Could Class D really be that good?

It's funny how you can find cables labeled as 6N, or even 7N/8N OCC or OFC all over Amazon and AliExpress, and they're incredibly cheap. It's amazing to see the quality control and production efficiency of the plants in China these days???  I am a bit being sarcastic.

The only way to know if a single component (or several) changes the sound and or measurement is to use the same amp and just change the parts.  Then you will actually KNOW something.

This statement contradicts this one:

I did not say it changed the measurements....

I have found over the years that when someone is willing to contradict themselves in an argument, its never about the truth of the matter; its only about trying to make the other person wrong.

This makes a noticeable sonic improvement......and no, I bet you cannot measure it. 

(emphasis added)

Despite the obvious syntax error, the above statement suggests that no attempt was made to measure any of the changes introduced. The reason you would want to do measurements after changing anything in a class D circuit is noise and parasitics can show up, which can interfere with other equipment, for example an FM tuner or digital equipment sitting nearby. Bruno, the designer of the Purifi and Hypex modules, is very emphatic about this in his papers.

@lanx0003 BTW, if anyone tells you they have 6N (6 Nines, IOW 99.9999% copper) copper wire, if you are in their presence it might be sensible to turn and run as hard as you can. 6Ns copper is a myth; it does not exist. Wire manufacturers just laugh if you suggest this sort of thing to them. What is real is OFC (Oxygen Free Copper), which was not developed for audiophile applications (see the link).

Does anyone know how the NuForce/NuPrime class D stuff stacks up? They seem like they've put more thought and research into the stuff than just using reference class D designs.

@eddnog  Same here. I made a last-minute switch to the separates based on my instinct. I recalled that the overall package deals were identical during the pledge.

@lanx0003 well…guess I should be glad that I went with the dual 48V/5A option, then! Granted, I always believed that in the spirit of actually running separate monoblocs, it would imply powering them with actually separate power supplies as well. It should not be a huge surprise that there are actual benefits to doing it this way. The detriment is always to the wallet, yeah? But in the grand scheme of our beautiful hobby, this specific cost difference isn’t all that huge.

-Ed

@eddnog  I didn’t see any results from anywhere testing the V3 Mono specifically with a 5A power supply versus a 10A, making it even more difficult to say.

Erin just shows the test result today. 'Single' means single amp fed by 48v, 5A psu and 'Double' means double amps fed by 48v, 10A psu.  Having individual 48v, 5A psu feeding to the v3 mono does have 10dB advantage in THD. Interesting. 

Ric,

I really admire what you do and wish I could learn that skill set someday. It's a pretty good review, and I congratulate you on the impressive 9.5 out of 10 score you received for your mod. I agree that measurements do not imply sound quality, and I believe the listening part of the assessment is greatly lacking in the ASR review. Nevertheless, it is suggested that you send your mods to ASR for his complimentary measurements. Coupling with the excellent listening review you have received, a potential outcome from getting more comprehensive measurements done is that the forum will get to know your mods, your sales will be boosted, and the community will benefit from it. A win-win situation.

I don't think you are subborn.  You are just checking out all possibilities.  However, ASR themselves say that super low distortion is not required for "perfect sound"  Only a certain measurement of SINAD is needed.  They like lower measurments but they never claim they sound better....because they believe everything that measures below their SINAD value is perfectly transparent.

It is not just the input stage that changes the sound in a Pufifi amp build.....it is the damping of parts and the input board.....the cables, the AC inlet, the output connectors, the quality of parts on the input board. etc. to infinity.  I modify the Sparko regulators and the Sparko pro op amps for better sound.  I modifiy the ouput filter caps on the Purifi boards by removing all 6 on each board and removing the distortion producing steel leads and soldering 6N copper leads using Wonder Solder.  I then test the caps for outside foil and put them all back in so the outside foil is to ground on all the caps.  This makes a noticeable sonic improvement......and no, I bet you cannot measure it.  Look at the reviews of the NAD Purifi amp and my modded Purifi amp on 10 Audio.......he says my amp is way better and even with the mods it is cheaper than the NAD.  This game is infinite......just like our own nature. Eveything makes a difference and most things that make a sonic improvement cannot be measured.

Alright, it is fun discussion but counter-productive (to me) talking these technical issues phyosophically.  Call me stuborn but, before making any major purchase / investment, I would like to see positive / superior objective measurements on the particular model I intend to purchase to get the maximal bang for the buck.  I will also do my due diligence to get the users' and revierers' (I thrust) feedback and get a feel how it sounds.  I won't make my decision just based on diyers claiming that all Purifi / Hypex based modules (including power supply) measured the same.  Different implementation in the input buffer makes significant difference!  And that is true!

