Actually it is not the power adapter but the power source (Electrical Outlet). You problem is a floating ground. As a temporary fix get a 3 prong to 2 prong adapter. That eliminates the hum from the ground. This not the best way to fix this but it is a quick way to prevent it.
If this is at a club you are playing at see if they can get the plugs fixed in the stage area or Radio Shack sells a device to protect against this problem (someone else will have to tell you what it is called....I don't remember

).