There are certain safety requirements for electrical appliances (including audio) that force either an earthed device (Class 1 equipment) or one with double/reinforced isolation (Class 2 equipment).
More here:
https://en.wikipedia.org/wiki/Appliance_classes
For smaller manufacturers the requirements of testing and approval for Class 2 equipment may be forbidding, so “boutique audio” often uses Class 1.
Either Class is legal and if designed and tested correctly safe. So it is no question of “should it have an earth?” no matter if devices have earth or not, problems with noise may arise.
If a system contains multiple Class 1 devices – the potential for earth/ground loops is created that can lead to measurable or audible hum and buzz.
If a system contains only Class II equipment – the potential for “missing earth” noise (hum, buzz, RFI etc) is created as now all the audio systems shielding is no longer connected to the local earth but floating, so the shielding becomes an antenna that picks up noise.
But if a system contains only one Class 1 device and any number of Class 2 all is well and happy.
Due to historical reasons, many people will identify the second case as an earth/ground loop, when in fact it is the opposite!
Disclaimer: This article outlines how we would diagnose. Not how you should diagnose. One should always ask a qualified electrician to troubleshoot as mains voltages can kill.
