Keeping Quiet
Learn to handle hum and buzz with a critical look at system noise from the ground up
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Properly designed balanced gear should tolerate shield current. Unbalanced gear, on the other hand, cannot. By design, a balanced input circuit is immune to most cabling noises by way of its common mode rejection ratio (CMRR). At minimum, CMRR sees noises common to both signal wires as out-of-phase and as such, cancels the unwanted intruders. Not all balanced outputs have signal on pin 2 and pin 3, but the source impedance at each pin must be the same for CMRR to be effective.

Another source of common-mode noise is a wall wart. In close proximity to a balanced audio cable, any transformer will induce hum into both the signal wires and the shield. For wiring that feeds unbalanced gear, pay strict attention to cable dress and run wires away from power transformers both obvious (wall warts) and hidden (in gear). Balanced gear can tolerate both common-mode (induced) noises as well as shield current noise.

Figure 2. A ground adapter doing its intended job—to update an old two-prong outlet.

Internal product grounding will be referred to as the pin 1 issue not only as it relates to XLR pin 1 but also other connectors (¼ inch, RCA, BNC). The correct approach is easily illustrated by observing the many ¼ inch connectors on a Mackie mixer, all of which are metal jacks on a metal chassis. This approach creates a low-impedance firewall that protects the internal, high-gain circuitry from external influences.

It was not so long ago that manufacturers chose connectors that were plastic-insulated from the chassis. Such a choice may be cost effective or even streamline the manufacturing process, but it undermines the internal ground scheme, decreasing the noise immunity of even good product designs making the resulting gear inherently vulnerable to any power- or transmission-induced (RF/TV) interference. The reason is simple. Printed circuit board (PCB) ground traces do eventually make contact with the chassis. If shields do not go directly to the chassis, the pin 1 ground current (and its noises) will infect the PCB ground and all amps referenced to it. Remember that a mic preamp can have 60 dB of gain, more than enough to amplify even the slightest ground noise. Higher ground and noise currents can also raise the copper's temperature, increasing its already high resistance and decreasing ground integrity within the unit.

I once received an e-mail regarding a mic preamp that picked up television interference (TVI) with a certain condenser mic and not another. Incidentally, TVI is often heard as a 59.94 Hz buzz, video's vertical sync rate, with harmonic variations that correspond to the transmitted video image. This was a clear case of pin 1 not being connected to the chassis at the point of entry. Resolving the problem initially seemed complex—how to fix the problem via e-mail without the customer opening the preamp to make any modifications.

Because condenser mics require phantom power, there is DC current flowing in the shield in addition to whatever external noise currents the shield acquires while trying to do its job. The phantom power requirements of condenser mics may vary, but all should be within the industry specification. The higher current requirements of this particular mic tugged on the ground enough to cause the preamp to detect the TV signal. In essence, piggybacked on to the DC current in the shield was the AC transmission of a local TV station, the most obnoxious of which was the vertical sync frequency.

To fix the immediate problem, I simply had the customer open the male XLR of the mic cable and wire pin 1 to a lug on the XLR connector designed to route the shield to chassis. (See Figure 1.) The customer was now happy, and I felt rewarded for the number of times I have attempted to improve a product's noise immunity on the bench. Time spent on equipment modifications may not always be profitable, but it sure is enlightening.

Perhaps you can now understand why, when customers ask about the relative merits of balanced power or any external ground scheme, I have difficulty giving a straight answer. I believe that if all gear were balanced with well-executed internal grounding, then no deviation from standard electrical power distribution would be necessary. That said, customers with enough cash to pay for noise insurance will reduce potential system noises with the aforementioned items. On the other hand, as systems grow in size, a multi-room or multi-floor facility with great cable distances, it becomes more difficult to implement and distribute a power and ground scheme. More important is to make good equipment choices. To installers who learn the hard way that certain gear is consistently problematic, please forward that information to the manufacturer.

There should not be any voodoo required to achieve low system noise, but mere knowledge of the noise devil does not solve the problem. In order to complete the circle of designer, installer, end-user and technician, we must all provide feedback to encourage manufacturers to take the lead.

A Critical Look
To sum up thus far, I have detailed the causes of hums and buzzes in audio and video systems, placing the blame on gear that falls somewhat short of being professional in terms of its inability to reject noise. Gear modifications might be the short-term answer to what ultimately should happen as part of the manufacturing process. Further, all of us have taken detours in an attempt to accommodate unhappy pieces of gear. As a technician, the worst that I have seen are user-installed systems where home-brew fixes include everything from the benign—rubber chassis isolators and plastic rack screws—to the illegal lifting of the third pin AC ground. All are band aids that make for an idiosyncratic installation. I will now focus on why various external fixes do not consistently solve noise problems. Hopefully, after finishing this article, you will have enough background information to assist those in the trenches who are moving too fast to understand or investigate the underlying problems fully.

Regarding the implementation of electrical power, the rules and regulations of the electrical code vary regionally. I am no expert. Finding a knowledgeable local electrician who is sensitive to the needs of a multimedia system is key. Using a ground adapter as a ground lifter is, by code, illegal. No matter whether the gear, the signal wiring or the power distribution is at fault, every temporary fix will eventually become an intermittent noise problem while also creating a potentially life-threatening situation.

The outlet-ground connection is simply to protect humans from the risk of shock (see Figure 2), and despite all the fuss about the dreaded ground loop, consider how may potential loops are created as soon as a piece of gear is rack mounted, plugged in, then connected to another piece of gear. It would seem as if we are doomed at the start and that meeting code and achieving a quiet system are disparate goals, but that is not the case.

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