5. Internal Adjustments
Speaker-Level Input
Impedance Adjustments
The speaker level inputs of the
GTS300 and GTS600 come factory
set with 100K ohm input impe-
dance. This will provide the lowest
distortion operation from the
speaker outputs of most modern
head units by reducing the power
the amplifier in the head unit must
deliver to practically nothing.
On some older, or lowerpriced
head units, this load will not
facilitate proper fader operation.
To provide for this, we have
provided the ability to change the
input impedance of the speaker-
level inputs to 15 ohms. This is
accomplished by connecting the
jumpers as shown in the diagram
below.
• If the head unit has 4 channels of
builtin amplification and/or an
electronic fader control, you
should leave the jumpers in the
factory set position.
• If the head unit has 2 channels of
amplification, with a speaker-
level fader, the jumpers should
be set to the 15-ohm position.
This will always be a rotary-type
control, not one controlled by
elecronic pushbuttons.
Crossover Frequency
Adjustments
The GTS300 and GTS600 ampli-
fiers include built-in frequency
selectable crossovers. One
crossover is connected in series
with the amplifier circuitry and the
other crossover is connected to the
preamp level output jacks. These
crossovers can be set in either the
“flat” (full bandwidth operation),
Low Pass (Subwoofer operation),
or High Pass (satellite operation).
• The crossover frequencies are
set by “chips” inside the
amplifier. These “chips” are
simply a set of resistors,
connected across the pins and
molded into a single package.
The crossover frequencies may
be changed to any value desired
by changing the resistor
network. JBL has “chips”
available in the popular values
listed in the table below. If none
of these suit your system, you
may pruchase compatible
resistor networks from a local
electronics store, or you may
build you own custom values
from discrete resistors mounted
on a 14-pin “DIP Header” using
the instructions which follow.
Frequency Resistor JBL Part
Value
Number
50 Hz
47 K
GTM 50
80 Hz
33 K
GTM 80
120 Hz
20 K
GTM 120
200 Hz
12 K
GTM 200
250 Hz
10 K
GTM 250
500 Hz
4.7 K
GTM 500
2.5 Hz
1 K
GTM 2500
5 Hz
470
GTM 5000
6
40
BALANCED
INPUTS
SPEAKER
INPUT
IMPEDANCE
SELECTOR
LEFT
ZIN
SEL
100K
15 OHM
100K
15 OHM
RIGHT
ZIN
SEL
GND
REM
OUT
FUSE
40A
BRIDGED
+
L
–
+
R
–
R
R
L
L
+
R
–
SPEAKER LEVEL
INPUT
PREAMP
OUTPUT
LINE
INPUT
BA
JUMPER SELECTOR
SELECT 100K OR 15 OHMS
