In commercial practice, this is decided for you by the marketing department. Even if you can please yourself, the power output capability needs careful thought as it has a powerful and non-linear effect on the cost.

The last statement requires explanation. As the output power increases, a point is reached when single output devices are incapable of sustaining the thermal dissipation, parallel pairs are required, and the price jumps up.

Similarly, transformer laminations come in standard sizes, so the trans- former size and cost will also increase in discrete steps.

Domestic hi-fi amplifiers usually range from 20Wto 150Winto 8 though with a scattering of much higher powers. PA units will range from 50 W, for foldback purposes (i.e., the sound the musician actually hears, to monitor his/her playing, as opposed to that thrown out forwards by the main PA stacks; also called stage monitoring) to 1kW or more.

Amplifiers of extreme high power are not popular, partly because the economies of scale are small, but mainly because it means putting all your eggs in one basket, and a failure becomes disastrous. This is accentuated by the statistically unproven but almost universally held opinion that high-power solid-state amplifiers are inherently less reliable than others.

If an amplifier gives a certain output into 8, it will not give exactly twice as much into 4 loads; in fact it will probably be much less than this, due to the increased resistive losses in 4 operation, and the way that power alters as the square of voltage. Typically, an amplifier giving 180W into 8 might be expected to yield 260W into 4 and 350W into 2, if it can drive so low a load at all. These figures are approximate, depending very much on power supply design.

Nominally 8 loudspeakers are the most common in hi-fi applications. The nominal title accommodates the fact that all loudspeakers, especially multi element types, have marked changes in input impedance with frequency, and are only resistive at a few spot frequencies. Nominal 8 loudspeakers may be expected to drop to at least 6 in some part of the audio spectrum.

To allow for this, almost all amplifiers are rated as capable of 4 as well as 8 loads. This takes care of almost any nominal 8 speaker, but leaves no safety margin for nominal 4 designs, which are likely to dip to 3 or less. Extending amplifier capability to deal with lower load impedances for anything other than very short periods has serious cost implications for the power-supply transformer and heatsinking; these already represent the bulk of the cost.

The most important thing to remember in specifying output power is that you have to increase it by an awful lot to make the amplifier significantly louder. We do not perceive acoustic power as such – there is no way we could possibly integrate the energy liberated in a room, and it would be a singularly useless thing to perceive if we could.

It is much nearer the truth to say that we perceive pressure. It is well known that power in watts must be quadrupled to double sound pressure level (SPL) but this is not the same as doubling subjective loudness; this is measured in Sones rather than dB above threshold, and some psychoacousticians have reported that doubling subjective loudness requires a 10 dB rather than 6 dB rise in SPL, implying that amplifier power must be increased tenfold, rather than merely quadrupled.

It is any rate clear that changing from a 25W to a 30W amplifier will not give an audible increase in level. This does not mean that fractions of a watt are never of interest. They can matter either in pursuit of maximum efficiency for its own sake, or because a design is only just capable of meeting its output specification.

Some hi-fi reviewers set great value on very high peak current capability for short periods. While it is possible to think up special test waveforms that demand unusually large peak currents, any evidence that this effect is important in use is so far lacking.

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