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Bridge Adaptor For Stereo Power Amplifiers

Would you like to connect a stereo amplifier in "bridge mode" in order to deliver double the power to a single loudspeaker system? This simple adaptor allows you to do it, without any modifications being necessary to the power amplifiers themselves.

By Leo Simpson

We regularly get requests from readers asking how to connect a stereo power amplifier in "bridge mode" but up until now we have not had a specific project article to suit the application. Then recently we received an email from a reader asking how to run the SC480 amplifier modules (SILICON CHIP, January & February 2003) in bridge mode, just as we were proof-reading last month’s article on the "Balanced/Unbalanced Converter for Audio Work". We immediately realised that half of that project would provide the needed adaptor.

Before going into the details, let’s briefly describe how a pair of power amplifiers can be run in bridge mode to extract more power. Fig.1 shows the set-up. For a start, you must have two identical power amplifiers and this is why this arrangement is often convenient with a stereo amplifier.

Click for larger image
Fig.1: this diagram shows how the Bridge Adaptor is connected to two power amplifiers to drive a single loud-speaker. Note that only the active output terminals of the power amplifiers are connected to the loudspeaker while the ground terminals are not connected.

Out-of-phase signals

The two power amplifiers are driven with signals that are out-of-phase by 180°. If we consider a sinewave signal (or any other audio signal for that matter), when one power amplifier is delivering the positive half cycle of the waveform, the other amplifier will be delivering the negative half-cycle. The amplifiers drive a single loudspeaker and the result is that the two amplifier voltage waveforms are added, ie, we get double the output voltage of one amplifier across the loudspeaker. And since power is "voltage squared", the resultant power in the loudspeaker is four times the power that could be obtained with one power amplifier driving that same loudspeaker. Well, that’s the theory anyway.

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