LearnTronics
Small Practical Audio Mixer
Recipe #22 — put the blocks together
We now have enough pieces for a recognizable small mixer: two inputs, an individual level control for each channel, isolation/mixing resistors, an op-amp summing stage, and a master output control.
Channel 2: IN 2 → P2 level → R2 → summing node.
Then: U1 sums the two channels → P3 sets the master level → MIX OUT.
What each control does
P1 and P2: decide how much of each source enters the mix.
R1 and R2: isolate the channel controls and establish the input gains with RF.
U1 and RF: form the active summing amplifier.
P3: controls the final output level without changing the relative balance between Channel 1 and Channel 2.
Another way to think of it
P1 and P2 are the individual faucets, U1 is the common collecting point, and P3 is the final valve controlling the combined output.
This is now a mixer rather than just a resistor network
The active summing stage makes the gain and loading more predictable than the original passive mixer. A buildable product would still need proper op-amp power, input/output coupling or biasing as required, decoupling capacitors, grounding, shielding, connector details, and checks for headroom and noise.
Stereo can later be built by using separate LEFT and RIGHT summing buses. Once those buses exist, a pan control has a clear job: it decides how much of a channel is sent to each bus.