The imperfect amp: Pass or Ayre?

I believe John Curl refers to 7th order harmonics as the “devil’s 7th” and attributes a lot of what is heard as harshness to excessive 7th order harmonics.  

This fact has been known since the 1930s and probably earlier. The 3rd edition of the Radiotron Designer's Handbook, published in the 1930s makes mention of this particular fact- and that it has to be in much lower amounts than the lower orders in order for the amplifier to be listenable.

It would be nice to me if I could go from my preferred amps to specs and go "aha! This is what I like!" and therefore, buy the cheapest that fits. :D :D :D

@erik_squires 
What is needed (and entirely possible right now) is a weighting system for the various harmonic spectra. The 2nd and 3rd, since the ear is least sensitive to them having a weight of  'one', something like the 7th having a weight of 500 and the 11th or 13th maybe 1000. If we had a system like that we'd know what we're dealing with in a heartbeat, but don't hold your breath- the industry collectively would be really hating on something like that since its bad for business- what- a spec sheet that tells you how it sounds?? Yikes!!

To add to @almarg 's comments above, specs *are* important; perhaps the one that can be the most counter-intuitive is THD, as a very low THD spec is often an amp that doesn't sound nearly as good as one with a higher THD spec. I've already explained why above.
But bandwidth is real, output impedance is important (which isn't to say that the lowest output impedance is the best). Some speakers **require** that the amp have a higher output impedance, such as ESLs, horns and single-driver full range loudspeakers. Or any speaker where the designer has sorted out that he doesn't like brightness and harshness and so has designed the speaker for amps designed with intention to not have these properties (and often that means no feedback, so a higher output impedance). For more on this topic see:http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
Now I discovered back in the 1970s that feedback wasn't all it was cracked up to be; that it could cause the amp to have a smaller soundstage (unlike the original recording; I know that from the use of master tapes) and be brighter than the actual recording as well. Since then I learned there is far more to it than just that simple concept- the use of feedback introduces about 7 different variables that the designer has to weigh out. FWIW the feedback formula doesn't offer a lot of guidance in that regard: in a nutshell most amps with feedback have a rather cursory application and ignore some factors (like RFI leaking into the amp via the speaker cables) entirely.


So of the specs that are suspect as to how much they contribute to good sound in an amplifier, IMO/IME the two top suspects appear to be THD and output impedance (IOW a better sounding amp might be one where these specs have higher rather than lower numbers).


Alternatively we also know that IMD is highly audible so low IMD specs are a very good sign but IMD is more pervasive than most audiophiles like to think about. In addition to the normal interactions between two tones, IMD also is a major issue in digital audio, but because the intermodulations are having to do with the scan frequency and the Nyquist Theorem, its called 'aliasing'. But as far as the ear is concerned, aliasing (the digital industry does not like to refer to it as a distortion, but that's exactly what it is) is an intermodulation (sometimes called 'inharmonic distortion' in the analog world) and so highly audible (the ear treats it as a brightness or 'crispness' and its still there if you turn down the treble, because the brightness is caused by distortion rather than frequency response). Of course if you look at the specs of a digital product, this form of distortion isn't listed. Again- the Emperor's New Clothes. Fortunately the digital world has been getting a better handle on this in later years; the more they do so the more natural digital is able to sound.

It’s advertising BS by Ayre to say "no Feedback!" on a solid state amp.

This statement is false. Although there are not many examples, it is possible if build a solid state amp that lacks any sort of feedback whatsoever.

Both Pass and Ayre are two amps which eschew technical perfection.


Both Ayre and Pass stand in contrast to say, the Halcro amplifiers, which were famous for their absolute dominance in technical terms. Ayre and Pass however have ended with different followers.

@erik_squires 
IMO/IME you've got this backwards. Ayre and Pass are great successes, because they sorted out what is important to the human ear and applied engineering to design a circuit that more closely obeyed the rules of human hearing/perception. This was done in the face of a test and measurement regime that ignores how the ear works and instead concentrates on measurements that have little to do with how the ear perceives sound.

Now if you are coming from the perspective of 'objective measurement' this will come off as a bit of heresy. But the simple fact of the matter is 'objective measurement' isn't objective; at best appears to be a mindset that just gets really riled up when things don't go its way. To this end if an amplifier circuit is designed to sound good to the human ear, it gets lambasted for 'high distortion' in the face of what we know at this time of how the ear perceives distortion. 

So a bit of a primer: lower orders (2nd, 3rd and 4th) are perceived as 'richness', 'bloom', 'warmth' etc. But they do more than that, but first its important to understand that the ear perceives the higher ordered harmonics as harshness and brightness- in particular the odd orders. Now the ear also has a masking principle- how a louder sound masks the presence of a quieter sound. In the case of distortion, the lower orders are useful in masking the higher orders. This is important as the ear uses higher ordered harmonics to sense sound pressure (which is very easy to demonstrate with simple test equipment).

So imagine what happens when feedback suppresses the lower orders (and for that matter, the higher orders too) but in the process **adds distortion of its own** through a process called 'bifurcation' (which, not by coincidence, is a common term to those familiar with Chaos Theory). Also not coincidental is the fact that some basic Chaotic systems have mathematical formulae that are strikingly similar to the feedback formula!

So in an amplifier that employs feedback and thus has low THD, the remaining THD that is *does* have is mostly there from the operation of the feedback itself (again this phenomena is well-known; see the writings of Norman Crowhurst from 60 years ago for more). So this distortion, while low, has nothing to mask it (the lower orders are suppressed) and so easily stands out to the human ear as brightness and harshness. Some people describe it as 'crisp'.


Put another way, an amp that has higher levels of higher ordered harmonic distortion will sound smoother and not as bright **if** it also has lower orders (in particular the 2nd or 3rd) to mask the higher orders. This is why tube amps have higher overall distortion across the entire spectrum yet sound smoother and not as bright; this having nothing at all to do with bandwidth, since the ear is converting that distortion into a tonality. In a sense the tubes are very good at obeying human hearing perceptual rules- that is why they are still around.


So neither Nelson or Charlie were/are going for a tube sound so much as they are going for a **natural** sound. The problem is simple to any pragmatic designer: its impossible to build a circuit with zero distortion! So next best isn't to make an amp that is as low distortion as possible, the next best is to make an amplifier that has the lowest possible **perceived** distortion, which is something very different. Since the ear is relatively insensitive to the lower orders. such a design makes use of that fact and the masking principle to create an amplifier that actually sounds neutral (IOW, more like real music).


Something peculiar here is that amps with a prominent 2nd or 3rd harmonic also are perceived as more detailed with a more palpable sound stage- better center-fill. Why this is so isn't well understood and might be an excellent topic of inquiry.


So you've singled out the two most commercially successful solid state amps that address these issues!. They aren't failures!- instead they are **leaders** which will be historically recognized as such when the rest of the industry sorts the above facts about human hearing. But I'm not holding my breath on that because the spec sheets are some of the best advertising any inferior amp manufacturer has at their disposal; 'see?- very low distortion- it has to be great!'; an excellent example of the Emperor's New Clothes. If things were otherwise we could simply look at the spec sheet and know how the amp sounds and we wouldn't have to audition it in our systems to know it would work. In a nutshell, the industry has been lying for at least the last 60 years- and we've all grown up with that so don't see it as weird that despite the spec sheets we still have to do an audition.