Simplistically amplifiers will have a power rating into wattage described in a number with some caveats in the bracket. For absolutely run of the mill amplifier, even the caveats will be very vague and not give a true idea of what is the amplifier really capable of. This is the exact reason why most of the lower end and mid-end amps have ‘similar’ specs, but sound miles apart.

A loudspeaker is a variable load with changing music, unlike a light bulb – this is what makes the power rating more complicated. In a gist – a good amplifier should be able to supply clean power to the speaker(s) in all the varied musical conditions.

Let us examine a technically correct power rating what tells us a complete story (and doesn’t hide any part of it):

This is truly an honest power specification of a very powerful home/semi pro amplifier. Let us examine the nuances one by one:

• Ohmage: (resistance to the current flow) of a loudspeaker varies according to the musical content. It is a loudspeaker’s reaction to the music thrown at it. There are difficult loudspeakers that reduce to 2 or 3 ohms sometimes and go up to 12 or 14 ohms on the upper range. Every halving of the resistance will demand double the current (meaning power wattage) from the amp. Going by this logic at zero ohms (a short-circuit or no loudspeaker electrically ‘visible’ to the amplifier but just a wire connected from its positive terminal to the ground i.e. red to the black coloured speaker output) the power supplied by the amplifier should be infinite – but this is truly impossible for any amplifier in the world – that is the exact reason the amplifiers clip and go into protection mode if they encounter an extremely low impedance (variable resistance is called impedance). Some of the amplifiers may not be able to cater to 2 ohms load at all and that is acceptable in the real world – however this one does.
• Channels: practically a stereo amp will have two channels driven always, but an amp that want to show spec-manship will give power rating for one channel at a time. Since the power-supply in the amplifier is shared in most cases, a single channel driven may yield larger wattage number! But that is a useless specification, as we seldom listen to a single channel!
• Frequency: At what frequency is the test conducted? Feed an amplifier one single frequency and it may give you a larger power output (like 1kHz in this case). Higher the frequency, more will be the power output; however, we do not buy amplifiers to listen to the test tones. It is more realistic to simulate the musical conditions, but to keep uniformity in the test a full bandwidth white noise is fed to the amplifier and then the power output is measured. This is the exact reason the wattages are lower than their counterpart measurements from rows 5 to 8 as compared to rows 1 to 4.
• Distortion: a test bench can let the sound go really dirty in the output to get higher number of wattage by cranking up the volume levels. If one plots the input vs output graph the difference in the shape of the input to the output curve is distortion – the shape is distorted, which is function of the amplifier. In an ideal world apart from size both the curves should be of the same shape! It is important to keep the distortion levels to acceptable levels while conducting the test, as a listener will tone the volume down to these acceptable levels if the distortion is huge. Many run-of-the-mill amplifiers test the amps at huge 3-4% THD (Total Harmonic Distortion), some of the minicompos may even have an audacity to test them at utterly nonsense 10% THD! 0.1% is a respectable figure that most of the good Hi-Fi brands will use.
• Duration: almost never mentioned, but is a very important aspect. Have you ever wondered why the first song sounded so good as compared to the 5th song? Most of the time one may blame the listening fatigue. But wait, as the amplifier heats it introduces distortions which were not present earlier. With very space efficient, cheap class D amplifiers where cost cutting is the key and keeping it compact is a forte – distortion due to component fatigue in short term (after an hour or so) or long term (due to prolonged degradation due to ageing and heat) are very common. Same new amplifier will sound better than this “aged and worn” old amplifier. A good design will have these tolerances already accounted for and will have heat and ventilation more than adequately provided for – and this is not cheap to do. In fact most of the Mil-Spec (meeting military standards) certified components will be a reliable consumer grade component but with very well engineered heat convection mechanism without perforations in the chassis – this will easily make the component 3-4 times expensive!
• Power Density: this is one of the features that is difficult to measure and is often lacking in the newer class D amplifiers. Have you ever wondered why an old-age pure class A amplifier measuring barely 20W per channel sounds more denser/thicker/juicier than the new-age 100W per channel class D amplifier? Well, the devil is in the details. If you plot the power delivery onto a sophisticated measuring device then the different will be evident. below those joined crests of class D ripples how many towers stand and how fat are each vis-a-vis a fully-filled area below a pure Class A wave definitely carries much denser power than a class D amp. Thus the promise that class D will make things cheaper has somewhat failed – a “good” class D amp is not much cheaper than a good class A/B amp today.

Basically, do not take the power rating on its face value. When two components with similar power specs are priced far apart – there is lot other than jut a number. Distortion-free clean sound, more power bandwidth, power density, being able to cater to difficult loads (speakers) could be the hidden fortes.