Brand new noise coming from G5 into audio signal

September 7, 2007 7:47 PM

I have a Mac G5 and it is connected via the analog audio output port to my stereo receiver, and a new humming buzzing just started.

I connected it last week so I could play music from the G5 through my stereo speakers. It worked fine. Today there is a loud humming noise that comes through the speakers whenever the G5 is selected as the stereo input. Any other stereo input does not have the humming noise. It's definitely an electrical type noise.

It's so loud that there is no point playing the music.

If I unplug the cable from the G5 and plug it into any other audio source, the humming goes away, so it's definitely the G5. It's not interference from a nearby object, because I can put a laptop next to the G5, and use it as the audio signal, and the humming goes away.

The noise is there whether the G5 is powered on or not. I've plugged it into different electrical outlets with no change.

The noise is still there if I move the audio out cable to the headphone jack of the G5.

There is no noise when playing sound through the G5 internal speaker.

There is a common problem with the G5 Mac power supply making noise that leaks into the audio signal, but that is a problem that is inherent in the power supply, not a problem that develops over time, so I don't think that's it.

There was no problem a week ago. I can't think of anything that changed since then.

posted by lockedroomguy to computers & internet (18 comments total)

It's called a ground loop. You probably changed how the stereo was plugged into the wall, or the laptop to stereo cable changed somehow, or something like that. Someone else will probably post something better than this, but you can get started by googling "ground loop".
posted by intermod at 8:09 PM on September 7, 2007

Also check whether any of the connecting audio cables are lying in proximity with a power cable (any power cable), as the EM field created can induce the hum just by being nearby. Especially if your stereo has any unshielded wires, when plugging in a cable you might have moved some wires aside or otherwise changed something such that an audio cable is now lying close by a power cable.
posted by -harlequin- at 8:24 PM on September 7, 2007

Try a ground lift, just to see if that helps. Also, Fluorescent and neon lights are famous for causing this, as are lamps with dimmer knobs, when they're partway dimmed.
posted by Devils Rancher at 8:30 PM on September 7, 2007

Also, there is one check you didn't mention - to determine whether the problem has appeared in the mac or the stereo, plug an mp3 player in to the cable (without moving the cable) instead of the computer and see if you still get the hum.
posted by -harlequin- at 8:33 PM on September 7, 2007

Sound like a ground loop, I've had this problem numerous times hooking computers into audio systems. First thing I'd try is making sure both the computer and stereo are on the same electrical circuit by running them off the same outlet. If that doesn't work, ebtech's hum eliminators are an easy fix. They're pricey at about $50 but I love em.

Alternatively, If you're handy with a soldering iron, you can make a cable with the shield disconnected at one end (might work), make a cable with the shield disconnected at one end and the signal wire in series with a ~.1uF or so cap (will probably work), or use an audio transformer, which will probably be audiophile quality and a bit more expensive ($100+) and will almost certainly work and sound better than ebtech's cheap transformers.
posted by waxboy at 8:35 PM on September 7, 2007

Also what -harlequin- and Devils Rancher said.
posted by waxboy at 8:37 PM on September 7, 2007

Harlequin, I think I did mention that check - I unplugged the audio cable from the G5 and plugged it into the laptop. Is there some reason plugging it into an mp3 player would be a different test?

Also to Harlequin, I think that covers your first point--whether the audio cable is attached to the G5 or the laptop, it's still in the same position, and running past all the same cables. But it works from the laptop.

Everyone else, can it still be a ground loop when it was working before, and everything is still connected identically?

There has been no change in the electrical outlet the stereo is plugged into, or the electrical outlet the G5 is plugged into.

I have no dimmers, and no fluorescent lights anywhere.

I can start treating it as a ground loop and try the fixes suggested--just checking that that still makes sense given the conditions. It worked, and now it doesn't.
posted by lockedroomguy at 8:55 PM on September 7, 2007

The fact that the noise only appears when the computer is connected to the stereo strongly suggests that a ground loop is the problem. Most other noise sources would be prevalent regardless of whether the cable between the two is plugged in or not. You can test for a ground loop by running a wire from a grounded part of the computer, such as exposed metal on the case or even the outermost ring of the audio out, to the outer ring of the RCA input on your stereo or some exposed metal on its enclosure. A friend will be of great help here. Ground loop noise travels via the cable shield which is connected as such internally, so you won't need to have a signal connection to produce the noise. If you hear the noise with this connection in place and not without, you've got a ground loop.
posted by waxboy at 9:07 PM on September 7, 2007

Thanks for all the answers. It does sound like a ground loop. The only reason I've been doubting it is that I have the stereo plugged in to one outlet, and the computer plugged into another. They have both been plugged into those same outlets for the past week.

