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Class D Switching Power Amplifiers and Power D/A conversion

Bluetooth noise problem.
Bluetooth noise problem.
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Old 12th September 2019, 01:31 PM   #1
jackstraw is offline jackstraw  United States
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Join Date: Jun 2016
Default Bluetooth noise problem.

Hi guys. I build these single speaker class D amps out of those 5L beer kegs. A couple years ago I made a new model with bluetooth. It's been great for a couple years, so I'm trying to make more for my friends. The bluetooth module that I used in mine is no longer available, and the ones i've tried are producing a lot of bluetooth signal noise. I've been assuming it's the cheap modules that is the problem, but after my most recent purchase of a more expensive module that is supposed to be the upgraded version of the module in my original amp, I have to accept that maybe i'm doing something wrong. So i'm here to ask for help from folks who actually know stuff. Below is a description of the problem and my setup & components.

The issue:
When the amp is powered up there is what I describe as signal noise for 5-10 seconds, and then it goes quiet. When a device connects (my phone) the signal noise comes back. If no audio is played the noise again goes away in 5-10 seconds. When audio is played the noise comes back and stays until the audio is paused or stopped. The signal noise scales with volume. Mostly you can't hear it over the music, but I can detect it in softer songs and audio books.


The basic setup:
Click the image to open in full size.

Specifics:
Power supply: 6x Panasonic Lithium 18650 wired 3S2P

Amp: TPA3118 PBTL Mono Digital Amplifier Board1X60W 8-24V

Bluetooth Module: Bluetooth 4.2 CSR64215

As of right now the bluetooth boards have not been modified to accept the necessary external antenna.

Thanks for any advice you can offer!
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Old 12th September 2019, 05:11 PM   #2
Nenola is offline Nenola  Finland
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Join Date: Feb 2008
Or maybe the upgraded version has something wrong? Sounds weird but...

I've tested few modules and this one has proved itself to be noise free:
CSR8645 APT-X HIFI Bluetooth 4.0 12V Receiver Board for Car Amplifier Speaker | eBay
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Old 12th September 2019, 06:09 PM   #3
TBTL is offline TBTL  Germany
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How did you connect the grounds?
You could try adding a small electrolytic capacitor to the power input of the bluetooth board.
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Old 12th September 2019, 08:52 PM   #4
jackstraw is offline jackstraw  United States
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Join Date: Jun 2016
Thanks guys. The grounds all connect together. I'm not running a ground line from device to device, but rather they all connect to a central point. I think so anyway.

When it comes to any wires that carry sound (bluetooth to pot to amp to speaker) i'm using shielded wire. I'm using the outside bare shield wire as the negative and the inner coated wire as the positive.

I should have some electrolytic capacitors around. What volt & mdf would you recommend? I have a 16v 100uF, and a 16v 220uF. Would either of those work? I also have some others if not.

Thanks!!
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Old 12th September 2019, 09:52 PM   #5
Turbowatch2 is offline Turbowatch2  Germany
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Both are good, capacity does not matter that much.
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Old 13th September 2019, 09:17 AM   #6
Mark Tillotson is offline Mark Tillotson
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Join Date: May 2018
Location: Cambridge UK
Any RF injection might be reduced with 100pF to 1nF ceramic capacitors across signal<->ground, and power<->ground. RF on the audio signal can get rectified in the amp's input stage, polluting the audio with RF envelope waveform.


Keep antenna well away from amp, add the ceramic caps at the connections to the amp board right at the terminals, preferably soldered direct to the terminals.


Does the amp pick up GSM packet bursts too?
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Old 13th September 2019, 09:44 AM   #7
jackstraw is offline jackstraw  United States
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Join Date: Jun 2016
Mark, thanks! I'm going to try the capacitors.

Currently, with this new board just in testing, I haven't modified the bluetooth board for the antenna. I have the cut the on-board antenna and the solder the lead wire. Anyway, currently the bluetooth module is sitting well away from the other components.

I had to google GSM. Without any experience, i'm not certain. To me the noise in question sounds like it could be this. It's not a steady hum, but rather an irregular jumble of noises that vary in pitch and volume. If the capacitor idea doesn't solve the problem, i'll record a sample of the sound. Thanks again!
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Old 14th September 2019, 02:41 AM   #8
jackstraw is offline jackstraw  United States
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Join Date: Jun 2016
Ok, so the capacitor thing didn't work out. So, i decided to remove everything and start as simply as possible. I'm only using the battery, the amp, the bluetooth, and the speaker. Along the way I found something on accident that might point to the problem. Or it may be perfectly normal... but it doesn't seem right to me.

First, i connected the amp to the battery and speaker. It powered up, and there were no unexpected noises in the speaker.

Then I connected the audio out from the unpowered bluetooth to the audio input of the amp. No problems there, but again, there's no power to the bluetooth module yet.

Lastly, i started connecting power to the bluetooth. I connected the positive lead first, and suddenly the bluetooth powered up and started making the interference noise through the speaker. I had not yet connected the negative power supply lead! This made no sense to me. The only thing i could think of is that the audio connection between the amp to the bluetooth was completing the circuit. So, i unplugged the 1/4" phone jack from the audio out of the bluetooth board... and it powered down, confirming my suspicion.

Is this par for the course? Should the audio connection between the boards be able to complete the bluetooth's power circuit?

Thanks guys.
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Old 14th September 2019, 02:58 AM   #9
horst303 is offline horst303  Australia
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Yeah it's a well known problem, give your bluetooth module it's very own battery and listen to it become silent.
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Old 14th September 2019, 03:11 AM   #10
jackstraw is offline jackstraw  United States
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Join Date: Jun 2016
Quote:
Originally Posted by horst303 View Post
Yeah it's a well known problem, give your bluetooth module it's very own battery and listen to it become silent.
Wow, really? So i need to have one set of batteries for the bluetooth and a different set of batteries for everything else?? How would that even work? I mean, all the batteries are rechargeable and contained in the unit. I guess i would have to have 2 different charging ports and BMS boards, and charge them separately? That's nuts!

What are my options? Are there bluetooth boards out there that don't suffer from this problem? Is there a way to isolate the power between the two modules somehow, like with diodes or something?

Thanks for the response!
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