this is an attenuverting mixer. one attenuverter for each input. when not
plugged the switching input sockets (switching type) provide 7.5V for processing purposes.
all attenuverted signals where summed, as long as the appropriate
single output is not used (switching jack sockets again). the summed signal
is switchable from AC to DC. in AC mode the signal goes thru a cap.
in dc mode the the cap is bypassed and the signal is also routed to
a LED driver indicating the positive or negative parts of the signal.