Behringer and no ground

[traidma]

A while ago I was asking for your help... my mixer started humming and would do that only for the mic inputs, not the stereo ones. Well, yesterday, after a whole year of thinking that the mixer is dead forever, I got this thing working. As some of you had suggested, it was a problem with the ground.
But the mixer's adapter doesnt have a ground pin at all.. is that normal? How does it get grounded if no power pin..
and yet.. as I plugged it into a grounded outlet.. the hum stopped..
I'm confused..