I’m doing an installation in a chapel, using a Van Damme Starquad multicore running along the roof void. Should I wire each pin‑1 to the XLR shell? I never have for any cable I’ve wired up to now, but that doesn’t mean I’ve been doing it right! What do you advise?
SOS Technical Editor Hugh Robjohns replies: The AES specifications do say that you should wire each pin‑1 to the shell, and there’s good sense behind that policy, but only if all of the equipment being connected together is built and wired correctly too.
The idea is that the case around each piece of equipment forms a Faraday Shield to keep any environmental RF (radio‑frequency) interference out of the sensitive electronics within. When an XLR cable is plugged into the device, that Faraday Shield extends out along the cable’s screen to whatever is connected at the other end of the cable, a microphone’s outer body or the case of another device, for example. In that way, all of the cabling and sensitive electronics are always enclosed within this Faraday Shield. To reach this Utopia, modern equipment should be built such that, inside the case, the XLR socket pin‑1 connections go straight to the chassis and nowhere else. With balanced connections between these devices, their own local audio reference grounds are not directly connected together, so no audible ground‑loop noises can occur.
Thinking about the metal shell of the XLR plug for a moment, when an XLR is plugged in it’s shell also connects to the chassis via a spring contact in the socket. However, as the shell can become dirty and corroded this connection might not be perfect, so the argument is that linking pin‑1 to the shell inside the connector itself provides more reliable grounding to maintain the Faraday Shield around the wires where they solder to the plug terminals outside of the cable screen.
I always avoid linking XLR cables together if I possibly can.
In a perfect world that all works well but, unfortunately, it was common practice in much legacy equipment to connect pin‑1 directly to the input/output channel’s audio reference ground. The inevitable result with this arrangement is that any noise currents carried on the cable screen are injected straight into the audio reference ground, and thus get into the audio electronics and become audible. In that kind of equipment, if the XLR shell is linked to pin‑1, the equipment’s chassis ground will also be connected to the audio reference ground, creating a ground loop within the device itself, with all the familiar hums and buzzes. So, while modern practice dictates that pin‑1 should be connected to the shell contact inside every XLR plug, if you’re using equipment which doesn’t conform to the current best‑practice wiring arrangement, linking pin‑1 to the shell can cause ground‑loop problems.
Not linking pin‑1 to the shell obviously avoids creating ground‑loop problems with non‑conforming equipment, but this practice potentially also risks allowing interference to get into the exposed wires inside the connector. That said, the risk is pretty small in most cases, since the XLR shell gets grounded anyway via the chassis socket, at least to some extent. The only situation in which this doesn’t happen is when two XLR cables are joined together to extend a run. In that case, with the XLR shells not connected to pin‑1, the exposed signal wires in both connectors are theoretically left unscreened and thus, theoretically again, prone to external interference. In practice, I’ve only known this to be a real problem in a couple of rare and extreme circumstances, but I always avoid linking XLR cables together if I possibly can, to minimise the risk.
One way to get the best of all worlds, if you have the patience and dexterity, is to link pin‑1 to the XLR shell via a small capacitor (100pf polystyrene or similar, the value is not critical). This capacitor maintains the shell’s grounding at RF frequencies, while presenting a high‑impedance path for circulating mains‑frequency ground currents. In this way, it works perfectly with both modern and legacy equipment.
If you don’t have the patience (or if someone else is paying!) an even better idea is to use Neutrik’s EMC XLR connectors. These take a similar approach, but with the capacitors built into the construction of the connector itself. Unfortunately they are three times more expensive than the standard XX‑series connectors, but they are very effective.