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Background
In much the same way that balanced audio lines can reduce the pickup of hum
and other types of electromagnetic interference (EIVII), the use of balanced AC
power lines in sensitive audio, video, or computer installations can make an
enormous difference in system noise. But power distribution in North America,
unfortunately, is not balanced. The distribution standards currently in use were
derived from practices established over a century ago, when electric power use
was limited to lighting and motors-long before any EMI-sensitive applications
existed. The emphasis then was on convenience (from the power utilities'
standpoint) and safety, but not noise cancellation. The result was a three-wire
distribution scheme in which 120V branch circuits have a hot wire and a neutral
wire, with the neutral tied to a third wire connected for safety to an earth ground.
The third wire does not carry any current unless there is a short circuit. This
unbalanced scheme causes hum in audio circuits for two main reasons: First, the
current flowing in the hot wire induces hum in any other nearby wires, which may
carry vulnerable low-level audio signals. Second, because the impedance of
chassis and cable shielding to ground is not zero, ground current flowing from
power supply capacitors and from EMI pickup causes a voltage drop at 60 Hz
and its harmonics which is added to the audio signals. With a center-tapped
isolation transformer, the AC power feeding a studio can be balanced at its
source. The current-carrying wires then are no longer "hot" (120V) and "neutral"
(OV), but two equal and opposite lines of 60V each (referenced to the safety
ground connected to the center tap), which add to 120V. This type of power,
when run around a room, does not induce hum into nearby audio wiring, because
the two power conductors induce equal and opposite voltages that cancel each
other out. Similarly, ground currents are all but eliminated by the same common-
mode cancellation effect. No longer is it necessary to adopt cumbersome and
expensive star-ground systems or use massive bus bars or heavy ground rods.
All such systems are doomed to yield only mediocre results because of the
impossibility of reducing the ground impedance to zero. The common-mode
rejection of a truly balanced AC supply is far simpler, cheaper, and more
effective. Balanced power is a technology whose time has come. Balanced, or
"technical" power, is now recognized by the US National Electrical Code (Article
530) for critical, low noise applications.
Installation
The power source to which the IT-1220 is connected must be adequate for use at
20 amps. We recommend a 30 amp circuit, wired with 10 gauge wiring, if
continuous operation near 20 amps is anticipated. A dedicated (nothing else
connected) 20 amp circuit is the minimum acceptable to comply with NEC
requirements. Consult a qualified electrician if in doubt. The IT-1220 is designed
for mounting in a standard 19-inch equipment rack. Because of its weight, the
best position for it is the bottom slot of the rack. Its toroidal transformer minimizes
magnetic leakage, but nevertheless, due to its high capacity, it will radiate an
appreciable magnetic field. Therefore, we recommend that it not be positioned
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