This is 4-input audio mixer using an integrated circuit 741 and two more operational amplifiers. The characteristics of the inputs are given by the diagram. The power supply of the integrated circuits must be symmetrical with 6 to 12 V. The input leads and output audio signals must be shielded. The resistor 741 between the output and the inverting (-) input determines the loop gain. The NE5534 and TL081 operational admit equivalents. For the TL081 can be used CA3140. The circuit has the following features:
Input 1 - maximum gain of +6 dB and 10k input impedance - source: line
Input 2 - maximum gain of 20 dB, and input impedance of 5 to 10 k - source: line
Input 3 - maximum gain of 46 dB and input impedance of 1 kΩ - source: low impedance microphone
Input 4 - maximum gain of 6 dB and input impedance 1 MΩ - source: high impedance microphone
We also recall that the cable inputs and signal outputs must be shielded, and the pots must be of logarithmic type. As a possible modification, one or more inputs can be eliminated, if not used in the desired application.
