PA Systems - Bridged Power Amps

Note: Class A amplifiers cannot be bridged, Some Class A/B amplifiers can be bridged. Most Class D amplifiers (Digital) already operate in bridged mode. If you have a Class A amplifier, do not attempt to alter its default speaker configuration

Speaker Voice Coil Motion

There are 3 states that a speakers voice coil can be in. Center - neither moving away from, or towards the speaker. Examples would be when there is no signal to the Speaker, or during the zero crossings of a signal being sent to the Speaker. The Sine Wave diagram shows the 3 states: Zero, Push and Pull. Most waveforms that occur in music are far more complex than this, however some instruments are nearly sinusoidal - The high E string, with a note plucked at the 12th fret on a Fender Stratocaster set on the (single coil) bridge pickup is closest that I've ever seen to a perfect sine wave from a non-electronic musical instrument.

The next state is having the voice coil pushed away from the magnet. The extent to how far the voice coil moves outward is dependent on how much voltage is moving thru the voice coil. If the voice coil stays out, and never comes in, then there is a positive DC offset from the power amplifier.

The last state is where the voice coil is pulled towards the magnet. Again, the amount of motion depends on how much voltage is moving thru the voice coil. If the voice coil remains pulled in to some degree, then there is a negative DC offset from the power amplifier.

A slight DC offset is fairly common with single power amplifier configurations, and not a substantial problem to deal with since the voltage is completely absorbed by the speaker system. In the case of a bridged amplifier configuration, a DC offset can be a serious problem that causes overheating or failure of one (or both) of the power amplifiers.

Regular Power Amplifier / Speaker Usage

Audio systems have a pre-amp stage that amplifies a signals voltage level 1,000 times (or more) its original signal level - this cause the power amplifier to push a speakers voice coil away from, or pull towards the speakers magnet. Most high wattage power amplifiers have dual power supplies, and as such apply either the available positive or negative voltage directly to the speakers voice coil in response to the pre-amp signal. Many other power amplifiers have a single power supply and simulate a middle level via a voltage divider circuit, or an output transformer - this allows approximately 1/2 of the available voltage to be available as it either pulls or pushes the voice coil in or out. Because of this simulation of the ground level, many Monophonic Single Power Supply Amplifiers do not have a well defined zero point and should not be used in a configuration other than originally intended. A number of Stereo Single Sided Power Amplifiers (notably car audio systems) can be bridged, provided that their DC offsets are accurately adjusted.

What may not be obvious is that during the push portion of the sine wave, the part of the power supply that pulls the speaker is loafing around, doing nothing. The same holds true when the pull portion of the sine wave occurs - the part of the power supply that pushes is doing nothing. Because of this, you can never really use more than 50 percent of the available power.

NOTE: This is not true for Class A power amplifiers, They actually use the available voltage range. This is one reason that they cannot be Bridged

Bridged Power Amplifier /Speaker Usage

To effectively bridge the 2 power amplifiers, you need to find a way to utilize the full available voltage provided by the power supply. In a Single Sided Power Amp, we only use 1/2 the available power at a time. In order to use the other half of the power, you will use a second power amplifier (of exactly the same power rating), however the second power amplifier has to have its signal altered such that its exactly the same signal, but 180 degrees out of phase. If you were to SUM the original signal and the 180 degree out of phase signal into an audio mixer, they should cancel each other out completely - If they don't then the signals are not properly balanced. When bridged, you feed the regular signal into one side of the first power amplifier, and then the output goes to the speaker. The signal that is 180 degrees out of phase is fed into the second power amplifier, and its output goes to the other side of the speaker. When this is done, you have bridged the 2 power amplifiers and they will operate as a single power amplifier. This configuration allows one power amp to push the voice coil while the other is pulling the voice coil at the same time - effectively doubling the amount of voltage swing that the speaker sees. In most cases the available load capability of the Power Amplifier doubles and you get 2 times the power, or you can put 1/2 the speaker load. For example, if the 2 power amplifiers can handle a 4 ohm load individually, when bridged, they can drive a 2 ohm load. See your Power Amplifiers User Manual for specific loading and power capabilities.

The following diagram shows the concept. It shows a single input from a pre-amp stage driving a power amplifier. The same single input is inverted (easily done with an Op-Amp) and fed to another power amplifier. the outputs of each amplifier directly drive the speaker.

Please note that the Original Signal Ground is no longer a part of the voltage path thru the speaker. You must be careful to never allow a ground connection to be made to a Bridged Speaker cabinet - doing so may destroy one or both of the Bridged Power amplifiers (usually in less than a few seconds). Use individual cables that have any metal external parts (such as the outside Plug Case) taped up to protect against accidental shorting to other signal ground connections.

This lack of Signal Ground usually means that the 2 power amplifiers should share the same power supply. Because DC offsets from either power amplifier into the speaker will cause the other power amplifier to draw current to compensate, its possible to burn out one of the power amplifiers, or at least have it run exceptionally hot when there is no signal to the power amplifier. Many large Stereo PA power amplifiers (and some car audio power amps) are designed to operate in a bridged mode and have been adjusted from the factory to have the appropriate DC offset compensation to allow their use in this mode. Always check that your system was designed to operate in bridged mode before trying it - if not, operate it only in single side fashion.

Note: Mismatched power amplifiers run in a bridge configuration can destroy each other in a matter of seconds, often frying the voice coils on your speakers as a part of the failure mode- always refer to the amplifiers owners manual for proper usage and configurations.

NOTE: We do not work on Home or Car Audio. We work only with Pro-Audio applications. We cannot help you with Home or Car Audio questions.

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