CMoy Buzzing

I recently purchased a CMoy headphone amp from a reputable builder on ebay, and I'm having some buzzing issues when using a DC adapter (regulated 12v with 3.5mm tip from Radio Shack).  I have the amp plugged into my computer's 3.5mm output. 

With everything plugged in, there is a loud buzzing sound coming through my headphones, even with the amp turned off.  When I switch the amp on, the buzzing usually goes away, though I have noticed some buzzing or faint popping sounds may linger or develop later on while listening.  The loud buzzing also goes away immediately if, when the amp is off, I unplug either the power adapter OR the audio cable coming from my computer.  It doesn't matter whether the computer is on or off at that point. 

I can hear the buzzing even with the amp plugged into the same power strip as the computer.  Does that mean it can't be a ground loop issue, or is that still a possibility? 

I'm really at a loss for what to do here...There are basically no problems with the amp when running on battery power, but I intended to use the DC so I wouldn't have to keep replacing the battery.  Any advice would be greatly appreciated. 

Thanks for the input! 

I've actually been in contact with the builder, I'm just trying to branch out to get as much input as possible.  I'm using a DC adapter that he recommended (with the correct polarity), and he built the amp to fit my Audio Technica A700s (should have mentioned that in my original post).  His initial thought was that it could be an issue with the power adapter, but I've since discovered that there are no problems when using the power adapter and an mp3 player.

I may try a ground loop isolator, just to make sure that isn't the problem.  I will also be ordering a new sound card very soon, which throws another variable into the equation.  If the issue persists, then I suppose its back to the builder :)