It's small and compact and can deliver quite a punch to your loud-speakers. Controls are simple, with bass and treble controls which can be used to brighten up your listening pleasure and a volume control to set you rocking.

Here's proof: with a 14.4V supply and a 4W load, the amplifier does indeed produce up to 18W, albeit with a fair bit of distortion. The horizontal scale is output power (in watts); the vertical is total harmonic distortion (THD).

It makes a great little amplifier for your Walkman, personal CD or mini-disc player, etc. And it can be operated from a 12V battery or good 12V power supply.

Whether you want an amplifier for your car, boat, caravan or for some other 12V application, it is often difficult to find a design which will produce very much power output.

It is just an unfortunate fact of life that at 12V the very absolute maximum power that can be delivered into a 4W load is 4.5W.

The reason for this is that the maximum voltage "swing" possible from the output devices with a 12V supply is 6V in the positive direction and 6V in the negative direction. This equals about 4.25V RMS (6/1.4142).

Power output equals the square of the RMS voltage divided by the load resistance (P=I2/R), so we get 4.25 x 4.25 / 4, or about 4.5W.

However, even this 4.5W is a theoretical maximum and the output power is more likely to be closer to 3W due to losses in the output devices of the amplifier.

As an aside, one of those little mysteries of life which make us smile (wryly!) is when we see consumer "hifi" claiming 50W output or more – and they take six "C" cell batteries.

Oh yeah?