R/E/P > Klaus Heyne's Mic Lab

Eliminate RF Noise And Buzz With Proper Microphone Cable Terminations


Is there a constant noise, like a ground hum, a mid-frequency static, a high frequency hash, or an annoying radio station signal hitchhiking on your recording chain? In almost all cases the cause is incorrect termination of the microphone cable.

Due to misinformation, negligence or a deliberate decision by the manufacturer, ground and/or shield of microphone cables are often terminated incorrectly.

A bit of background.
3-pin XLR cables are sold as mic cables (between mic and preamp) AND interconnects (between AC-powered components like preamps, compressors, recorders). Though there is only one correct way to terminate mic cables (detailed below), there are cases where a cable’s shield when terminated at both ends of powered units may introduce ground hum.*

Mics, being at the beginning of the recording chain will act as antennas picking up stray radio waves or electro-magnetic noise, unless all housing parts that envelope the audio signal - from the mic’s head grill to the shell of the XLR connector that’s plugged into the mic pre - are 100% covered by a shield that is also grounded.

Here is the rule for terminating mic cables. It has no exception:

1. Terminate Ground AND shield of the cable at BOTH connectors to pin #1 (XLR 3-pin connectors) or to the corresponding ground pins of a multi-pin microphone cable.

2. Make a wire connection from the connector’s ground pin to the connector shell, on BOTH connectors. You can solder a piece of wire (any type will do) between the ground pin and the little solder tab or loop that connects to the housing of the connector. Or you can twist the cable shield into two strands, soldering one to the connector’s ground pin, the other to the housing tab (see first photo).

Tube mic cable connectors rarely have a grounding tab, so use the connector’s strain relief clamp to attach a section of the shield, or the bare end of a piece of wire (the other end is soldered to the ground pin) second photo.

* Powered components connected by an interconnect cable have their own ground supplied from the AC outlet. A slight ground voltage differential between these components and the cable’s ground can introduce a 60 Hz hum.
Yet, even with interconnects, I always start with the mic grounding/shielding scheme: if there is no ground hum, RF protection will be optimal. Only if there is audible ground hum, I cut the shield on the receiving end of the cable.

Klaus, I'm sorry, but that does not always work, and can cause problems. Having worked on PA systems for years, I can tell you that grounding the shell of the connectors on the cable can cause problems. In a working environment, at the microphone end, the body of the microphone should be grounded. When you plug a connector into this microphone, the shell will then be grounded by the microphone body. At the other end, in a mixer, if you connect the shell to pin one, you may introduce an unwanted ground loop at that end. And if cables are used to connect pieces of equipment, the the problems multiply.

I invite critical posts. They elevate the discussion.

In this case I disagree with you and don't think you will be able to demonstrate to me even ONE case where the grounding scheme I shared for microphone cables will result in ground loops. I have helped many desperate mic owners over the years who had tried about everything else to eliminate noise and hum on the line. In all of these cases that noise was eliminated through proper cable termination, and no noise reminiscent of a ground loop hum was introduced.

This wiring scheme has been adopted as industry standard by major microphone and cable manufacturers, including Neumann, Gotham and Dörfler, all of which terminate dedicated microphone cables since the 1960s in that manner.

An interesting aside: a batch of mic cables marketed by a major manufacturer who even advertises proper mic shielding on the packaging was erroneously terminated with an open shield and sent out. After users complained and contacted me, I asked them to open the connectors and describe the termination. Long story short: the manufacturer took back wrongly terminated cables and replaced them.

I repeat myself: ground loops can only happen between powered units. Or, if by sheer bad luck or sloppiness, a shell of a properly terminated mic cable touches a powered unit other than the pre it is plugged into.

As I mentioned, the mic is at the beginning of the chain. Because it's not otherwise connected to a second powered device, the rule is ironclad, with no exception for mics and few exceptions for interconnects.


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