A bridge-tied load (BTL), also known as bridged transformerless and bridged mono, is an output configuration for audio amplifiers, a form of impedance bridging used mainly in professional audio & car applications. The two channels of a stereo amplifier are fed the same monaural audio signal, with one channel's electrical polarity reversed. A loudspeaker is connected between the two amplifier outputs, bridging the output terminals. This doubles the available voltage swing at the load compared with the same amplifier used without bridging. The configuration is most often used for subwoofers.
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Power output
The available voltage swing across the load is doubled for the same power supply voltage, but the maximum current output ability of the amplifier remains the same. Consequently operating a pair of existing amplifier channels in bridge mode doubles available power output to the load. The BTL load needs to be twice the impedance since it sees twice the voltage at the same current.
Impedance
Driving the load between two signals of opposite electrical polarity makes each amplifier see only half the load's electrical impedance.
Benefits and drawbacks
Since two amplifiers are being used in opposite polarity, using the same power supply, there is no need for the use of a DC blocking capacitor between the amplifier and the load. This saves cost & space, and there is no power reduction at low frequency due to the capacitor.
Bridging an amplifier increases the power that can be supplied to one loudspeaker, but it does not increase the amplifier's total available power. Because a bridge amplifier operates in mono mode, a second identical amplifier is required for stereo operation. For bridged amplifiers, damping factor is cut in half. Because the amplifier's bridged output is floating, it should never be grounded or it may damage the amplifier.
Uses
This output configuration is useful in applications where battery size dictates a lower supply voltage, e.g., automotive or handheld applications.