I have had some testing with ground loop isolators/filters in the audio signal line. But with no effect.
@Jzad - You are right about placement in proximity as a potential problem source. I have now enlarged the distance between the Up2Stream Mini and the PSU. The PSU is now in the same area as the PSU of the active speaker I am using in this project. This already makes a difference. Also I have shielded cables now both for the audio (between the Up2Stream audio-out and the amplifiers audio-in. But also for the DC 5v coming from the PSU to the Up2Stream Mini. This also makes a difference.
After that I tested other PSU’s (230AC > 5vDC) and found that the iPhone USB PSU is less ‘noisy’ than the others I tested.
Still I believe I have to eliminate the 230AC>5vDC PSU for the Up2Stream Mini. To avoid potential ground loop, and noise from a switching PSU.
I am currently looking at a more audio-grade, low-noise/ripple DC>DC converter. So taking a DC voltage point from the amplifier/power side of the active speaker itself. The module I am considering for that is: https://bit.ly/3jqizNr
I would appreciate any comments/advice on this approach, and whether this module is advisable or if to choose for a different one.
Let me know!