The SPM120 is an active converter for personal monitor application, with a built-in battery. An XLR or 1/4” jack can be connected to the personal monitor. The volume knob adjusts the output level and powers the unit off when turned counterclockwise. The adjacent green LED will light when the power is on. The inner switch should be set to MONO when used with a mixer’s auxsend or any other mono source and set to STEREO when connected to a headphone output or other stereo source.