Power amplifiers are uneventful devices. They are usually big and heavy, take up a lot of rack space, and feature very little (or sometimes nothing) beyond input and output sockets. Because one tends to ignore them, it is all the more important that they are chosen and used with due care. Coming in a variety of shapes, sizes and ‘generations’, they are all required to do the ostensibly simple job of providing voltage amplification – converting line levels of up to a volt or so into several tens of volts, with output currents in the ampere range to develop the necessary power across the loudspeaker terminals. Given these few requirements, it is perhaps surprising how many designs there are on the market.