Here the simple mixer circuit with 4 inputs and 2 operational amplifier (op-amp) modules:
A basic mixer suitable for mixing microphones or even effects outputs. The overall gain from input to output is one if the pot related towards the input is full up. You can make this a net gain of ten (or any other reasonable gain) by reducing the input resistor towards the second op amp. 10K in this position gives a gain of ten, or 20db. In case you are mixing effects outputs that have an output level control constructed into them, you are able to dispense using the input level controls, or make some have level controls, some not. Each channel use a potensiometer to adjust the volume level of each channel. Audio taper potensiometers are possibly much better, but linear will do the job.
For the op amps, choose a JFET input dual or singles, such as from the National Semi LF3xx series, or something such as the TL072 or TL082. This circuit operates with 9V power supply.
Download the schematic diagram Simple Mixer 4 Inputs circuit in PDF document: