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Interesting topic...some thoughts

Posted by morricab (S) on September 15, 2016 at 02:13:14

In Reply to: How much power do you need? posted by PaulF70 on September 9, 2016 at 17:25:09:

I have long been a proponent of using lower power amps and SET in particular whenever possible.

more than 10 years ago I was using a 30 watt KR Audio VA350i on a pair of Acoustat Spectra 2200s that had a sensitivity rating of 84db. It worked beautifully up to the upper 90s db range, which as you rightly mentioned in a room is LOUD.

I have also seen demonstrations where the speakers were not getting so loud but were sucking hundreds of watts from the speakers as visible on their front panel power meters. The speakers in question had a similar sensitivity rating to the electrostats (perhaps a db or two more even).

My current horn speakers (Odeon La Boheme) will rock the house with 20 watts and probably a lot less (my SETs are all 20+ watts so I don't know about significantly less than this). THey are around 98db/watt and I sit around 3 meters (10 feet) away from them. My small Odeons (Rigoletto) will play quite loud with the same 20 watt amp in a bigger room despite having a sensitivity of "only" 93db/watt. Both speakers, even going full tilt have nearly zero driver movement as the bass driver of each (both are two-ways) is back horn loaded and the tweeters are horn loaded as well.

So, what gives here? I think power compression and crossover losses are the main culprits for a majority of "normal" box speakers.

This box speaker that was still playing at tolerable levels even though the meters were reading hundreds (like 500+) watts seems to strongly indicate that the speakers were not increasing in loudness by 3db with every doubling of the power. Probably at some point they simply were not getting any louder and the energy is wasted as heat.

Why my electrostatic speaker probably worked ok with 30 watts was a few reasons: 1) They do not suffer power compression as there is no voice coil to heat up, 2) They were line sources and as such the SPL drops more slowly with distance than a point source 3) THe room was not huge so there is likely significant room gain. I actually measured the drop from the front of a panel to my listnening spot (just over 3 meters) and it was only about 1db. With two panels you get then about 87db for 1 watt, 90 for 2, 93 for 4, 96 for 8, 99 for 16 and 102 for 32. I measured peaks with my Behringer DEQ2496 and calibrated mic in the high 90s without any audible distortion (I didn't have a scope hooked up, alas).

As speaker with significant power compression might be fine for the first few watts but then as the demand to go louder it will compress. This is happening both on steady state and to some degree on sudden peaks as well when the current goes surging through that voice coil it will heat up very fast. A complex crossover will exaccerbate the losses before getting to the voice coils.

I experienced this first hand in the past with Dynaudio speakers. I had a pair of Dynaudio Contour 1.8MkII speakers with about 85db/watt sensitivity. They would not play soft well at all, sounding lifeless and gray. Turn the wick up a bit and they would sound quite good...for compressed pop/rock music and some jazz but not classical at all...the soft stuff just got lost. Turn the wick up some more though and the speakers started to sound compressed with dynamic peaks lacking dynamic punch...the soft stuff would work better but crescendos became a compressed congealed mess. This was the same with several amps I tried...from 500 watt beasts to 35 watt EL34 tube amps.

This speaker has a "comfort" zone of around 80-90db where it sounded quite good and this meant it was tailored to pop music and some Jazz. Rock music played really loud didn't work that great either. Classical was hopeless. Switching to electrostats (AudioStatic ES100s) fixed the low level resolution issues I had and allowed unrestricted dynamics up to the physical limits of the speaker, despite having about the same nominal sensitiity. Jazz and classical became a joy to listen to for a change.

For me, the speakers that "need" 500+ watts are the ones to avoid because they are actually not really delivering the sensitivity they claim beyond 1 watt where they were tested. The normal math (3db for every doubling of power) shows you can get quite loud with a handful of watts but this is only true IF the speaker is really getting 3db louder with each doubling...I suspect most are compressing badly and don't get close to this.

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