Bridged Amplifier
Example. Two amplifiers, each rated 100 watts maximum into 4 ohms, in bridge mode they will appear as a mono amp rated 200 watts into 8 ohms. This is the most commonly misunderstood mode of operation and it requires additional circuitry to implement if the pair of amplifiers does not have the facility built in. The image shows two identical amplifiers A1 and A2 connected in bridge mode. The signals presented to each amplifier of the pair are caused to be in anti-phase. In other words, as the signal in one amplifier is swinging positively, the signal in the other is swinging negatively. If, for example the maximum output voltage swing of each amplifier is between a peak of + and – 10 volts, when the output of one amplifier is at + 10volts the output of the other will be at –10 volts, which means that the load (a loudspeaker) now sees a 20 volt peak difference between the “hot” (normally red ) output terminals.
The provision of the anti-phase audio input signal can be provided in several ways:
- by the built in facility in an amplifier pair provided with the bridge mode option;
- by an internal modification such as that described by Rod Elliot at http://www.sound.westhost.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.
Options 2, 3 and 4 require appropriate knowledge and skill. The bridge mode option is often used in PA systems and especially in car audio applications to feed bass loudspeakers at high power. Car audio amplifiers commonly have only a 13.8 volt supply and obtaining the voltage levels in the amplifier circuit required for even modest powers is expensive. Bridge mode operation helps provide the power required at lower cost.
Read more about this topic: Bridged And Paralleled Amplifiers