The Model 588C Switching Mixer is a high quality automatic switching device designed for use in professional audio applications. The unit can function as a 2 x 1 mixer with mic/line inputs, a 2 x 1 switching mixer, a summing amplifier with gated input, and an automatic voice-over-signal priority switching device. Multiple units may be daisy chained to provide additional levels of automatic priority switching. The initial circuit starts with two individual channels, each one capable of accepting either a mic or line input signal. An input select switch on each input channel, allows the user to select which type of input will be used. Each input is balanced, transformer isolated. In the microphone position, 15 volts phantom power is supplied to the input circuit. The gain of each input channel is individually adjustable. The second portion of the circuit consists of a voltage controlled amplifier (VCA) placed into a parallel position on each input channel. The two VCA's are controlled by a common logic circuit. The logic circuit is controlled by the signal level present at the output of the channel one line/preamplifier. When this output level exceeds the predetermined level (adjustable via the threshold trimpot), channel one switches on, thru to the output of the card, while channel two turns down or off, to the output of the card.