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 problemthe
primary topic of this articleand 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 unhappinesshums 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.
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.
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.
to Page 2
Reprinted with permission fromSound & Video Contractor Magazine, June,
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved