new GAN amplifier

GaN are faster and present lower drive capacitance which theoretically allows for a higher switching frequency - which has encouraged many people including George. However, regardless of power device used efficiency is always inversely proportional to switching frequency.  Class D will get a bit better but will never quite be able to outperform the best traditional linear class A / AB designs above about 5kHz. However, in terms of bang-for-buck they are increasingly hard to beat as full range amplifiers and the best on offer are close to absolute best below about 500Hz. As was pointed out they don't have problems driving highly reactive loads - apart from treble range and up due to output filter interactions. 

In my current setup I'm using hypex NC500 amplifiers to power my woofers and an ultra LD class AB amplifier for mids / tweeters. A best of both worlds implementation.


 

A lower damping factor will, and especially in the case of the Alexia where one just has to look at the frequency response/impedance curve to see it, often correct peaks (valleys) in the frequency response. It is like a built in equalizer :-)

And .... most people don't know this, so I will not fault you, but high damping factor often results in more distortion in dynamic drivers. So no, high damping factor is not always better. Different yes, but better ... not necessarily so.

 A poor damping factor will raise the designed loudspeaker system Q to beyond the point of optimum damping, and because low frequency impedance is typically modulated by the signal itself distortion will always be lowest when driven by an ideal voltage source. Noting too, if damping factor is close to "1" by definition half the power is also being dissipated in the source impedance. That's definitely not desirable. 

Not surprisingly perhaps, 99.9% of high-end commercial speakers are not particularly amplifier friendly and are clearly designed to be driven from a very low impedance source (ie. ideal voltage source). 

Please compare data for your favorite top A/B amp U-LD Mk4, given by the creators at www.siliconchip.com.au (August 2015) and the newest D-class from Bruno Putzeys - Purifi 1ET400A (www.purifi-audio.com).

Best to use third party measurements such as these... https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-purifi-1et400a-amplifier.7984/page-1

Distortion (IMD) is still higher than a decent classic blameless AB design above 10kHz.

The Hypex modules I'm using sound pretty good as full range amplifiers, but with the AB driving mids / tweeters the fidelity goes to another level.  

As Putzeys himself has stated, Class D amplifiers sound good in spite of being class D, not because of it. 

>100kHz response?  Some species of bats indeed can detect 200kHz but they can't hear the human range sounds.
Then there is the question of transducers, I think I came across once a lab specimen that goes high ultrasonic. Then there is the problem of mics, top studio Telefunken U47 (9k $) goes only up to 20kHz.  So, other than Batman music recorded with lab mics and played by lab transducers it might be a tall horse to climb.

Of course we can't hear 100kHz. The desirability of a flat response out to >100kHz has to do with minimizing the side effects from the cascading of low pass filters starting all the way from the recording process itself - as you correctly identify includes tranducers too.  

Always need to be mindful of the time domain. 

As for subjective perceptions of audio, to paraphrase the local classic, "there are many, me including, who like tubes best".

Horses for courses. I don't care too much for tubes but I'm a fanatic for pristine treble, and that includes a ruler flat response out to >100kHz.   

Measurements of the purify look good but keep in mind those IMD measurements conducted at relatively low power and with a LPF in line. 

Btw, the new Halcro Eclipse is now the new "benchmark" for low distortion.   

All digital audio is built around the sampling theorem, not the other way round

I wasn't singling out digital audio, but it's worth mentioning the use of oversampling to remove the necessity for analog brick wall filters in AD-DA conversions. Class D amplifiers don't require brick wall filters but they do still add multi-pole low pass filters to an already band filtered signal path for playback of a wide range of recorded music that's already passed through a diverse range of low pass filtering even before it leaves the mixing desk for digital recording and mastering.  

For the record (no pun intended), my highest fidelity recordings are all high res 24/192 digital. To my ears they're the closest thing to real live music. 

Thanks George, my bad - typo.

Typically, in "eng" speak impedance is expressed as a value with a phase angle (polar form) or real + reactive (rectangular form).

EPDR is obviously a means of expressing power dissipated in the amplifier based on instantaneous values of voltage and current at a specified frequency. In other words, a reverse engineered value. 

EPRD isn't a term that's used in engineering circles, even if it's a more valid descriptor than than the invented term  "RMS power".

Any amplifier that relies heavily on NFB to present a low output impedance can be brought undone by a nasty capacitive load, and that includes Bruno's hypex modules, particularly when knowing that the output filter is also within the feedback loop. 

When time permits I'll hook up the sig gen and scope and run some tests to see how well the NC500's perform on a typical square wave test Vs a class AB design. 

It's probably worth making the point that a person who owns a pair of Alexia's is probably driving them with a decent amplifier. Most, if not all high-end class A-AB designs are usually very over-engineered to avoid device failures and to be able to drive the sorts of difficult loads that will inevitably be presented with ease. Also, when the difficult load is isolated to a relatively narrow bandwidth as is typically the case it's really not a significant issue.    

Like the Halcro Eclipse  with its incredible specs; 110kg/250 pounds, 135,000

In line with laws of diminishing returns, but it is the best if you think least distortion = best. Distortion @ 100W into 4 ohms is on par with small signal performance of the best op-amps.

If / when Halcro goes class D for their flagship amplifier, only then can it be said that class D is as good or better than AB. 

I once made the class D > DSD analogy on another forum and was met with enormous hostility from the Class D faithful.  

The believers provide the missing bias.  

I think I just saw a pig fly over my house.

A Class D amplifier is a "analog amplifier" with a two state (binary) output stage modulated by the output of a comparator that compares an analog signal with a reference waveform.

Ultimately I guess it boils down to the definition of "analog", but there is quite a big difference from a linear amplifier that simply "amplifies" a signal, and class D which recreates or reconstructs the signal.

In the strict sense, class D isn't an amplifier it's a "reconstructor".