Hummmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm

I strongly recommend getting a hum filter to connect between your laptop and PA system.  Radio Shack makes one (Cat #270-054) for about $20.00.  It filters the annoying 60 cycle hum that can squirrel its way through the most filtered power strip and shielded cables, making your music sound...amateur.   The best part is how it strips the hum from the laptop power supply.  Trust me, you need one.