Since the late 19th century, with the appearance of electrical and electronics, the audio has been developing constantly. In recent years with Digital Electronics new methods of transmission and preservation of quality were explored, but in the end everything is analogic, and there are many people who are adept of using only analogic equipment because they believe that it has a superior quality. Thus the bases and fundamentals were always preserved and this Project’s intention is to analyze the circuits and how one of the basic and essential equipment within the professional audio works, the Audio Mixer. This project will describe the operation of balanced and unbalanced signals, preamp and gain, EQ, pan, volume control and channel addressing.