This is primarily used for running additional 250/1’s
in a slave configuration from the master amplifier
as shown in Appendix C (page 14).
If the “Output Mode” switch is in the
“Low-Pass” position and the “Amp LP Filter”
switch is in the “Off” position, the preamp output
will be a 12 dB/octave low-pass with the
“Bass Control” processing added to it.
This is not a preferred operating mode. Do not
use this mode for master/slave configurations.
3) “High-Pass”: In this mode, the preamp
outputs deliver a high-pass signal with a cutoff
frequency identical to the low-pass frequency
selected at the “Amp LP Filter” and a filter
slope equivalent to the one selected with the
“Amp LP Filter” “Mode/Slope” switch .
Although it is not a preferred application, this
high-pass output feature will continue to operate at
a 12 dB/octave slope when the “Amp LP Filter” is
set to the “Off” position.
The signal level of the “Preamp Output” is
affected by the setting of the “Input Voltage Range”
switch (of the input section chosen by the
“Signal From” switch). See Appendix B (page 13)
for details on “Input Voltage” settings. All “slave”
amplifiers should get this switch set to “Low”.
See Appendix C (page 14) for details.
BASS CONTROL SECTION
The 250/1 includes a basic bass processing
section consisting of two primary components: a
fixed-frequency, 24 dB/octave infrasonic filter and a
fixed-frequency/“Q”, single-band equalizer centered
at 48 Hz.
1) “Infrasonic Filter”: The infrasonic filter is a
24 dB/octave high-pass filter, with a fixed cutoff
frequency of 30 Hz.This filter is designed to
conserve amplifier power and protect subwoofer
systems without audibly affecting the sub-bass
output. With ported enclosures, the use of the
infrasonic filter is highly recommended to protect
the speaker(s) from excessive excursion below
box tuning.With sealed enclosures, the use of the
filter is less necessary, but can still help protect the
speaker system.
The infrasonic filter can be completely defeated
by selecting the “Off” position on the “Infrasonic
Filter” switch.This bypasses all signal from flowing
through the circuit.
2) “LF (Low-frequency) Boost”: This feature
allows the user to control the boost of a fixed “Q”,
fixed-frequency equalization band centered at
48 Hz.The “LF Boost” control knob determines
how much boost (in dB) you are adding to the bass
signal. A range of 0 - 15 dB of boost is available.
If no boost is desired, rotate the “LF Boost” knob
fully counterclockwise.
The “Bass Control” section will only operate
when the amplifier's filter is activated with the
“Amp LP Filter” switch in the “12dB” or “24dB”
position. It will not work with this switch in the
“Off” position.This is to prevent cascading the
processing of multiple amplifiers when configured
in a master/slave arrangement as shown in
Appendix C (page 14). If you are using an external
active crossover and would like to use the “Bass
Control” features, set the “Amp LP Filter” switch
JL AUDIO 250/1 9
CROSSOVER CONTROLS
Crossovers are groups of individual electronic filters
which allow only certain frequency ranges to pass
through them by attenuating frequencies outside
the selected range.These filters allow the user to
specify what frequency range will be sent out of
each channel section of the amplifier.This, in turn,
allows each speaker system to only reproduce a
range of frequencies it is well-suited for, resulting in
reduced distortion and improved fidelity.
AMPLIFIER LOW-PASS FILTER
The 250/1 employs a sophisticated, state-variable,
low-pass active filter for its internal channel.This
feature is designed to attenuate frequencies above
its filter frequency, so that the system's subwoofers
do not reproduce any audible midrange content.
1) Filter Operation: The low-pass filter in the
250/1 is fully variable between 40 Hz and 200 Hz
via the “Filter Freq.” control knob and features the
ability to select between a moderate “12dB” per
octave or a steep “24dB” per octave slope via the
“Mode/Slope” switch.
Depending on the subwoofer system and the
vehicle, different slopes may be required to produce
a smooth transition to the mid-bass speakers in the
system. Experiment to find the slope which best
matches the acoustic requirements of your system.
Hint: A trunk mounted sub whose output has to
"fight" through a rear deck or a back seat often
benefits from the 12 dB/octave slope which lets
more upper bass content pass through. A sub that
fires directly into the listening environment is more
likely to benefit from a 24 dB/octave slope.
The above hint is not “set-in-stone”…
You should always listen to the system carefully to
determine the best choice as vehicle acoustics and
other factors play a big role in choosing the most
appropriate filter slope.
2) Precise Frequency Selection: The filter
frequency markings on the front panel of the
amplifier are for reference purposes and are
generally accurate to within 1/3 octave or better.
If you would like to select the filter frequency with
a higher level of precision, consult the chart in
Appendix A (page 12) of this manual.This chart
gives you a more accurate frequency for each of
the forty detented positions of the frequency
selection control.This method can be very useful if
the amplifier is mounted in a location where you
can’t see the front panel markings easily.
3) Defeating the Amplifier Filter: The Low-
Pass filter can also be defeated completely, by
switching the “Mode/Slope” switch to the “Off”
position.This is useful if you are using an external
active crossover in the system. Keep in mind that
turning the internal crossover off also defeats the
“Bass Control” section processing (see page 9 for
details). With the internal crossover turned off, the
250/1’s upper frequency response limit is 500 Hz,
due to its bass-specific Class D design.
PREAMP OUTPUT SECTION
The 250/1 incorporates a flexible preamp output
section, designed to make multiple amplifier systems
easy to set up.
The Preamp output can be configured in three
different “Output Modes”:
1) “Full-Range”: This is a pass-through mode for
the preamp output, delivering the same signal that is
being fed to the “Amplifier Input Section” (If the
input signal is full-range, the preamp output will be
full-range).This signal is not affected by the “Bass
Control” processing selected for the amplifier.
2) “Low-Pass”: The preamp output delivers the
same mono-summed signal that is feeding the
250/1's amplifier section when the “Amp LP
Filter” is engaged, including all the low-pass filtering
and “Bass Control” processing that is selected.
8 JL AUDIO 250/1
