That could many reasons why that may be the case. Off the top of my head it could potentially be due to:
- Similar distortion in your other DACs
- Your system doesn't reveal enough detail to hear the distortion
- Your electricity supply is less noisy than mine and many others
- The distortion is different for 50Hz/240v and 60Hz/120v electricity (I'm 50Hz/240v)
- All your USB adapters just happen to be much cleaner than all of mine and many others
- There is some ground loop in my systems that I can't resolve. However, my desk system is fully balanced now, which unfortunately doesn't appear to improve anything that I can hear over single ended apart from extra volume on the headphones, so I doubt that's the cause.
- You don't know what to listen for or are used to the sound of the distortion
- Your hearing is not good enough to hear the distortion
There's plenty of testing that's been done on noise levels of 5V USB power. There's also much much more anecdotal reporting of the audible differences that electricity supply noise has on DACs in general, and on the E50 specifically. I don't think it's reasonable to ignore all of that just because nobody has performed thorough testing on electricity supply noise on the Topping E50 yet.
I can personally attest to that. I was not very impressed with the sound coming from the E50 when I ran it off the 1A USB port on two of my amps, then found a noticeable improvement when using a random USB charger that I had lying around, then noticed a further improvement when using an Apple iPad charger. Higher frequencies are less "harsh" and "grating", and sound more precise. It's not 100% resolved yet and so I'm looking at what I can do to improve it further. I've ordered a USB battery charger after hearing reports of that helping a lot, so will see how that goes.
My desk setup is Pi4 -> USB -> E50 (DAC mode, balanced) -> TRS to XLR balanced cables -> Singxer SA-1 (XLR input) -> 4.4m balanced headphone cable -> HD660S