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

Hello,

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

Comments

[bassbindevil]

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]

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.

[mpilchfamily]

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.

[steveastrouk]

+1 on ground loops. Almost certainly the problem.

We need to see a proper wiring diagram.

[faxwork]

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]

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]

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...

Tor

[steveastrouk]

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.


Steve

[torclausen]

I'm pretty sure I have it right, but I will look more into it.

t

[steveastrouk]

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.

Steve

[torclausen]

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.

t

[torclausen]

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.

T

[tgferreira184]

why didn't you just insulte that? you put in a project box and cover it with a sound insulator?

[torclausen]

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.

[Computothought]

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.

[torclausen]

I've lifted the ground to the audio leads between the Bluetooth and the amp... Zero change in the noise.