Digidesign MBox 1 high-pitched hum

[Chief Pronto]

I'm not sure in what section this should be posted, so I did it in a few spots...

In short, my MBox emits a highpitced hum when Pro Tools starts up and I listen through headphones (both 1/8" on the front and the 1/4" on the back). It's weird though, the hum gets "choppy" when I move the mouse in Pro Tools, and the hum goes away completely when I go into menus like Hardware Setup. Furthermore, when I change the clock from Internal to S/PDIF, a different type of hum comes out.

I tried changing USB cables, but the same thing happens. When Pro Tools is not running, it's MUCH quieter and it just sounds like air. When Pro Tools is running, it's incredibly loud. It's like when the computer is "thinking" the hum goes down a bit. Like say while I'm loading files and I see a status car, the hum gets quieter.

Any ideas?

 

[jimmy2sticks]

This is what is known as frame rate noise and USB switching. The grounding and power supply design cause this noise. A good example of a unique solution to this problem is in the Line6 Tone Port product. Line6 actually used a small transformer and 555 timer to make a switch-mode power supply for the audio section. This power supply brought the USB power into the primary side of the transformer and used the 555 timer as the PWM (switching waveform) generator. This allowed creation of +/-5V audio power rails referenced to the audio ground point on the secondary. This alone will not solve all the problems, but this in conjunction with careful ‘star’ grounding techniques allows the noise floor to be pushed down to very acceptable levels. Most current products on the market do not use these techniques and you can hear the 1kHz USB frame rate in the noise floor, especially with headphones. This is true of even high-end products from Digidesign and Lexicon.