I have a Sony VAIO with Windows 7, Edirol UA-101 USB external sound card. When the laptop is on mains power I get a persistent (fairly loud) rustling noise through the audio outputs. The noise is not present when I run off the battery.
Why is this? How to prevent it?
Thanks.
We used to have this problem at my church before we moved to a desktop computer.
Granted, we plugged directly into the line-in of the sound card.
I think it’s a grounding issue, or rather lack of grounding issue.
Some laptops aren’t grounded (as their transformer is usually external).
Unfortunately, the only advice I can offer is to run everything off the same circuit, move the laptop’s power brick away from the interface, etc. Other than that, I don’t know.
Thanks Shinta. Sony VAIO online troubleshooting says its grounding issue also. But it then says if you do what it suggests (which is actually not clear) I would run the risk of being electrocuted (!). That’s the opposite of what I thought!
Didn’t follow what you meant by ‘running off the same circuit’. You mean plug both laptop and soundcard into same bank of power sockets? I thought I had already tho.
Yeah, don’t do that…
Basically everything in the same outlet, powerbar, surge protector.
Looking at the back of your unit, though, I see two things:
You could run a wire (bare or insulated, gauge doesn’t matter) from that screw to something that is grounded (usually something that has a 3-prong cord and a metal chassis).
The laptop and interface probably run from whats called a double insulated supply/s that is there is no direct connection to ground. I’m assuming powered monitors! they most probably have a three prong connection and are grounded.
If your monitors have an external ground connection then connect that to the back of the interfaces ground bus. 1 from each speaker to the ground lug on the interface.
No bare wires please 
Hmm…
I figured bare wires would be fine for a ground connection (little current, well, except for when it grounds).
That’s not the point! bare wires are just bad practice, ground wire should be covered with yellow/green sleeving. Also helps to stop it connecting with any other bits of metal (or skin) that may be around.
What is connected to the audio interface? Powered studio monitors?
You need to make sure you are using balanced connections for EVERYTHING possible. Three conductor TRS 1/4" cables must be used to connect you monitors etc. If you are still having problems when using balanced cables, you can cut (lift) the ground at ONE end of each balanced audio cable - this would probably require a pair small diagonal cutters. The technique is often called a “telescoping” ground, although it does not strictly meet the definition.
Do NOT lift any of the AC/mains grounds!
A detailed explanation of your equipment and exactly how it is all interconnected would be immensely helpful.