I'm using the above combination. Currently, there's a constant hissing when the player is not outputting any mp3. Now I saw that the PAM8403 has a "Shutdown" Pin that is active low, and the DFPlayer has a "Busy" pin that goes high when playing. Obvious solution: Wire Busy into Shutdown. It works, too.. except the hiss is still there. Shouldn't the PAM8403 be "dead" on the output when in shutdown? Or does it not completely cut off the output pins?
The hiss is more than likely interference on the TX/RX pins. Try putting a 100K resistor on BOTH the TX and RX lines, that stopped the hiss for me.
I have a 1k on TX, at least. But shouldn't there be no audio output at all when the amp is disabled via shutdown pin?
The PAM8403 datasheet says the supply current drops to < 1µA on shutdown, so the oscillator must be stopped,
but it doesn't say if the output devices are fully off - if not then there could be supply rail noise across the output.
Adding more rail decoupling could help if so.
However I would have thought shutdown meant fully shutdown withouts floating. Multimeter would let you know.