A simple multi-channel audio mixer circuit using LM3900 quad amplifier is given below. The circuit consists of 4 channel quad amplifier (LM3900). two mic audio inputs and two direct line inputs are offered in this circuit. By adding constant circuit parallel with this, you'll be able to increase the quantity of inputs according to the applications.Each input is connected to the inverting terminal of LM3900. The built in amplifier of each section amplifies every audio input separately and is fed to the output terminals. The output terminal from every channel is connected to one output line with a resistance not greater than 680K and produces a mixed audio at the output with terribly low noise. This audio mixer circuit doesn’t use a low impedance input to combine ideal sources. Capacitors C1 to C4 are the decoupling capacitors for the corresponding channels. C5 is the output decoupling capacitor.
