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High pitched hum through studio monitors with new iMac

S

slicecom

macrumors 68020
Original poster
Aug 29, 2003
2,065
98
Toronto, Canada
I recently upgraded from a MacPro 1,1 to a 27" i5 iMac and this is driving me nuts!

I get a high pitched hum through my Yamaha HS80M's and it seems to be related to the graphics card in the iMac (gets worse and changes pitch when playing games).

My speakers are connected with balanced 1/4 plugs to a USB Lexicon Alpha which I bought to eliminate noise (and it did with my MacPro), but now I'm getting another annoying noise with my iMac!

Anyone have any suggestions?
 
Try a different USB port? That seems odd, I'd be pissed too if something was ruining the perfection of my HSM80s. (Still haven't gone over to the dark side, by noise out of my KRK's when I have an external device would piss me off too)
 
ChrisA said:
I can think of several reasons why this might happen. But you can't do anything to address any of them. About the only option is an external USB audio interface.
Click to expand...

That's what I'm using. The Lexicon Alpha is connected to one of the USB ports on the back of the iMac. I tried all 4 ports and they all make the same noise.
 
I've come to the conclusion that this is caused by the CPU throttling down employed by the iMac's firmware.

When running a benchmark test that revs up the CPU, or just opening an app, the noise disappears completely. I've written a bug report to Apple. Hopefully this can be fixed in the next firmware update. It's unacceptable to have a constant beeping noise in my studio.
 
This computer is going back to Apple (even though I think its a flaw in the iMac design and not just this specific computer).

In the meantime, I've found just keeping folding at home running keeps the CPU busy, which prevents this noise from occuring. Of course then the fan noise increases on the iMac, but honestly its much less annoying than the high pitched beeping!
 
Ok after 2 weeks of pain and suffering I figured out a solution!

When plugging the USB Audio Interface into a POWERED USB HUB, the noise is completely eliminated. This only works with ONE of the 3 Powered USB Hubs I tried, but I can now happily listen to my music with no noise. The hub that worked, happened to be the cheapest hub I tried, so it doesn't appear to be related to the quality of the hub, and probably has more to do with the amount of power that the hub gives.

I also found that the USB cable seems to be very important. I've ordered a cable with 2 ferrites on it, but even when comparing 2 usb cables without ferrites on them, one would make the noise in the speaker and one wouldn't.

So in conclusion, it would appear that the new iMac's have weak USB ports that don't feed enough power through as this doesn't happen on even my 2003 12" PowerBook.
 
Well, I have the exact same problem with these monitors and my edirol ua-25 external sound card. I'm going to try a usb hub and I hope it indeed solves the problem cause it really annoys me.
 
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