JBL Grand touring series User Manual page 33

speaker, such as those sold for
use with computers or Walkman-
type portables, 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
reducing this noise. If there is no
noise in this signal, it can be
eliminated as the source of your
problems.
Ground Loops - The vast maior-
ity of noise problems are caused
by inadequate or improper
grounding. The head unit, the
amplifier and any other compo-
nents must be grounded to a
maior metal member of the vehi-
cle's frame. Make sure to choose
a solid metal ground point, as
some new vehicles contain struc-
tural elements made of plastic.
Most often the noise level in the
system will be lower if any
amplifiers and signal processors
which do not have isolated SUP-
plies are all grounded to the
same point on the chassis.
Amplifiers such as the GTSlBOx,
which internally isolate the sis-
nal ground connection from the
power ground connection, gener-
ally do not benefit from this.
Usually the head unit does not
need to be grounded to the same
point as the rest of the system,
but in some instances, depending
on the design of the signal
processors involved, grounding
signal processors to the same
point as the head unit will also
help. If this does not adequately
reduce the noise level, try an-
other ground point on the car
frame. Because of current flow
patterns within the vehicle chas-
sis, some ground points are nois-
ier than others. As a general
rule, try to keep grounds away
from the main flow of current
between the battery and alterna-
tor. For example, in a vehicle
with a battery at the right rear,
and an alternator at the left
front, you are most likely to find
a quiet ground at the far left rear
than anywhere between the bat-
tery and alternator. In some rare
instances with plastic-bodied
vehicles, grounding the eauip-
ment directly to the battery will
provide the best results, although
usually this provides poor re-
sults with most cars and is not
recommended.
In complex systems involving
components from different manu-
facturers, it helps to know the
type of power supplies used in
each component. For low-level
signal processors such as eaual-
izers or electronic crossovers, a
manufacturer may use either a
power supply which is isolated
through a DC-to-DC converter, or
a simple regulated supply from
the +12V vehicle battery. The
important characteristic to know
is how well the power ground is
isolated from the signal ground
(phone-plug shield) in each com-
ponent. Many simple signal
processors or low-powered
amplifiers have no isolation.
These can be identified by a
direct connection from power
ground to the shield of the RCA
iacks (measured with an ohm-
meter with no other connections
present). For these components,
the best ground connection may
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