Atma-Sphere Class D Mono blocks

[DonH50]

You are on to something.

The variable to which you refer is called 'Gain Bandwidth Product'; you need a lot of it to support a lot of feedback. If you don't have enough, distortion will rise with frequency since the feedback is decreasing with frequency. Almost all amplifiers made prior to year 2000 have this problem and it results in harsher sound.

If you've wondered why tube amps with feedback have that 'sheen' on the strings and the like, this is part of why.

In a class D amplifier you can run a lot more feedback than you usually can in a conventional solid state amp. This is because the output devices are so fast (even if MOSFETs) that the GBP is really quite high. The kind of amp we make is of a class called 'self oscillating' . How a self oscillatiing amp works is you apply so much feedback to the amp that the phase margin of the amp is grossly exceeded and the amp goes into oscillation as soon as its turned on. The feedback network is designed so that a certain frequency is favored for oscillation, and this frequency is the switching frequency. Now we have a whole lot of feedback to use, and unlike most tube and solid state amps, the feedback isn't applied to a non-linear node in the circuit that can distort the correction (feedback) signal before it can do its job! Its mixed thru a resistive divder network instead, so the feedback is not only more linear so more able to do its job, but there is also so much of it that its able to clean up a lot of the problems that lessor amounts normally cause (which is why feedback has such a bad reputation).

The thing is though that the output filter prevents the amp from really having actual wide bandwidth. It also introduces phase shift in the upper region of the audio band. But since we are running so much feedback the phase response does get correction. So generally speaking, even though a self-oscillating amp has a very good GBP figure, it really does not have a lot of bandwidth.

The non-linearities caused by the deadtime in the output section tends to result in lower ordered harmonics. The same is true of the non-linearities in the encoding scheme. So this, combined with accurate feedback and a lot of it, results in an amp that really doesn't make a lot of the irritating higher orders.

Thanks Ralph, that matches my understanding, and hypothesis. My class D designs (mostly not audio, RF) were before self-oscillation became widely used so I have very little direct experience with that. I do have experience with their harmonic structure and that's what led to my thoughts vis a vie the audio world. Thank you for a very thorough explanation and covering a lot of the details I glossed over or left out of my post (my bad, not thinking of the wider audience).

 

[DonH50]

Professor Don,
I don't' mean to be a smart aleck.

You are not, that is a very good question!

By most accounts the absence of the harmonics you describe would be counterintuitive the notion that they sound tubelike? Or did I sleep through that class?

A good tube amplifier doesn't have much distortion, albeit more than typical SS designs; that was one of the benefits of early Audio Research designs, for instance. They had lower distortion than many (most?) competing amps and were praised for that. One of the things we anal engineer types have to keep in mind is that 1% (-40 dB) distortion was for decades the standard for "high fidelity", and is still a reasonable threshold for music and such. It is hard to hear distortion much less than that for most music (for me it takes pure tones like a flute or some such). We see numbers in the -100 dB range (0.001%) and below now, but that is well below audibility. Speakers exhibit much higher distortion...

Back on topic, Ralph (@Atmasphere ) said the deadtime* of a class D output stage results in lower-order harmonics, so they are not zero and thus add some "tube-ness" (though probably lower than a typical tube amp -- he'd have to answer that). Whilst some studies have shown a little 2nd harmonic can be "pleasing", other studies dispute that; outside my ken. But higher-order harmonics, say 5th or 7th and up, lead to both harmonic and intermodulation distortion that sounds harsh to everyone. So if his new class D design (like several others) has much less of those higher-order harmonics, it will sound better, and potentially more "tube-like". Lower low-order harmonics, yes, but the lower high-order harmonics are key IMO/IME to "good sound" and a failing of many SS amps.

HTH - Don

* The deadtime is required because you have two devices switching between negative and positive voltage rails (one for each), or the positive rail and ground, so you must allow a little ("dead") time to allow one device to switch off before the other switches on. If they overlap, thus are on at the same time, you are shorting the power supply rails to each other (or to ground). Bad idea. You'd like that dead time to be as short as possible, and that is one reason Ralph and others are using these fancy new GaN (gallium-nitride) transistors -- you can switch them faster, and they have less charge storage, so less dead time, smaller switching glitches (which are well above the audio band), and faster switching speed. It also makes the amplifier more efficient, though class D is already pretty durn good.

 

[PYP]

I've had my system fool me a few times. Its spooky when it does- especially when its someone singing. I like to think I don't have strangers in my house when I'm at home listening alone...

While engagement with music is my main goal, it is interesting when that kind of realism occurs. I was in another room while the music was playing and a musician introduced the song. I thought: who the hell is that? Took me a few seconds to realize there wasn't a stranger in the house (the cat did a double take too). That was something like a party trick, but it does give an idea of how realistic vocals can be via a well-designed DAC and Class D amps.

