8. Power Amplification

Our pre-amplifier passes the signal it receives from the music source through to the ‘power amplifier’. If this process is combined into a single unit, this is simply called ‘amplifier’. If a tuner is added in the same housing, the unit becomes a ‘receiver’. While these combined units are often cheaper and more convenient, they also limit the options in customising our system, e.g. when changing to new speakers. - Without doubt, there are great units to be found in either discipline.

A power amplifier takes the relatively weak electrical current coming from the pre-amp and uses it to operate a regulator that controls a high current coming from the power grid. The regulator can be in the form of a transistor or a tube, or in that of a digital switch. Both transistors and tubes radiate lots of heat during operation. Tubes mostly because of their internal heating, and transistors because of their relatively slow adjusting of a high energy source. Classic transistors are round and shiny looking, similar to tubes.

An amplifier needs a strong, low radiation transformer (often toroidal or encapsulated) that delivers both the operating and the amplification current, and lots of excess energy that is stored in the large capacitors (similar to ultrafast batteries) to provide music burst power that the power grid could not otherwise provide fast enough. It also needs one operating board and at least one transistor (or similar) per channel, usually mounted on large heat sinks for cooling.

From the combination of these parts results the amplifier’s power rating. This is usually provided in two numbers: watts & ohms. The watts is the ability to provide power into a load resistance that is rated in ohms. Theoretically, when you lower the load resistance on the side of the speaker, the ability to deliver watts on the side of the amplifier increases. There is a limit to this, however, because reduced load also means more back current to the amplifier which at some point either becomes unstable or overheats and dies, perhaps taking the speakers with it.

When I first looked at the subject of power amplifiers for my system, I was surprised that some of the most highly esteemed amp designs displayed great simplicity in terms of both: components and layout. Having had the privilege to sonically compare some older with more modern designs, I am currently very pleased with the well balanced sound of the 1989 B&K ST140 amplifier that I purchased from a member of the local audiophile community. The amp's ability to deliver into a 2 ohms load was among my considerations.