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.

The ferrite will only block HF noise. Load on the RpiS creates LF noise on the supply line.

If mono is an option for you, use differential signalling on a stereo output. The right channel has to send the inverted signal of the left channel. Connect the speaker between right and left channel of the amplifier (ie nothing to ground). This also allows +6dB since you are like in bridge amp mode.

Otherwise, try a big capacitor to smoothen the supply.

Best practice is separate analog supply and differential (balanced) signalling.

Thanks for the reply and the interesting solution.
I’ve moved to another platform that supports out of the box I2S class D HATs.
The signal processing being digital, there’s no noise whatsoever.