Multiple electronic amplifiers can be connected such that they drive a single floating load (bridge) or a single common load (parallel), to increase the amount of power available in different situations. This is commonly encountered in audio applications.
Overview
Bridged or paralleled modes of working, normally involving audio power amplifiers, are methods of using a two or more identical amplifiers to drive the same load simultaneously. This is possible for sets of mono, stereo and multichannel amplifiers since the amplifier outputs are combined on a per load basis. Depending on the method of combining separate amplifiers, bridging or paralleling, different amplification goals can be served. The result is an amplifier that can be further combined with bridging or paralleling. This approach can be beneficial for driving loads for which using a single-ended amplifier is impossible, impractical or less cost-effective.
Bridged amplifier
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.
For a given output voltage swing, the lower the impedance the higher the amplifier load. Bridging is used to allow an amplifier to drive low loads into higher power, because power is inversely proportional to impedance and proportional to the square of voltage, according to the equation . This equation also shows that bridging quadruples the theoretical power in an amplifier, however this is true only for low enough loads. For example, for loads where the amplifier reaches its full potential in single-ended mode, there is no gain to be made with bridging. This is because an amplifier can have current limitations. In practical applications, inadequate heating dissipation and power supply also limit the power increase, although if the bridged amplifiers use separate power supplies, which is typical, the limit increase for high loads is power doubling, since every amplifier only delivers half of the total power.
Typical circuit
Example: The image shows two identical amplifiers A1 and A2 connected in bridge mode. The system is arranged in such a way that the outputs from the amplifiers are inverted with respect to each other. In other words, as the signal in one amplifier is swinging positively, the signal in the other is swinging negatively. When the output of one amplifier is at +10 volts the output of the other will be at −10 volts, and vice versa. The load is connected between the “hot” (often red) outputs of the two amplifiers, and is subject to the potential difference between them. If the instantaneous potential at the output of one amplifier is at +10 volts, then the output of the other one will be at −10 volts, and the potential difference across the load will be 20 volts, or double the potential available from a single amplifier.
The provision of the anti-phase audio input signal can be provided in several ways, which require appropriate knowledge and skill.
by an internal modification such as that described by Rod Elliot at https://sound-au.com/project20.htm; by a simple active phase splitter circuit, external to the amplifier; by a phase splitting audio input transformer, external to the amplifier.
Benefits and drawbacks
Since two amplifiers are being used in opposite polarity, using the same power supply, the bridged output is floating. This makes a DC blocking capacitor between the amplifier and the load unnecessary, saving cost and space and avoiding the power reduction at low frequency due to the capacitor. For the same reason the amplifier outputs should never be grounded or it may damage the amplifier. Damping factor is cut in half, which is beneficial to the power delivery.