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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 PC 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)

enter image description here

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:

enter image description here

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?

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  • \$\begingroup\$ Are you using a small cigarette lighter USB outlet ? \$\endgroup\$ – D-on Aug 21 '16 at 12:56
  • \$\begingroup\$ Yes I am, the 12V - 5V is a cigarette USB outlet \$\endgroup\$ – KiralyCraft Aug 21 '16 at 12:57
  • \$\begingroup\$ There's your problem. I have one that causes interference with an FM transmitter. I would wait for the engineers that can explain further however I would imagine it's poor EMC from that and not ripple on your power. \$\endgroup\$ – D-on Aug 21 '16 at 13:00
  • \$\begingroup\$ I wonder if a 5V regulator would do a better job, in spite of the fact that it would generate a ton of heat \$\endgroup\$ – KiralyCraft Aug 21 '16 at 13:02
  • \$\begingroup\$ All you need is just a simple dc noise filter, a Ferrite Coil and a cap. You can make from ferrite bead , toroidal core, or even just wounded wire on something metal plus one 1000uF/16V caps. Power source > the filter > the device. \$\endgroup\$ – Bianca Aug 21 '16 at 13:06
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Are you creating a ground loop (https://en.wikipedia.org/wiki/Ground_loop_(electricity)) in your second setup ? You could try a ground loop isolator.

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    \$\begingroup\$ This is the problem, you should use an isoltaion transformer to eliminate the grund loop. \$\endgroup\$ – Stefan Merfu Aug 26 '16 at 13:48

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