Internal PC power amp mod

So I'm working on a project of mine to integrate a power amp into my comp and run it off the PSU. The amp is this 25watt x2 by parts express: http://www.parts-express.com/pe/showdetl.cfm?partnumber=320-332

So here is my dilemma: it works, but there is lots of noise. Little more detail first though. I currently have it wires to run off pin 1 and pin 2 (12v and ground) from a 4 pin molex connection. The input is a RCA to 3.5mm headphone jack. I've used this connection on my Marantz 2325 with no issues, so I know the cable is good, and even tested it against another one. The power adapter is solid with a good connection that I made myself. 

What at gets me on this though is that the static is only present when I use a computer PSU and connect it to my PC's headphone jack (ASUS crosshairs V Formula Z motherboard). If I use my laptops power supply, and connect it to the PC audio, there is no noise. If I use my PC's power supply, and connect the amp to my phone, again, no noise. However, PC power supply and PC audio means noise. I even tried another power supply to simply power the amp while my computer ran on its own PSU, and again, noise was present. 

I'm im thinking it might be a grounding issue with the PSU on the desktops that won't let this work, or perhaps even the audio on the ROG motherboard is still dirty, and putting it with a PSU which may also have a dirty signal is only adding to the matter. 

 

Im at at a loss here. Any ideas?

A friend just brought up that it could be a ground loop issue, and getting an isolator may help. Ordered one, and it should be in on Tuesday.

I also think it is probably a grounding issue. It may well be the ROG output is filtered (to keep out the digital noise). If you then connect the audio jack GND to your PSU GND, you 'short out'/eliminate this filter and get noise back in.

Or the audio output is offset from GND (like its 2V DC plus 1V AC "audio"), but that seems unlikely to me, because then it wouldn't "normally work".

I think it's best to isolate the audio signal. Or use something optical like S/PDIF.

Just a small update; the ground loop isolator worked! Also ended up getting the Xonar Essence STX which did clean up the signal a bit, but not completely, but with the isolator, everything is crystal clear.