Noisy audio output

I’m using the RPiS as a spotify connect client, connected to a small amplifier.

When both the RPiS and the amplifier are connected to the same 5V power source, the audio output presents digital noise from the RPiS: the busier the RPiS is, the louder is the noise.
When the two devices are powered by different chargers there’s zero noise.

I’ve tried adding a ferrite coil to the 5V supply line, as well as to the audio output, without any noticeable improvement.
Any suggestion to remove/reduce the background noise?
Thanks in advance.