Why amplifier power is important

For Musical Fidelity, one of the markers of genuine hi-fi is a decent dynamic range. To produce a decent dynamic range, the system needs to be able to produce peaks of at least 100dB in your listening position, and comfortable peaks of 105dB to 110dB are better. However, 100dB is the bare minimum requirement, and any system that produces less than this does not deserve the name of hi-fi. It takes an amplifier with a lot of power to produce peaks of 100dB or more.

Many amplifier manufacturers produce units with low power and call them 'high-end.' Low-power amplifiers will produce a surprising continuous level, but unless they are coupled with extremely efficient loudspeakers, they cannot possibly produce a decent dynamic range. No matter what a manufacturer says, or how much it costs, a low-power amplifier is not genuine hi-fi.

Unless you understand the relationship between amplifier power and peak listening level, which most audiophiles do not, you are likely to be sold something that costs you a lot but is not really a proper hi-fi system.

One reason that few audiophiles understand the relationship between amplifier power and peak listening level is that loudspeakers and amplifiers are specified differently and there is no obvious way to use the information to find out what kind of peak levels you can expect from a particular system. Most manufacturers are loathe to clear up the confusion, for reasons that will become clear.

Loudspeakers are usually specified by their efficiency; they produce a certain number of dB for 1W at one metre. However, as you move away from the loud- speaker, you lose about 5dB for each metre over one metre. You'll see in a minute why you need to know this.

Amplifiers are usually specified by their power rating in watts.

To use these two pieces of information, you need one more number - the amplifier's power in dBW This is a measure of how much sound (dB) is produced for a certain amount of power (W). As you will notice on the table, the amount of power needed for each increase is not linear - each increase in dBW requires a steep increase in power. A low- powered amp will never be anything other than a low-powered amp: we are dealing here with the laws of physics.

However, armed with this information, you are now in a position to find out whether or not your system is capable of peaks of 100dB or more.

To find out what a system can do, take the loudspeaker's efficiency, add on the amplifier power in dBW and deduct for the listening position. You have now worked out the system's peak level capability.

For example, to work this out for a system with loudspeakers of 87dB efficiency and an amplifier power of 200W and a listening position about three metres from the loudspeaker: 87 + 23 - 10 = 100 dB peak level capacity. This assumes that the amplifier's power really doubles into 4 Ohms, a can of worms that we will not open at the moment, and that the amplifier is stable. In this example, the system would be just about reasonable.