• Power amplifier modes : stereo, parallel and bridge mono

    In general, two-channel power amplifiers for professional use default to stereo mode. That is, each amplifier channel receives a signal from its input connector and its volume is controlled by that channel's volume control. So, really, although the name of that default mode is 'stereo', it is more of a two-channel mode, since those two channels do not necessarily have to be right and left. For example, they could be two separate channels for the stage monitors of different musicians. Or the high and midrange bands of a multi-amp system.

    Both channels are often required to carry the same signal. We achieve this by connecting the inputs of the channels in parallel. To do this we can make a so called "Y-cable" with which we exit the signal source (e.g. a mixer) and reach the two input channels. Another way to achieve the same result is, if the amplifier has more than one signal connector, to take the input signal from one channel to the other channel using a short cable that comes out of an unused input connector on one channel and goes into a connector on the other. It is common for amplifier manufacturers to simplify our lives and avoid the complication of the Y-cable or the short loop-through cable. So they provide a switch that activates the parallel mode that simply links the inputs of the two channels of the amplifier so that they carry the same signal. Normally, when selecting the parallel mode, the amplifier takes the signal from channel 1 and the input to channel 2 is disconnected (or should not be used). Depending on the amplifier model, each channel will still maintain its volume control, else channel 1 will control the signal level going to both amplifier channels.
    NOTE: Some amplifiers brands such as Crown/Amcron and Camco have traditionally deployed a parallel mono mode that is totally different from the one described, since the output connections are also paralleled. The manual of our amplifier will give us details and connections for the parallel mode.

    Something similar happens if you want to use an amplifier in bridge mode, i.e. combining the two channels of the amplifier into a single, more powerful, channel (hence the name "bridge mono"). To do this we need to bring the same signal to both channels, except that channel 2 should have the opposite polarity to channel 1. The loudspeaker signal will then come out of the two positive output terminals. To avoid the complication with the wiring, just like with the parallel mode, many amplifiers that allow for bridged use (not all of them do!) provide a bridge mode switch that conveniently makes the required connections internally for us. Depending on the amplifier model, each channel will still maintain its volume control, so we should always use the amplifier with the two volume controls at maximum. Else, channel 1 will control the signal level going to the amplifiers of both channels, so we can use the volume control of channel 1 as the level control of what is now a single amplifier. Please consult your amplifier's manual for details and connections for the bridge mode of your particular model.

    Since the output voltage is doubled, a conventional amplifier will try to deliver four times more power (in theory, in practice it would be slightly less, of the order of only three times more) than in stereo mode per channel (that's around 5 to 6 dB more), so it is common for the minimum allowable impedance for the amplifier in bridge mode to be higher (typically twice) than in stereo mode. For example, a conventional amplifier that delivers 1000W per channel into 4 ohms that connected to a single 4-ohm load in bridge mode will attempt to deliver 4000W (in practice more like 3000W), which would exceed the power capacity of the amplifier, so the manufacturer may specify that the amplifier should only go down to 8 ohms in bridge mode. Similarly an amplifier that goes down to 2 ohms in stereo mode would only go down to 4 ohms in bridge mode. More modern and sophisticated amplifiers may be less restrictive on this.

    In any case, it should be noted that these mode switches that provide many amplifiers are only a convenient way of doing the same thing that we could do with input wiring. Also, muti-channel amplifiers will offer one mode switch for every pair of channels (e.g., and 8-channel amp may have four of these switches).


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