No Power on Earth - Grounding in Audio Circuits


 There's quite about of conflicting advice to be found online about how best to ground/earth audio equipment and audio connections. Most of it arises from considerations of mains-powered equipment and the avoidance of ground loops. But these days a lot of equipment is battery powered, or the mains power comes from a switching power supply (like a phone charger) with no connection to the mains ground. In those circumstances, things are rather different.

On Balance

I'm only going to consider balanced connections here - unbalanced connections are yet another story. If you have a 3-pin XLR connector, such as that above, pins 2 and 3 are the balanced signal and pin 1, well, isn't. Unless you're using phantom power, pin 1 isn't really necessary; if you are, it's the 0V reference for the power that's applied to both pins 2 and 3. But there's actually a fourth connection: XLR cables are usually screened and the screen will be connected to the shell of the XLR connector.

We don't have to be too concerned about the balanced signal - the whole point of it is that it's fairly robust in the presence of interference. However, the two other wires in the cable are rather more problematic as both will pick up both mains and RF interference, the cable attached to pin 1 rather less so as the cable screen will protect it to some extent.

We need to distinguish between a 0V reference and ground/earth. A true earth connection - to the earth pin of the wall plug - is the ultimate current sink, but a 0V reference is just that: the potential against which a different potential is measured. Traditionally, circuits would distinguish between the "chassis ground" (usually connected to earth for safety reasons) and the "signal ground" (the 0V reference for audio signals). The two would usually be connected, probably at a single point. If we had a true earth, connecting the two unbalanced connections to it would conveniently sink away the potential interference. But if we don't?

Floating Volts

It's probably not a good idea to connect the cable screen to the local 0V reference (what might be considered the "signal ground") since this risks coupling the interference from the cable directly into the 0V power line. Indeed, the screen will act as a "Faraday Cage" in the absence of being connected to anything at all. The same applies to any metal enclosure. You could connect such an enclosure to the cable screen, but there's probably no major benefit from doing so.

In the absence of phantom power, leaving pin 1 unconnected, too, is probably the best course. If you need phantom power, you don't have that option.

If your phantom power supply is floating relative to power to the rest of the circuitry (there's an isolating transformer), then you should connect its 0V reference to pin 1 (and consider switching it so there is no connection when the phantom power is not required) and leave it at that. If the power shares a 0V reference with the signal circuitry, then you have little option but to connect the shared 0V line to pin 1. But that's merely one of the many compromises that phantom power will require...