Buzz from DC-DC buck step down converter - how to isolate?

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I just installed a Raspberry Pi inside an old 80's boombox using a DC-DC buck step down converter power supply to deliver the needed 5v, like this one:
Amazon.com: 5 pcs LM2596 DC-DC Buck Converter Step Down Module Power Supply Output 1.23V-30V: Car Electronics

I wired the input of the buck converter from the 12v DC side of the boombox's AC-DC power supply, just straight positive to positive and negative to negative. As soon as I turn on the box, there is a great deal of noise and I can actually hear the Raspberry Pi doing its thing with the screeches from the wi-fi and sd card reader.

Obviously I needed to do something different with the ground on this but how? There is no third, ground lead on the AC unfortunately. Thanks!
 
Hmmm. Well the truth is I don't know very much about electronics yet. I've just been winging it by reading online and soldering modules together. I went with the buck because I plan on running this box with batteries part of the time and I read that the switching PS is much more efficient than the linear.

Is there any way to solve this with the stuff I already have soldered in?
 
Unfortunately the kit you chose may be upsetting the Rasberry due to inherent HF noise. Make sure you have chosen the correct ground point. Experiment with different points starting from the power supply smoothing common ground between the + & - smoothing caps if dual rail or the main ground point on the main smoothing if single rail supply. Also the positioning of the module may affect the noise.
 
Thanks for the tips- Unfortunately, I don't have time to investigate this today. If moving the ground point doesn't fix this, would I be better off (efficiency-wise) to use a LM7805 as described above or just wire in a simple 5v AC-DC converter, like the guts of a cell-phone charger?
 
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