Tubelike? I don't know. Even though I've had tubes in the past, my brain doesn't make that comparison. Close to the live event? A good enough simulation to be highly enjoyable.

 

[PYP]

Regarding preferences. The topic reminds me of a study in which babies were fed either a sweet or salty formula. The babies liked the particular formula they were given. But when the formula was then switched (a baby accustomed to sweet was now given salty), they were not happy. Perhaps tubes vs. solid state is like that.

Following that logic, I should prefer Classical music played in mono via a turntable and tubes, concentric tweeter/midrange driver and a highly simplified "crossover" (if it could be called that) for a single, very large bass driver in a washing machine sized enclosure. Not my thing, but surely the quest for a very dynamic and realistic presentation has stayed with me.

 

[Holmz]

If I want bling, I can go with the McIntosh or D'agostino meters. Form and function. I must admit a flagship of Atmashere tubes does warm my heart. The meters are not that pretty.

You know what sounds tubelike? Tubes.

I am not sure I agree.
The last two preamps I had were tubes, but they did not have any obvious tube sound.

However the last two power amps did.
(But I am not sure either of them were anything to write home about.)

Or… maybe it is easier to make tube gear with a higher distortion? And especially so with power amps?


What is a tubelike sound?
I know what a grainy and sibilant sound is, which largey went away with tube preamps.

 

[Ron Resnick]

Ralph, unboxed,

The fact is that I have not heard Ralph’s Class D amp. I just have trouble believing a priori that I will hear from the Class D amp the liquidity and breath of life I hear from tubes.

But without doing the audition I concede that my presumption is mere prejudice.

 

[Gregadd]

No, but I would ask the judge to quote the sentence in full and according to WBF quoting system!

Hardly anyone quotes in full. I should have put usual text omitted}. I did not think it was necessary since my post immediately followed yours. I am happy to edit my post.

My substantive point is I love audio equipment too. without music it would be merely expensive "boat anchors." IMO YMMV

 

[Gregadd]

Ralph, unboxed,

The fact is that I have not heard Ralph’s Class D amp. I just have trouble believing a priori that I will hear from the Class D amp the liquidity and breath of life I hear from tubes.

But without doing the audition I concede that my presumption is mere prejudice.

Or you may simply be influenced by inconsistent arguments. You don't need to circumnavigate the earth to know it's not flat.
The question is why does Ralph need to compare his class D amp to tubes? Is the goal tubiness or musicality?

 

[Gregadd]

I am not sure I agree.
The last two preamps I had were tubes, but they did not have any obvious tube sound.

However the last two power amps did.
(But I am not sure either of them were anything to write home about.)

Or… maybe it is easier to make tube gear with a higher distortion? And especially so with power amps?


What is a tubelike sound?
I know what a grainy and sibilant sound is, which largey went away with tube preamps.

I think we are quibbling over semantics. I could care less whether an amp is based on tubes, solid state or class A/B, B or D. I heard there may be a Class G amp in the works.
However, in my decision to purchase I have no desire to start from scratch. Thus, certain designs don't make it pass the first round.
I will not bother to name them. Some tube designers have managed to work around some of the faults of tubes. But they have retained thier benefits.

 

[Hear Here]

But without doing the audition I concede that my presumption is mere prejudice.

Yep - there's far too much of that regarding Class D. Many won't even consider the notion. More fool them.

 

[Holmz]

I think we are quibbling over semantics. I could care less whether an amp is based on tubes, solid state or class A/B, B or D. I heard there may be a Class G amp in the works.
However, in my decision to purchase I have no desire to start from scratch. Thus, certain designs don't make it pass the first round.
I will not bother to name them. Some tube designers have managed to work around some of the faults of tubes. But they have retained thier benefits.

^Well put^ sir.

Most of the nice stuff, just ends up sounding similar to my ears.

 

[Gregadd]

Just one need to repeat myself. I often get dragged various versions of the tube v solid state debate. The other side often is willing to concede that tubes add a certain pleasing warmth to the sound, albeit as a product of distortion. Tubes and what they provide have been detailed over my audiophile career.
The good news is that the "ears have it." Theory is nice but sound eventually triumphs.

 

[Gjo]

Ralph, unboxed,

The fact is that I have not heard Ralph’s Class D amp. I just have trouble believing a priori that I will hear from the Class D amp the liquidity and breath of life I hear from tubes.

But without doing the audition I concede that my presumption is mere prejudice.

Ron, since you're in the same metro area as AGD Productions (located in Marina Del Rey), have you ever auditioned any of Alberto's amplifiers? Doing so might put a new lens on any prejudice you have. I know it did for me.

 

[Atmasphere]

We see numbers in the -100 dB range (0.001%) and below now, but that is well below audibility. Speakers exhibit much higher distortion...

