Hum from PC driving me crazy!

drummer86

Scenario: In my classroom at work I have a computer connected to a Yamaha portable PA system (StagePas 400i) to run sound for various purposes. I have a Roland keyboard, my Taylor guitar, and the PC all hooked up to the built in mixer. Instruments using standard instrument cable (no issues here) and PC using a 50' 3.5mm to RCA from Monoprice. Whenever I use the RCA connected to the PC I get a VERY loud buzz/hum from the speakers that is not present at all with other sources.

Things I've tried:
-A different source connected through same cable: no hum
-Different cable from the same source: hum still present
-Unplugging the computer from the wall: Hum still present!
-Switching power strips/surge protectors: Hum still present
-Using both the Audio Out and Headphone jacks: Hum still present

It boggled my mind when the buzz didn't stop when I unplugged the computer from the wall.

Any other ideas or probable causes? I'm going positively mad here. The stagepas is new and works/sounds great with everything except this PC.

drummer86

One more thing: When I plug the PC into the small desktop stereo I have it DOES NOT hum.

KurtBJC

Hm. I'd figure a ground loop, which you could break up with an isolation transformer (or two if stereo), but for the unplug-the-computer bit. When you unplugged the computer, did you also unplug anything it was connected to (e.g., the monitor)? You could be completing the ground loop through the grounding pins on the monitor cable.

Kurt
Blue Jeans Cable

drummer86

I turned the power strip off but didn't unplug everything. I did, however, unplug each thing one at a time from the back of the tower and nothing stopped the hum. Could it be that the computer and amp are plugged into different outlets on opposite sides of the room?

steveinaz

Intermixing of power sources can really complicate isolating a ground loop hum. Wish I had some ideas.

Have you ever tried chasiss-grounding the computer?

drummer86

Doesn't help that part of the building was built in 1970 and partially renovated in 2000 and that the computer isn't mine to work on. Was hoping there would be something obvious that I was overlooking. I'll keep farting around with it.

Speedskater

What happens when you use a long AC extension cord, so that everything can be powered from the same wall AC outlet?

drummer86

Speedskater wrote: »

What happens when you use a long AC extension cord, so that everything can be powered from the same wall AC outlet?

That's a good idea. I'll probably try that next. Trying to avoid a bunch of cords everywhere but I'd rather trip on a cord than listen to that buzzing.

KurtBJC

drummer86 wrote: »

I turned the power strip off but didn't unplug everything. I did, however, unplug each thing one at a time from the back of the tower and nothing stopped the hum. Could it be that the computer and amp are plugged into different outlets on opposite sides of the room?

I'd unplug everything, all at the same time. Bear in mind that the ground prong isn't going to be switched either in your power strip or in your devices, so as long as there is some path to ground on that end, you're going to have a ground loop. Turning devices, or power strips, off won't help.

If it's a ground loop, the easiest solutions are either (1) find a way to plug things in that doesn't result in the hum, or (2) use isolation transformers on the audio lines to break the ground connection.

recoveryone

Radio shack use to sale a patch cord with some type of dc isolator in line to cut out any humm from laptops. I use it when I am asked to do retirement parties and use a laptop with my external HD to play music through a old 12 channel mixing board. Before I got that patch cord you could hear the HD spinning up and a consitent hum in the system.

Found a pic of one

Speedskater

Those little Radio Shack isolators work way better than the printed specs would lead you to believe.

drummer86

Speedskater wrote: »

What happens when you use a long AC extension cord, so that everything can be powered from the same wall AC outlet?

Update: I plugged the PA system into the same outlet as the computer and the humming is all but gone. Still a hint of it when I'm not playing any sound at all and really crank the volume but I'll never have it this high for playing music/videos.

I guess I dismissed this idea when I was troubleshooting because the hum wasn't present on my keyboard or guitar. That led me to assume that the wall outlet I was using was clean enough and that the culprit was the computer. I'm no electrical engineer so can anyone explain why this happened?

Glen B

You probably had a ground loop from the PA system and ground loop plugged into different AC outlets. The two outlets could have been on circuits powered from opposite 120V legs in the electrical panel, or the two circuit grounds could just be at slightly different ground potentials. Either one would cause ground currents to flow between the two branch circuits, and result in noise.