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Hi guys, got a 4pre to use for my students in college and its started to make a high pitched sound. Even when i plug in just the 4pre and turn the p supply on the noise is present. ive tried connecting to the mac via firewire alone and the sound is present. When i turn off the power supply the noise goes away but at the mo its a real pain. i can record and playback ok.
Any ideas.

computer in class is a quad core 17 imac running osx 10.7.4 & Logic 9

Sounds like environmental interference to me, such as to something connected to the unit.

For example, fluorescent lighting, crt monitors, too close to power lines etc...

sample rate mismatch?

Not sample rate issue as it makes the noise even when the computer is turned off.
Mtec