I don't think its the sound card. But, try using the LINE (analog) output from the mixer to your sound card, rather than the USB interface. Also, make sure your input is set to MIC.
Also, what mic are you using? You say condenser mic, but not all condenser mic's are created equal. I used to use MXR condenser mics in my studio but was having a lot of sound quality issues which turned out to be the MXR mic's. I've since moved on to other brands that are much better.