Hi there,
I recently got an old GoldStar GSM-6330 and wanted to add an internal Bluetooth-receiver to it, without sacrificing any of the existing functionality. I decided for some cheap Bluetooth receiver board.

You can see on this image, how I wired everything. (I stole the +-12V from the phonograph power and tied the LGR 3pin output of the receiver to the AUX-input so I can still use the RCA input like normal. -> I also tried plugging the 3pin phonograph audio cable into it; same result.)

Sadly once I connect power and audio to the receiver I get some LF-noise and some "beeping" but can still hear the transmitted audio through the noise. Once I power the module from USB or a second power supply, the noise disappears. I looked around and found a video talking about the exact same problem, saying that its a ground loop and that you can fix it with an DC/DC isolated converter. But (i would have to look again, I don't really remember right now) the power consumption of the module is exceeding the rating of every (SMT) isolated converter I could find.

So my question is, if a ground loop is the problem, and if yes, how can I fix it?
Also, I can supply audio clips, images of the wiring / PCBs / power supply or whatever. Just ask if you need anything to help me out.

Thanks in advance

 

If you want to avoid the isolated power supply, you might get lucky and find an audio transformer of a suitable high impedance.

Before undertaking any major changes, you might want to try using a battery pack and a monolithic (linear) regulator as your isolated power supply. If that doesn't fix it you're looking in the wrong place.

 

If you want to avoid the isolated power supply, you might get lucky and find an audio transformer of a suitable high impedance.

Before undertaking any major changes, you might want to try using a battery pack and a monolithic (linear) regulator as your isolated power supply. If that doesn't fix it you're looking in the wrong place.

I did not try a battery pack but I tried using a random 26V 0.5A charging cable to get power from somewhere else than the built in power supply of the GSM-6330. This worked perfectly without any noise which leads me to believe, that it has something to do with the build-in power supply and a ground loop seems logical.

 

Hi Defin,
Ref this image, measure the voltage and also confirm a LED series resistor
E

 

Hi Defin,
Ref this image, measure the voltage and also confirm a LED series resistor
E
View attachment 337739

The voltage is around 12V and stable. Can't do much about the LED tho since its built into the switch (https://amzn.eu/d/7cj9AcC). I opened one up and there is a resistor in series with "392" on it. So 3900Ω if I remember correctly.


Here is the updated image.