Battery run bluetooth class d amp - help me fix the noise problem.


I am designing a small bluetooth battery powered portable speaker enclosure/boombox.

At the core of this design is a cctv li-ion battery that has 12v and 5v USB output ports. Its a great start to a boombox, or at least I thought it was.

From that battery, I am powering two things. A parts express Stereo Bluetooth Module, (running on the 5v USB) and a Lepai 2020 tripath amp, running on the 12v

Perfect you say. Thats what I said.

Plug it all together.... huge huge huge garbage noise, with digital clicks and beeps. All three components work fine on their own. The amp plays cleanly with bluetooth modules that have internal lithium batteries, the bluetooth module sends a clean signal to amps not sharing a power source.

I had a proficient audio repair man attempt to filter out the noise with resistors placed in numerous places, but it only slightly lessened the noise. Unfortunately I didn't document what he tried.

I have attached a photo of the setup (without the speaker attached)

I noted one thing... that if one was to connect the bluetooth using the rca input to the amp, if you only touched the internal rod to the inside of the female rca jack on the amp, keeping the outer shielding free from the amp, it would clearly make the noise, with no change in the noise when you pushed the plug all the way on, touching the outer shielding. That seems of interest. Yes?

Let me know if I can clarify any more of this. Any help would be most appreciated!

Thanks, Tor

Picture of Battery run bluetooth class d amp - help me fix the noise problem.
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StephenF8912 days ago

I have the same problem with my bluetooth receiver that I've connected to my amplifier TDA2030. As I understand it, it's the DAC in the dongle or BT receiver that is causing the problem. We are actually hearing the conversion done by the DAC getting into the audio signal. I suppose it's the device build itself is the problem. The converter works but not totally eliminating it. The only solution I did is to have a separate 5V supply for the BT receiver. I modified a cheap powerbank so that when I charge the portable speaker, it will also charge the supply for the BT. I just added a 5V relay to do the work without a manual switch. I put everything inside the boz and it works fine.

af35564 months ago

If anyone else come across this kind of problem: eliminating ground / earth loops can be challenging. One way to test if you have a earth loop issue is this: does the problem go away - completely - when you power the sub-systems from totally separate power supplies? If so, you likely have an earth loop issue. If not, or only partially, you have other issues to contend with, e.g. RF interference.

On earth loops: ground loop isolating transformers can help, but can impact audio quality. An alternative approach for low-power devices such as BT receivers is to isolate the power supply via an isolated DC-DC converter (hit Google for suppliers).

Yes caps will help, but you need to coordinate the resistors with the caps to block the correct frequencies. Also recommend taking power line and loop it thought a ferrite core several times.

i am having a similar problem with the following setup as drawn on the diagram (BT=bluetooth modul, AMP= mono amplifier). i dont have a noise problem since i got rid of noise by adjusting the resistors on the amp input. Everything worked fine on separated power supplies. Once I connected it to a common 5V source, the BT board won't start up anymore. It should be a kind of ground loop problem. When i take away the signal connection between BT and amp, the BT module starts up as it supposed to. Once I establish signal connection, it crashes again. Any idea?


i just added a Ground Loop Isolator (GLI) between BT module and amp, see drawing. It doesn't do no good.

The only thing I noticed is, when i establish audio signal connection after startup of BT module, it works. But when audio signal connection is established before, BT module won't start up. Oh man, this is supposed to be an Xmas present, I really need to fix it fast. Any help appreciated.

HansV131 year ago

I had the same problem as described with my KRC-86B bluetooth module. My solution was to wrap the board in electric tape and after that I wrapped it in a sheet of aluminium foil, not covering the antenna on the board. All digital clicking and cracking noises disappeared since then.


did the wrapping in aluminium foil really help for you?

i do have the same problem with my krc-86b module, but your solution didnt really help...

Do the noises dissapear when you hold the board between 2 fingers?

Not all of them. But the noise is quieter.

It also depends on the device that is connected to the bluetooth module. There is more noise if a device with bluetooth 4.0 is connected. I still don't get it.

torclausen (author) 5 years ago
I've lifted the ground to the audio leads between the Bluetooth and the amp... Zero change in the noise.

I know this thread is over 2 years old, but I was having a bluetooth noise issue and this thread was the one that kept coming up in Google searches. In my situation, I was powering a bluetooth audio adapter and a small 7-watt mono amplifier from the same power source. I was getting very loud digital noise... beeps and clicking and such as soon as the bluetooth established a connection. The previous posts in this thread pointed me in the right direction... it was a ground loop issue. I was able to solve the noise problem by installing a 1:1 isolation transformer on the audio wires between the bluetooth receiver and the audio amp. This isolated the audio circuits and eliminated 99% of the noise. The part I used was a Neutrik NTE1 1:1 Audio Adapter Transformer and can be found here : https://www.parts-express.com/neutrik-nte1-1-1-aud...

