Audio Mixer Inverting Summing Circuit Diagram An inverting summing circuit (see figure) is the basis of an audio mixer (combining or mixing, routing, and changing the level, timbre and/or dynamics of audio signals). In this circuit design, a single-supply voltage is seldom used for real audio mixers. Designers will often push an op amp up to, and sometimes beyond, its recommended voltage rails to increase dynamic range. Here is the calculation:

Vout = – R2(Vin1/R1 + Vin2/R2 +Vin3/R3) = R1A||R1B||R1C||R2, for minimum error due
to input bias current

Noninverting summing circuits are possible, but not recommended. The source impedance becomes part of the gain calculation. (source:

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