Making a simple audio mixer is actually an easy task. According to our tutorial article on how to build an audio mixer, a simple audio mixer is actually a summing circuit, based on an operational amplifier.
The circuit we present here is probably one of the simplest audio mixers that can be ever made. It consists of a single operating amplifier, some resistors, capacitors and potentiometers. This mixer has five inputs but you could add more or remove any of them. This circuit is monophonic. For a stereo version you will need to make two identical circuits, one for the right (R) audio channel and another one for the left (L) audio channel. For a stereo version, you will also need to use dual - stereo potentiometers.
Image 1. The circuit of the simple audio mixer
The operating amplifier is used on a classic summing circuit. The gain is set to be exactly equal to 1. This means that the mixer does not amplify at all. Actually, the gain for each input depends on the ratio of the feedback resistance, R4, to the input resistance of each channel (ie R5, R6, R7, R8 and R9 for the first, second, third, fourth and the fifth audio channel, respectively). Since all resistors are identical, their ratio is equal to 1 and therefore the voltage gain for each input is also set to 1. If you wish to set the gain for any input at any different value, you may use different resistors in place of any of the R5, R6, R7, R8 or R9. For calculation formulas, you may refer to the theory of the summing amplifier circuit.
The C5 capacitor and the R3, C4 network, are used to limit the frequency response of the amplifier at very high, not audible frequencies, in order for the noise level to be minimized.
Resistors R1 and R2 are used to DC-bias the non-inverting input of the operational amplifier at a voltage level equal to the half of the supply voltage. This is essential, in order to the circuit being actually able to operate from a single supply voltage. C3 acts as a short circuit for the AC and thus, the non-inverting input of the op-amp is grounded on AC.
The supply voltage should be in the range of 12 to 30V according to the specifications of the TL081 op-amp. Maximum supply voltage ensures greater dynamic range and better linearity.
The basic audio mixer circuit would be sufficient for most audio sources. However, for some special cases of audio sources, such as microphones or magnetic cartridges of turntables, you will need to add preamplifiers to the inputs.