Breaking conventions

When we think of a power amplifier we visualize a conventional design where the speaker is attached to the red (+) terminal and the black (-) terminal for each channel. The red is the output of the power amplifier, the black is ground. But there is a different type of amplifier called a bridged amp. In this design there are two amplifier outputs for each channel, one for the red (+) terminal, the other for the black (-). To build a stereo version of this amplifier we need four channels of amplification, one for the + and - of each channel, rather than the two we commonly use in a conventional design. Stereo amplifiers that can be switched from stereo to mono move between conventional design to bridged with the switch, which assigns two channels into one when mono is selected. This is why a switchable bridged amplifier requires the loudspeaker to be rewired when you select mono: the speaker is placed between the two red terminals, rather than red and black. You've probably seen this arrangement before. Here is a picture of the rear panel of one of our older designs that featured a stereo mono switch. Note the red lines on the back panel showing where to place the speaker cables, between the two red terminals. Also note the red line pointing to the left channel input. When the amp is switched into mono mode, only the left channel may be used for connecting to the preamplifier. This is because the right channel is now fed the same input the left channel receives, just flipped upside down - when one channel goes positive, the other goes negative. Doing this doubles the voltage and if the power supply is big enough, increases the wattage as well. The use of bridged amplifier technology is not new and its use is increasing. But these increases are coming not from more stereo amplifiers with a stereo/mono switch, but rather from designers purpose building bridged amplifiers; like the BHK Signature which internally has four channels of amplification to make stereo. An amplifier that is designed as a bridge cannot be bridged again for greater power. To create a mono amplifier from a bridged design, we need to take a different approach: parallel, which we will discuss tomorrow.