Questions re:  GaNfet technology vs other designs.

you are twisting my words to attempt to win an argument. Clearly I said your OTL amplifier cannot be neutral compared to a high damping factor amplifier. You have said nothing that disproves that.

I'm not twisting your words, simply pointing out that without direct experience you simply are not in a position to know the truth of the matter; i.e. you are wrong. Not a judgement or anything, just didn't get the 'facts' right is all (and still abundantly clear you didn't read the article at the link I provided).

The reason the two amps sound similar is the distortion spectra is very similar (although fir entirely different reasons). The reason the class D sounds a bit more transparent is the distortion is quite a bit lower. You have to listen closely for the difference; the easiest way to hear it is that the class D is a bit better at bringing out detail in the rear of the soundstage (distortion obscures detail so this is no surprise). I'm by no means the only one to say this; our customers that have had both amps in front of them say the same thing.

To be clear there are many speakers were this won't be the case! But I didn't stay in business nearly 50 years selling amps to people when I knew they wouldn't work. That leaves a lot of speakers out there on which our OTLs are quite neutral. I refer you to reviews, as there are quite a lot of them...

Does it sound loud. Of course it does, even if the distortion is low,

A lot depends on what is meant by 'low'. In the case of whatever you were thinking of, apparently not low enough. When its a system you of course can't ascribe the 'loudness' being caused by the amps or speakers, but its pretty safe to say that in a proaudio situation where sound pressure is favored over fidelity, its going to sound loud.

I can't help you with not getting the point of my comment about loudness; FWIW a lot of people reading this thread will get it.

My statement is absolutely true. Yours is not. I said it is impossible for your OTL amplifier to sound neutral compared to a high damping factor amplifier with any speaker that does not have a flat impedance curve.

And the fact of the matter that given the right speaker which may or may not have a flat impedance curve the OTLs can sound perfectly neutral (and to be clear those speakers are the exception rather than the rule). Just FWIW, I have the advantage of you on this point- we make an amplifier that has an output impedance that's difficult to measure and we can compare it to our OTLs and do so on a daily basis. Clearly you didn't read the article at the link I posted. Maybe you should. Or not... Tempest in a teapot, that sort of thing 😁

Let us unpack this fully. You are claiming that harmonic distortion 105 db below the fundamental is audible. Let us take an example of a 90db/watt efficient speaker, with 200 w/channel, what most would consider pretty loud. That is 90db/watt at the speaker. Let’s say 8 feet distance in a typical room.

Ever hear a system that sounds loud? If you work to really get rid of those pesky higher orders, you'll find that even at high sound pressure levels on a meter that the system no longer sounds 'loud'.

Put another way the sign of a good system is that it does not sound loud at any volume- it will always have a relaxed character. What you are forgetting is the ear converts distortion to tonality and it also perceived higher orders as loudness. This is why SETs can sound so 'dynamic' and its the source of brightness in most transistor amps. Easy to demo, FWIW... and is also why the right measurements on paper have a directly line to our listening experience (and likely why most of the time they are either not made or not published; I think the industry really wouldn't like people knowing what equipment sounds like by looking at the measurements...)

Its funny how so many people think the ear stops using its perceptual rules beyond an arbitrary point on a bit of paper.

Distortion is audible when a high enough level, end of story. I have also been clear that distortion is less detectable with music. I have also been clear that frequency plays into distortion audibility. There is no contradiction in anything I have said.

@theaudioamp Hm. On that basis we’re on the same page. Since you seem to think we are not, I’m disposed to think that you’ve not stated your position all that clearly in the past. There is also the issue of what is meant by ’when a high enough level’. IME that means well over 105dB down. Most amps can’t do that. The bit that I think gets ignored here is the fact that the ear uses the higher ordered harmonics to sense sound pressure, and due to a +130dB range is far more sensitive to them than most people think.

Bruno Putzeys points out that we’ve been hearing the brightness of solid state since its inception. So if we go back about 20 years or so its safe to say that at that point no amp prior employed enough feedback as that was the basis of his statement; in fact largely the reason he wrote the article!

No one disputes this is the case. However, you fail to provide evidence to support this is at all an issue.

😁 If that were the case Crowhurst might have spent more time at golf or something. Do you see how this statement contradicts itself?

You also ignored my very salient point that masking of lower order harmonics coupled with hearing degradation and the slope of Fletcher Munson curves means out past 4KHz, if not lower, higher order harmonic distortion becomes less of an issue.

You can’t be serious! IME/IMO you got it backwards. Making sure the harmonic content is correct in the Fetcher Muson range is crucial. Not only do you have a lot of higher ordered harmonics in that range but its also the region at which the ear is most sensitive!

w.r.t. Bruno, I will link his article here: https://linearaudio.net/sites/linearaudio.net/files/volume1bp.pdf which is a good dissertation on distortion and why negative feedback is good, which I am certainly not disputing, and which most SS amplifier vendors use enough of (as noted in measurements by Stereophile and ASR).

OMG... Bruno is essentially pointing out that most of those amps don’t have enough feedback, and shows just exactly why. Did you not read the article??

This is why as engineers off the cuff statements should be avoided as most Purifi amps are front ended by just those same type of circuits buried in op-amps with enough feedback not to be an issue.

