The Model 901 Modular Matrix Mixing Card is designed for use in professional audio applications. Each unit features four (4) selectable music inputs, three (3) paging inputs, one (1) line level output, and a switch closure circuit for control of priority bypass relays. Typical applications are public address systems in buildings such as airports, factories, hospitals, courthouses, casinos, convention centers, libraries, hotels, racetracks, and office buildings. The mechanical construction of the Model 901 allows mounting of up to ten (10) units in a standard Model 857B Card Frame Package. Each Model 901 card unplugs from the front of the card frame, without the need for removing any wires from the rear of the card frame. Spare units may be placed into service in a matter of minutes. Each Model 901 has three user adjustable features, in addition to the actual wiring hierarchy. There are two level adjustments, one for the page output level, and one for the music output level. There is one four position DIP switch for selecting any of four music input channels, or no music input at all. The Model 901 Modular Matrix Mixing Card is built using two separate printed circuit board assemblies. The two boards are connected together using a high quality, gold-plated 15 pin connector. The main printed circuit board contains all audio input and output connection points, the page output level adjustment, and all relay control circuitry. The smaller printed circuit board is piggy-backed to the main printed circuit board, and contains all background music channel circuitry, including the channel selector switch and the background music level adjusting trimpot.