Hello, I'm setting up an amateur home studio at home mainly for recording acoustic guitar and vocals. I have some AKG mics (a small diafragm P170, a large diafragm P420 and a dynamic D5). I have an Avid Fast Track Pro properly configured on Yosemite too. Now I also picked up a good (to my ears) Mackie 402 VLZ4 2-channel mixer/preamp. This said, I'm not very satisfied with Mackie preamp connected to the Fast Track Pro, since its preamps add some noise to Mackie output, even if I do all the procedures to mitigate noise, like activating switches pad, line and turning all the interface gain to 0. In some manner it seems that Fast Track Pro is messing with the Mackie's signal. So my question is: is there some way of using the built-in Retina Macbook A/D converter, that is, does the headphone output work as a line input? Which plugs should I use? Can I use an Y cable for allowing headphone monitoring? Thanks in advance!