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Are you happy when your power amplifier produces 10% distortion?

The amplifier you bought may claim a distortion figure of 0.03%. But what if you knew that often it could be producing as much as 10% distortion?

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There are lies, damned lies and statistics, according to the saying. At an even lower level of truthfulness are audio specifications.

We could be talking about any item of audio equipment, but let's stick to the good old power amplifier. In this day and age it's easy to produce the equivalent of a piece of wire with gain, is it not?

Well, it depends how you look at the specifications. The main problem with power amplifiers occurs when the signal crosses the zero volts boundary, exactly halfway between the positive and negative excursions.

At this point, the current switches from being delivered by an npn-type transistor to a pnp-type 'complementary' transistor or vice versa. Inevitably there is at least some slight mismatch and this causes 'crossover distortion'.

Still, distortion of this type is at a low level compared to full signal output. As a percentage, it could be as low as 0.03%. But remember that this is a percentage of full output level. Your amplifier is not going to be working as hard as this all the time.

Expressed as decibels, this equates to around -80 dB (-80 dBFS) in comparison with full output level (0 dBFS). But what about when the level drops to something rather more quiet - say -60 dBFS, which is still perfectly audible?

Now the distortion is only 20 dB below the signal level. Express this as a percentage and you get the mammoth distortion figure of 10%!

So just consider that when the music gets quiet and peaceful, a full 10% of what you are listening to could be pure distortion. Yurggh!

By David Mellor Thursday November 30, 2006
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