ozorn

hi there, i just got a spider IV 15. i am using the CD/mp3 input to jam - using the phones output with headfones. But the signal from my mp3 player coming though the headfones output is only mono. Mp3 player is connected via stereo 3,5 mm and headfone are connected via 14" stereo. why is there only a mono signal coming though?why ist there no stereo? it doesn make sense to provide a "music line in" if it`s not stereo. And why ist there a stereo line out, if there is no stereo signal?