How to mixing stereo input to mono output

Here , I will give a circuit of schemes that are used to mixing stereo input to mono output to boost  gain output . Why 2 inputs in to one because, if we need a stereo amplifier we want to become a stronger by combining the two input into one input, so that a higher power output. Actually without any circuit above we can combine stereo amplifier into mono, but the sound output less than the maximum , need for this additional combiner circuit.

Part List =
R1______150K
R2______920R
R3______150K
R4______920R
R5______920R
U1______NE5532

For balance _: R1 / R2=R3 / R4
For balance _: R1 = R3
Gain ______: R5 / R1 = R5 / R3