What is a Watt?

Written by: Tony Harris, September 7, 2014



I recall my physics teacher (Kevin Conway, at the British School of Brussels) looking at me and saying: watt is a unit of power. Placed on the spot I was mentally sifting through possible answers. We had just learned about Volta and Ampere, so I knew it was neither of them. I was thinking of offering up horses, but thought it sounded silly, when he either tired of my deliberations or took pity and explained he was making a statement; not asking a question.

So we learned if you take an Italian and multiply him by a Frenchman you get a Scotsman. Or a Watt is a Volt times an Ampere – it is indeed the SI unit of power. James Watt had a lot of influence over the industrial revolution. There are several statues of him in the city of Birmingham, England where he lived and coincidently where I was born. Strangely enough though, there are no statues of me there.

The main specification of a power amplifier is their output power. But within the operating range of an amplifier, where should the measurement be made? In an apparent attempt to confuse our customers, amplifier manufacturers all use different points. At E&I we believe that if we are talking about a linear amplifier, we should specify the output power at the 1 dB compression point. So our 2100L, for example, we call a 100 watt amplifier and we guarantee that it will produce a minimum of 100 watts at 1 dB compression. Also this is where we specify the harmonic content and distortion.

A quick look at a few web sites and I see that some 100 watt ‘rated’ amplifiers have specifications that only guarantee, 60 – 75 watts at 1 dB compression. There is one that says it is a 175 watt amplifier, but in the detail of the specification it is actually only produces 50 watts within the linear range. (In each case it was the 1 dB point that the harmonic distortion is specified).

So I apologize to you, on behalf of all amplifier manufacturers for the confusion. I do wish that James was around to add clarity to the situation and come up with a standard for when we say ‘rated’. In the interim, I would suggest that if you are comparing amplifiers, compare them at the 1 dB compression point – not rated, not saturated – but at a useful operating point.



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