ido1957 said:
That sounds like a pretty simple solution, now all I have to do is find one of those adapters and try it. I think I remember hearing they were discontinued because of safety reasons but I'll check around....
They're a terrible idea. Most computers were NOT designed to operate without a ground, and will produce substantially elevated EMI/RFI if you do. The few laptops that were designed for such operation come with a two prong plug on their power adapter from the factory (and maybe not even then...
). For that matter, in general, lifting the mains ground from any piece of equipment is not a safe thing to do. Those third prongs aren't just there for looks....
First, you need to understand why you're getting noise. The noise caused by a ground loop (which what you describe probably is not, BTW) is caused by the grounds not being at the same voltage. This can be caused by a leaky power supply circuit, a poorly grounded device, etc., and is really common on devices with two prong adapters.
When you float a ground like that, you have a 50/50 chance of actually increasing the noise. If you float the ground of the device whose ground wasn't sufficient to begin with, every bit of electrical energy that would normally have passed harmlessly to the mains ground (as a result of power supply leakage, etc.) is now being shunted through your audio cable's ground in addition to all the energy that was there before. Since this is a relatively high resistance path running parallel to your signal wires, you're basically dumping additional noise into your signal....
The only way this won't happen is if you get lucky and lift the ground of the device that the current is flowing towards, in which case you're really just floating an electrically hot ground, and thus potentially creating a shock hazard if you touch any "grounded" metal part of that device while touching something that really is grounded. Oh, and you've also turned the case of the machine into a giant antenna....
The right place to lift the ground is in your AUDIO cables, where the amount of current in the line is mostly harmless, and where you really don't want any current (apart from any noise induced in the cable from your environment) to be flowing. By doing this, you are preventing current from running between one device's ground plane and the other device's ground plane. In theory, the best place to cut the shield is probably in the middle of the cable, for minimum average resistance to ground, but nobody actually does that. They lift one end. Easy enough to do.
That having been said, what you are seeing is probably a very different issue. A high pitch whine in a laptop power supply is almost certainly caused by a noisy battery charge circuit. That shouldn't be helped at all by floating the mains ground, but I suppose anything is possible....
The correct solution for this problem is to physically isolate the power (and possibly ground) between the USB port of your laptop and your audio interface. The easiest way to do this, if it works, is to simply purchase a powered USB hub. In all likelihood, that should provide the isolation you need, assuming the power rail is at fault (which it probably is).
If you find that the noise still continues, then the problem is likely the computer's ground not being sufficiently well grounded. The easiest solution for that is to force the computer's ground to earth ground by connecting a heavy gauge wire from a grounded part of your audio interface (silver screw, ground shield of an unused input or output, etc.) to a building ground (the ground pin on a wall plug). 10AWG should solve the problem.
Forcing the ground plane of the offending device to earth ground with a suitable heavy gauge wire will take care of most ground loop issues, as well....