filtering USB bus power?

[egasimus]

I'm not satisfied with the noise performance of my external audio interface (E-Mu Tracker Pre USB 2.0). I'm powering it off a laptop's USB port, and that might not be the cleanest 5 volts in the world, so do you think a couple of caps (100u+100n) across the USB power rails, or maybe a LC filter, would help any?

[Gurner]

It might, but the problem with a lot of computer USB supplies is a noisy ground......& if you've got a poor (noisy) USB ground kind of screwed cos noise from it will permeate into your signal at every available opportunity throughout your circuit! Think of your ground as like the foundations to a house - it needs to be rock solid to build upon (rock solid here being ultra low impedance, no noise etc)

[Gus]

http://en.wikipedia.org/wiki/USB  The power section

USB 2 has 100ma and up to 500ma available. 
Have you tried the E-Mu with a desktop or another laptop? 
Do you know how much power or current your laptop can delver via the USB port.

[DavenPaget]

http://en.wikipedia.org/wiki/USB  The power section

USB 2 has 100ma and up to 500ma available. 
Have you tried the E-Mu with a desktop or another laptop? 
Do you know how much power or current your laptop can delver via the USB port.

Meh , most USB 2 controllers don't follow the scheme anymore , most can do 1000ma without tripping a polyfuse or having to require negotiation . Now it's changed , because manufacturers think 500mA is nothing .
USB 3 mostly provide up to 1.5A now .