Back on topic, Ralph (@Atmasphere ) said the deadtime* of a class D output stage results in lower-order harmonics, so they are not zero and thus add some "tube-ness" (though probably lower than a typical tube amp -- he'd have to answer that). Whilst some studies have shown a little 2nd harmonic can be "pleasing", other studies dispute that; outside my ken.

I am firmly convinced that -100dB isn't far enough down! I think if you ignore the distortion spectra (as a designer) then it has to be more like -110dB. The problem being that the ear uses higher ordered harmonics to sense sound pressure and has a range of about 120-130dB. Add to that the simple fact that harmonics sculpt the sound of instruments and you find that the typical amp with 0.001%THD still sounds bright and harsh.

The question is why does Ralph need to compare his class D amp to tubes? Is the goal tubiness or musicality?

I wouldn't except for the fact that we've been in business for nearly 50 years (next August will mark the gold) and all that time have been known for making tube amps (although we think our preamps are a much bigger statement than our amps are...). So right away I've been getting pushback (which is entirely understandable; tube aficionados like their tubes and anything not tubes can threaten that worldview) so the best way to describe what we've been hearing has been in relation to our own tube products, to which people actually ask if we've done comparison! As you pointed out earlier, there have been a number of solid state amps reviewed over the last 30 years that have been described as 'tubelike' and at this point its a boy that cried wolf thing.

My goal with the OTLs has always been neutrality and the class D is another step in that same direction but I can't stand 'neutral' if it is irritating (that seems to be of no concern to some designers...). The bottom line there is whether I have any street cred or not.

 

[iansr]

I’ve come to understand that there are 2 categories of amp that have the potential to float my boat. One of the key ingredients of the first type is that it has extremely low levels of distortion. I have some of Tom Christiansen’s Neurochrome amps. They are the most neutral and transparent sounding amps I’ve ever heard and that kind of open window on the music has its own set of charms. These DIY amps would embarrass many commercial amplifiers costing orders of magnitude more. The Wolverine amp recently designed by a group of enthusiasts on DIYA may pull off an even better disappearing act, if that’s possible. (I have the PCBs to build the amp.)

The other category comprises amps that have a particular distortion profile, specifically one where the 2nd harmonic is dominant and the higher order harmonics successively decrease in amplitude. I believe the BBC did some research in the 40s or 50s that concluded that this sort of profile was favoured by many test listeners. Jean Hiraga conducted his own experiments and reached a similar conclusion. You’ll find modern day designers who also subscribe to this school of thought. Hugh Dean (AKSA) designs all his amps with this goal in mind and Nelson Pass has even designed a DIY distortion generator which you can use to add variable amounts of 2nd and 3rd harmonic distortion to your audio chain.

So Ralph’s comments about his amps resonate with me and I’d certainly like to hear them. Not sure that’s possible in the UK though . . . .

 

[Gregadd]

[text omitted] right away I've been getting pushback (which is entirely understandable; tube aficionados like their tubes and anything not tubes can threaten that worldview)" Atmasphere
Pushback is putting it milady. I have been avoiding this long enough. It is you who touted tubes as the best. Some lof those "tube aficionados" own your amps that you still market. It is you who are going to have to justify your 180-degree pivot. A difficult but not impossible task.

 

[Atmasphere]

[text omitted] right away I've been getting pushback (which is entirely understandable; tube aficionados like their tubes and anything not tubes can threaten that worldview)" Atmasphere
Pushback is putting it milady. I have been avoiding this long enough. It is you who touted tubes as the best. Some lof hose "tube aficionados" own your amps that you still market. It is you who are going to have justify your 180-degree pivot. A difficult but not impossible task.

I agree.

And that is what I have been doing, for some time now. As I've mentioned before, I have come to the opinion that class D now offers a means of having a system as musical as that of any tube system, but more transparent, making power tubes obsolete. I still think there is a place for small signal tubes in case anyone noticed my wording.

Of course the ultimate 'justification' will be to simply listen to a set, properly burned in. The only area where this can be dicey is if the loudspeaker used is one that favors amplifiers with a high output impedance (see http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html for more information). If the comparison is done on such a speaker, there may be a bit more bass with the tube amp due to the impedance curve of the speaker. This will result in a warmer presentation. But its not because the class D is lean, its simply the speaker interface. We don't run into that problem with conventional speakers, which is 99% of the market. But that 1% has been where a lot of our market has been!

 

[the sound of Tao]

Ralph, unboxed,

The fact is that I have not heard Ralph’s Class D amp. I just have trouble believing a priori that I will hear from the Class D amp the liquidity and breath of life I hear from tubes.

But without doing the audition I concede that my presumption is mere prejudice.

This has been a painful issue of this thread where much much more than usual there has been the claims and repeated insistence that only already fully signed on new Class D acolytes can be the only ones who know the truth in how these Class d amps are demonstrably better than all tube amps and all the rest of us according to some are apparently fools for not believing and just following suit in processing ourselves into new class d converts.

