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Op-Amp Summing Mixer

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Recipe #21 — op-amp summing mixer

An op amp can do more than buffer the passive mix node. In the summing-amplifier connection, several input currents meet at the inverting input and the feedback resistor converts their sum into the output voltage.

OP-AMP SUMMING AMPLIFIER — ACTIVE MIXER INPUT 1 R1 10 kΩ INPUT 2 R2 10 kΩ − + MIX OUT RF 10 kΩ with R1 = R2 = RF, each input contributes approximately equal gain simplified signal circuit — op-amp power connections not shown
VOUT = −RF(V1/R1 + V2/R2)

Equal-value example

When R1 = R2 = RF = 10 kΩ, each input has a magnitude gain of about 1. The output is approximately the negative of the sum:

VOUT ≈ −(V1 + V2)

Why this is better than the passive mixer

The input resistors still keep the source outputs separated, but the op amp's feedback holds the summing node near a stable operating point and gives the circuit a defined gain. The output is also low impedance compared with the passive mix node.

Another way to think of it

Each input resistor contributes a measured amount into one common accounting point. The op amp continuously adjusts its output so the contributions add in a predictable way.

The output is inverted

This common summing connection reverses signal polarity. For one isolated audio signal that reversal is usually not heard as a change in pitch or loudness, but phase becomes important when related signals are later combined.

A real design also needs a suitable op amp, power supply, input/output headroom, and—in a single-supply design—a proper bias reference.

Try these yourself

What determines how strongly each input contributes to the summing amplifier? OP-AMP SUMMING AMPLIFIER — ACTIVE MIXER INPUT 1 R1 10 kΩ INPUT 2 R2 10 kΩ − + MIX OUT RF 10 kΩ with R1 = R2 = RF, each input contributes approximately equal gain simplified signal circuit — op-amp power connections not shown
The ratio of RF to each input resistor sets that input's gain contribution.
With R1 = R2 = RF, what is the approximate output?
Equal input and feedback resistors give each input a magnitude gain of approximately one, and the inverting connection makes the summed output negative.
Why are separate input resistors still useful?
Each source reaches the summing node through its own resistor, which isolates the sources and helps define gain.
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