The red and black leads connected to the bluetooth side, while the yellow and white leads from the transformer connected to the audio amp side. This gave me the best bass response and rolled the high end off a little bit. Reversing the transformer seemed to roll of the low end while maintaining the highs. You can try and both ways and see what sounds best to you. Hope this helps someone else having bluetooth audio problems when running from the same power source.

Why not just use a cable? Bluetooth audio is lossy (like mediocre MP3). Headphone jack outputs are just fine for feeding amplifiers.

Apart from that... I'd want to confirm the whether the DC commons are the same between the BT power and audio output. Also with the power source; is 12V common the same as USB common? When it comes to "made in China", assume nothing. The 5V USB output is probably generated by an MC34063 switching regulator; the output may have some electrical noise on it (not be pure DC).
torclausen (author)  bassbindevil5 years ago
Bluetooth is amazing. a) When it runs without noise, I can't tell the sound difference from it and an aux cable, if a2d2. and b) everyones doing it! many of us youngsters don't even put the thing down, because we text so much. A cable is a nuisance.

I had the battery apart, and the grounds are common. I also as a test set up an well made belkin usb 12v cigarette adapter on a straight battery, and it made the same noise.

Have you tried the setup with a different power source? Are the data lines from the USB connected to the blue tooth board?

Is it just me or is the ground from the battery to the amp missing? The problem sounds like a ground loop issue. Have you double checked the RCA wire. The center rod should be the signal and the shield should be the ground. If you have this reversed it could be causing the ground loop.
+1 on ground loops. Almost certainly the problem.

We need to see a proper wiring diagram.
I'm pretty sure this is a ground loop problem.

What I'm thinking is that the ground for the amp and module are at different slightly different potentials giving that noise. I'm not sure how much work it will take, but try to connect the module to the same ground point as the amp and not the ground from USB.
torclausen (author)  faxwork5 years ago
I'll try that, thanks! I did purchase a ground loop isolator and have removed much of the noise ( not enough) . I appreciate the thought! T
torclausen (author)  steveastrouk5 years ago
I am unsure of common diagram etiquette... did the best I could.

Different power source tested, noisy still, but slightly different sounding.

The RCA's are all correct. The amp is wired with a proper ground. (the wire is white in the photo.... good eyes on you mpilchfamily)

I have had a second technician look at the setup and sleuth around. He had it on an oscilloscope and was able to get the noise, which he said was very spikey, to show up clearly on the scope. He confirmed it was wired properly.

I hope the video is viewable as well, it shows the sound...

You're a ground missing by the look of it.

1.) Ground the amp to the -ve terminal of the battery.
2.) Ground the bluetooth module to the -ve of the battery.

torclausen (author)  steveastrouk5 years ago
I'm pretty sure I have it right, but I will look more into it.

If you HAVE, then try removing the ground on the analogue out from the BT module to the amp. I'm going to guess that the 5V supply is noisy out of the battery pack, and that's where your problem's may start from. Hard to say for sure, without access to your kit and my oscilloscope. Try putting some big electrolytic caps on the power connections to the amp too, it looks like its signally lacking any.

torclausen (author) 5 years ago
I bought a ground loop isolator from Radio shack and placed it on the audio feed to the amp, from the bluetooth. It cut 95% of the noise out. Its still too loud, and the sound quality overall isn't as good as it is on a separate power source. But I now believe I can get it to work.

I am getting a few other bluetooth modules in the mail. A griffin Bluetrip and a Xtrememac Incharge auto BT. I am hoping that these are quieter out of the box, as they are designed to be on a single battery... the car battery. Wish me luck.

I realize that the drawing above leads one to believe I have only one wire leaving the battery.... Two wires are there, that was an assumption for clarity.

torclausen (author)  torclausen5 years ago
Oh man, even that 5% is driving me nuts. If listened to quietly, its a staticy mess.

Still in need of help! What can be learned by the ground loop isolator being so impacting on the sound? What can I do to lessen the last bit?

I miss the sound quality of the little self powered bluetooth. I hope I can get there again. But it is great to plug this thing in at night and have the whole thing charge up with one external port.

why didn't you just insulte that? you put in a project box and cover it with a sound insulator?
torclausen (author)  tgferreira1845 years ago
The sound is coming from the speakers, and distance and insulation from the amp has zero effect on the noise. I do not believe that insulation is the problem.