Help with Bluetooth audio receiver board CSR8675

So I bought this cheap bluetooth receiver along side a class-d amplifier board in hopes of connecting the two together to make a portable bluetooth speaker. This chinese amplifier board uses 12-24V dc and has a power out connection which provides 5V dc (probably specifically designed for a bluetooth adapter, since it also has an additional audio in connection right next to it). So I went ahead and connected them together. Upon turning on the amplifier board the reciever board light comes on but it doesn't really turn on and enter pairing mode as it should. The light just stays on and nothing happens. But if you disconnect the audio connection between the amp and reciever, the reciever turns on, pairs and functions normally. The reciever will also work if the power is coming from a different power supply and only the audio connection is in place (tested it with a phone charger).

In conclusion the bluetooth reciever only seems to work if it's powered by a different device than the one it's outputing audio to. How could I fix this? I have no idea what is going on, any help would be much appreciated!

Bluetooth receiver:
**broken link removed**

Amplifier board:
**broken link removed**

Perhaps you require DC blocking capacitors between the two modules?, both might be DC coupled and thus confuse each other.

Connect them together with just the power connections, then measure the DC voltages on the outputs of the BT module, and the input of the amplifier module. If they aren't all zero volts, then you need capacitors.

Thank you for the reply, it appears the voltages between the amp input and BT output are between 2-3.5v depending on which pin I touch. I will buy capacitors and attempt this again.
Thanks again!

I happened to find a few 100nF capacitors laying around and soldered one to the amplifier audio input ground lead. The reciever now actually turns on, pairs and works, but there are new problems now. There is a very loud hum coming from the reciever output now and also if you proceed to increase the volume on the BT connected device past around 50% while playing music, the reciever just cuts off and then turns back on again in a few seconds. It seems as if it's trying to draw more current than the amplifier board will allow and failing.

You don't want a capacitor in the ground lead, just in each of the multiple signal leads.

So I followed your advice and replaced the single 100nF capacitor on the audio-in combined ground for two, one on right and one on the left channel. And now im back to the original problem, it again doesn't turn on and work properly, just the light turns on.

Try connecting ground (directly) and audio (via capacitors), but not making the 5V power connection until after the power to the amplifier has been on for a few seconds.

It may be that the 5V output from that does not just "switch on" and rises slowly instead. That can cause problems with some types of electronics.

Waiting until the amp power has settled before connecting the 5V wire may allow it to work if that is the problem.

If so, it needs something to add a permanent delay when the amp i switched on; or a separate 5V supply regulator for the BT module.

The sales sheet for the amplifier does not show a +5V output that you used.

Yeah I noticed that too, but then again most ebay listings are lacking a lot of info... I measured the voltage and no it doesn't slowly rise it instantly jumps to 5V when turned on. Basically now it works with a single 100nF capacitor on the audio-in combined ground, but makes terrible whining noises so I ordered a ground loop isolation transformer and hopefully that should fix the noises.
I also ordered a dc step down converter so I can run it straight from the 12V battery, to see if that fixes the crashing of the reciever when you turn up the volume too high.

In the attached picture you can see the 5v output (yes it looks awful since it has been resoldered like 3 times while troubleshooting)

I've had some of those issues about receiver as well, and it was mostly because of wrong voltage measurements. By the way, there are a lot of situations occur, when the reason is audio receiver. I've been facing such issues with my old one, and after some struggling with that I've a new one Onkyo TX-SR383 ( one from the list: https://chooserator.com/best-av-receiver-under-200 ), and now I'm pretty sure about my audio stuff and connections.