EDIT: Wait, so how could you get a negative voltage with a DC power source?
Come on, think a little 
A given DC power source has two output wires, one is positive and one is negitive, can you hook them up in either direction? Will your car battery work in your auto if you hook the Red and black battery cables backwards to the battery?
Grasshopper needs to slow down and think, I think ;D
Lefty