Help fixing power line interference with car amp & power supply

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I made my own boom box for fun. I already had a small pair of B&W speakers, so I made a amp/woofer module to power them.

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Inside is an Alpine 4-channel car amp, and a 15-amp power supply. I wanted to be able to run it either in the car via 12V DC, or plug it into the wall and run it via 120V AC. So it works great in the car, and also works fine via 120V, UNTIL the music source is also plugged into the wall, at which point I get an annoying, loud hum. Let me give an example: I plug the unit into the wall, turn it on, plug in my phone via the headphone jack to play some music. No problem, sounds great. But when I then plug my phone into the wall to charge it, I get loud hum. Same thing happens when using a laptop as the music source - works fine unless the laptop is plugged into the wall to charge.

Any thoughts on how to fix the hum? Do I need a different power supply? The one I bought was pretty cheap - similar to this 15A one on ebay: DC 12V 5A 10A 15A 20A 30A Regulated Transformer Power Supply for LED Strip USA | eBay

Thanks!
 
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This "ground lift" or "loop breaker" resistor is a feature of many mains grounded audio products, particularly musical instrument amp. gear and DIYs all come up against it at some time. The solution is not particularly safe and fault currents can be large, vaporizing the resistor and leaving the gear ungrounded.

BTW, 10 ohms resistance in a circuit impedance of ~ 10k makes no difference - none that can be heard or can't be fixed by turning the volume knob a gnats whisker, anyway.

Even if a 5c resistor can solve the problem, read this first (suitable parts ratings are also listed). Earthing (Grounding) Your Hi-Fi - Tricks and Techniques
 
Thanks for the info, Ian. I'm basically a noob with this stuff, so a lot of the info in that article went over my head. Below is a diagram for how I would be adding the resistor, if i'm understanding the suggestions correctly. Does this look correct? Am I grounding everything correctly or should I remove any of the grounds?
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Thanks again!
 
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