So I just got this amp from my friend, who said volume knob didn't work, just played at max volume. I set it up, nothing but a rather large humming (coming out of speakers), and a burning smell. Any thoughts? I thinking the no sound is from the volume knob being down, but I was a bit concerned by the humming, and a lot concerned by the smell. Also, whats with the inputs (this is a mono amp) labled AC and DC? Any ideas on which I'm supposed to plug into?