Wiring Schematic Diagram

Here is a  simple 4 channel audio mixer  that has proved useful. Using this mixer alleviates the need to unplug and plug the various audio sources from the recorder input.

The 4 inputs are connected to a summing network consisting of R1 to R4, each input has its own level control ( VR1 to VR4 ). The output of the summing network is connected to the inverting input of the 741 op amp which is configured as a inverting amplifier with a voltage gain of exactly 1, the feedback resistor R5 in conjunction with R1 - R4 ensures this. As the inverting input of the op amp is a virtual earth, each individual mixer input is completely isolated from all the other inputs. Up to fifty or so inputs could be used here providing the sum of all the voltages present on the mixer inputs never exceeds the supply rail of the op amp ( within two volts of supply using this op amp ). The non inverting input of the 741 is biased to half supply by R6 and R7. All of the summed or mixed signals appear on the output of the op amp pin 6 and they will be 180 degrees out of phase with the input signal, for this application the phase inversion is of no consequence here.

The output of the mixer is also fed to D1, D2, and TR1 which is a simple peak indicator. D1, D2, C7 and C8 form a charge pump circuit which rectifies the AC signal present on pin 6 of the 741 to an average DC level, this in turn is used to illuminate the LED via TR1. I adjusted the preset pot so the LED just starts to glow for an input of 2 volts peak to peak