A friend of mine loves to sing. I wanted to make something for her that'd help her do so.
What it does
Songs are typically mixed with a center, right, and left track. This device allows you to force the left and right channels to use each other as ground, thus canceling the center track via destructive interference. By flipping the switch, you can either enable or disable this behavior, letting my friend enable or disable a song's vocals easily.
How I built it
A little bit of janky 3-way soldering plus a careful handheld dremel job to drill holes.