’just those same type of circuits buried in op-amps with enough feedback’ as you put it does not seem to make sense. With any of his modules you simply need an opamp circuit that can drive the input of the comparitor; you don’t need that ’same type of circuits’ thing (which seems to suggest more than just opamps) you came up with. Its hard to interpret what what you meant there; FWIW Bruno spent a bit of energy in that article explaining why a certain capacitor was used in conjunction with degenerative feedback in the differential input amplifier of conventional amplifiers, none of which would be used in an input buffer to one of his modules. That’s the only interpretation I can come up with for your remark; if I misinterpreted that I apologize. Otherwise I agree that if a purely opamp circuit is used as a buffer, the feedback on them renders them entirely neutral. But we are not talking about Bruno’s amps, FWIW...

Unless you have a speaker with a near flat impedance over frequency, it is impossible for that to sound neutral compared to a high damping factor amplifier.

This statement is simply false- we’ve compared exactly that side by side. I concede that you do have to be careful about the speaker that is being used. But it does not have to have a flat impedance curve! As I pointed out in the article to which I previously linked, the speaker needs to fall into the Power Paradigm of design rules (an example is a speaker designed to work with SETs, which have a similar output impedance to our OTLs).

Last, keep in mind, that many of these claims of distortion, at the levels of the amps in question causing tonal changes, has never, what 50 years later, be shown to be fully factual even though experiments are readily done.

@theaudioamp I see you dropped the idea that a GaNFET amplifier by virtue of GaNFETs must have load dependent frequency response. That's good- Occam's Razor wasn't supporting that idea at all. 

We seem to be seeing two messages from you- that harmonic distortion is and at the same time is not audible. If  you will set aside the handwaving for the time being, the ear's sensitivity to higher ordered harmonics has been known for a very long time- I refer you to the Radiotron Designer's Handbook, 3rd edition, published in the 1930s. The tonal effects of something like the 7th harmonic are easily explained in musical terms, for the obvious reason of the 7th having a very specific tonality with regard to the fundamental tone.

Regarding the effects of insufficient feedback, I refer you to Norman Crowhurst, who wrote about how feedback adds distortion of its own (including IMD) back in the late 1950s. At the time, no amp existed in which sufficient feedback could be applied, so he wrote about this in the context of all amplifiers. He also explains why: non-linearities at the feedback node. You can find his work on Pete Miller's reference book website.

Crowhurst's position on this topic is reinforced by that of Bruno Putzey's excellent article on the same topic. Seems to me I've referred you to that article before...

Finally, the tack we've taken over the years with our OTLs seems to have worked. FWIW they sound very much like our class D, which is to say quite neutral. From your comments it appears that you did not read the article at the link I provided.

Read my words carefully. "MAINLY" comes from a non-flat frequency response. That is especially true of most tube amplifiers such as the high output impedance of your own OTL amplifiers.

@theaudioamp Our OTLs are intentionally designed to work without feedback and so do not operate on the principle of voltage driven loudspeakers. Like SETs, you have to be careful of the loudspeaker used to get proper response that isn’t colored. See http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html for more information. This is done with intention since its impossible to apply the kind of feedback you need to really get rid of distortion that is otherwise generated by the application of feedback itself (this occurs because the feedback node is always non-linear, which means that the feedback signal itself gets distorted before it can do its job). As Norman Crowhurst pointed out decades ago this causes generation of higher ordered harmonics. You need +30dB to get around that problem and since tubes won’t have the gain bandwidth product to allow for 30dB at all audio frequencies its a Sisyphean task. So zero feedback is how the harshness of feedback is avoided since in a tube amp you still have pretty good linearity if you’re careful.

So the OTLs were not a good example. If the amp has enough feedback or else a low enough output impedance to act as a voltage source then you really have to ask yourself why one can sound bright while another does not- because if its acting as a voltage source there is inherently no frequency response variation.

If you’ve not found any information about why the higher ordered harmonics are so audible when in such small amounts, its easy to demonstrate with simple test equipment. IOW you’ve not looked all that hard. The ear uses the higher orders to sense sound pressure. If there are higher ordered harmonics added to a signal, they will be perceived by the ear in two ways: louder, and also harsher. Because of ’louder’ that also means brighter (and Mr. Fletcher and Mr. Munson don’t help). If you need to know the procedure to demonstrate this to yourself let me know.

The ear/brain system has a variety of tipping points in its perception of sound. Since it assigns a tonality to all forms of distortion (the ’warmth’ of tubes for example being caused by the 2nd and 3rd harmonic) it will pay more attention to this sort of tonality than it will actual FR errors. You might be looking in the wrong place if you go to ASR and Stereophile; as best I can make out they don’t seem to have made the connection (yet) between measurements and audibility.

Don't let yourself fall into the trap of "musical" / engaging. Mainly that comes from a non-flat frequency response

@theaudioamp This statement is incorrect. The ear converts distortion into tonality; actually the reason some amps sound musical and others do not is the distortion signature of the amp (often referred to by audiophiles as the 'sonic signature') rather than a frequency response error.

This is why you can measure two amps on the bench that are equally flat in frequency response but one will sound bright and the other doesn't. Higher ordered harmonics (the 5th and above) are perceived by the ear as harshness and brightness if they are not masked by lower ordered harmonics. This is a problem with most solid state amps made and why tubes are still around. But if the solid state amp has the right distortion spectra (lower orders dominating) it can sound as smooth and musical as a tube amp, yet will have nice flat frequency response.

I noticed there are several GaNfet technologies power amps available but not many integrated amps. I wonder why. 

@hgeifman The most likely reason is that they will be seen soon.