Noise issue
If you tried everything and still get noise there is the possibility that there is a problem with one of your pieces of equipment. Let's assume they are working...
The settings of your pre-amp and compressor can certainly cause lots of unwanted noise if they're not properly set. If you don't have noise problems when recording direct through your mixer, the mixer settings are fine. I would try sending the mic through the preamp, then compressor, then mixer to sound card. Try bypassing the compressor (there should be a bypass button) and record with the preamp only. If you're getting noise try lowring the input, gain, output, etc., see how it sounds when they're all set at a medium level. If you can get the preamp to sound good without noise the compressor is the problem...
If the threshold is set really low you will be compressing a lot! the ratio should not be too high either, that will cause heavy compression also. The attack & release can be set to medium (or auto if it has that feature). Try setting the threshold to just below 0dB, and the ratio to 2:1. Sending sound through your mic you will see the compression increasing (gain reduction meter) as you turn the threshold down into the negative range. Over-compression can cause a lot of noise (pumping, room noise, breaths, etc.). Playing with the threshold is the key, it should only be reducing gain (compressing) on the loudest vocal parts. Hope this helps...
By the way, a bus is just a way to send a signal from one place to another. Like your vocal on channel 1 can be bussed to another channel, sound card input, etc.
YZ
http://www.soundclick.com/ztrax