^ Ground Loop Isolator (gets rid of 'feedback' hum from the computer).
It's that shiny little box behind the mixer and works a treat.
For those interested (Rich wig): My Blackjack has XLR mic, L & R outputs, headphones, and USB from Mac, all always connected.
Important to always zero the input/monitor levels from a computer before either Sleep mode or Shut Down, otherwise BANG!!!! when starting up again.