In 1996, the vast majority of domestic amplifiers were two-channel stereo units. Since then there has been a great increase in other formats, particularly in multichannel units having seven or more channels for audio-visual use, and in single-channel amplifiers built into subwoofer loudspeakers.

Multichannel amplifi ers come in two kinds. The most cost-effective way to build a multichannel amplifier is to put as many power amplifi er channels as convenient on each PCB, and group them around a large toroidal transformer that provides a common power supply for all of them.

While this keeps the costs down there are inevitable compromises on interchannel crosstalk and rejection of the transformer’s stray magnetic fields. The other method is to make each channel (or, in some cases, each pair of channels) into a separate amplifier module with its own transformer, power supply, heat-sinks, and separate input and output connections – a sort of multiple-monobloc format.

The modules usually share a microcontroller housekeeping system but nothing else. This form of construction gives much superior interchannel crosstalk, as the various audio circuits need have no connection with each other, and much less trouble with transformer hum as the modules are relatively long and thin so that a row of them can be fitted into a chassis, and thus the mains transformer can be put right at one end and the sensitive input circuitry right at the other.

Inevitably this is a more expensive form of construction. Subwoofer amplifi ers are single channel and of high power.

There seems to be a general consensus that the quality of subwoofer amplifi ers is less critical than that of other amplifiers, and this has meant that both Class-G and Class-D designs have found homes in subwoofer enclosures.

Subwoofer amplifiers differ from others in that they often incorporate their own specialized filtering (typically at 200 Hz) and equalization circuitry.

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