In fact I have a problem to test because one Zynthian is “on the raod” for tests and the second “disco” one is mounted deeeeeeeeeeeep inside a rack
My contribution was also not directly related to the use of Zynthian, but rather a general hint that USB power supplies can cause a lot of (audio) trouble. The Raspi/Hifiberry seems to compensate this quite well so far.
I only filtered the power supply of my TeensyMIDIAudio board (for MicroDexed) relatively simple. It’s ok, but you really have to pay attention which power supply you use. With the Aqvox power supply it is absolutely silent - and luckily 1A is enough.
Hm, probably both is ok, although a step-down converter can produce additional “noise”. That really depends on a try and I tend to work as little as possible with converters. Don’t forget, the Raspi addidionally converts 5V->3.3V.
Try Google for interference suppression solutions for audio systems to be powered via USB: Yes, there are a lot of ideas, but none of them satisfied myself for 100%. I think you just have to try (and have a scope within reach ).