Keeping Quiet
Learn to handle hum and buzz with a critical look at system noise from the ground up

By Eddie Ciletti

  For sound and video systems in-stallers, voodoo and wives tales abound about how to achieve minimal system noise. By noise, I mean hums and buzzes, which are power- and grounding-related issues. Hiss, on the other hand, is a gain-structure issue not covered in this article. Troubleshooting noise can eat into profit margins for installers, technicians and end-users, ultimately consuming an unpredictable amount of time and money. This article is intended to help everyone who has ever been burned by a problematic system.

The lack of noise immunity in some equipment is an insidious problem—the primary topic of this article—and the industry's dirty little secret. Long before software and hardware developers started pointing fingers at each other, all eyes turned toward the installer when the system powered up for the first time humming a tune. At least bad analog gear has somewhat of a voice to express its unhappiness—hums and buzzes for audio and a vertically creeping, horizontal bar for video. Digital gear lets us know only when it is too late, but it has increased manufacturer awareness regarding noise immunity, yielding a heightened consciousness that has trickled down and into some modern analog gear.

Figure 1. Wiring male XLR pin 1 to the chassis lug will safely direct shield noises to chassis

The first part of this article will cover systems basics and what should happen. Later in the article, I will address the fixes that have been attempted to accommodate bad gear and why they do not always work. Some are valid as insurance policies; others are borne out of paranoid overkill and mostly unnecessary.

I could not have written a single word about system noise without experience in the field. I have admittedly made mistakes and am lucky to have a tenacious, curious nature. All this makes me knowledgeable, but I am not an expert. In a parallel universe, brains far more brilliant than ours have wrestled these thorny issues. You can find more detailed information from Bill Whitlock at Jensen Transformers (www.jensen-transformers.com), including a library of reference books for your suggested reading.

Another authority is Neil Muncy, who chairs the AES Standards Committee (SC-05-05) on the subject. You can find his condensed overview of suggested practices at the Rane site (www.rane.com/note110.htm), and honorable mention of Muncy appears at the Philip Giddings site (www.engineeringharmonics.com/papers/pin1.htm) specifically addressing the pin 1 issue (more on that later). At www.josephson.com/audiofaq, there is a wonderful FAQ compiled from the rec.audio.pro newsgroup by the late Gabe Weiner.

Typical solutions
There are four solutions typically applied to circumvent system noise problems. It is not my intent to debunk what some may have embraced as religion, merely to shift the focus to the real problem, which involves the gear itself. The first solution is power distribution, an important part of every system's foundation. Before considering noise insurance options like balanced power, an isolated ground or star grounding, all of which require considerable effort and cash, it is important to remember how easily installation costs can exceed the cost of the gear. Few customers can claim that money is no object, and none of the aforementioned items are an absolute guarantee of long-term silence.

The second common solution for noise problems is the use of balanced gear and avoidance of troublesome and, most likely, unbalanced equipment whenever possible. The third fix requires the installer to tame the problem children with transformers or an active-balanced interface. Necessity being the mother of invention, the fourth, only the most tenacious, uses gear modifications to drive the evil spirits away. Some of these modifications are simple; some are not.

Many designers and installers would choose to start from scratch rather than perform surgery on a system with a preexisting condition. It may be hard to convince a potential customer of this approach, but the experienced traveler will avoid the dark road that leads to wasted time, lost profits and customer dissatisfaction. It is too easy to put the blame and the curse on a previous installation and installer, respectively, so avoid the habit of flaming the competition.

Let us examine the foundation of the problem. Power from a standard wall outlet is an alternating current (AC) at 60 Hz. At minimum, this hum radiates from the power cable at the moment a device is plugged in and turned on. Sound converted to electricity is also AC, but batteries produce direct current (DC). Note that most equipment ships with a longer-than-necessary power cable. In cramped spaces, power cable bundles make for a crowded, unserviceable installation. Belden makes an 18 inch (457 mm) IEC power cable (part number 17002A-B1-10) available from Newark Electronics.

Additionally, electronic equipment is typically built on a metal chassis that is referenced to the earth via the round prong of a three-conductor AC power plug. If you are barefoot, pregnant and standing on a slab of damp concrete, all properly grounded equipment is safe to the touch.

Hum and buzz are often attributed to the elusive ground loop. This phenomenon might more accurately be described as ground current in that no two pieces of gear can be exactly at the desired zero-voltage reference (the earth). When connected by audio or video cables, an extremely small amount of AC current flows in the shield and ground wiring. Inside each electronic device, leakage currents from power transformers induce an electrical charge on the chassis. Even though the unit is grounded via the power plug, the length of all power cabling (back to the breaker box) amounts to enough resistance so that no chassis can be held to exactly the same voltage as la terra firma. Because this difference in potential (a measurable voltage between any two chassis) exists in nearly all instances, you might think we face insurmountable odds, but in reality, ground current affects only flawed gear. The typical external fixes, such as running ground wires and flying the shields at one end (of audio cables), solves the immediate problem; ground wires lower the impedance of the path to ground, reducing shield current. Disconnecting the shield at one end eliminates current flow, but neither is the long-term solution.

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Reprinted with permission fromSound & Video Contractor Magazine, June, 2000
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