9 February 2017
My daughter is getting old enough that I want to start exposing her to lots of music. I hooked up an MP3 player to some speakers and everything works great. The issue is that the MP3 player requires power, through a USB wall-wart. These little transformers consume a power socket, and they emit unwanted heat. Fortunately the speakers have an internal 13 VDC power rail, so I added a 7805-based power regulator to create a parasitic power feed for the MP3 player.
Although this works, there is a shocking amount of interference. One can hear the processing on the MP3 player's CPU. Extensive debugging has failed to identify the cause.
If you have any ideas, please post them in the YouTube comments. I'm going to be out of the country until September, but will resume debugging then.