JBL 408GTI, Installation Manual

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System Connections
There are a variety of system options
available with the 408 and 508GTi. The
simplest configuration is to use the 408
and 508GTi full range. Although this will
yield good sound, the best performance
will be achieved in a high-end system
which includes subwoofers and, optionally,
midbass transducers. Because the 408
and 508GTi are specifically designed
for mid and high frequency operation,
they should have their extremely low
frequencies blocked by electrical means in
order to achieve the best system fidelity
and the widest dynamic range. Full range
use may be obtained at slightly lower
power handling levels.
For the best performance, we
recommend that an active crossover with
a minimum of 12dB per octave slope be
used for the transition point between the
Subwoofer and the 408 or 508GTi. The
optimum crossover frequency for this
transition is dependent on the specific
components used, the vehicle, and other
installation details. As a general rule, the
crossover frequency should be as low as
possible to achieve the best system
imaging. You can fine-tune the crossover
after the installation is complete for the
best sound. A good place to start your
tuning process is between 60Hz and
150Hz when using the 508GTi and
between 80Hz and 150Hz when using the
408GTi. In 4-way systems using a midbass
driver such as the 600GTi, the crossover
point may be raised higher without
detrimentally affecting imaging as long
as the woofer/midbass drivers can
be mounted reasonably close to the
woofer/midranges. This higher crossover
frequency will allow the system to play
louder. Since no crossover is perfect, it is
advisable to keep your crossover points
away from the range where your hearing is
most sensitive. Therefore we recommend
that you try to keep the high-pass
frequency below 250Hz. This lets the 408
or 508GTi reproduce the entire range of
frequencies covering the most important
vocal and lead instrument fundamentals
and their harmonics.
If you prefer to use all passive
crossovers, you will need to construct
or purchase a high-pass filter for the
subwoofer to woofer/midrange transition
point and install it in-line ahead of the 408
or 508GTi Crossover. For the best system
dynamic range, we recommend that a
minimum slope of 12dB per octave be
used.
The most advanced system designs
use active electronic crossovers such as
the JBL GTX2, GTX4 or GTX47 at both
the subwoofer (or woofer/midbass)-to-
woofer/midrange transition point and at the
woofer/midrange-to-tweeter transition. Set
the high crossover frequency at or above
5kHz if a 12dB per octave crossover is
13
OdB
–2dB
–4dB
Tweeter Level Adjustment Switch
Figure M
408/508 MANUAL.QXP 7/17/98 11:21 AM Page 13