I've seen a lot of amplifiers that don't have this and a few that do. What's the difference electrically? Also what's the implication for an end user?
The crux with unbalanced connections is that there must be
at most one (and preferably
exactly one) earth connection
in the entire system for it to work flawlessly, avoiding the dreaded
ground loops. That may be rather more easily said than done in practice. You may want to connect a PC, or something that uses an external antenna of sorts (most of which are required to be earthed for lightning protection alone), or broadband cable. So if you see an amplifier with RCA inputs and a 3-pin IEC, you may want to tread with caution.
I would assume that the larger model Rotel's transformer had rather more mains leakage capacitance than they (or regulations) were comfortable with. An outfit on the smaller side (and Rotel aren't a super large company) may have difficulty sourcing some suitable mains transformers with a shield winding. That's really the kind of effort it takes to avoid issues with unbalanced connections though. Mind you, you can "cheat" by avoiding a "hard" connection to protective earth... a few 10 nF of capacitance alone will tend to greatly reduce issues with leakage currents while still being sufficiently "open" compared to audio connection shields. So seeing a mains earth connections isn't necessarily
always bad news, though it
may indicate that the designer didn't really know what they were doing.
The world of
balanced connections is quite different - here all the devices being earthed is actually
preferred, and not at all problematic, at least if proper wiring guidelines are followed (see: pin 1 problem, AES48-2005).
