I hooked up a pair of Genelec 8050A monitors (with built in power amplifiers, provided with balanced XLR inputs) to a Mac Pro´s 3.5 mm stereo output on the front using an adapter cable with stereo 3.5 mm mini-tele in one end and dual RCA in the others. Then a RCA to XLR converter.

I got some irritating noise from the loudspeakers. It is not very strong but at a fairly constant level, not affected by the volume control in the computer.

It is mostly some hum and higher friquency hiss but not strong. You don't notice it when playing music but when there should be silence, it is rather irritating.

I also hear the little scroll wheel on top of the mouse when rolled, it produces click sounds in the speakers. Mouse dragging also gives some weak noise.

If I pull the 3.5 mm plug the speakers are dead silent.
If I connect a separate wire between the Mac and the speakers the 50 Hz hum are gone but higher frequency noise is still there and also the scroll wheel and mouse drag sound.

The noise is not audible if I use headphones only.

What is the cure for this? An audio board with XLR outputs?
Anything you could recommend? I will edit HDV video with Final Cut Studio with this setup and have not invested in any video card so far. Could video and audio get out of sync with different boards?

Thankful for suggestions!! /Johan

That problem is in the new computers? Discouraging.

First try the audio output on the back of the computer. If that doesn't help you might want to invest in a USB to audio adapter. The digital audio out is great but there's nothing economical to hook it up too that I'm aware of.

The hum is either from the RCA-XLR adapter being miswired internally or you have a bad wire.

Double posting removed.

Dear William, thank you for your answer. I had overlooked the 3.5 mm output on the rear between the many firewire outputs and the ethernet connectors when I first examined the computer.
You were right, the 3.5 mm line-out on the backside is definitely better than the 3.5 mm headphone output on the front! The higher frequency hiss disappeared.
The mouse wheel sound from the speakers are now down to about the same level as the mechanical sound from the mouse itself.
–Hey, this mouse wheel sound disappeared when I later moved the audio wiring further away from the DVI video cable to the LCD monitor.
The 50 Hz hum is low but will almost disappear if I connect a heavy gauge wire directly from the speaker input to the computer.
So overall, with your help it is about as good as I need now. Perhaps using the digital opto output via an adapter to XLR outputs could provide us with an even better output. We will keep our ears open for such stuff.
This community is really appreciated and helps many, thanks Chris Hurd.