I need to mix pre-recorded sounds with the output from my laptop. That allows me to do sound layering and smooth transitions during laptop dead air. Running a SanDisk mp3 player into a Behringer FCV100 pedal gets me foot control of the mix-in. Running both sources into a standard mixer works fine, but makes the rig more cumbersome. My first attempt to simplify was a basic summing cable, but that got too clunky when I needed a mic level mix output for recording. My next attempt was a proper passive mixer, but the mic level attenuation wound up too quiet for the recording device I was using, so here it is with a variable pad.
Here's a pic showing the active contacts for the pot. The center pin is the wiper variable output. This happens to be a Taiss 1K Ohm 6 Pins Split Shaft Rotary Linear Dual Taper Audio B Type Potentiometer, but I can't recommend it because it makes an audible woosh when you turn it. Good enough for a trim knob I'll adjust once at soundcheck. A 1 kΩ logarithmic would probably be better but couldn't find one.
This is the (kinda too small) project box with the inputs and pot mounted with lead wires.
The wiring diagram for the mixer describes the connections:
Board pin map (7x8):
Finished wiring. Getting the project box closed was a trick but it works!