GTQ 190 (serv.man2) - JBL Car Audio User Guide / Operation Manual. Page 14

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the battery and alternator. 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 battery and alternator. In
some rare instances with plastic-bodied
vehicles, grounding the equipment
directly to the battery will provide the
best results, although usually this
provides poor results with most cars and
is not recommended.
In complex systems involving
components from different
manufacturers, it helps to know the type
of power supplies used in each
component. For low-level signal
processors such as equal-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 

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12V

vehicle battery. The important
characteristic to know is how well the
power ground is isolated from the signal
ground (phono-plug shield) in each
component. 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 jacks
(measured with an ohmmeter with no
other connections present). For these
components, the best ground connection
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, such as
amplifiers, are best connected with their
ground wires connected directly to the
chassis of the head unit.
The trickiest grounding task is created
when some components of this type are
mixed with other components using
isolated supplies. For this type of system,
the following grounding scheme will
usually work:
1. Connect the head unit chassis to a solid
vehicle ground using a short, heavy
gauge wire (AWG10). Do not connect this
to the vehicle’s wiring harness, but go
directly to a metal part of the vehicle.
2. Connect all signal-processor grounds
directly to the head unit chassis at the
same point. Run each wire individually.
Due to wire resistance, connecting
multiple wires to a single wire, then
running the single wire to the head unit,
is not the same!
3. Connect all amplifier grounds directly
to the vehicle chassis but not necessarily
to the same point as the head unit.
Power-line Noise – 

The built-in power

supply filter of the ampli-fier makes
external filters unnecessary. In some
cases, power supply noise can enter the
system through the head unit power
supply or the supply of an equalizer or
other signal processor. Putting a filter on
the head unit or signal processor power
supply input may then be helpful. Using a
portable powered speaker as described in
the “Source Noise” section can help you
isolate an individual component which
may be sensitive to this. Start at the head
unit, and work back, checking each
component individually.

Power wires carrying high currents may
induce noise in nearby signal wires.
Make sure that power wires and signal
wires do not run together for long
distances. When power and signal leads
must cross, they should cross at right
angles. If you suspect that power line
noise is being induced in the signal
leads, you can repeat the test for source
noise described previously, but perform
the test at the amplifier end of the signal
cables. One power wire that will be
impossible to stay away from is the
vehicle chassis. The chassis is one giant
ground wire! Use high-quality shielded
cables, twisted pair, or a balanced-line
system for the long front-to-rear signal
runs to reduce the likelihood of noise
pickup from this source.
Other Noise Sources –

Common noise

problems will be solved by proper
grounding and power supply
connections. However, there are impulse-
type noise sources which may require
suppression at the noise source. Many
noise suppression devices (such as spark
plug and coil lead suppressors, and rotor
and coil bypass capacitors) are available
at auto parts or car stereo stores. There
are also noise suppressors that can be
connected directly to the alternator that
are effective in some situations. The use
of any such suppressor should be
discussed with a JBL authorized
installation specialist after the basic
grounding scheme and power
connections are confirmed to be correct.
Certain vehicles are particularly noisy,
especially models that have solid-state
ignition systems or that have non-metal

bodies. Such vehicles may require
electrical noise suppression devices
which are not normally needed.
Antenna –

A common noise problem is

generated by a ground loop produced by
the antenna shielded cable being
grounded at both the antenna mounting
point and at the head unit input. In this
instance, insulate the antenna ground
from the chassis of the vehicle at the
antenna mounting point so the antenna
shield is grounded only at the radio’s
antenna input. Commercial antenna
ground-loop isolators are also available.
Switching Noise –

GTQ amplifiers have

highly developed switching power
supplies which generate some RF
interference as a result of their switch-
mode operation. Although this is
internally filtered and shielded by the
GTQ ampli-fier chassis, some unusually
sensitive installations may pick up
switching noise, especially when listening
to weak AM radio stations. If this unusual
situation occurs, one of the following
installation corrections will typically
eliminate the problem.
1. Relocate the amplifier to a position
farther away from the radio or radio
antenna.
2. Move the electrical ground of the head
unit and/or amp to a different point on
the vehicle’s chassis.
3. Keep the amplifier power supply wiring
away from the radio or antenna wiring.
4. Wrap the 

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12-volt power supply wires

for the radio/tape deck and/or the GTQ
amplifier with metallized shielding tape
or ground braid, and ground the tape to
the chassis of the vehicle.

GTQ 190 - English  06/03/98 11:50  Side 14