Best not to get hum in the first place. Because you used your PA and computer, it may be that the earth/ground connections for the computer and PA equipment are separate, and the various earth/ground connections form a loop which act as a sort of aerial to low frequencies.

Avoiding this is a bit of a black art:

Keep leads short
Use balanced good quality leads/connectors if possible
Don't plug items in at opposite sides of the room
Don't run signal and power cables alongside each other
Fluorescent lights are often an electrical noise source
Use ground lift only as a last resort
Listen carefully in the recording room for eg fridge noise, or central heating pump or W.H.Y.

I'm sure this list is not exhaustive...


Stu