Hello,
 
I have the exact same issue,
 
A profane hiss and buzz... I have already given up the idea of recording any clean sound.
Roland should really get into this problem and try to solve it somehow...
 
It is driving me nuts actually, thinking more and more about replacing the Quad Capture for some other device.. enough is enough!

wait a minute, isn't this post like 10 years old?!
 

JazzSinger
Turn HI-Z (INPUT 1) on the rear of the device ON. (VERY IMPORTANT)
Turn SENS 1L up to about 3 o'clock. At least one segment lights.
Turn MIX to INPUT
Turn OUTPUT to 12 o'clock.

Yes, there is buzzing sound (more hum). If I move the mouse at some web pages, the noise gets louder, like a wind noise recorded with an open mic.
 
Might try out the Ebtech Hum X.. as suggested by thefyn.

Probably similar to the problem I posted a solution to at
http://forum.cockos.com/showthread.php?t=113721

hiii...
       Well this is quite useful site for particular forum.Yes my Quad Capture the buzz all around.
      

sounds like some of you guys do not know how to hook your stuff up properly.
 
ground loops, bad shielding, proximity to RF generators, all kinds of reasons to get buzz that has nothing to do with the unit.
 
there are people all over the world, recording with this thing, and no buzz issues.