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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 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 can double the voltage swing at the load as compared with the same amplifier used without bridging. The configuration is most often used for subwoofers.

Power output

Because the available voltage swing across the load is doubled for the same power supply voltage, bridged output theoretically enables the design of an amplifier producing four times the power output on the same supply voltage, since.

$P=\frac{V^2}{R}$.

However, operating a pair of existing amplifier channels in bridge mode only doubles or triples power output to the load, since the amplifier retains its existing current rating.

Impedance

Driving the load between two signals of opposite electrical polarity (often incorrectly called antiphase, or 180° out of phase - one signal is inverted, there is no phase change) makes each amplifier see only half the load's electrical impedance. In practical terms, an eight-ohm loudspeaker could be safely connected to a bridged amplifier that was capable of driving four ohms per channel.

Benefits and drawbacks

Since two amplifiers are being used in antiphase, using the same power supply, there is no need for the use of a DC blocking capacitor between the amplifier and the load. This way, it will be more linear and less expensive, use less space, and there is no power loss through 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, so speaker cables should have a large diameter conductor or be kept as short as possible for minimum resistance. 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.

See also

• Amplifier
• Bridged and paralleled amplifiers
• Electronic amplifier
• Single-ended signalling

References

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