P 4020 (serv.man3) - JBL Car Audio User Guide / Operation Manual. Page 5

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4. In some systems it may be desirable to

have two sources play simultaneously
through the audio system . For instance, if
you have a factory installed radio in your
car, and you want to add an aftermarket
CD player, you may connect the CD player
to the RCA inputs of the amp, and the
factory radio to the Universal Interface
Speaker Inputs. Note: Front and rear
channels of the CD and radio must be
driven into the front and rear inputs of the
amplifier. Use Y adapters if necessary. The
Universal Interface circuitry will isolate the
two signals from each other while
allowing them to drive the amp
simultaneously. Therefore switching from
one source to the other is as simple as
turning the unused source off, and the
other on. (Note: you must connect the
remote turn-on wire from the CD player to
the Rem terminal on the amp. Install a
diode in the remote line from the CD
player as shown in the diagram below.)

Electronic Crossover
The electronic crossover in the P-4020 and
P4040 should be used to route the
appropriate signal to speakers intended to
play only bass, or only midrange and treble
frequencies. The crossover slope is 12dB/oct
and the frequency is fixed at 80Hz. 
Pre-amp Outputs
Connect these to the input connectors of
other amplifiers or processors in your
system. Refer to the Applications section for
system expansion possibilities.
Setting Input Sensitivity
Proper input sensitivity control settings on
Power Series amplifiers are critical to ensure
optimum performance, low noise levels and
maximum system reliability. As a general
rule, controls at the front end of a system
(source, equalizers, and other processors
should be set as high as possible with the
amplifier input sensitivity set as low as
possible while still providing adequate
volume levels. Using a high signal level and
a low sensitivity setting will help keep
background noise in the system to a
minimum.

Initially, turn the input sensitivity GAIN
control to its minimum (counter-clockwise)
position.

1. Reconnect the (–) negative lead to your

vehicle's battery. Apply power to the audio
system and play a favorite music track
from CD or tape. 
NOTE: After the source unit is on, the JBL
badge (on the top panel) will illuminate,
indicating the amplifier is on. If not, check
the wiring, especially the remote
connection from the source unit. Also refer
to Troubleshooting on page 7.

2. On the source unit, increase the volume

control until it is approximately 3/4 of its
maximum output level. Slowly increase
the Input Sensitivity control (clockwise)
towards three o clock and, at the same
time, listen to the quality of the
reproduced sound. At some point, you'll
hear distortion on the music peaks. Stop
the adjustment and turn it back slightly.
This is the maximum undistorted output
level of your system, and should not be
exceeded during use. 

3. After adjusting the level of the main

speakers in your system, you should
adjust the remaining channels (if you are
using a P-4040 or a multi-amp system) for
the desired system balance. You'll find this
easiest to do by adjusting the channels in
the following order: 1) front speakers, 
2) rear speakers, 3) subwoofer. Elaborate,
multi-channel systems may be complex to
adjust. Your local JBL installation specialist
is the best person to help with such
adjustment. 

For your convenience, we've included
several application diagrams to help you
plan your own system installation. Figures 1
through 3 show how to configure the JBL 
P-4020 or P-4040 for stereo, bridged-mono,
and tri-mode operation. 
For system expansion ideas, see Figures 4
and 5 on the next page. 
NOTE: For simplicity, Figures 1 through 3 do
not show power, remote, and input
connections.