Bridge-tied load
Encyclopedia
A bridge-tied load also known as bridged transformerless and bridged mono, is an output configuration for audio amplifier
s, 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 subwoofer
s.
.
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.
. In practical terms, an eight-ohm loudspeaker could be safely connected to a bridged amplifier that was capable of driving four ohms per channel.
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.
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
s, a form of impedance bridging
Impedance bridging
In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal to the load...
used mainly in professional audio
Professional audio
Professional audio, also 'pro audio', refers to both an activity and a type of audio equipment. Typically it encompasses the production or reproduction of sound for an audience, by individuals who do such work as an occupation like live event support, using sound reinforcement systems designed for...
applications. The two channels of a stereo amplifier are fed the same monaural
Monaural
Monaural or monophonic sound reproduction is single-channel. Typically there is only one microphone, one loudspeaker, or channels are fed from a common signal path...
audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
, with one channel's electrical polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
reversed. A loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
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 subwoofer
Subwoofer
A subwoofer is a woofer, or a complete loudspeaker, which is dedicated to the reproduction of low-pitched audio frequencies known as the "bass". The typical frequency range for a subwoofer is about 20–200 Hz for consumer products, below 100 Hz for professional live sound, and below...
s.
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..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 (antiphase, or 180° out of phase) makes each amplifier see only half the load's electrical impedanceElectrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
. 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
Damping factor
In audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
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
- AmplifierAmplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
- Bridged and paralleled amplifiersBridged and paralleled amplifiersMultiple electronic amplifiers can be connected such that they drive a single floating load or a single common load , to increase the amount of power available in different situations...
- Electronic amplifierElectronic amplifierAn electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
- Single-ended signallingSingle-ended signallingSingle-ended signaling is the simplest and most commonly used method of transmitting electrical signals over wires. One wire carries a varying voltage that represents the signal, while the other wire is connected to a reference voltage, usually ground....