LearnTronics
Passive Audio Mixer
Recipe #17 — passive audio mixer
Two audio signals can be combined with nothing more than two resistors.
Why not just tie the two outputs together?
Each source may try to drive the other source. The mixing resistors provide isolation and limit that interaction.
Another way to think of it
The two resistors let each source contribute to a common meeting point without connecting the source outputs directly together.
The tradeoff is signal loss
A passive mixer does not provide gain. The output is usually smaller than the original inputs and depends on the source and load impedances. A buffer or op-amp stage is often added after the mixer.
Where the passive mixer leads next
The two-resistor mixer is intentionally simple, but it reveals the next problems to solve. We still need a convenient way to set each signal level, we may want simple tone control, and the passive mix node can lose level or be affected by whatever circuit follows it.
The next few recipes therefore build toward a small practical mixer in this order: volume control → passive tone control → op-amp buffer → op-amp summing amplifier → complete small mixer.
Stereo buses and pan controls make more sense after the basic mono mixer is working, so they will wait until later.