M-AUDIO Thru 3x8 - Manual (Page 3)

The Thru 3x8 uses a 9 volt, center pin positive wall adaptor for
power (300 milliamp minimum). To power the unit up make sure
that a suitable adaptor is plugged into the power jack on the back
of the unit and also plugged into the wall. Next, depress the
power switch on the front left panel. If everything is done correctly,
the power LED over the power switch will light.
Thru 3x8 Hook-Up Diagram