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the amplifiers and signal processors are
all grounded to the same point on the
chassis. Usually the head unit does not
need to be grounded to the same point
as the rest of the system, but in some
instances grounding the head unit to
the same point will also help. If this
does not adequately reduce the noise
level, try another ground point on the
car frame. Some ground points may
work better than others. In some in-
stances with plastic-bodied vehicles,
grounding the source unit and/or
amplifier directly to the battery will
provide the best results, although,
usually, this provides the worst results
with most cars.
In complex systems involving compo-
nents from different manufacturers, it
helps to know the type of power sup-
plies used in each component. For
low-level signal processors such as
equalizers or electronic crossovers, a
manufacturer may either use a power
supply which is isolated through a DC-
to-DC converter, or a simple regulated
supply from the +12V vehicle battery.
Components using the latter type of
supply can be identified by a direct con-
nection from power ground to the shield
of the RCA jacks (measured with no
other connections present). For these
components, the best ground connec-
tion may be one where the power
ground wire is not connected at all! The
RCA cables will provide the ground
connection to the source unit. Please
note that this is only appropriate for
units which draw less than 500mA of
current. Higher power units of this type
are best connected with their ground
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ment is more likely to pick up such
noise than conventional equipment
because it has wide frequency band-
width and high gain (amplification) cir-
cuits. The GTS100 has built-in power
supply filters to help prevent noise prob-
lems. If noise occurs it is probably the
result of improper installation. The
following suggestions will help you
eliminate most noise problems.
Source Noise
– Often noise in a system
is picked up by the signal source.
Before attempting to eliminate noise
from the “amp” be sure it is not being
picked up by the signal source and then
passed on to the amp. To do this, con-
nect the signal source output to an
external amplifier which has no other
connection to the vehicle except for the
audio signal leads. A battery-powered
portable with line-inputs works well for
this. Listen to see if the noise is present
in the signal from the source unit. If so,
consult the manufacturer of the source
unit, or your JBL dealer, for help in elim-
inating this noise. If there is no noise in
this signal, it can be eliminated as the
source of the noise in your system.
Ground Loops
– The vast majority of
noise problems in car audio systems
are caused by inadequate or improper
grounding. The head unit, the amplifier
and any other components must be
grounded to a major metal member of
the vehicle’s frame. Make sure to
choose a solid metal ground point. In
many new vehicles some structural ele-
ments are made of plastic.
Although not usually necessary, practi-
cal experience shows that often the
noise level in the system will be lower if
JBL 1394 Manual 7/17/98 11:49 AM Page 22