Primarily a live sound term, “bridging” is a means to configure a 2-channel amplifier to drive a single loudspeaker with more power than the two original channels. For example, a 100-watts-per-channel amp may output a single channel of 300 watts after bridging.
To operate an amp in bridged mode, a single input signal enters the amplifier and is split into two identical signals. One has its polarity inverted. The original signal is sent to one channel of the amp and the inverted signal is sent to the other channel. The end result is two channels of identical signal with one having reversed polarity. Connecting a speaker between the two positive speaker terminals of the amp (the negative terminals aren’t used) adds the two channels for more power to be sent to the speaker.
Typically, amplifiers operating in bridged mode can only do so with speakers that have twice the impedance of the minimum rating load on the amp. For example, an amp rated at 4 ohms running in normal mode will generally require 8 ohms in bridged mode.
WARNING: Always be sure to check your power amp’s manual before attempting to bridge an amplifier.
See also Word For The Day “Mono Bridged.”