A audio mixer circuit is used to combine output from several microphones/channel into one o more common outputs, usually for public address purpose or while recording. The function of the audio mixture circuit, as the name suggests is to ‘mix’ the different audio signals which are fed to the input for the mixer. In this article we include various types of audio mixer circuit from the simplest possible transistor audio mixer circuit to the high end audio mixer preamplifier using operational amplifier.
The circuit of simplest transistor audio mixture circuit is shown in figure 1. This circuit has a low input impedance and the three channels are not isolated prior to mixing. Thus the impedance of each channel is dependent to some extent on the setting of the potentiometer controlling the other channel. Here resistors R1 through R4 setting the voltage gain at unity. Capacitor C1 through C4 is the DC blocking capacitor and potentiometer VR1 through VR3 are the feeder control for channel 1 to 3 respectively. Audio mixer circuit is work as the current adding circuit.
An improved transistor audio mixer circuit is given in figure 2. Here each input is fed to a separate transistor. Bootstrapping ensure a high input impedance more than 1 M. All the input transistors share a common collector to load resistor. The following emitter – follower stage ensure a low output impedance (less than 100). As in above circuit capacitor C1 through C3 is the DC blocking capacitors.
An IC equivalent to the simplest transistor audio mixer circuit of figure 1 is shown in figure 3. This circuit is basically that of an inverting amplifier, but with each input channel having its own input resistors. With just one input signal fed to the amplifier, it acts as an inverting amplifier with R1 and R6 setting the voltage gain at unity. C1 is the DC blocking capacitor and VR1 is the feeder control for channels.
When two or more inputs are used, it functions as a sort of current adding circuit. The voltage at the output is the sum of the input signals fed to the inverting input of the op-amp (pin 2). The audio mixer circuit works as a high quality mixer.
Another audio mixer circuit using the LM349 is given in circuit 4. Here each input is fed to a separate op-amp: the input is given to the inverting input of op-amp through its own input resistor as shown in circuit diagram. After suitable buffering and amplification they are mixed by the last op-amp (N4). The output is taken from pin 14.