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Sound Advice

Good Amps and Power Efficiency

PA amplifiers need to combine the delicacy of a good hi-fi amp with the robustness and reliability of a farm tractor, blending (increasingly) with the low weight and compactness of aeronautical gear.

Good-sounding power amps (ones which add minimal colouration or distortion to the signal, purely making it louder) require great sophistication to enlarge and deliver the signal very precisely over a wide ‘canvas’ of levels and frequencies, while also delivering high currents and voltages.

And these quantities are not delivered into docile power-absorbing elements, but instead into speakers, which are quite complex and ‘reactive’ in the way they interact with the amplifier.

No power amplifiers are 100 per cent efficient – even the best manage only about 80 per cent in reality. The best speakers, meanwhile, only approach 25 per cent efficiency. Best overall efficiency is consequently about (0.8 x 0.25) = 20 per cent.

The average overall efficiency figure is more often between five and ten per cent. Taking ten per cent as an approximate figure, this means to get a certain amount of acoustic power – in other words music at a suitable sound level – in the room, we have to provide about 10 times that power from the electricity supply. And so this is the amount that an audio power amplifier has to handle and ‘process’.

We’ll also want to have some power capability in reserve – since inadequate power results in amplifier overload and bad sound. In general, erring on the side of over-rating is better than under-rating.

And remember that the relationship between watts and loudness isn’t proportional in the way you might imagine. As a reminder, a rule of thumb is that you need to increase the power delivery into any particular speakers by at least tenfold (x10) to attain about twice (x2) the audible level. This appears on a sound level meter as a 10dB higher SPL (sound pressure level) – so, for example, if 100 W gives 90dB SPL, 1,000 W will be required to increase the level (where nothing else is altered) up to 100dB SPL.

In short, much, much more power is needed than you might expect.

This article is reprinted with permission from The Live Sound Manual, published by Backbeat Books, www.backbeatbooks.com. All information is copyrighted and cannot be reprinted without the permission of the publisher.

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