MacBook Pro audio out buzz ONLY when connected to extra monitor?

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MacBook Pro audio out buzz ONLY when connected to extra monitor?

Hi all - just wondering if anyone here can help diagnose a problem...

I have a (late 2006) 15" MacBook Pro. I've been happily using it for 2 years with the onboard sound. Now I find myself wanting to connect it to an extra monitor (using the Apple DVI-VGA connector), only...

...when I connect it to the extra monitor a buzzing sound comes over the audio out (connected to a Mackie 1402 and from there to speakers/headphones). When I plug headphones directly into the audio out of the laptop the buzz isn't there so I assumed it was some kind of ground loop problem and I got a DI box (Samson S Direct Plus). This hasn't made the blindest bit of difference though, so what could the problem be?

There is a noticeable "pop" when I turn the monitor off and part of the buzz goes away, only to return when I turn the monitor back on again. Thinking it was a problem with the monitor I used an Apple DVI-S-video connector to plug the laptop into a TV with the same result so I don't think the monitor is at fault (which runs perfectly fine with my other computers).

If I run the audio out straight through the DI box into my speakers the buzz isn't there so that would indicate the mixer is at fault, but I've tried running the audio through a different mixer - same problem occurs. The volume of the buzz is not affected by the output volume of the audio out.

I'm in the middle of a discussion with Applecare but I thought I'd also throw it out here as there seems to have been some other similar problems - perhaps one of the people with this problem has found a solution?

http://discussions.apple.com/thread.jspa?messageID=7087760&#7087760
http://discussions.apple.com/thread.jspa?messageID=4190022&#4190022
http://discussions.apple.com/thread.jspa?messageID=3558070&#3558070
http://discussions.apple.com/thread.jspa?messageID=4200671&#4200671

Anyone fancy taking a stab at a diagnosis?

Thanks in advance,

Martin

Message was edited by: mosso

Macbook Pro (late 2006), Mac OS X (10.4.11)

Posted on Jan 7, 2009 12:47 AM

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Answer 1

i.3d

Level 2

420 points

Jan 7, 2009 1:18 AM in response to mosso

No diagnosis... I've been battling the same problem for a while.
MBP early 2008 + 23" ACD.
The only difference is that my buzz is intermittent and goes away for a few minutes if I unplug and re-plug anything connected to any of the USB ports.
It seems as if the MBP "charges up" like a condenser and the starts buzzing and USB seems to "discharge" it.
Very scientific, I know 😉 but can't think of a better explanation.
Mabe directly grounding the MBP's body would help.... will test.

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Answer 2

Jan 7, 2009 10:34 AM in response to mosso

You might want to check out http://discussions.apple.com/thread.jspa?threadID=1653861&tstart=0 to see if you're experiencing the same audio problem many other Macbook Pro owners are - a poorly shielded audio card that results in whining, bleeping and blooping when connected to low resistance speakers or headphones. Are you sure it only makes the noise when connected to the external monitor?

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Answer 3

mosso Author

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Jan 8, 2009 12:55 AM in response to josh.lawless

Thanks for the replies. I'm don't think my problem is the same as that as I definitely don't have a problem when headphones are plugged directly into the MBP - only when it's connected to a second monitor AND a mixing desk. As for the buzz going away when USB devices are unplugged - that doesn't work on my machine so I guess it's not quite the same problem as that even though the symptoms are similar!

Applecare told me to take my machine in to see a genius, which I will do when I can manage without it for a week. I currently have very little hope of getting this problem resolved though, as I received a reply on a pro music forum that said this is a problem with the machine and the way around it is to get a soundcard.

Thanks again!

Martin

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Answer 4

louie

Level 4

2,950 points

Jan 8, 2009 7:08 AM in response to mosso

IMO, you have the same ground problem, but your Samson DI is not a transformer isolated box. It's an active DI without a transformer, according to the schematic, and that ground lift switch might help IF the problem is an actual ground loop, with multiple ground paths causing the problem. You still have at least one ground path directly connected from the unbalanced input, or you wouldn't hear anything.

You should try completely isolating the audio ground system from the computer/display system with a true audio transformer isolator. In the States, the cheap answer is the Radio Shack device previously mentioned in my other post. A DI that actually has an isolation transformer should work, too.

I also have no problem with external display and headphones, or external speakers and no display. However, I describe my sound interference as low level digital noises that vary with the external display screen content, not a constant buzz or hum. I conjecture that could be a related issue - or not, I think it could depend on the exact grounding scheme used by the audio and display devices connected.

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Answer 5

mosso Author

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Jan 8, 2009 7:47 AM in response to louie

Thanks very much Louie! I've ordered a ground loop isolator through Amazon (a bargain at £5 provided it works). I'll report back once I've received and tested it.

Best wishes,

Martin

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Answer 6

louie

Level 4

2,950 points

Jan 8, 2009 8:47 AM in response to mosso

OK, hope it works. Do you have a link for this item?

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Answer 7

mosso Author

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Jan 8, 2009 8:58 AM in response to louie

Yes I do...

http://www.amazon.co.uk/exec/obidos/ASIN/B000NVWB9O

They are also stocked by Maplin in the UK (our equivalent of Radio Shack)

http://www.maplin.co.uk/Module.aspx?ModuleNo=33172

I went for the Amazon one because of they are out of stock at my local Maplin and charge for delivery. Any reason to think that this might not be up to the job?

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Answer 8

louie

Level 4

2,950 points

Jan 8, 2009 9:55 AM in response to mosso

Yes, that should work. IMO, it is fine for the quality of the analog computer output. For fullest fidelity one should use a digital audio interface and high quality D to A converter.

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Answer 9

mosso Author

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Jan 8, 2009 2:05 PM in response to louie

Thanks once again for the advice - I'm already saving up for one! Up until now the MBP has been only for sequencing duties (which is why this wasn't a problem 'til now). I've currently got my eye on the MOTU Ultralite or Apogee Duet but until I can get either this will tide me over nicely (I hope).

Message was edited by: mosso

Message was edited by: mosso

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Answer 10

mosso Author

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Jan 12, 2009 9:41 AM in response to mosso

Just got the ground loop isolator today and...

...it works!!!

Thanks very much for all the help!

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Answer 11

louie

Level 4

2,950 points

Jan 12, 2009 10:07 AM in response to mosso

Very good - spread the word.

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