I love my old MB Pro, but for some reason whenever I connect it to AC and I put my Beats by Dre noise-canceling headphones, there would be a nasty hissing sound in my ears (which is perfectly unacceptable). Obviously, mac adapted do not connect the laptop to the ground, and a difference of potential between the battery powered headset and the AC powered laptop are the cause of this sound. Also touching the laptop stops the sound. If you’re experiencing this issue, I have an easy fix : take an ipod or micro-usb cable and stick it to something that’s at the ground (lik another laptop) and there you go : perfect shielding !
Any one motivated to actually build a “usb-to-ground” adapter ?