Boy you just keep sawing on that limb.......you cannot say anything but the same thing over and over and you know you are wrong and cannot prove otherwise. You alone hold your opinion. I have been around since 1975 and in the industry since then. I did not say it changed the measurements.....but if you are going to look for a different measurement than that is the way you would do it......using one amp or device.

Please prove you are correct. Show us the measurements......You know you cannot or you would have done so by now. You know you are wrong and just keep saying you are right. Would you rather be LOVING or RIGHT. Some would rather be dead right then ever admit that they were wrong. Are you one of them?

You are not your thoughts....you are not your opinion. You are infinite love and joy....and always will be....forever and ever. You can let go of your opinions because next life you won’t even remember them....you will obsessed with something else you need to let go of....he he.

When you really truly love yourself at your core.....you cannot do or say anything that is not in harmony with your soul.....you speak truth. Please love yourself more. I would love you to be more happy. Having pretend opinions...to be right...is not happy creating. Growing up and admitting you are wrong is the hardest thing to do. We are all so stubborn (me too). It is so easy to be bad......to be righteous......it is hard to be loving. It takes great courage to be loving. For we fear rejection....as we reject ourselves. Look in the mirror when you first get up in the morning and smile at yourself and tell yourself out loud that you love yourself and are beautiful. Do this several times every day.....Caress your face like your mother did when you were a baby. I do it when I drive. Wave at people. Tell people how beautiful they are and thank them for sharing their light with the world. Treat yourself and everyone as if they/you are your pet or baby. This is the way to really start to feel love for yourself. There is just God loving itself. You are your mother....the mother of all.....loving your self and everyone. Forever. This is the truth of your soul. Bless everyone.....forever.

I have NEVER in my entire life heard of any high end audio designer, engineer, manufacturer say this

Hm. Not been around, apparently.

The only way to know if a single component (or several) changes the sound and or measurement is to use the same amp and just change the parts.

Wait- what? So if you change parts it does change the measurements?

Sorry, wrong.  You have no idea what is producing those different numbers.  You just want to believe in something.  Those Wima Polyester caps that they use (both Fosi and Ayima now use the in all their latest gear) are crap.  I replace them with modified Wima polyprop caps that are oriented so the outside foil is to ground.  The Ayima Max and all the latest Fosi and Ayima amps all use the same inductors and capacitors yet still measure different.  Lots of things in the design can influence the measurements.  Fosi is now adding feedback around the output coil and this lowers the distortion and makes it less load variant (ie....measures flat at 8 ohms and 4 ohms).  The only way to know if a single component (or several) changes the sound and or measurement is to use the same amp and just change the parts.  Then you will actually KNOW something.

I don't believe it's necessary for components to be of poor quality to show disparities in measurement. Take the Fosi V3 and Aiyima A07 as examples. Both of them use the TI TPA 3255 module, but Fosi utilizes quality passive parts like Japanese/German capacitors and inductors. Both use the same stock op-amp (active). These disparities in the quality of passive parts are sufficient to produce audible differences in THD, as shown in ASR measurements.

Yes, some will laugh at their own folly.  I would llike to see you prove this statement: "You certainly can measure a change in distortion when passive parts are replaced!"  Show us the beef!  Where are your graphs?  Show us the volumes of statements from other high end audio designers who have tested this and show graphs.....please.......I have NEVER in my entire life heard of any high end audio designer, engineer, manufacturer say this except for you.  If this statement were true then every time a manufacturer made the sound better by upgrading the jacks, fuses, wires, resistors, damping of parts, capacitors, power supplies, power supply parts, etc. into infinity.....they would state it.....and they would show graphs to prove it. They would say.  We found a better cap.....we replaced our super last years cap with a new one and not only does it sound better but the distortion went down .001 percent of distortion.  But no one has ever said this....because you CANNOT measure the distortion of most parts. 

A Furutech AC inlet sounds better than the one you use.  If you replaced yours with a Furutech would your amp measure better?  (By the way, the Furutech ones with Rodium plating sound different from the ones with gold plating....same with their NCF technology.....do they measure differently?  does Furutech claim such?).  Removing the steel hardware on your power supply transformer and raising it off the chassis improves the sound.  Can you measure that?  I listened to three different brands of tiny .1 percent nichrome surface mount resistors that all looked the same and all of them had slightly magnetic end caps.  This was a 75 ohm resistor on the output of a Crystek clock.  One brand sounded much better than the others.  Did that change the measurements of the Oppo I was modding?  Hardly.