Perhaps it would be good for Ralph to set up a new year put up or clam up all round Class D is new best amp type challenge.

The amps are very light and manageable. Ralph could reasonably and I’d imagine affordably send a fully burnt in pair to do the rounds at perhaps PeterA, Tima, Ron and MikeL’s for proper objective forum respected home evaluation.

This would be a great marketing opportunity for Ralph given his absolute faith in the inherent superiority of his new amps. The claim these new class d amps are better sounding, more transparent and more musical than any existent tube amp would make them more than fair comparison competition for Peter’s Lamm SET based horn system, Tima and his Atmasphere experience and Lamm gear and previous AR amps and Mike’s sota SS amps and Ron’s top end trad VTL push pull tubes. All four of these well established members have had very real exposure to the sorts of tube amps that this thread claims should be simply not as good or even as musically engaging as Ralph’s Atmasphere new class d amps. Why not simply put this to the test. If these amps are indeed good enough to put all tube amps out of the market within a decade this would be a good and transparent way to allow them to show their stuff and open the conversation up properly. If all the players were willing sign on I am sure Ralph’s amps would get a genuine fair hearing.

 

[Atmasphere]

This has been a painful issue of this thread where much much more than usual there has been the claims and repeated insistence that only already fully signed on new Class D acolytes can be the only ones who know the truth in how these Class d amps are demonstrably better than all tube amps and all the rest of us according to some are apparently fools for not believing and just following suit in processing ourselves into new class d converts.

Perhaps it would be good for Ralph to set up a new year put up or clam up all round Class D is new best amp type challenge.

The amps are very light and manageable. Ralph could reasonably and I’d imagine affordably send a fully burnt in pair to do the rounds at perhaps PeterA, Ron and MikeL’s for proper objective forum respected home evaluation.

This would be a great marketing opportunity for Ralph given his absolute faith in the inherent superiority of his new amps. The claim these new class d amps are better sounding, more transparent and more musical than any existent tube amp would make them more than fair comparison competition for Peter’s Lamm SET based horn system, Tima and his Atmasphere experience and Lamm gear and previous AR amps and Mike’s sota SS amps and Ron’s top end trad VTL push pull tubes. All four of these well established members have had very real exposure to the sorts of tube amps that this thread claims should be simply not as good or even as musically engaging as Ralph’s Atmasphere new class d amps. Why not simply put this to the test. If these amps are indeed good enough to put all tube amps out of the market within a decade this would be a good and transparent way to allow them to show their stuff and open the conversation up properly. If all the players were willing sign on I am sure Ralph’s amps would get a genuine fair hearing.

I take umbrage with your opening remark, which does not at all reflect the situation with accuracy. I'm not suggesting anyone is a fool and adamantly deny that without reservation! Obviously there is a flip side to your comment which is anybody can simply put up or shut up...

However your suggestion is a good one. The problem is we have been beset by supply side shortages for the last 2 years and the amps have been popular enough that they have been on constant backorder. You're suggesting that we put a set of amps aside over a customer that's already paid money down and has been waiting for however long.

But I think the idea is good and I will try to implement it. In the meantime we have a 2 week return policy- if you buy the amp you have two weeks to return it if you don't like it.

 

[morricab]

This has been a painful issue of this thread where much much more than usual there has been the claims and repeated insistence that only already fully signed on new Class D acolytes can be the only ones who know the truth in how these Class d amps are demonstrably better than all tube amps and all the rest of us according to some are apparently fools for not believing and just following suit in processing ourselves into new class d converts.

Perhaps it would be good for Ralph to set up a new year put up or clam up all round Class D is new best amp type challenge.

The amps are very light and manageable. Ralph could reasonably and I’d imagine affordably send a fully burnt in pair to do the rounds at perhaps PeterA, Ron and MikeL’s for proper objective forum respected home evaluation.

This would be a great marketing opportunity for Ralph given his absolute faith in the inherent superiority of his new amps. The claim these new class d amps are better sounding, more transparent and more musical than any existent tube amp would make them more than fair comparison competition for Peter’s Lamm SET based horn system, Tima and his Atmasphere experience and Lamm gear and previous AR amps and Mike’s sota SS amps and Ron’s top end trad VTL push pull tubes. All four of these well established members have had very real exposure to the sorts of tube amps that this thread claims should be simply not as good or even as musically engaging as Ralph’s Atmasphere new class d amps. Why not simply put this to the test. If these amps are indeed good enough to put all tube amps out of the market within a decade this would be a good and transparent way to allow them to show their stuff and open the conversation up properly. If all the players were willing sign on I am sure Ralph’s amps would get a genuine fair hearing.

I offeredRalph something similar but I was told to buy them first…