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Speaker hum when I connect my external computer monitor

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Sean McPhillips
Speaker hum when I connect my external computer monitor
on Aug 3, 2008 at 12:45:11 am

Hi,

I've just bought a pair of KRK Rokit 5 powered speakers to plug into my MacBook Pro (via the unbalanced audio/headphones out plug) for monitoring audio out of Final Cut Pro (for example).

They sound great until I plug my external monitor (Samsung Synchmaster 205BW) into the MBP - at which point I get a loud hum through both speakers.

According to the Rokit 5 manual any video monitors have to be on a different circuit than the speakers, so I went and got myself a couple of extension leads and proceeded to plug the Synchmaster into every power point I could find in our office (seven in total), and got the hum every time.

Now there are four power circuits in this office so unless all the points are coming off only one of them for some reason, that doesn't seem to be the problem.

Has anyone else had this issue?

Any help appreciated,

Sean



Sean McPhillips


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Ty Ford
Re: Speaker hum -- An AC Power Tutorial
on Aug 3, 2008 at 3:28:20 pm

Hello Sean and welcome to the Cow Audio Forum.

My guess is that you have a ground loop, but this could be a symptom of a very dangerous AC power problem. You can have four outlets in the same room, all connected to the same branch of power, or on different branches. A ground loop happens when there is even a minor difference in ground potential between two different power outlets. If you measured voltage or resistance between the little screws that hold the cover plates on the outlets with a volt/ohm meter, you'd get a voltage and/or resistance reading. That's not unusual, but there also may be a wiring problem in the walls or at the panel. Be very careful when trying to find the gremlin. Consult with a professional electrician.

First try plugging everthing into the same outlet or strip. That will eliminate the possibility of ground potential differences from separate outlets. I'm not sure why they say to plug in somewhere else, perhaps because the device uses a lot of power and they are concerned that you'll blow a breaker. Try it anyway with just as much stuff on as you need to see if the hum goes away.

Here's another simple fix. If your power plugs are only two prong (like your Mac Book power supply), try rotating each one (unplug, put the plug in the other way) one at a time. That sometimes solves the problem.

Try running the Mac only on battery. When I did that a few years back with my Ti book, the buzz went away because trash from a funky DAT external power supply was coming back up the power cable to the laptop.

You may find that an ISOBAR power strip with transformer separated banks will remove or reduce the hum.

I use an Ebtech Hum Eliminator (by Morely) between my G5 and audio rack. It's designed to transformer isolate two unbalanced audio lines. That really helped a lot in reducing the hum I had when interfacing the unbalanced audio to and from my G5 with my rack. And it's SAFE.

SImple three prong-to-two prong converters on grounded power plugs may work, but be EXTREMELY careful. The grounds are there for your protection. I have an FCC license that says I'm an engineer. I know what I'm doing (most of the time), so I can break the rules a little. If you don't know what you''re doing you could get a nasty shock, kill yourself and/or burn the house down, not necessarily in that order. :)

We have all heard of musicians and vocalists who have been killed by an improperly grounded system. These systems were operated by professionals who thought they knew what they were doing.

You may find you need to employ one or more of these tricks to get rid of hum and buzz and you may still have a little left. Let's have great audio and let's be safe out there.

Regards,

Ty Ford




Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Sean McPhillips
Re: Speaker hum -- An AC Power Tutorial
on Aug 5, 2008 at 1:10:57 pm

Hey thanks for your advice Ty. I'll take your points one by one...

First try plugging everthing into the same outlet or strip

Everything was coming off the same strip to begin with - Computer (laptop plugged in with three pin Australian plug), 2 x powered speakers and external monitor. I then tried plugging just the monitor power into different outlets.

try rotating each one (unplug, put the plug in the other way)

Australian plugs are different from American ones - you can't rotate them. Also, everything was plugged in with three pin plugs.

Try running the Mac only on battery
Tried that - it changed the hum but didn't remove it.

ISOBAR power strip with transformer separated banks
I haven't been able to find any in Australia.

Ebtech Hum Eliminator
That's a possibility, is it anything like this?
http://www.minidisc.com.au/xitel-ground-loop-isolator-p-423.html

SImple three prong-to-two prong converters on grounded power plugs
Is this the kind of thing you're talking about? I'm a bit suss on this solution and am definitely not comfortable messing around with power plugs.
http://www.filmtools.com/grliadco41.html

I'm thinking the isolation transformer is my best bet at this stage.

Is a ground loop inherently dangerous or just a fact of life in some buildings?

Thanks again,

Sean

Sean McPhillips


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Ty Ford
Re: Speaker hum -- An AC Power Tutorial
on Aug 5, 2008 at 3:19:36 pm

Hello Sean,

back to you point by point.

ISOBAR power strip with transformer separated banks
I haven't been able to find any in Australia.
Tripplite makes them. Their website shows this in Australia.
http://www.tripplite.com/EN/products/model.cfm?txtSeriesID=324&CID=1&txtMod...

Ebtech Hum Eliminator
That's a possibility, is it anything like this?
http://www.minidisc.com.au/xitel-ground-loop-isolator-p-423.html
Yes, like these. Don't know about quality differences.

SImple three prong-to-two prong converters on grounded power plugs
Is this the kind of thing you're talking about? I'm a bit suss on this solution and am definitely not comfortable messing around with power plugs.
http://www.filmtools.com/grliadco41.html
Yes, these.

I'm thinking the isolation transformer is my best bet at this stage.
You may need a combination of things and find each reduces a bit here or there. Some combinations may work better than others.

Is a ground loop inherently dangerous or just a fact of life in some buildings?
Fact of life (not dangerous) but potentially dangerous if someone has carelessly wired the structure.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Sean McPhillips
Re: Speaker hum -- An AC Power Tutorial
on Aug 6, 2008 at 12:12:31 am

Thanks a million Ty, I'm gonna try to track down a transformer isolator.

I'll let you know how I get on.

Sean

Sean McPhillips


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Ty Ford
Re: Speaker hum -- An AC Power Tutorial
on Aug 6, 2008 at 2:00:30 am

Sean, you may find it's a combination of things that does the trick, but these are pretty much all of the tricks I know.

Yes, please do let us know.

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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