If you are using junk parts....like bad electolytics, ceramics, very inductive resistors and put them in a sensitive place then not only will the device sound bad but it will measure worse than using a better made part.  But what high end manufacturer uses junk ceramics as a feedback or coupling cap?  The knowledge on caps and their sound and distortion was first published in Audio Magazine by Marsh and Jung back in 1980....I am sure you remember that.  That article changed what high end manufacturers did.  They realized that bad dialectric caps sounded bad.....so they all changed to film caps and the high end electrolytic caps market (Elna, Blackgate, etc.) started.  Conrad Johnson started using all film caps in their power supply (no electrolytics whatsoever).  However, every brand of super high end film caps sounds different (including all the various models from the same company).  For instance....Clarity cap makes many models of film caps.  They claim the better ones "sound better".  There is no mention of lower distortion measurements on their site.  If they could measure lower distortion....they would brag about it....obviously.

Ralph, I wish you the best.....but if you are going to sit all by yourself on that tiny branch and saw away......well, eventually.....you fall.  

Here is an article about using really bad parts in portable audio devices......yes, those seriously bad parts can be measured.......However, we cannot measure the parts that are used in high end audio......unless you are Ralph....he must have super sensitive test equipment no manufacturer of better sounding parts has.

Did you not read what I said?  I said the amps will all measure the same.....this is absolutely true.  While it is true that when changing parts the distortion changes....because all noise and veils are distortion.  However, you CANNOT measure a change in measured distortion when changing passive parts. 

@ricevs Oh I read what you said. But it was false, as is the above statement. You certainly can measure a change in distortion when passive parts are replaced! The idea that this isn't so is laughable.

I first heard about the 1656 from a description of the LSA Discovery Warp 1 amp. I bought some and put them in place of the 1612s in the Purifi factory input board. Did not like them, not as musical as 1612. You might love them....but trust only your OWN ears.....not what others say. Most people act like sheep.....just following and repeating what others say somewhere. How about the "You just need to have the impedance 10 times higher for a preamp to amp match".....another words, a 100 ohm output impedance is fine driving an amp with 1K input impedance. Who the heck made up that statement? But you see it repeated on forums over and over. My 50 ohm output impedance source sounded better with a 150K load than it did with a 50K load. How about bypssing caps....."The rule is that you need a bypass cap that is one tenth the value". What a bunch of nonesense. You need the value and type that sounds best with the main cap. Many use .02uf to bypass large caps. You must listen to know. You must love to really know love. Please Love more......more often and more deeply. Makes you happy.

Whilst Discussing OP Amps, the OPA 1656 is one known to be selected for use in place of other OP Amp's.    

I definitely have no experience with op amps but believe in their ability to change a piece of equipments performance. After building the Nilai the first things that come to mind are wanting to upgrade the jacks along with internal dampening. The caps could be a major hassle due to space..the wiring not so much.

As much as I love the beautiful inaccuracies or pleasant mild distortions/coloring of tubes I do appreciate low noise linear solid state gear as well. I pulled a tube pre off the Nilai yesterday and enjoying now straight from dac. With certain music I prefer the visual perceptions I can get from tubes and a clean power amp. Kinda funny how I strive to find & appreciate an amp like the Nilai due to its engineering and impressive specs & then look for different ways to add changes to its sound. Not with just passive parts. 

Did you not read what I said?  I said the amps will all measure the same.....this is absolutely true.  While it is true that when changing parts the distortion changes....because all noise and veils are distortion.  However, you CANNOT measure a change in measured distortion when changing passive parts.  I can hear the difference in 2 inches of wire.....try measuring that!  We have been down this road many times.  You are one of the few people on the planet who BELIEVE what you just said.  Again, you cannot measure the difference in distortion by changing passive parts, damping, fuses, cables, etc.....no way Jose!  Lift your cables off the floor.....you get lower distortion....try measuring that!  Try removing the steel plate and bolt off your toroidal transformer and raise it off the chassis with a one inch piece of wood and be amazed at the sonic difference....try measuring that.  Please, come over to the "dark side".....it is way more fun than re-stating the same BS.  You will learn a lot and your products will get even better and you will make more money.  A win for everyone.  But many will take their "opinion" to the grave.....It is called being "dead right".  I am wrong many times.....I change my mind....it is fun....it is an ever changing game....infinite and full of surprises.....I learn new sonic things most every week.

Yes, the amount of distortion and its spectrum has some correlation to sound. However, it is not the major or only thing that makes and amp SOUND the way it does. You can give me 3 pairs of Atmasphere amps and I will change some of the parts in one to really bad sounding parts.....leave one of them stock and in the 3rd one I will change a bunch of parts and execution that I know will make the sound better. They will still all measure the same. Ralph or anyone would be able to pick them apart..........even on a double blind test.

