USB-audio interface - need your advice

[Jerome]

Hmmmmm - I am not sure if this answer is going to make sense or not, but I will try to explain. Your domestic power supply determines everything. If you have an external audio interface (i.e. it has it's own power supply unit) and only connects to the PC via USB/FW then it probably has a crap/dirty supply. If you have an internal PCI card it is subject to your PC's regulated power supply - which is better. If you have an external audio interface, but which is powered VIA USB then it is subject to the same clean supply that your PC has. That's a good thing. Some basic measurements with a relatively accurate oscilloscope will bear me out. Remember one thing. Computer power supplies have to be stable or the operating system will crash on a regular basis. External audio interfaces are not subject to the same stringent demands. They do not actually run software, just signal processing, unless you are talking about the high end stuff like UAD.

[Experimentonomen]

Actually yhe voltage rails inside a PC are extremely dirty and noisy compared to the output of a wall wart, though the power in a usb/fw port is cleaner than anything a pci/on-board chipset would ever see.

[Jerome]

Sorry, but I disagree. Put an oscilloscope on any wall socket (which by the way is connected to all the other devices/appliances on the same circuit) and see the response. The power supply on a PC is designed to be stable. It has to be to provide the correct voltages to the different IC's. A wall socket is just utility power and is subject to all sorts of interference, spikes & dips. That's why it's recommended you use a UPS/regulated supply to your PC. As for the USB/FW and chipset issue - they are being fed from the same regulated supply in your PC.