AMPLIFIER INPUT SECTION
The 250/1 employs a differential-balanced
input topology that provides the user with a high
degree of input flexibility while retaining superior
noise rejection.This type of circuit also allows the
250/1 to accept high-voltage inputs from factory
source unit outputs without excessive distortion
or noise problems.
1) Input Connections: A standard left/right pair
of RCA type jacks is used for input on the 250/1.
You may run a stereo or a mono signal into the
inputs of the amplifier.The amplifier's input section
automatically sums stereo signals to mono for
the internal amplifier section and for the “LP”
output mode of the “Preamp Output” section. In
“Full-Range” or “HP” mode, the “Preamp Output”
delivers a stereo signal if one is present at the
amplifier inputs.
If you plan to use the “Preamp Output” in
“Full-Range” or “HP” mode to feed a stereo
amplifier, you must connect a stereo signal to the
input of the amplifier. A mono signal into the
amplifier will result in a mono signal out of the
preamp output. (It's a great amplifier, but it
doesn't do magic).
The amplifier will operate with only one input
connection (left or right), but will require an
increase in input sensitivity to overcome the loss of
signal. If a mono input signal is to be run, we
recommend that you use a “Y-adaptor” to split the
mono signal into both inputs of the amplifier.
2) Input Voltage Range: A wide range of signal
input voltages can be accommodated by the
250/1's input section (200mV – 8V).This wide
range is split up into two sub-ranges, accessible via
switches located in the “Amplifier Input Section”
of the amplifier.
The “Low” position on the “Input Voltage”
switch selects an input sensitivity range between
200mV and 2V. This means that the “Input Sens.”
rotary control will operate within that voltage
window. If you are using an aftermarket source unit,
with conventional preamp-level outputs, this is most
likely the position that you will use.
The “High” position on the “Input Voltage”
switch selects an input sensitivity range between
800mV and 8V. This is useful for certain high-
output preamp level signals as well as speaker-level
output from source units and small amplifiers.To
use speaker-level sources, splice the speaker output
wires of the source unit or small amplifier onto a
pair of RCA cables or plugs.
The output of the amplifier will decrease for a
given input voltage when the “Input Range” switch
is placed in the “High” position. Conversely, the
output will be higher with the switch in the “Low”
position.While this may sound counter-intuitive, it is
consistent with the descriptions above.
3) Input Sensitivity Adjustment: Located next to
the “Input Voltage” switch, in the “Amplifier Input
Section”, is a rotary control labeled “Input Sens.”.
Once the appropriate “Input Voltage” range has
been selected, this rotary control can be used to
match the source unit's output voltage to the input
stage of the amplifier for maximum clean output.
Rotating the control clockwise will result in higher
sensitivity (louder for a given input voltage).
Rotating the control counter-clockwise will result in
lower sensitivity (quieter for a given input voltage).
To properly set the amplifier for maximum clean
output, please refer to Appendix B (page 13) in this
manual. After using this procedure, you can then
adjust the level of the amplifier by adjusting the
input sensitivity downward, if the amplifier requires
attenuation to achieve the desired system balance.
Do not increase the “Input Sens.” setting for
any amplifier in the system beyond the maximum
level established during the procedure outlined
in Appendix B (page 13). Doing so will result in
audible distortion and possible speaker damage.
JL AUDIO 250/1 7
TURN-ON OPTIONS
The 250/1 can be turned on and off using two
different methods:
1) A conventional +12V remote turn-on lead.
2) A signal sensing turn-on circuit.
To select between the two modes there is a
switch, marked “Signal Sensing”, located in the
“Amplifier Input Section” of the front panel.
1) Conventional Remote Turn-On Method:
uses a conventional +12V remote turn-on lead,
typically controlled by the source unit's remote
turn-on output.The amplifier will turn on when
+12V is present at its “Remote” input and turn off
when +12V is switched off. If a source unit does not
have a dedicated remote turn-on output, the
amplifier’s turn-on lead can be connected to
+12V via a switch that derives power from an
ignition-switched circuit.To use this method, select
“Off” on the “Signal Sensing” switch in the
“Amplifier Input Section”.
The 250/1's “Remote” turn-on connector is
designed to accept 12 AWG – 8 AWG wire.
12 AWG is more than adequate for this purpose.
To connect the remote turn-on wire to the
amplifier, first back out the set screw on the top of
the amplifier, using the supplied hex wrench. Strip
1/2 inch (12mm) of wire and insert the bare wire
into the receptacle on the front panel of the
amplifier, seating it firmly so that no bare wire is
exposed. Smaller wire than 12 AWG can be used,
but it may be necessary to strip 1 inch of insulation
from the wire and fold the bare wire in half prior to
insertion.While holding the wire in the terminal,
tighten the set screw firmly, taking care not to strip
the head of the screw and making sure that the
wire is firmly gripped by the set screw.
2) Signal Sensing Turn-On Method: A valuable
feature of the 250/1 is its ability to be turned on
and off by the presence or lack of signal at its audio
inputs.This allows you to operate the amplifier
without having to locate a remote turn-on lead at
the source unit.This can be very useful if interfacing
the amplifier with some OEM source units that do
not have conventional turn-on leads.
The sensitivity of the signal sensing turn-on circuit
has been designed for high-level (speaker level)
signals, not for low-level (preamp level) signals. Using
this feature with low-level (preamp level) signals is
not recommended.We do not recommend this
method of turning the amplifier on and off as a
default.Whenever possible, use the conventional
remote turn-on lead method.The signal sensing
circuit is designed to detect midrange frequency
signal presence. If the signal feeding the amplifier is
not full-range (for example, if there is an active
low-pass crossover in line before the amplifier),
this circuit will not operate properly.
To activate the signal sensing turn-on feature,
place the “Signal Sensing” switch in the “On”
position.This should only be done if there is no
conventional turn-on lead connected to the
amplifier's “Remote” connector. When in the “On”
position, the signal sensing circuit will monitor the
audio input signal and turn the amplifier on when
signal is present. After about 25 seconds of no
signal, the amplifier will shut off.
Turning on other amplifiers in signal sensing
mode: When the signal sensing circuit is turned on,
the “Remote” Connector on the amplifier may be
used as a remote turn-on source for other
amplifiers in the system. +12V will be present
at the connector when the amplifier turns on in
signal sensing mode and will not be present when
the amplifier turns off.
You cannot use the turn-on output to turn on
processors that are in the signal path before the
250/1. (Signal will not pass through most
processors when they are not powered up,
meaning that the amplifier will not turn on until
that processor is active).
