And so we move neatly to the subject of amplification. The amplifier in a system is generally dictated by the speakers, as different speakers have different requirements.
Broadly speaking, the majority of modern commercial speakers are in the region of 86-90dB/W/m, which is what I'd class as "mid efficiency" units. These can be driven to modest levels by almost any amplifier, but ideally 20 watts or more, depending on how wild the impedance curve is.
Heading into the realm of drive units designed for high SPLs (a typical example would be for PA use) are units in region of 95-100dB/W/m... I class these as "high efficiency" drive units, which typically have very powerful magnet systems and lighter diaphragms. There is a penalty (other than the typically high price) in that the lighter cones often lack the critical damping of lower efficiency units, which can lead to considerable colouration in the sound... however, the gains can be worth it.
Once you are beyond 100dB/W/m, you are in "super high efficiency" territory. While even a 100dB drive unit may only be technically a few percent efficient in terms of converting electrical power into sound, these units are vastly more efficient than a conventional unit due to the log scaling. The use of horns in particular can allow (given enough space!) very high SPLs from only a handful of watts. If you are prepared to spend serious money, then 110dB/w is feasible! At this point, an amplifier is practically unnecessary - a liability, even.
Back in the real world, we have limited resources, and without the space for large horns (or wanting to deal with their own set of issues), "high" efficiency is a reasonable goal.
My own speakers are essentially divided into two. A bass driver operating essentially in free air, which is very inefficient, and a midrange and HF unit of relatively high efficiency. While the bass driver requires a powerful amp, ideally of at least 100-200W in power handling, the midrange and HF unit are never likely to see more than a watt in typical use.
You can further lower the workload on the MF+HF amplifier by taking advantage of the fact that there tends to be more musical energy at the low frequencies than the high. By moving the high pass filter from the speaker crossover to before the amplifier input, you can reduce the load by 3dB or more, depending on the music. Do note that this is fine for a midrange like the TD15M Apollo which has insane (>500W) power handling as even if the amplifier goes DC, the speaker won't care, but it's a really bad idea to DC couple any kind of high frequency driver without suitable protection in place. Be warned that it can be an expensive lesson...
The high power demands of the bass driver realistically limit choices to a Class AB or a Class D solid state amplifier, which aren't very interesting from a purist point of view, and this drive unit is only covering a few hundred Hz with fairly quick rolloff, so let's not concern ourselves with that for now. What is interesting is the watt for the MF+HF, which covers roughly 250Hz onwards... the bulk of the sonic spectrum.
So we want a good watt... how to get it? The immediate answer is Class A operation... whether it be a single output device or a push pull pair, "always on" operation yields the lowest distortion, but unfortunately also the lowest efficiency. For a single clean watt, we can sacrifice efficiency and still keep power consumption within manageable limits.
(Do note that I've seen a Class D amplifier that idles at over 20 watts, so
it can be dangerous to make assumptions purely based on topology!)
There are some very well known Class A designs that put out a few nice watts, and I've been endeavouring to build them up to try with the speakers... time to build, listen and measure... in that order. :)
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