Stereo Mixer

With this circuit you can mix four separate audio inputs. Each input will accept high or low impedance microphones, phonograph, tape or aux signals. You can adjust the gain of each input by adjusting each respective pot. With the Stereo/Mono switch you can combine just inputs 1,2 and 3,4 or in the Mono position combine all four inputs. Signal response is 20 Hz. to 20kHz. with very high input impedance and very low distortion with complete output short circuit protection. This circuit will drive any power amp with an input impedance of 2,000 ohms.

From : http://home.maine.rr.com/randylinscott/may20.htm