2-INPUT AUDIO MIXER
As simple as it gets. The mixer consists of two transistor-based preamplifiers. The first one (designated "Mike in") offers higher gain. it works well with a regular dynamic microphone. The second one can be used to control audio input from a tape recorder/CD player.
There can't go much wrong here. Do not place the circuit too close to a mains transformer to avoid any hum.
I built this mixer for my 4W FM transmitter. To hear what I'm transmitting, I also added a LM386 headphone amplifier.