Switching noise that is usual for switching supplies (this is a low-noise SMPS brick).
Shouldn't bother us at all if these spikes are so low, definitely not doing anything to the output sound, especially because of their very high switching frequency.
SMPS ripple and noise after XL4015 boost-converter (AC RMS: 6.175 mV at inaudible frequencies)
Headphones-out noise when powered from the above SMPS (AC RMS: 3.7mV at inaudible frequencies)
The headphones amplifier was somehow affected by the switching noise of the boost-converters, but this noise becomes barely audible on sensitive cans only, like 16 Ohms IEMs. It was probably related to to lower harmonics of the original switching noise (300-500KHz perhaps?). This was the first PCB revision 1.6 of PLAY. With ver. 2.1 and above there's no more switching noise present...at least nothing to look like in the above pics. Also, no more background noise present with the new boards.
Linear PSU noise after swapping XL4015 with LM7815 regulator (AC RMS: 280uV)
Headphones-out noise when powered from the above LPSU (AC RMS: 309uV)
Removing the boost-regulators from the board and adding a DYI dual-linear PSU instead improved the output noise of the amplifier by
3.6dB, as per below pic (compare columns 2 and 3 only):
Column 2: SMPS is powering the audio equipment, column 3: LPSU is powering the equipment.
About the same 3-4dB difference was found by Headfonia in their measurements between the first version PLAY V1.6 and the second version PLAY V2.1:
https://www.headfonia.com/review-burson-play/3/. That means that the new boost-regulators from PLAY V2.1 do compete with my DIY linear PSU, which is a good improvement.
All my findings were already uploaded here:
https://www.audiosciencereview.com/forum/index.php?threads/burson-play-mods.3346/. The linear PSU did improved the background noise with sensitive IEMs, but THD remained about the same (low enough anyway). The LPSU mod might be done for PLAY V1.6 only, as the newer revision has already a very low background noise, inaudible, so not worth the time spent (I own both revisions, I A/B tested and measured both).
My own thoughts and conclusions: if a SMPS is well designed and implemented, then it should be just fine (see 2'nd rev. of PLAY), otherwise LPSU might worth trying.
L.E.: Column #1 from the above RMAA test is the original PLAY playing with solid-state SS V6 Classic op-amps in I/V and LPF. Seems that the noisy SMPS boost-converters are affecting how these big opamps measures. However, after swapping the 1st mainboard revision with the newer one the issue goes away and RMAA numbers become similar with the ones from column #3. Same numbers were achieved after swapping internal boost-converters with linear regulators (without swapping the mainboard). Column #2 has the MUSES8920 and the same first ver. of mainboard (smaller opams from MUSES is not affected by the noisy boost-converters). Column #3 has the SS V6, but with the new mainboard (same results with MUSES8920).