Thank you, Erik. Yes, my dac already employs "galvanic isolation to the outputs" but not really sure if this means I shouldn't be concerned about noise coming into the dac. Some of these isolators are inexpensive while others such as the Intona or ISO Regen are >$300. I am currently using a codex.
Does my laptop need to be grounded when charging?
I generally leave my MacBook Pro attached to its charger while listening to music files played thru my dac via USB. I have noticed that when I am barefoot and gently rub my finger over the metal surfaces of the laptop that I feel a slight fuzziness as if there is stray voltage going through me. It is not a shock per se and the sensation goes away when the charger is unplugged. The charger does not have a ground prong. Should I ground the metal cover of the laptop? Should I get into the habit of disconnecting the charger when I listen to music files from the laptop? Is this normal?