When you change parts, you change the distortion so the sound changes. This is easily measured and correlated to what we hear. So they would not measure the same. Distortion is literally the sonic signature of any amplifier.

All op amps have an intrinsic sound.....and how you know this is to listen. There is no monolythic integrated circuit IC (that I know of) that sounds as good as a really great discrete circuit. This is from testing (and designing and making my own) op amps for 25 years. If you have an integrated circuit op amp......you need to upgrade it. So, which discrete op amp sounds the best?......I personally like the LKS and the modified Sparko Labs 2590 the best. When I was modding Oppo 205s I changed the stock 4562 to Sparko 3601 but then went to the LKS (I had modded the LKS DAC so I knew those op amps were probably really great). When I saw them for sale on line I emailed LKS and they sold some of them directly to me. They were much better than the 3601s....Of course, I removed the connectors on the opamps and soldered my own 6n copper leads to the op amp.

I find the Weiss very detailed but lacking depth and musicality. The Sonic Imagrery, and New Class D too transistory sounding. I have not listened to the latest Burson or the Staccato, and I am sure there are others that might be better.

Adding a tube buffer will ADD tube sound. That is what it is for. It will not measure as good, as no tube has the low noise of a great op amp. The tube buffer in the VTV has an op amp after the tube.....so it really is just adding tube sound. Warren at VTV loves how it sounds. It is his reference. He uses the stock 2950 as the op amp on that board. Warren has never heard any of my mods (to the op amp or to his amps). If you are buying the tube buffer it is because you want tube sound....not for its measurements. Tubes add liquidity, space and palpability.....they make for a more "musical" sound. I personally think that if you want a tube in your system.....that it be only one. Too much tube sound makes it slow. This is why people like a tube preamp with class D...or other solid state ammps. A nice combo. I tweak my whole system enough that even without a tube I get very musical and revealing sound. Most people do not know how to tweak that well so a tube piece of gear is a nice way to get to heaven.

The Nilai amps have built in buffers and regulators....so the only tweaks you can do is to mod the output filter caps and change all the wiring and jacks and damp the chassis (yes, it is a worthwhile improvement).

@lanx0003 I have not listened to the Apollon but I had given it some research. Just on looks & the couple extra features & upgrades I would have preferred that model. But I really didn't need a new amp..was just curious about the newer Nilai tech & power supply. The Stereo DIY was the cheapest I could find & I figured worst case scenario (no difference from Ncore) I could resell it with minor loss. Basically I wanted a taste of the newer tech at cheapest price. Personally I like the look of the Apollon. 

Now that I know firsthand about the Nilai I at least wish I would have gone for the mono blocks. Although I certainly don't "need" them for extra power.

Definitely there are ways to change the sound of these amps with whatever op amp or buffer. There are quite a few selections to choose from depending on the company. Problem is..which one or ones sound best? Is there a popular opinion on which one is subjectively "best"?

Going back to the Elac class D amp I mentioned the input stage made all the difference. I had heard the basic Ucd modules ​but Elac did a Jfet or Class A input driver that took that mediocre module to a new level imo. 

I while back when I was looking at different amps I liked the idea of the VTV tube buffer but it seemed a little pricey & I don't recall seeing any specs for it. My opinion was I'd be better off going with a tube preamp then an add on tube buffer. Which I'm a big fan of pairing Tube pre's with Class D...or really any amp lol. I've experimented with a few different pairings. Tube pre with low power supply noise & a smidge low harmonic distortion sounds good to my ears. I just started & almost finished with a new preamp build to pair with the Nilai. Minimal circuit design & the designer provides specs. Small parts list so upgrading to V-caps, mundorfs, AMRG carbon films & such wasn't too bad. 

@ricevs

Thank you for the write up / advices. I pretty much agree with what you said there regarding the sound signature heavily depending on how the modules are implemented. The reason I put so much effort there investigaing measurements is because there are still significant variations in the performance specification among the diy or commercialized products based on the same Hypex / Purifi modules. A few notes / questions here if you don’t mind:

- I am interested in VTV Purifi with tube buffer as well but I still want to see the detailed measurements there like ASR; I won’t bet / invest 2-3 grands just for experimentation. Hopefully ASR could have a chance to evaluate the product.

- Discrete op-amp like Sparko S3602 is certainly top notch but, according to FFT freq. spectrum analysis, the TI LM4562 is no slouch either and only costs 1/6. I know you might throw in the similar comment like they will sound differently even measurement is alike BUT I wish to hear your opinion in case if you had some experience with it.

