I have a circuit that samples the audio from an MP3 player, this audio is also split to an amp/headphones. There's is also a USB port to provide power to the MP3 player from the 5v circuit supply.
The issue occurs when the MP3 player is plugged into charge - I can hear intrerference in the audio. It's fine when not powered.
For comparison an iPhone doesn't have this problem, so I suspect it's the cheap MP3 player.
Is there anyway I can stop this interference? Using an additional dedicated 5v supply for the MP3 player works but I would like to avoid that.