DIY car audio noise when charging from the same power source
I have a hobby in electrical engineering so I built my own car audio system given the fact that it didn't have any before. It's basically two pairs of CDM-12008 speakers adapted with some resistors and regulators to work on DC 11-15V from my car. In order to eliminate any ripple and noise I have added a 4700 uF capacitor in parallel with the power wires. Like this (excuse the paint drawing)They are low power enough (around 30W max) to work without any noise when the car is running (noise induced by the alternator) as well as when it's on battery only (of course, no noise). My problem is when the device i'm using is charging from the SAME power lines, all the noise-filtering the capacitor is doing suddenly vanishes. This is when my problem occurs:My skills aren't advanced enough to figure out what's going on, so I'm asking here after 1 month of research: How do i eliminate the ripple from when my device is charging and plugged into the audio system at the same time?
I didn't find the right solution from the internet.