I returned to DIY electronics in 2009 after a 20 year pause by building a few kits to get in shape. This blog is for me to keep track of my progress.
How much power is enough for an amp?
Posted 2nd January 2012 at 03:09 AM by alexcp
Bruno Putzeys posted:
About power ratings, well... Normal "critical listening" levels hover around a long-term average of 85db SPL at the listening position. If we're assuming uncorrelated signals (rather inaccurate assumption but it'll do for the purpose) that is 82dB SPL for each speaker alone. Crudely calculating back from a 2m listening distance to the 1m standard measuring distance that's 88dBSPL, one speaker, one meter. It's not unusual for speakers to have an efficiency of 88dB/1w/1m so 1W is roughly the required amplifier power.That is, with Heresy III's 99dB/1w/1m, 1W into each speaker produces the SPL of 96dB at 2m. 25W is 14dBW, so 25W into each speaker produces 110dB of SPL. The normal listening level of 85 dB comes at -11dBW, or 80mW. That's less than 1V at 8 ohm.
The current crop of unlistenable hypersquashed pop music has a peak to average range of around 4dB. So all it takes to reproduce pop at a reasonable level without clipping is a 4dBW (2.5W) rated amp.
Before the loudness war got into swing (say until the early 90's) peak-to-average ranges up to 14dB could be found in mainstream music. To play Sultans of Swing at the same average level requires an amp rated at 14dBW or 25W.
Classical music tends not to have dynamics compression on it (although I hear the loudness war has leaked into classical ranks too) but usually the dynamic range is manually managed (by gain riding) to about 16dB. You need 40W to deliver such recordings at a comfortable volume. Classical albums do exist though that have no gain riding and peak-to-average ratios of 20dB or more do occur. Wagner lands us into triple digit territory.
Note that I said comfortable volume, not realistic volume. Symphony orchestras can get very loud. An oft quoted number is 115dB SPL. That's 30dB over our 85dBSPL reference level, indicating a 1kW amp is in order.
First conclusion: Nelson Pass is a Dire Straits fan.
Second conclusion: clipping often goes unnoticed. It pays to build a clip detector into an amp just to get an idea of whether it happens. You'd be surprised how some kinds of material can sustain a lot of clipping before anyone notices. You can slice almost 10dB off the top of a snare hit with not much more than some fattening of the sound.
I think the above should make it clear that people who need more than 25W are not doing anything wrong. The only reason why you can say that more than a few 100W is overdoing it, is because at such levels the occasional clipping event isn't going to spoil the fun.
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