I've had two problems (both in the past):
. . . Noise from USB connections;
. . . Noise from computer power supplies.
I don't know if a Firewire connection could be a noise source. I think the Focusrite uses its own power supply.
So:
. . . If you disconnect the Firewire connection, between the
. . . Focusrite and the computer, does the noise stop?
That will make the system go quiet -- no audio feed from the computer into the Focusrite. If the noise stops also, it suggests that some hash from the Firewire connection is getting into the audio wiring, somehow.
In that case, I'd try putting some ferrite cores around the Firewire cable. That has sometimes helped me with USB cables.
And:
. . . If you're using a laptop computer with its own battery,
. . . and you unplug its power supply and run it off its battery,
. . . . . does the noise stop?
I know this sounds like a blind man trying to find the elephant.<g>
Thanks --
Blind man or not, I'm grateful for the company in my big game hunting. Firstly, the computer is a desktop so it can't be worked off a battery. Second, I think FireWire is the same as USB regarding digital noise problems. But there is no noise whatever when the system includes DP, PC and sound interface and microphone, headphones, monitor speakers (balanced). Even with FireWire connection, there's no noise.
The only time the noise comes in is with the xenyx mixer connected and the noise does not effect the main output from the Focusrite - that remains clean.
It's strange because the the xenyx has no USB, it's analogue only. And when I was using it as my main output mixer with my previous setup (with internal soundcard connecting to the xenyx for monitoring), there was no noise. So it really looks like I've made a ground loop somehow.
Some people have reported good results with the ground loop isolator unit, but it doesn't make a ha'pth of difference in this set up. But it's ok for the moment. It's not of critical importance as it's only for backup, second headphones.
