PART III Preamplifiers, Mixers, and Interconnects Miscellaneous features Professional -quality microphones have an output impedance of around 200 ohms, and the balanced microphone inputs will have an input impedance of between 1000 and 2000 ohms ("2 k ", k thousand). The outputs should have an impedance of around 200 ohms or lower. The headphone output impedance will typically be 100 ohms or so. Small mixers usually have a separate power supply which plugs into the mains. This typically contains a mains transformer, rectifiers and regulating circuitry, and it supplies the mixer with relatively low DC voltages. The main advantage of a separate power supply is that the mains transformer can be sited well away from the mixer, since the alternating 50 Hz mains field around the former can be induced into the audio circuits. This manifests itself as "mains hum" which is only really effectively dealt with by increasing the distance between the mixer and the transformer. Large mixers usually have separate rack-mounting power supplies. FIGURE 5.3 A compact stereo mixer: the Seemix "Seeport." (Courtesy of Seemix Sound AS) The above-described mixer is very simple, offering few facilities, but it provides a good basis for the understanding of more complex models. A typical commercial example of a compact mixer is shown in Figure 5.3 . A MULTITRACK MIXER Overview The stereo mixer outlined in the previous section forms only half the story in a multitrack recording environment. Conventionally, popular music recording involves at least two distinct stages: the "track-