At my wit's end

[Epovo]

Sorry, I could not find a descriptive subject line, because I need help with something I don't really understand.
I am connecting a device which outputs an audio signal to an amplifier, and then to the speaker. It works fine.
Then I wanted to power the device with the same power source of the amplifier (this power source is a set of Li ion batteries). The device powers on ok, and it outputs a nice test sine wave as it should.
But when I connect this audio signal to the amplifier, something strange happens. The speaker starts to jump, the power led on the device starts blinking. Eventually the amplifier breaks down (I think this is what broke the amplifier, though I can't be sure).
I have installed a brand new amplifier and it behaves the same, although I disconnected it before it broke!
In summary, the amplifier works fine, the device works fine and they also work fine together as long as they are individually powered.
I have attached a diagram to show what I want to do. I can't do much testing for fear of the new amplifier will  break too.
Any suggestion will be appreciated!

[brucehoult]

Why do you have two wires from audio device to amplifier?

Perhaps you should try AC coupling them e.g. via idk maybe a 5 or 10 uF capacitor followed by a 10k resistor to gnd?

[Epovo]

the two wires are the two wires from a mono jack connector: signal and ground.
I will do as you suggest - would you do the AC coupling on both?

[bateau020]

Indeed, AC coupling is probably the solution. AC coupling only the signal wire, and NOT connecting the ground to the audio source, could be OK. If the sound quality is bad, some more measurements and probably more photos would be needed.

[pcprogrammer]

There is a good change the amplifier makes a virtual ground and when you connect both devices to the same battery and then connect the signal ground to the virtual ground of the amplifier things go wrong.

In this case an AC coupling won't help. Try not connecting the signal ground to see what happens.

[PA0PBZ]

The TPA3110 datasheet gives you the answer:

[Epovo]

Thank you guys!
Both solutions (not connecting the signal ground and the AC coupling) worked fine.
You guys are wonderful.