I haven't installed any new equipment, or plugged anything else in to those sockets, or anywhere else for that matter.

Shouldn't the ground loop exist as soon as you plug the components in, not after a week of use? Could the ground paths have changed by themselves?

I understand that this argument kind of applies to anything that could be wrong (if NOTHING changed, then nothing would be wrong)... just checking to see if this makes anyone think of any other possible issue.
posted by lockedroomguy at 9:38 PM on September 7, 2007

Shouldn't the ground loop exist as soon as you plug the components in, not after a week of use?

No. That is not the case. If you plugged something else into either circuit since then, that could change the circuit dynamics and cause the grounds to move relative to each other.
posted by Steven C. Den Beste at 10:12 PM on September 7, 2007 [1 favorite]

Thanks Steven, but as I said:

"I haven't installed any new equipment, or plugged anything else in to those sockets, or anywhere else for that matter."

Which is why I'm asking whether they could have changed by themselves, or by some other means that hasn't been suggested yet.
posted by lockedroomguy at 10:23 PM on September 7, 2007

You know, you could probably rule out a ground loop just by temporarily plugging both the stereo and the computer into the same 6-outlet strip side by side, yes?
posted by davejay at 11:29 PM on September 7, 2007

Harlequin, I think I did mention that check

Oops, I misread that paragraph. Sorry.
posted by -harlequin- at 2:44 AM on September 8, 2007

You know, you could probably rule out a ground loop just by temporarily plugging both the stereo and the computer into the same 6-outlet strip side by side, yes?

That might work, but you can still get a ground loop with everything plugged into the same outlet if there are internal resistances contributing to the effect, although that is not very likely. Plugging everything into the same power strip cures most ground loops.

It does sound like a ground loop. Cable position is not the issue as when you plug it into the laptop you do not get noise. The laptop is isolated absolutely from ground when you have it unplugged, and the low voltage portion is likely isolated even when it is plugged in.

There was no problem a week ago. I can't think of anything that changed since then.

It could be something else you plugged into the same circuit as either the G5 or stereo. It probably is not a wiring issue in the house wiring, but if a ground is loose that is quite a bad thing. If it goes away when you plug the G5 and stereo into the same outlet you might consider having your wiring checked. A wire that is not well connected at an outlet can lead to a fire or shock. If it doesn't go away when they are both plugged into a common outlet it is likely something in the stereo or G5. It could be a solder joint starting to fail, although that is not common. If you have a sound card it could be oxide on the connectors. If you have a multimeter you can check, with the unit unplugged, the resistance between the ground of the sound card output and the ground on the plug and similarly check the stereo. It should be zero or very close. With the unit plugged in you can check for voltage differences between these points and also between the G5 and the stereo, but make sure you know which is the ground hole on the outlet. If you can't get the voltages in line the hum eliminator will do the trick.
posted by caddis at 6:20 AM on September 8, 2007

It's probably a dirty plug. Whichever end has the 1/8" headphone-style connection is just cruddy, and one contact isn't making good contact. Plug it in and out a few dozen times, or use an alcohol-moistened pipe cleaner to get the crud out. See if wiggling or torquing the plug makes it go away.

Could also be a damaged cable. Got a spare?
posted by Myself at 6:53 AM on September 8, 2007

Davejay, that's a good suggestion but a major undertaking.

Caddis I haven't plugged anything else into any circuit at all. And it cannot be something in the stereo since when I use a different audio source with the same cable and stereo input selection it doesn't happen. Thanks for the rest of your suggestions, though, they're good.

Myself, it can't be a dirty plug or jack. It worked a week ago and the audio cable has been plugged into it the analog out jack that whole time.

I think what I'm going to do is get one of those little ground testers and see if the socket went bad in the past week. It's the cheapest and easiest thing I can do. If that is it, I'll have the ground problem fixed. If that isn't it, then I think I'll unplug the G5 from the stereo and get a couple of amped speakers, since that will be cheaper than any other path I can take from here.

Thanks everyone!
posted by lockedroomguy at 7:43 AM on September 8, 2007

it cannot be something in the stereo since when I use a different audio source with the same cable and stereo input selection it doesn't happen

That doesn't rule out the stereo. The other things you plugged in were not grounded to the house wiring ground (laptop).
posted by caddis at 8:07 AM on September 8, 2007

Caddis, that's a good point. I didn't plug the laptop in when I did the test. Since it's a Mac it does in fact have a ground prong so I'll try that and report back.
posted by lockedroomguy at 8:29 AM on September 8, 2007