box with the GL in the wrong position will cause hum.

You could alson mute all of the chanels with the equipment still on and see if the hum is present. If that is the case I would look for a GL switch in tha amps, or the mixer. You don't want to lift both, just one. If you lift both you lose the ground to the shield if it is shielded cable. Try lifting the ground on the amps first, or other siganl processing equipment after the mixer.

Hums can also be poor connections. We have a portable system for our church that gets set up and torn down every sunday. You's be suprised how often there is a loose connection when setting up a system.

Hope it helps

 

[skorper]

I will try your suggestions after Sunday service tomorrow.
Thank you very much for all of your help.

 

[Rockyd]

Besides the usual ground and nuetral being tied together at all the usual wrong places, been there with a milligauss meter?

Mike Holt had an article in the miscellaneous when you first click into the forum - here's the address ~

http://www.mikeholt.com/mojonewsarchive/All-HTML/HTML/Audio-Hum-Solutions~19990709.php.

Other possibility might be to check with Karl Riley (he's one of the moderators, that is also an EMF guru) for possible external problems messing with the church (stray voltage and such).

Good luck in your pursuit.

Sounds like there is already better help with the particular issue than what I can be, without looking at the actual problem (I make no claimto be an audio espert).

 

[RayS]

well, if it worked fine at the last install, and all the speakers hum now, I'd try to find the common link to the amps and start disconnecting from the amp end till you find the hum stops. Then, try to find the problem with that link. 2 possibilities come to mind- 1. ground loop, which you can try to temporarily eliminate by using isolation transformers on the signal line (hopefully balanced) at each end (radio shack or guitar center). If this works, try lifting the ground on one end (generally leave the sending end on, but try both) so you don't need to leave the transformers inline.
2. induced noise, with the run too close to something like power wiring. First and best solution is to move it- don't have it parallel, and if you have to cross it try to go at a 90deg. angle. If it's not balanced, make it so.

 

[skorper]

There are two mixers in two separate rooms. Their speakers both hum. Even if I turn one of them off, the other still hums. I have also shut off all the breakers, one at a time, in trying to find the source. No luck.
Could the water pipe being used as a ground buss cause this problem?

 

[RayS]

another thought- maybe power circuits in the area that are not run correctly to cancel their fields- like a 3-way switch run that doesn't have a neutral routed with it to cancel the EMF. Sounds like (no pun intended) something nearby is radiating into the system, rather than a power issue.

 

[brian john]

This is most likey a neutral grounded downstream from the main service. Check for neutral to ground shorts.

 

[LarryFine]

I would start by eliminating the building wiring by temporarily powering everything in the system from a single circuit using extension cords if necessary.