4-channel mini-mixer query

[effbiae]

i'm studying the mini mixer on ggg:
  http://generalguitargadgets.com/index.php?option=content&task=view&id=95&Itemid=123

here's the schematic:
  http://generalguitargadgets.com/diagrams/mixer_sc.gif

why does the output jack have two contacts plus ground?  isn't the battery's negative terminal connected to ground?

thanks again,



jack

[choklitlove]

it disconnects the battery when unplugged.  plug something in, it completes the circuit and it turns on.

oh yeah, it uses a stereo jack with 2 contacts plus ground.  the extra contact is shorted to ground when plugged in.  in this case (and most), it connects the battery's negative to ground, thus completing the circuit.