SA-3052A
Owner’s Manual
6-10
®
Chapter 6 - Getting the Last ¼ dB
Grounding practices
CAUTION
Ground Isn’t Ground!
Remember those words. Any time that you think that grounding is a trivial matter
recite those three words.
The basic reason that we ground anything at all is to supply a negative return for the
electric current that operates a device. In boats we use wire and have many fewer prob-
lems, if you’re an automaker, basically anything that is metal and somehow associated
with the vehicle body is fair game. If you’re a state-of-the-art autosound installer, that
belief is the first and last mile on the road to ruin.
A second reason that things are grounded is so that their metal enclosures can act as
a shield for incoming EMI. Remember...the metal enclosures only act as a shield when
they are grounded. This helps keep portions of the circuitry that weren’t designed to be
radio receivers from becoming radio receivers.
Ground loops cause problems by allowing multiple paths into the grounding system.
This wouldn’t be such a problem if it were not for the finite and measurable resistance of
the ground system. When you combine the multiple current paths into the ground system
caused by the automaker’s indiscriminate definition of the word ground and the finite
resistance of the ground system you get voltage drops. These voltage drops cause trouble
when they get into an audio system via a ground loop. What happens is that the audio
system tends to amplify any noise present in the ground system. The net result is noise.
Here’s some (heh-heh) ground rules:
1. There should be one and only one path to the negative side of the vehicle elec-
trical system. If you elect to use the vehicle body as this point, scrape the paint
from under the contact point, and use an internal star lockwasher on both sides of
the terminal lug. This grounding method is known as a single point ground.
In extreme cases, you may need to run a heavy gauge wire to the battery’s nega-
tive terminal. Alternatively, you could use the point where the battery negative
hits the vehicle frame. Again, clean everything and use toothed lockwashers. If
the connection point is subject to vibration (like on the engine), then be sure to
use stranded or braided wire (braided is better), and leave enough slack so that
the entire length of wire can vibrate. This will help keep it from breaking.
2. If you mount everything on a board, connect the chassis of each component to a
good, solid electrical ground. The vehicle body is a good choice here because
what you want is a good RF (radio frequency) ground. The wire running to the
negative side of the electrical system has inductance which raises it’s impedance
at high frequencies. In effect, at radio frequencies, the impedance could be high
enough that wire would look like an open circuit. Braided wire works best here
because it has low inductance.
3. It’s good insurance to use an ohmmeter to make sure that the chassis of each
component isn’t already tied to its internal electrical ground. Sometimes the case
is tied to circuit ground through a low to moderate value resistor. If the case to
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