PS Audio: Why watts ain’t watts

Writes Paul McGowan: Before we get started on different amplifiers and why some seem to sound more dynamic than others, I wanted to clear up a couple of points about yesterday’s post showing how many watts it takes to get appropriate dynamic range.

All the measurements we described were anechoic chamber measurements: meaning there was no room involved.  As soon as you add a room to the mix, the loudness levels at your seat are very close to the loudness levels near the speaker – certainly greater than if the speakers were outside or in an anechoic chamber – the good news is the extremes I showed aren’t quite that dramatic.

The crest factor I describe is the difference between the average level and the peak level over a specific slice of time.  Many of you asked what was the difference between crest factor and dynamic range: the answer is that dynamic range describes the difference between the loudest and the softest passages of music in a complete piece – where crest factor is describing the difference between an averaged level (not the softest) and the peak level over a constant interval.

What’s really important to understand is that the needs for a power amplifier really are dependent on the type of music you listen to, the type of loudspeakers you have and the size of your room – among other factors.

If, for example, you listen to a lot of high average loudness rock music in a medium sized room with reasonably efficient speakers, then you’d want to find an amp with fairly high power, lots of heat sinks and a very stout power supply.

If, on the other hand, you listen only to chamber music or relatively low average loudness in the same size room, you could get away with far fewer watts and an amp with a fairly “spongy” power supply (like many tube amps have) if you don’t listen too loudly.

lastly, watts ain’t watts.  Loudspeakers are very complex loads and there’s far more than driving a resistor with a power amplifier and simply measuring the wattage that results.  Loudspeakers have different impedances at different frequencies, they are also inductive so the phase and current requirements can be off the charts.

Your power amplifier may be doing ok or it may be clipping or getting close to clipping without you ever knowing it.  VU meters on the front of a power amp are pretty much meaningless to measure peak current and I would never trust what I saw with them.

Tomorrow we’ll start to look at some different types of amps, I’ll try and explain about current vs. voltage etc.

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