If you plan on using a Nextion in a project that involves audio, I can tell you these displays are disgustingly noisy!
I could hear all this low level “crap”, which as it turned out, came from the Nextion/TJC display when the display brightness was reduced down from 100%.
This “crap” was traveling back down the PSU line into the audio equipment, also powered off the same mains to 12V PSU.
The backlighting level is controlled by PWM, and that’s the source of the problem.
You’ll hear a low level ~300Hz (and harmonics ie; 300Hz, 600Hz, 900Hz… ), and it’s quite evident on an audio spectrum analyzer.
For my project, I tried powering the Nextion off a separate 5V regulator to the MCU, but that didn’t fix the noise problem. The noise was still getting back into the audio equipment.
When I temporarily powered the Nextion-based project and the audio equipment off separate 12v PSUs, the problem went away.
The solution was to have a relatively large inductor (10mH wire wound with a suitable current rating, 270mA in my case) and a large electrolytic on the input of the 5V regulator feeding the Nextion. This prevented the noise traveling back down to the common 12V PSU.
Typical wire wound 10mH inductor:
A 12V rail feeds this 5V regulator which then feeds the MCU board and Nextion: