How to Bridge an Amp (Guide and Diagrams)

Bridging an amp allows doubling the power of an amp by using both channels to make only one. A stereo amp of 2 x 100 W can deliver 1 x 200 W in bridged mode, the amp is then mono (one channel).

Bridging an amp means putting the two amp channels in series and connecting the loudspeaker directly from “one edge to the other” without going through the common ground of the two channels.

This tutorial will show you how to bridge an amp.

How a Bridged Amp Works

The sound engineer or DJ can “bridge” an amp when more power is needed to feed a big speaker, like a subwoofer, for example. The power is doubled but beware of the minimum impedance to respect (which will also be doubled). We will understand why this constraint on the impedance is due to the “bridged amp” operation.

Example of a Sound Amplifier

Imagine an amp of 2 x 100 W effective at 4 Ohms. When this amp is at full power on its two channels, each delivers 100 W RMS at an impedance of 4 Ohms. For example, here are the characteristics of a bridgeable sound amp:

– 2 x 100 W effective at 4 Ohms

– 2 x 70 W effective at 8 Ohms

– Minimum impedance of the speakers: 4 Ohms (in stereo mode)

The power of 100 W at 4 Ohms corresponds to an RMS voltage of 20 Volts and an RMS current of 5 Amps. (20 V x 5 A = 100 W and Ohm’s law: 20 V = 4 Ohms x 5 A).

Similarly, the power of 70 W at 8 Ohms corresponds to an RMS voltage of about 24 Volts and an RMS current of 3 Amps. The voltage delivered at 4 Ohms is slightly lower because the current is higher. It is like a car battery or any generator: the more current you draw on it, the more the voltage drops.

The maximum output voltage that an amp can deliver drops a little when it has to deliver a large current. This is due to the voltage drop in the transformer and a bit in the output transistors, and the voltage ripple in the filter capacitors.

How to Bridge an Amp

Principle of the Bridged Amp: Create a Double Voltage

Here is an example of the back of a bridgeable sound amp. You have to switch the button on “BRIDGE”.

Sound amp : stereo mode (2 speakers) or bridge mode (for a single more powerful speaker)

In fact, this amp is not able to give more than 5 Amps as output current. When you bridge this amp, one of the two channels delivers a voltage identical to the first one, but with an inverted sign. Between the two outputs, there is therefore a double voltage:

Principle of the amp in bridge mode: double the voltage at the speaker terminals

Digital Example of an Amp in Bridge Mode

The power of a hi-fi or sound amp is determined by the voltage it delivers and the current that the loudspeaker will require (4 or 8 Ohms most of the time). If we consider a given moment, the instantaneous voltage can be 20 V at the output of channel 1. Channel 2 gives -20 V, so there is 40 V between the two red terminals (outputs) of the amp:

Bridge mode amp: double voltage

An amp that gives 20 V on each channel can create a voltage of 40 V between its two outputs. Like when you put 2 1.5 V batteries in series to make 3 V. To avoid exceeding what the amp can give as maximum current (here 5 A), you have to double the impedance of the loudspeaker: for one channel (classic stereo amp) :

– a single output: 20 V / 5 A = 4 Ohms

– in bridge mode: 40 V / 5 A = 8 Ohms

If we put a 4 Ohms speaker on the bridged output, we would have a double current (40 V / 4 Ohms = 10 A), and the amp would be excessively stressed. This sound amp will therefore be specified as: 4 Ohms minimum on each output (in stereo mode) and 8 Ohms minimum in bridge mode.

Bridgeable Amp

The French translation of “bridgeable” is “pontable.” A bridgeable amp can be bridged to obtain double the power on a single loudspeaker, the 2 channels participating for half of the electric voltage generation. A teacher may explain in an electronics class that “a bridge is a kind of bridge…”

Bridge = bridge

Bridgeable = pontable

Bridged = bridged

Now here is what you should avoid doing.

Wrong Connections in Bridge Mode

Suppose you leave 2 speakers connected while the amp is in bridge mode. In that case, the 2 speakers will receive the same signal (controlled by a single potentiometer, the other potentiometer having no influence) but in phase opposition. The sound will be strange, the stereo weird, and the bass almost non-existent! There is no danger for the amp or for the speakers.

It would help if you did not connect a speaker with a too low impedance, especially in bridge mode otherwise, and the amp will overheat and break.

Conclusion

Bridging an amp allows getting double power on a single speaker. The impedance of this single speaker must be correct (often 8 Ohms minimum). Not all PA amps are bridgeable.