This one was just for fun. The amp itself is nothing special - just an inexpensive, low-power headphone amp on a chip, designed around the chip's reference design from the data sheet. The challenge was the enclosure. I had the wireless mouse leftover from my first CMOY project, and I I wanted to see if I could design and build an amp into it, using the mouse wheel as the volume control, powered from the AAA batteries the mouse was already designed to hold. Most of the enclosure mods were done with nothing more than an Xacto knife and some trial-and-error. Unfortunately, it was either lost or stolen during an office move a few years ago.