When it comes to making both stereo and mono amplifiers one solution is the easiest for manufacturers and consumers alike: the switchable amplifier. In this clever scheme customers are thrilled because they can turn their stereo amp into a mono with the click of a switch. Manufacturers are happy as well. Instead of separate models and the supporting materials needed, there is only one that doubles as stereo or mono. It's an enticing solution to a problem few have, and even fewer benefit from. But it makes for great sales appeal.
To make a switchable amp there are two means: parallel and series. I'll cover parallel first, series second.
Picture a stereo power amplifier as I have previously described: two identical channels of amplification, one power supply, one chassis. If you wish to make a mono amplifier all you need to do is place the two channels, left and right, in parallel with each other, creating a single channel with double the current available. But as soon as you try this you run into trouble. The first problem involves the way amplifiers share: they do not. Because each amplifier has its own feedback loop, input, output and control circuitry, paralleling the two simply causes them to fight each other, like brothers and sisters. Secondly, you get no more watts.
Remember in an earlier post I mentioned the need for marketing people to write something different on a spec sheet to sell mono amplifiers? Imagine their difficulty convincing you to buy a stereo amp with a mono/stereo switch - and neither switch position produces more watts. Your first question might be, to what benefit? But there is a simpler way that resolves both issues: placing the two channels in series, a technique known as Bridging.
To bridge an amplifier it is necessary to flip the polarity (phase) of one channel relative to the other and place the speaker in series with the two channels. What happens next is interesting. If you place a loudspeaker in series, across the two red (hot) terminals of the amplifier, the voltage across the speaker doubles if the two channel's phases are inverted. This is because one channel is going one way while the other is moving in the opposite direction, increasing the distance between the two (measured as voltage) by twice. Like two cars speeding away from each other at the same speed, distance is doubled. When you double the voltage of an amplifier the wattage figures increase by a factor of 4. Thus, a 100 watt amplifier with a large enough power supply becomes a 400 watt amplifier. Neat, eh?
It is a cool trick but it is not without consequences, which we will cover tomorrow.