One reported problem with the audio injector is with the header pins that seem too long and flexible.
I purchased an Audio Injector Zero, for the tiny Raspberry Pi through Amazon, it’s $5 cheaper, but comes without potentiometers or header pins, it appears to be the functional equivalent to it’s bigger brother.
After installing header socket with extended pins like the original model has, I made the pins more stable by slipping over a piece of 1/10" hole spaced header board, it still leaves enough length for to plug into and makes the pins ‘feel’ a lot more rigid, on either Audio Injector model.
Although far less powerful, the Pi Zero combined with some tiny display and simplified interface could fit in a standard stomp box case to provide simple MOD processing.
The provided audio connector header boards accommodate 3 different connector types and volume potentiometers and are not a bad beginner soldering project.
Soldering on the header plug is another story, particularly if using the extended stacking pins. There are tiny surface mount resistors very close to the pins.
Some of the worst conflict pins are unused and could be left unsoldered.
According to the Audio Injector support pages, these are the pins used by the card.
3, 5, 9, 17, 25, 35, 39
2, 4, 6, 12, 14, 20, 30, 34, 38, 40
you probably don’t need to solder every ground pin, the grounds are:
6, 9, 14, 20, 25, 30, 34