Troubleshooting:
• Correcting Hum Problems
Reading this will help you avoid hum problems in any Synthesis Home Theater System installation. It really isn’t
very difficult to do, but you will need to take some preventive measures during system installation to prevent it.
Why Does It Hum In The First Place?
Anytime you interconnect two analog audio signal devices, and they each have their own AC cord, you can
potentially generate some hum. When you interconnect several devices, as is the case in a Synthesis system,
the potential is many times greater.
All audio signal devices must reference their internal circuitry to “ground” in order to establish a zero voltage
“reference” point for all active circuitry. In reality, it is not necessary for any isolated piece of equipment to be
connected to “earth ground”. An airplane is a good example of such a situation. The need to connect home equip-
ment to earth ground is based on safety concerns. It is essential that a path to earth ground be provided, as a
drain of unwanted potential. An example 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.
When two or more audio devices are interconnected, they must have precisely the same zero voltage reference
point or you may encounter problems. When you hear hum, this 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 system 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.
What You Can Do During Installation
First, do not isolate the third wire ground at the AC plugs. This is essential for safety as well as proper hum
reduction. Make certain this connection is as good as possible. The goal is zero resistance to the ground outlet.
Theoretically, this should be enough to prevent hum problems, but it isn’t always. Occasionally, the house wiring
does not provide a low enough resistance between components to be totally effective. Particularly when more than
one AC outlet is used. High resistance house wiring can be caused by oxidized or loose wiring connections, or
even improperly wired outlets. Correcting the house wiring may not be an option, and even then you may
encounter difficulty. You do not need to correct the house wiring to get the desired results.
It may be necessary to tie all of the hardware together electrically so there is no ground potential difference
between the various devices and third wire ground from the AC outlet(s). Even a few microvolts 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.
One very effective way of accomplishing this is by mounting the devices into a metal equipment rack. Most equip-
ment racks and the hardware you are installing are painted or anodized. These finishes will prevent a good
ground. When installing the equipment, file or sand the rack and the panel where they will contact each other until
you have a clean, metal to metal contact. Alternatively, you can put an unused external shakeproof lock washer
(sometimes called a star washer) between the rack and the panel with each mounting screw. This will cut through
the finish as you tighten the hardware.
4.7
CALIBRATION WALK-THROUGH
55
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