The mike input should be balanced, and suitable for 600 ohm (or so) microphones, usually using an XLR connector. For a line input it's normal to have a mono jack socket, and the switch on this disconnects the first microphone stage and goes in at a higher level.
A passive guitar really requires a higher impedance though, and it's normal to have a specific guitar input - with around 1mohm input impedance.
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