Articles for Audiophiles by Steve Deckert
AUDIO PAPER #143
WHY THE SET & HIGH EFFICIENCY SPEAKER APPROACH WORKS
by Steve Deckert
People who are unfamiliar with low power amplification, which is the majority of audiophiles, have a real hard time with the concept of 2 watts -- it seems like a complete joke to them. Perhaps this is you. It was me when I started the hobby so I know first hand the conviction behind the assumption that 2 watts couldn't get out of it's own way let alone control a large bass speaker or have any dynamics or headroom. All you have to do is read social media comments on the subject of low power amplification and you'll know this is a popular position.
Problem is, it's mostly wrong. Here's why:
Due to the gross inefficiencies of power hungry loudspeakers much of the first watt is not heard. A fair portion of it is spent as heat in the complex speaker crossovers and what's left is pretty hard to hear since the average efficiency of the woofers is 85dB 1w/1m.
How many loudspeakers have you heard that didn't really get with the program until you hit them with a little power... this is why. So when you consider most amplifiers are 100 watts or more, and most speakers are 86dB or less, you can assume that show is starting more with the second watt, not the first.
Why is this is big deal?
Consider this: a pair of 96dB speakers playing with one watt of power against the average noise floor in your listening room (41dB) is 50+ dB of dynamic range (96 – 41 = 55 dB). Adding a second watt of power from your amplifier only increases the dynamic range by an additional 3 dB. And for every additional 3 dB you add you will have to keep doubling the power. So if you think about it, there is over 10 times the dynamic range in the first watt as there is in the second and it dwindles rapidly from there.
Another way to look at it is that the 51dB of dynamic range in the first watt with a 96dB speaker would become 41dB of dynamic range (half as loud) on an 86dB speaker. Take away the losses in the crossover and the dynamic range can easily drop to 20dB. That means that with the 86dB speaker we can have lost around 20dB of dynamic range before we even get started.
How do you get that 20dB of dynamic range back that you lost? You get it back on the other end, you turn up the volume! That's not actually as easy as it sounds. 20dB would require over 120 watts of power with efficient speakers and in many cases your listening room's acoustical knee point is going to be well below 100dB meaning that at that pressure your room starts wrecking the sound quality. This means that you normally can't even turn it up loud enough to get 20dB of dynamic range back before the total SPL reaches the knee point of your untreated or casually treated listening room.
Now, I don't know about you, but the math suggests that if you're getting over 50dB of dynamic range from the first watt with 96dB crossoverless speakers whereas the 86dB speakers regardless of power can have as low as 30dB and in some cases even less than that pending the knee point of your room...
There is another element that you probably didn't consider.
Speed. Speed is how fast a loudspeaker cone can stop and change directions. Consider that you can have a 100dB 15 inch woofer that has a moving mass of only 17 grams and imagine how that would do compared to the average 86dB 15 inch hi-fi woofer with it's higher moving mass of 100 grams or more...
My guess is that the 100dB low-mass driver would stop 588% faster. That would mean that it wouldn't need butt-loads of power and tons of negative feedback from the amplifier to raise damping and 'control' the woofer because the low-mass high-efficiency woofer has a stiff suspension that barely moves. It doesn't need help controlling itself. It also has an under-hung voice coil where the voice coil is always within the magnet gap. A hifi woofer has a long-throw voice coil where 2/3rds of it is always outside the gap. This by itself becomes a filter, smearing time.
Without the negative feedback in the amplifier smearing the time and without long-through voice coils, leading edges of notes and transients in general can become superior to a point where there really is no comparison. If you want linearity at normal listening volumes in your home, than this is the most cost effective and best sounding way to get it.
Another way to look at it is that we want to reproduce the exact movements of the microphone diaphragm with a loudspeaker. That is the ideal. Sadly, the diaphragm in the microphone has such low moving mass compared to the speaker that it's almost silly to think the two would be anywhere near alike. A one gram diaphragm/coil assembly vs. a one hundred gram speaker cone/voice coil assembly would mean the speaker is 100 times slower in the bass frequencies.
In the case of the 17 gram moving mass high efficiency speaker, we are only 17 times slower than the 1 gram microphone. That's a rather profound improvement over 100 times slower. Of course this speed advantage shows up in the bass. Once you get into a 3 inch midrange driver or a 1 inch dome tweeter and the like, the speed advantage usually flips.
So in summary, the 2 watt amplifier on high efficiency speakers has:
A) almost twice the dynamic range. B) is almost 6 times faster. C) can be operated by superior sounding triode amplification without negative feedback.
This is why it sounds better. This is why the bass is better. This is why it hits harder. This is why it can sound convincingly real with a real sense of speed and dynamics. Some will make the argument that 900 watts of cost-no-object amplification into similarly priced multi-driver speakers turned up in a good room can get close or even match the dynamics, but sadly all too often there will be lots of negative feedback in the amplifiers keeping you from hearing deep into the music so even if the speaker sounded better- the amplification sounds worse --and the sound quality you chase is always limited by the weakest link in the audio chain.