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Need advice on noise in 12v supply

Not a problem.
 
That is a real possibility and one that I have long suspected. But the fridge stays.
I wasn't suggesting you remove the fridge, but was building on what David was suggesting....find the source. Is there a way to put a filter on the fridge to prevent the noise at the source? But first you would want to make sure it IS the source....
 
I wasn't suggesting you remove the fridge, but was building on what David was suggesting....find the source. Is there a way to put a filter on the fridge to prevent the noise at the source? But first you would want to make sure it IS the source....

In testing I turned the fridge off and there was no change. But that may not have disconnected the inverter that may be in its circuitry. The fridge is on its own run from the battery switch. The stereo is on the factory house wiring as is the USB power supply. There is perhaps 30' of wire separating the two. I have also tested drawing USB power from my accessory switch panel which has its own positive and negative runs to the battery.

I have tried connecting the USB power supply to the same power wires as the stereo. That did not remove the ground loop.
 
There are endless sources of noise on a boat, including alternator/stator, trim, fan, switch and relay transients, and on and on.
Noise modulated on the DC supply is completely normal. What is not normal is for the noise to actually find its way into the audio signal path.
The fact that the noise is limited to one audio source only under a limited condition, points to the audio accessory power supply, and not an external cause. The power supply conflict is not something you can likely correct. You are probably going to have to apply a bandaid of sorts.
My best guess based on the info at hand.
 
The ground loop isolator removed the hum. The 12 volt noise filter did not change anything. I am going to open it up and see what is inside.
 
Two (stereo) tiny transformers. The ground shield is now magnetically coupled with no hardwire pass-thru. Transformers pass AC but will not allow DC to pass. The noise is actually modulated DC current. Stop the flow of current and stop the noise. The center signal wire is unaffected and unaltered.
 
I ordered a noise suppressor from Amazon that had excellent reviews and claimed to boost the output level. It worked to block the noise but unfortunately reduced the signal level many decibels.

The USB sound card that I am using with the Raspberry Pi to play audio has a Toslink optical output. In September I ordered a Digital Toslink to Analog adapter that came with a USB power cable and a Tosllink cable for $9. I finally got around to connecting it today. The output level from the adapter is equivalent or higher than that from the sound card. So this may be the perfect solution for my noise issue.

Now that Pi is working correctly I will writeup the complete system which stores music and allows anyone on or near the boat to choose music and control volume from their phone.

I had a number of threads when I was looking to suppress the noise so I am updating all of them.

The UPC of the unbranded Toslink Adapter that I am using is 713665000703.

toslink.jpg
 
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