PS AUDIO: Compression explained

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Paul McGowan writes: In our quest to figure out why we might need a big power amplifier vs. a smaller power amplifier I’ve mentioned a lot of numbers: some scary big.  Many of you have written to me that the huge wattage numbers I’ve been mentioning don’t really stack up with your real world experience – where a much smaller amp seems to work just fine. Indeed, most of what we listen to never even comes close to needing the extreme amounts of power I mentioned.  Let me give you an example.  Take a look at this snippet of music.

This is from the loudest passages of music from Pink Floyd’s Welcome to the Machine I wrote about earlier.  The top trace is one channel, the bottom the other.  The dark blue areas are the peaks and the light blue areas are the average levels.  Remember we talked about peak vs. average? This somewhat shows what I was referring to.

But note in particular how most of the very top and bottom of each channel is basically empty – very few peaks come near the top and bottom edges, which means we’re almost never approaching anywhere near the clipping point.  That’s somewhat normal from what I’ve seen.

So, let me ask you a question – and this is something I ponder a lot.  How often do you find that you listen to a “live level” track and the sustained peaks of that track sound strained or compressed relative to the quieter passages?  I can tell you that on all but a very few systems I find that slightly compressed sound, bordering on hard and harsh, happens on many recordings during the loudest passages of music.  It’s particularly noticeable when that loud passage is sustained as opposed to a quick note or two.

As an experiment, I can turn the overall level of the sound down a bit and that compressed harshness goes away.  If I return the volume level back again to where it was sounding compressed on the peaks, and double the wattage of the power amplifier, the compression and tendency towards harshness is reduced significantly.

The first amp probably never got close to clipping and certainly the second more powerful amp hardly strained at all.

What we can learn from this is that there’s far more than simply clipping that meets the ear when it comes to wattage ratings.