Digital noise in battery powered system
while building a Bluetooth speaker based on this instructable, I ran into some problems regarding noise. There already exists a question regarding this problem here, but it is a few years old, and it gave me no solution.
The power source for this project is a self constructed battery pack consisting of 4 18650 battery's soldered to a battery protection circuit (just like in the instructable). The PAM8610 (black version) amplifier board, however, can't take 16.8 volts so a voltage regulator is placed in between set to 12 volts. Same story for the Bluetooth receiver board (KRC-86B V4.0), but with a voltage of 5 volts.
All fine and dandy till there. The KRC-86B has an input for a 3.5mm jack connection and an output. The output is a pass-trough of the 3.5mm input unless a Bluetooth connection is present, then the Bluetooth signal will overwrite.
Here's the problem: whilst the connection using the 3.5mm jack works fine, creating a Bluetooth connection results in a very annoying digital noise. This noise is there instantaneously and is the same on all volumes of the host/transmitter. If no music is being played by the host for around 15-20 seconds the device seems to enter a power saving mode and the noise changes to a regular beep. This is way less bad, but of no use because it is replaced again by the bad noise when the music starts again.
I have tried a diversity of things, listed here with their effects:
Action: wrap the Bluetooth receiver in electrical tape and aluminium foil.
Result: nothing much, it didn't even block the music. Signal was only blocked by three layers, a few meters distance and my body in between my phone and the receiver.
Action: lift ground in signal between Bluetooth receiver and amplifier.
Result: noise gets slightly worse, no change in tone, only amplitude.
Action: moved away from my computer, as to avoid interference.
Result: nothing at all.
Action: tried another Bluetooth transmitter, PC instead of phone.
Action: broke the previous board at the start of this project by heating it too much in the attempt to change a wire.
Result: noise was also present at the first board which I did test by hooking it up to the line-in of my computer. Though that could also have been by the ghetto set up I used. The 3.5mm cable was poorly connected, trough a breadboard.
Taking my multimeter and measuring the difference between negative of the battery and GND of the voltage regulator for the amp gave a 6.0 mV reading. The same result is achieved when measuring between battery and GND of the regulator for the KRC-86B. The potential between GND of the signal and the GND of one of the regulators was 0.1 mV.
I understand that you might want to hear the noise, so I recorded it: link. It is very soft, but audacity doesn't read .m4a files, and turning up the volume works just as well.
Please let me know if you have any idea's on what could solve it and/or if you need more info.
Thanks in advance.