This 2 channels audio mixer is based on the 2n3904 transistors .This two transistor and other components make two preamplifiers stage . The first preamplifier , firs in of the audio mixer has a high gain and can be used for microphone in , and the second one can be used to control the input of the audio level . The schematic circuit of the 2 channels audio mixer is very simple requiring few external components  .  This two channel audio mixer require a power supply with the output voltage between 9 to 12 volts .  For the audio signal you can use a CD player , mp3 player or other audio device and for the microphone you can use a normal dynamic microphone .