I have a portable speaker, with a rechargeable battery, it takes in
power via a uUSB, it also offers power out via a USB type A.

I can play audio to it over bluetooth, or into a 3.5mm aux socket, the
bluetooth quality isn't the best (no AptX) and neither is the range.

I have a chromecast audio, this also takes in power via a uUSB and has a
3.5mm aux output[*].

If I plug the chromecast aux into the speaker aux, and power it from an
external USB charger, and stream music while the speaker is running on
battery, all is good.

If the speaker is powered from one USB charger, and the chromecast is
powered from another USB charger, all is good.

I've got two 13A sockets nearby the speaker, but need three devices
plugged in.

However if I power the chromecast from the speaker's USB power out, even
when the speaker is running on battery, I get all sorts of horrid
"motorboating" and clicks like on an 1980's international phone call.

If I power both the speaker and the chromecast from a single USB charger
with multiple outlets, or using a uUSB "Y" splitter cable, I get the
same interference.

I presume the chromecast is spitting out interference on its power
input, which the speaker objects to as it's relative to the audio ground?

I looked to see if there's such a thing as a USB isolator, they do seem
to exist, but are north of £50 which seems excessive,
If I pared-down a USB cable, so it only has the power +/- pins, no data
pins and no shield, is that likely to help? or do USB sockets generally
link the -pin to the shroud?


[*] Cunningly the same 3.5mm aux socket is also an S/PDIF optical
output, which I suspect would help with isolation, if I fed it into a
DAC (I have a spare) but then it doesn't help overall because the DAC
would require yet another power source that I don't have a socket for,
and would add to the spaghetti.