Input Fader   

The input fader is typically of the sliding type which allows fine adjustment of the amount of signal going to the output section of the mixer. For optimum signal quality the fader should be operated at the design centre which is about 3/4 of the way to the top of its throw. This is usually marked on the fader by the manufacturer.
Too high and the sound may overload the input channel and distort. 

Too low and any inherent noise in the mixer itself may become apparent.