This is a passive filter with volume control. With a 1/4? mono input on the left and output on the right it is simple way to add control to devices that may be lacking.

The circuit is the same used in other devices I build like the Maximal Drone and Auto-Step Synthesizer. Since it is completely passive it requires no external power.

It is housed in a very snug yet extremely tough die-cast aluminum enclosure.

Here is a video demonstration playing a Casio through it...