In the past many thought all audio amplifiers sounded alike, some played louder than others and in general it was accepted wisdom that valve amps had a warmer sound than their solid state brothers – and that was about it.
Even the late, great Peter Walker of Quad is supposed to have remarked “if it measures the same – it will sound the similar” and Peter is famous for describing a properly designed audio amplifier as “a straight wire with gain” – which from an engineering point of view is correct.
In the past power output measurements were obtained by playing a single test tone (usually at 1kHz) and using one of the stereo channels to take the measurement, however, if both channels are measured at the same time – just as if you were listening to your music – the results were usually dramatically different!
For example, if the first measurement showed 50 watts RMS with only one channel in operation, the second measurement taken in stereo might well only show 20-25 watts per channel. Depending on how well designed the amplifier is, especially in the power supply area, the figures usually being a lot worse at the frequency extremes.
Nowadays, reputable manufacturers test stereo amplifiers – in stereo with both channels in use. This gives a far better idea of what the amp is capable of, and they use a range of test material containing signals from low bass to high treble because that’s what the amplifier will be used for.
To complicate the matter, the amplifier must “drive” a loudspeaker and some speakers are easily driven while others less so. Being less sensitive, there’s quite a big difference between a claimed 86dB or for example 89dB, which in theory means the latter should be easier to drive – until we factor in the speakers impedance, usually shown on the back panel as 4, 6 or 8 ohms. The problem is, speaker impedance is not static and it varies with frequency and some difficult ones can go from as low as 1.5 ohms to 30 ohms when in use – while at the same time reproducing an ultra rapidly changing music signal, which should be reproduced as accurately as possible – then we begin to realise audio amplifiers do not have an easy life!