I have been using my Echo Indigo IO laptop soundcard with headphones for some time now with no problem at all - it is always clean and crisp.
Today, I plugged the output of the Indigio into my Behringer UB502 mixer which is connected to my monitors. Straight away there is a horrible raspy noise on the speakers even though the mixer is turned right down. If I run Cubase SX3.01 it makes more noise, like digital interference, again even with the desk output turned right down. If I unplug the cable that runs from the Indigo to the mixer, the noise stops and the system is clean. If I plug headphones into the Indigo instead (using the same socket) the sound is fine and clean.
I have tried different cables, different mixer inputs, unplugging everything else but it makes no difference. Is it possibly an impedance issue baring in mind that the output of the card is designed to be both for headphones and line out.
1) Make sure your patch cord from Indigo to mixer is
shielded, and properly grounded, and not a speaker
cord(non-shielded). Also no longer than 6-10 feet,
and not touching an AC cord
2) You may have a ground-loop between your laotop
and the mixer-use a 3 pin to 2 pin AC adapter on
your laptop power supply, to disconnect the ground
pin-you could first try to unhook the power supply,
and run off of batteries, to see if the noise is still
3) 1/8 inch plugs have a small amount of contact area,
so make sure your Indigo connection is secure and
use a quality cable, such as the Monster brand, that
come premade with 1/8 inch male on one end, and
RCA(phono) on the other-you may still need to adapt
to 1/4 inch, so make sure all connections are firm
4) Experiment with the combination of soundcard volume
and mixer input volume-match impedance-is your
Indigo set to -10 db in it's software mixer and if so,
does the harware mixer input match?