CORRECTING HUM PROBLEMS
This information will help you avoid hum problems in any Synthesis Home Theater System
installation.
WHAT IS THE ORIGIN OF HUM?
W henever you interconnect two or more AC powered analog audio devices, you have the
potential to generate audible hum.
All audio signal devices must be referenced to ?ground?. This connection establishes a zero
voltage ?reference? point for the entire system. An "earth ground" connection is not necessary
for an isolated piece of equipment. A portable tape player would be an example of such a
situation. The need to connect stationary equipment to earth ground is based on safety
concerns. It is essential that a path to earth ground be provided, as a drain of unwanted,
potentially lethal current. Examples of this would be a lightening strike or an insulation failure
within a piece of equipment. Since the system must be earth grounded to maintain safety (and
in many states, to be legal), this is the logical place to begin.
W hen two or more audio devices are interconnected, they must have precisely the same zero
voltage reference point or you will encounter problems. Audible hum is an indication that one
or more of the devices in the audio chain has a slightly different ground reference potential.
This is what we must correct to eliminate audible hum. There are many ?bad science?
solutions out there. Please resist the temptation to experiment with them, ultimately, they will
not be fully effective, in some instances equipment can be damaged, they may actually be
illegal and in many cases are downright dangerous.
DURING INSTALLATION:
DO NOT isolate the third wire ground at the AC plugs. This is bad science! As stated
above, this connection to earth ground is essential for safety. Instead, make sure this
connection is as good as possible. The goal is zero resistance to the ground outlet.
Sometimes this requires tightening of the GREEN ground wire within the junction box.
W henever possible, try to connect all system power cords to the same AC feed. Connecting to
another feed means the ground connections have to travel all the way back to the power
distribution box before they meet. This adds resistance, and really increases the likelihood of
encountering problems. Correcting the house wiring may not be an option, or may not prove
effective. You do not need to correct the house wiring to get good results.
If necessary, tie all of the system hardware together electrically so there is no ground
potential difference between the various devices and the third wire ground of the AC outlet(s).
Even a few micro-volts (?V) difference will be audible. This is why you must create the lowest
possible impedance ground path between all of the equipment. Zero impedance = zero volts.
If the system components are mounted in a metal equipment rack, a low impedance ground
circuit can be achieved by using the rack itself. Most equipment racks and their hardware are
painted or anodized. These finishes will prevent a good ground. W hen installing the
equipment, file or spot sand the rack rails and each panel where they will contact each other,
giving a clean, metal to metal contact. Be sure the panel hardware is tight.
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