In previous posts, I've mentioned my TB-03 synthesizer and TR-8 drum machine. While both offer analog audio input and output to chain the sound, it gets tricky when trying to record since there are multiple volume nobs essentially daisy-chained. Adding even more devices breaks down the whole setup.
So I needed a simple audio mixer, and decided to put together something myself. The only requirement was to adjust the volume of the stereo out of each device, and mix it together into one signal. A simple solution adds a potentiometer on each input line (right and left), and combines each line down to one output. I used 10k ohm dual logarithmic rotary potentiometers (which I found at a local electronics shop - which means they're probably quite old). The input and output connectors are all stereo mini-jack.
As the pictures show, it's a simple small box, with four inputs, four rotary knobs, and one output. I liked the knobs, however, had wished they could be flush with the surface of the box. They were not made for the potentiometers I got, and it took some Dremel work to make them fit at all. The wiring got a bit messy, and combined dual or even three wire would have made it look better on the inside.
The mixer and volume adjustment work, however, I'm not sure if 10k ohm is enough to adjust the devices. It is not enough to mute the sound. This article suggest adding a 1k resistor in series, which I might try later. It would also be interesting to see at what resistance level the sound is completely muted. Other implementations connects the third pin of the potentiometer to ground from the input. In my box, all ground wires from all inputs and output are combined directly, which I'm not sure if is correct.
Oh, and as can be seen, there's no scale to indicate the setting of each knob. These can all go to 11 if you like!