AudioControl SA-3052A, Руководство по эксплуатации

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SA-3052A Owner’s Manual
6-3
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In autosound, this problem is exacerbated by the rather callous belief of automakers
and certain other folks that the metal frame of a vehicle makes a good ground. While this
may be true for cigarette lighters and tail lights, applying this belief to audio systems is a
gilt-edged invitation to disaster.
Figure 6.1 Anatomy of a ground loop.
Figure 6.1 shows what happens. Generally, the audio wiring has higher resistance
than the power wiring. Since the electrical system of the car uses the body/frame struc-
ture as it’s negative return, the non-zero (yes, it’s small, but not small enough!) resistance
of the car body allows small voltage drops to be created between various points in the car
body.
The alternator in the battery charging system makes things worse because the fre-
quency of it’s AC output (alternating current...that’s why it’s called an alternator, not a
generator) is easily within the audible range. The low impedances involved (high avail-
able current means low impedances) make filtering out alternator noise even more diffi -
cult. Anyway, these voltage drops occurring between various points in the car body usu-
ally have alternator noise riding on them, which gets into the sound system via a ground
loop.
Until you’ve traced out a noise problem, and found it to be a cleverly concealed
ground loop, the phrase “Ground isn’t Ground” just sounds like random noise from
another audio fanatic. All it takes is one good ground loop problem to turn the hardiest
skeptic into a believer.
Chapter 6 - Getting the Last ¼ dB