A passive resistor mixer sums multiple audio signals without amplification and is inherently bidirectional

Collins’ passive mixer connects each input jack’s signal through a pot and then through a summing resistor to a common output node. All ground/shields share a common bus. No power is required. The design is expandable: adding another jack, pot, and summing resistor adds another channel. Because it is passive (no gain), plugging a source into the output jack distributes the signal to the input jacks — the circuit is bidirectional. Audio-taper pots give a perceptually smoother volume law than linear-taper. The circuit is not suited to low-level microphone signals without a preamp stage, but works well for summing oscillators, CD players, and line-level toy outputs.

Examples

Three pots (20k ohm audio taper), three 10k ohm summing resistors, one output jack: a three-channel passive mixer for a trio of Schmitt oscillators.

Assessment

A performer runs two CD players and three oscillators through a passive mixer. They notice the combined output is quieter than any individual source. Explain why this happens and suggest a fix.