EDN magazine has a very good article series on audio electronics related to ground loop issues (issue I have written myself in this blog and my ground loop documents). The article series is based on material that has originally appeared in Linear Audio, a book-format audio magazine published half-yearly by Jan Didden.
The G word: How to get your audio off the ground article starts by describing the grounding issues in the audio devices. According to GND Gurus the root cause of all hum and buzz problems is current flowing through “the same ground” as that used as voltage reference. So, they suggest, we use “different grounds.” The supposed solution is called a “star ground,” a common point where “different grounds” connect. It looks nice at first glance and its practitioners defend it as though it were a fundamental truth. Practically speaking though it’s a nonstarter.
The problem in star grounding is that it only works at all when it’s rigorously done and on any circuits larger than tiny amplifiers you’ll run into stability problems (try switching circuits and all assumptions go out the window). Stars are a sticking-plaster to try to make flawed assumptions work. Fortunately there is solution on balanced signals. Sadly enough, this too is riddled with confusing semantics: Balanced, differential, symmetrical, what shall it be?
The G word: How to get your audio off the ground (Part 2) article tells that the ideal differential input/output would use a transformer (but audio transformers have their own set of challenges elsewhere). This article describes basics input and output circuits using active electronic components (opamps).
The G word: How to get your audio off the ground (Part 3) article goes to more details on differential interfacing circuits. It shows the problems on some traditional designs, and gives details of circuits with improved performance. The article has also details on cabling, cable shielding and cable shield connections to equipment.
The G word: How to get your audio off the ground (Part 3) article claims that the XLR connector has been something of a missed chance: It should have been a round shell with just two pins in it. Nobody would have doubted that the shell should connect at the chassis. But now it’s got the third pin which has misled people into thinking that it was some kind of “audio ground” connection that should connect somewhere other than the shell. What happened is that a lot of people connected pin 1 to their internal zero volt reference (infelicitously called GND): Instead of shunting away circulating currents into the chassis, this actually invites them in to have an all night party inside sensitive audio circuits.
Pin 1 problems drive users mad! I have personally used countless of hours to track down humming problems cause by pin 1 issues on many audio systems. Tracking down those problems is frustrating even when you have all the needed tools in hand (including sensitive clamp multimeter to measure currents flowing on audio cable shields).
At some point the problem became so prevalent that the AES had to enshrine the obvious into a standard. Called AES48, it patiently explains that the shield should be connected to the chassis via the shortest possible route and that connections between the PCB ground and the chassis should be made elsewhere.