Bridge Amplifiers Stereo to Mono Double Power Amp Subwoofer

A stereo power amplifier is limited in its output power by two main factors - the impedance of the load and the internal power supply voltage. To obtain more power, one has very limited choices - other than the purchase of a more powerful amp.

The load impedance can be lowered, but if the load happens to be a pair of standard loudspeakers this is not viable, since the impedance is set by the drivers themselves. Increasing the power supply voltage is generally a bad idea, since most commercial amps do not have a wide safety margin with component ratings, and will probably be destroyed if the voltage were to be raised sufficiently to obtain even 50% more power.


The bridging adapter shown can make an amplifier produce almost 4 times the power for the same impedance - but beware of the pitfalls.

• The amplifier must be rated to drive a load impedance which is half that of the speakers to be connected
• Although some forms of distortion will cancel, the primary form of amplifier non-linearity - crossover distortion - will be worse because ...
• Both amplifiers in bridge will cross the zero volt point at the same time
• The impedance is lower, there is more current, so each amplifier's contribution will be greater

Construction is not critical, and the adapter has unity gain for each output. Naturally, 1% metal film resistors should be used, and the choice of opamp is not too critical - the TL072 is perfectly acceptable in this configuration, and will have low noise and a wide bandwidth. Note that if interconnect leads are to be used from the adapter to the power amp, the 100 Ohm resistors shown must be placed in series with each output to prevent instability - this is important, as an oscillating adapter will inject an AC voltage of perhaps hundreds of kiloHertz into the amp's input, with the very real possibility of destruction of the output transistors.

The power supply may be taken from the preamp supply (this should be +/-12V to +/-15V). The preamp output is connected to the adapter's input, and for the sake of convention, connect the +OUT to the Left power amp's input, and the -OUT to the Right amp's input. For opamp pinouts, go to the link on the main projects page, or have a look at one of the other projects (such as the Audio Preamp - Project 02) where this information is to be found.

Naturally, for stereo two circuits are needed, as well as a second (preferably identical) stereo power amp. This arrangement is also very useful to convert an otherwise mediocre stereo power amp into a perfectly acceptable sub-woofer amplifier, having plenty of power (depending on the power of the original, of course).

Quality is not so much of an issue for a sub, since only the low frequencies are reproduced, and amplifier distortion is as nothing to the distortion generated by a loudspeaker at low frequencies.