If its the 3 head,maybe the monitor/record button?I can't seem to find a manual.I think most 3 heads let you do this on the fly while recording.I think it did it through the speakers also,I don't remember whether the headphone would let you monitor the bias change during recording.I hope someone else chimes in that has one.[http://en.wikipedia.org/wiki/Nakamichi] [http://forums.cnet.com/7723-7808_102-312487.html] [http://www.youtube.com/watch?v=49jC_vauAvU&feature=related]