Ground Loop with USB 5v adaptor

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I have built myself a portable boom box consisting of a lp-2020a+ amp and a 9ah sla battery. I wanted to put bluetooth in it so I bought a cheap 12v to usb car adapter and a cheap usb bluetooth receiver. The problem is that the 12v - 5v adapter creates a ton of noise through the speakers when anything is plugged into it whether it be the bluetooth adapter or a cord to charge my phone. Will a different more expensive adapter fix the buzzing or will I have to run it from a separate battery (prefer not to do this as this box is suppose to cost me the least money possible). Are there any other ways to fix this ground loop? I have tryed a ground loop isolator but it made no difference which makes sense because there doesnt have to be anything plugged into the amps inputs and there is still noise through the speaker when the adapter is powering something. I know it is the 5v adapter because if I power the bluetooth adapter from a wall charger then there is no buzzing.
I just did nearly the same thing with an old boombox and a Raspberry Pi. I experienced the same issue when connecting the 5v step down module. To remedy, I went to Radio Shack and bought their $20 Ground Loop Isolator and -BAM- problem solved!

There was one catch, the overall volume is greatly diminished. Turning the volume all the way to full blast sounds about the same as it used to on less than half volume.

gmarsh, do you know if the Pyle you linked to uses a different filtering method or component quality compared to the RS? I'd like to try this again with a different model but don't know what to look for to get that next level of quality.

Here's the RS unit, it looks like a D-cell battery:
Ground Loop Isolator : Isolators |
Hey Foxx- if it is indeed switching noise alone, what step down converter would you recommend. I want to keep things efficient because I plan on running this off batteries a good amount of the time, which is why I didn't use a linear regulator like a LM7805.

This is just like the converter I used, with a LM2596: LM2596 DC-DC Buck Converter Step Down Module Power Supply Output 1.23V-30V: Car Electronics

What would cause you to think the regulator is the issue when the GLI stopped all the noise issues?
I tried to do a little reading on the USB Isolators. It looks like it does the same thing as a ground loop isolator but instead of interrupting the audio path, it interrupts the digital path, correct? I don't see any reason the output from the DAC would be degraded with this.

How would I measure to find out if it is a impedance mismatch and what would remedy that?
Actually if the problem is coming from the usb power the usb isolator probably won't fix that.

I don't think you can measure the impedance of the the isolation transformer. You'd hope that it was in the specs though. I know some MP3 players don't like high impedance loads, and this might be part of your problem.

Sorry, I'm not being very helpful am I.
OK, so I temporarily gave up on getting this to work with a single power source. I found an old RC car battery pack in my parts bin and ran that to the Raspberry Pi. The buzzing problem is gone.

However, I'm still not getting enough volume and I wonder if this is the impedance mismatch you mentioned. I have this setup so that the output from the DAC is soldered into the "Mic" inputs on the back of the boombox. I'm not sure what kind of mics people were plugging into these things back in the day but I'm guessing they had a different output than the line level coming from my DAC.

When I switch from the Pi (at max output) to FM, the volume is noticeably louder and crisper. On the LED recording level gauge, the Pi never gets above about 1/3 of the scale, while FM consistently reaches near the peak.

So the next question is what kind of mics were people plugging into boomboxes in the '80's and what was their impedance? The mic inputs on this unit are dual mono 3.5mm jacks.

The final question is how would I step from my line level to this mic level?

Thanks again for your responses!
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