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Description Although the modular Portable Mixer design available on these web pages has become a hit for many amateurs, some correspondents required a much simpler device, mainly for mixing mono signals. This design should fulfil their needs, featuring three inputs with switchable high/low sensitivity and unusual level-control circuits, providing high overload margins and low-noise figures, proportional to gain-level settings. Low current consumption due to a simple, five-transistor circuitry, allows the Mini Mixer to be powered by a common 9V PP3 battery for many hours.  Circuit diagram:  Parts P1 = 5K P2 = 5K P3 = 5K R1 = 180K R2 = 2M2 R3 = 750R R4 = 1K R5 = 15K R6 = 220R R7 = 1.5K R8 = 820R R9 = 150R R10 = 100K R11 = 180K R12 = 2.2M R13 = 750R R14 = 1K R15 = 180K R16 = 2M2 R17 = 750R R18 = 1K C1 = 1µF-63V C2 = 100µF-25V C3 = 220µF-25V C4 = 100µF-25V C5 = 220µF-25V C6 = 1µF-63V C7 = 100µF-25V C8 = 1µF-63V C9 = 100µF-25V Q1 = BC550C Q2 = BC547 Q3 = BC557 Q4 = BC550C Q5 = BC5...

A basic mixer suitable for mixing microphones or even effects outputs. The overall gain from input to output is one if the pot related towards the input is full up. You can make this a net gain of ten (or any other reasonable gain) by reducing the input resistor towards the second op amp. 10K in this position gives a gain of ten, or 20db. In case you are mixing effects outputs that have an output level control constructed into them, you are able to dispense using the input level controls, or make some have level controls, some not. Audio taper pots are possibly much better, but linear will do the job. For the op amps, choose a JFET input dual or singles, such as from the National Semi LF3xx series, or something such as the TL072 or TL082. Source  http://circuitdiagram.net