AS-21 series: what the Mic/Line input switch does
The Mic/Line switch on the Gemini Sound AS-21 lets you match the input to your source, whether that source puts out a microphone-level or a line-level signal. Setting it correctly gives you the right amount of gain and a clean signal.
Mic setting
A microphone puts out a very weak signal, much lower than most other audio gear. On the Mic setting, the input runs that signal through a built-in preamp that boosts it up to a level the speaker can process and amplify.
Line setting
A line input accepts a higher-level signal from sources such as synthesizers, drum machines, and CD players. On the Line setting, there is no preamp boost, because the input expects the signal to already be at line level and ready for processing and amplification.
Which one to use
Set the switch to Mic when you plug in a microphone, and to Line when you connect a line-level source. Matching the switch to your source keeps the level right and avoids a weak or distorted signal.