I've built an audio adapter for car stereo system. It contains microcontroller, a connector to stereo system (a few data pins and line-level audio output), Bluetooth audio module (RN52) and power connector.
I've successfully tested it with a separate, floating power supply, and audio noise falls within my expectations.
However, when I power the adapter with a power supply which is also referenced to audio amplifier/speaker system, I am hearing significant noise. This happens in car, but I also hear the same issue on my desk, if I power the PCB from PC's USB port, and speaker system is also referenced to my PC.
I kind of suspect grounding issues on my PCB. When designing it, I did not pay a lot of attention to routing GND pins of the modules/ICs used- I relied on having ground fills on both sides of the PCB.
This is a high-level overview of the PCB I made:
I recall having heard about "star ground" concept, where all the ground points should connect in a single place, power input connector.
The questions are:
1) How applicable the star grounding is if there are multiple connectors (each with a separate ground wire), which may, or may not be referencing a "common" ground somewhere further.
2) Is it possible to improve noise with a different/proper grounding on my PCB, or should I isolate either the audio output or power input?
Images of the actual PCB layout (warning, hairy stuff):