Power Amp Buzz

[BassMan Butch]

Hi All,

I am a newbie to the forum so I hope I found the right Forum . The issue I have is I am getting a buzz from my monitor speakers. I believe it is ground loop related. I have a Peavey amp for monitors and two crown amps for the mains. I have separate outlets for both. I disconnected all the inputs from the amps but I still get a AC hum from the monitor speakers. The amps are switched on via a hospital grade switches in the sound room. All of the amps are rack mounted. I did noticed that when I leave the main amps off the monitors are a lot quieter. I have ground lifts on all of the plugs. If I don't use them the hum is 100% louder. The mains drive a Bose Sub woofer and arrays. I also noticed that as I was bolting the monitor amp into the rack I could hear noise as I turned the mounting screw.

[Garydean]

Have you tried plugging everything into one outlet? Also sometimes when you stack or rack mount, just the proximity can create a buzz from other transformers. I would check that first. It's going to be a process of elimination as the buzz could be coming from a lot of things.

[Enzo]

If the amp hums with nothing plugged into the inputs, I start to doubt ground loops. WIthout the input cable, there is no path for a loop.

Pull the monitor amp out of the rack and sit it on a chair or something. No input connected. Does the hum still come out the speaker? If not, then slide the amp slowly back into the rack. Does the hum grow?

SOmetimes the field of the power transformer on some other piece of gear will couple into your amp circuits. I have had situations where simply changing the order of the stack in the rack made the difference. If moving the amp away from the others makes the hum die away, then that is what is happening.

Yes, sometimes you need to insulate the gear from the rack.

[BassMan Butch]

I might need to clarify the connection. I have 2 outlets that are feed from switches in the sound room. They are on separate breakers but from the same panel. I took the amp out of the rack and the hum got worse ( I didn't remove it completely just set it on some books. The amps are not humming, it is a hum in the monitor speaker. If I shutoff the power to the mains outlet the hum is minimal. Do I need to have both amps on the same leg in the panel ? What about the power to the mixer ? I will pull the amp out completely and see what happens. I did notice that when I bolted the amp back into the rack, I could here a sound when I was turning in the mounting screws in

[Enzo]

I realized you meant the hum came from the speakers, if I said the monitor amp hummed, I meant through the speakers.

Can we assume the amplifier volume control has no effect on the hum?

If the amps make hum sitting there with no input connection, then the mixer is irrelevant. If connecting the mixer increases the hum, then we can worry about the mixer.

If the two outlets are not off the same breaker they may well bo on opposite phases of you0r mains, and that could be involved.

[BassMan Butch]

When you mean turn up the volume on the amp do you mean the input levels.

I will check that out. Should I disconnect the input lines to the main amps to see if that makes a difference ?? Thank You for all you input I am going to pull the amp today and set it up separately.

[Enzo]

Sure, the more potential things we can eliminate the better. Yes, the input level controls. If they have no effect, then the hum is coming from the amp itself. If they turn the hum up and down, then the noise is coming in the input stage somehow.

[BassMan Butch]

OK,

Here are the results ....

1. I plugged the monitor amp into the same outlet as the main amps. The hum was noticeably reduced. It sounded just like when I only had the monitor amp on and the mains off.

2. I returned the monitor amp back to is original circuit and the hum got worse.

3. I cranked the main amps up all the way it did not affect the monitors.

4. With the inputs connected to the monitors I cranked the input levels up all the way. The hum got very loud. I then disconnected the inputs and cranked up the monitor input levels they were whisper quite (a slight hiss).

5. I will pull the monitor from the rack and repeat the test. I did not have any channels ON from the mixer just powered up.

Looks like I am making progress. Do you thing I should go to the breaker panel and check the power and neutral connections ??

[Enzo]

Well you found that if the amps are on the same branch they run quieter. That isn't a fault of the service panel. You could maybe swap breaker positions for your other outlet so it ran off the same phase as the first amp.

[BassMan Butch]

OK I will check the branch circuits. I believe they are on the are on the same phase.

Here is the latest....

I pullled the amp and set it apart from the rack. I plugged it in where the mains get their power. I still had a hum but it was considerablely less.

I decided to turn the inputs down and pull out the TRS patch cord from the amp. I have one channel AUX 1 going from the mixer thru a EQ and then to the amp. These are outputed to the floor monitors. The AUX 2 channel goes directly from the mixer to the AMP and then goes to hotspots. I found that when I pulled the AUX 2 patch cord the hum was minimal. I checked the cable end at the mixer and found a broken wire. I repaired the wire. I plugged it in and all seemed good.

I decided to put the amp back into the rack and after getting it all back together I power it up and the HUM is BACK!!!

I also noticed that the EQ unit had a slight buzz so I disconnected it and went directly to the AUX 1 channel out of the mixer. I checked the AUX 1 and AUX 2 AFL's via the head phones and they were whisper quite. I even cranked the gains up.... SO it appears that I am sending a clean signal.

The next step was to pull the inputs from the amp and I cranked the amp up to the max . The monitors were whisper quite... SO I obivious have a wiring issue somewhere !!
between the Mixer and the AMPS

Any Suggestions ????

[Enzo]

If the hum only exists when you have the signal cord connected between the signal source and the power amp, then it seems you have the classic ground loop. A ground loop is not a defect in any piece of gear, it is simply an artifact of the connections. You probably need a "loop buster." A unit designed to pass the signal without a complete ground path.

[BassMan Butch]

ENZO, Thank You for all of your help!!. Here is the final chapter.

I brought a AMP in and connected it to the AUX 1 and AUX 2 inputs then connected the monitors one at a time. I then switched AUX channels and the sound moved with the AUX 1 cable. This AUX was going thru a EQ Unit. I bypassed that switched to another output cable on the snake and YAHOO quite as a mouse !! I pulled the orginal amp from the rack and placed it in a chair by itself.... It was quite (not as much as the amp I had brought in but the hum was only noticable if you put your ear right up to the monitor.

Conclusion : I had a bad connection on the Aux 2 cable. I had a bum EQ unit on the AUX 1 channel. I also repositioned all of the power cables away from any signal cables. I am going to insulate the amp from the rack and reinstall it. I tighened down all the electrical connections. Hopefully I am good to go.. Wish I could afford a digital mixer