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NOTE 1:
Always turn the ALC assembly off (switch down)
when adjusting any system gain controls, including
mixer inputs, master gain control, tone controls,
system equalizers, and amplifiers.
If this procedure is not followed, the ALC may fade
the gain back up when someone speaks softly, and the
system may go into feedback.
NOTE :
The automatic level control card assembly has
been aligned at the factory. Normally, no further
adjustments are necessary.
How It Works -
The ALC's function is to fade the master gain
down when the speaker is too loud, and hold it
there during pauses. When a quieter speaker talks,
the gain will be raised again. The ALC compares
the level of the mixer's output, measured with a
VU meter characteristic, with a threshold set by
the TARGET trimpot control. When the output
level is lower than the threshold, the ALC will fade
up the master gain. When the output level is higher
than the threshold, the ALC will fade the master
gain down. The speed of the fade is set with the
RATE trimpot control.
The gating action of the ALC freezes the master
gain when no one is talking. Normally it is con-
trolled by the logic outputs of the automatic
mixing input channels. Whenever a yellow logic
LED on any channel is on, the ALC action is
allowed to adjust the output level. The yellow
ALC ENABLE LED will come on when any
input's logic LED is on, and indicates when the
ALC is active.
Factory Settings- Recommended positions.
RATE Trimpot = 12:00, Counterclockwise =
Slower
TARGET Trimpot = 9:00, Counterclockwise =
Downward
WINDOW Trimpot = 12:00, Counterclockwise =
More Dynamic range
AUTOMATIC LEVEL CONTROL OPTION
Alignment Procedure -
Unplug the power supply card. Unplug and remove
the master output card assembly (with ALC card
assembly). Plug the extender card into the master
output card slot. Plug the master output card assem-
bly into the extender card. Plug in the power supply
card
With the ALC switched to "OFF", adjust all input
gains, master output gains, tone controls, and adja-
cent channel input controls, as well as any other
equipment in the signal chain (equalizers, power
amps, etc.). Be sure to test with speech at all micro-
phones after any gain adjustment. Because of the
automatic mixing action, a microphone that may be
ready to feed back, will not have it's gain turned up
until someone speaks into it.
Turn the ALC "ON". Have someone speak loudly at
a microphone. Adjust the TARGET trimpot down-
ward (counterclockwise) until the volume is reduced
to a comfortable level. The middle green LED is
illuminated to indicate where the target level should
be. Now the ALC will fade the master gain down
whenever the output level is higher than the TAR-
GET level. When a quieter speaker talks, the gain
will fade up again.
Next adjust the RATE trimpot to 12:00 or lower.
This will increase the ALC fade time so that it doesn't
"pump". 100% counterclockwise is the slowest
setting.
If more dynamic range is desired, more difference
between loud and soft speakers, turn the window
control counterclockwise. This will create a "dead
zone" below the threshold level, where the volume
can vary naturally without ALC action. A WINDOW
setting of 50% gives a dead zone of 5dB, and 0%
(full counterclockwise), about 8dB.
Exempting Line Level Inputs From ALC -
Since the ALC works by varying the master gain
when a logic indicator is active, removing R23
prevents the ALC from changing the gain on the line
input, whenever any input becomes active. Setting
the MODE switch to manual, also disables the ALC
from changing the gain on a channel.
