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10
Bridge Mode
The Servo 120a provides a rear-panel switch that allows it to be used in a
Bridge
mode. When this switch is placed in the “RIGHT” (Stereo) position, the Servo 120a
functions as a true stereo amplifier, where both of the two independent amplifier
channels (left and right) can receive different input signals and produce independent
output signals. However, when the switch is placed in the “LEFT” (Bridge) position,
both amplifier channels process only the signal present at the left input, thus produc-
ing a single output signal with a true 120 watt output (into 8 ohms).
The illustration on the left shows how this works. In bridge mode, the polarity
(phase) of the right output signal is reversed relative to that of the left output signal.
Both channels then process the same signal, with the speaker load connected so that
power is derived from both channels. The effective voltage swing seen by the load is
thus doubled so that the power output is doubled.
When using the Servo 120a in Bridge mode, be sure to connect each loudspeaker as
shown in the illustration below (and as silkscreened on the rear panel), with the red (+)
terminal of the left channel connected to the positive input of the speaker and the red
(+) terminal of the right channel connected to the negative input of the speaker.
Do
not use the black (ground) output terminal of either channel (the speaker load
must “float” away from the amplifier chassis).
+
-
WARNING: Bridge mode is to be used only when the Servo 120a is connected to
an 8 ohm speaker load. Use of Bridge mode with speaker loads of 4 ohms or less
can result in severe damage to the unit due to excessive heat and current and
will void your warranty!
INPUT
LEFT(+)
OUTPUT
RIGHT(+)
OUTPUT