Yes, the amount of distortion and its spectrum has some correlation to sound. However, it is not the major or only thing that makes and amp SOUND the way it does. You can give me 3 pairs of Atmasphere amps and I will change some of the parts in one to really bad sounding parts.....leave one of them stock and in the 3rd one I will change a bunch of parts and execution that I know will make the sound better. They will still all measure the same. Ralph or anyone would be able to pick them apart..........even on a double blind test.

All Hypex amps sound different from each orther. All Purifi amps sound different from each other. Please read the review on 10 Audio on the NAD Purifi based amp and then the review he did a little later on my modified VTV Purifi amp......way different and better sound. All parts and execution change the sound. I mod all these amps so I know that they all sound good stock.....but way, way better with mods. Yes, some do not believe it....they only believe in measurements. My mods do not change measurements.....they change the way an amp sounds.

I have been modding and manufacturing gear since the 1970s.....Everyone likes what I do......everyone (I mean, those that listen). If you change the coupling cap in a tubed piece of gear.....or brand of tube....or brand of resistor.....you change the sound.....but you do not change how it measures (obviously, the tubes must have the same gain). All op amps have a sound. Many amps out there right now (all the Fosi and Ayima amps,etc.) with sockets in them so you can "opamp roll". All the various op amps measure such low distortion that they do not change the measurement of the amp......but everyone can hear the difference in sound between the different op amps. This is why the best versions of Purifi and Hypex use discrete op amps and super low noise regulators (because they sound better). All the discrete opamps have a sound, as well. VTV sells various op amps for those that like different flavors of sound. I modify the Sparko labs Pro opamps to give better sound (one of my favorites, so far, along with the LKS discrete op amps). Warren at VTV sells various amps and he slightly prefers the Purifi (and especially the high powered Purifi) modules to the Nilai and NCX500 that he sells. He is a tube guy so he really likes the tube buffer in front of the Purifi module. Listening to a very modded VTV Purifi amp as I type.....Wow, what a great sound.

I am probably going to have a tour with a stereo dual mono high powered modded VTV Purifi amp soon. This way, this less than $3K (delivered including mods) amp can be compared to Atmasphere, Orchard, Hypex, Nilai and others. This dual mono stereo amp will do 222 watts into 8 ohms, 436 watts into 4 ohms and 750 watts into 2 ohms. I will warrantee the modded amp for 2 years. Fun stuff. Low distortion class D modules with serious tweaks equals serious goosebump factor.

@mattw73

I’m impressed with both the Nilai Mono and Apollon St in terms of their specifications. However, I lean slightly towards the Apollon for a couple of reasons: (1) it offers 25% more reserved maximum power in both 4 and 8 ohms (despite a slight sacrifice in dB noise in 4 ohms), and (2) it has the lowest measured power on/off noise, which is inaudible. Additionally, while the Nilai Mono has the advantage of being a stable 2-ohm load amplifier, the Apollon Stereo is half the cost of a pair of Nilai Monos. If you or anyone else has experience with these two amplifiers especially how they sound, I would greatly appreciate any advice or insights you could offer. Thank you!

Most agree that the 2nd harmonic is innocuous to the human ear; the real issue here is are the 2nd and 3rd able to mask higher ordered harmonics. I've heard the argument before that if they are 90dB down you aren't going to here them, which I think is false due to the fact that the ear uses higher ordered harmonics to sense sound pressure and it has over a 120dB range.

But if the 2nd or 3rd is masking them, then you have a 'tube amp' signature. The lower the distortion overall though, the more transparent the amp becomes.

@atmasphere So if a class D has the same distortion signature as a good tube amp, it will sound like a good tube amp simply on that account.

Agreed. But I want to emphasize, with the low 2nd HD found in these Purifi / Hypex class D, you won’t be able to have distortion signature comparable to a tube amp and possibly produce the tube-like smooth sound. Let me give you an example of sound color signatures built in the Smsl Su-8. The 2nd, 3rd and other higher order spikes in the standard setting (upper left) are insignificant (-120dB, similar to Purifi / Hypex. Users could elect the "Tube 1-3" sound signatures to experience the emulated tube sound, where you could find the 2nd-order HD boosted to -90dB in Tube 1 (light) setting and -70dB in Tube 3 (strong) setting. That tells me the 2nd order HD needs to have high enough ’dominant’ spike (relative to 3rd & higher-order) to exhibit the tube-like smooth sound.

Here is the FFT plot for a real good tube amp, McIntosh 40, which confirms with the above. But, interestingly, Mc40 actually has slightly higher non-dominant 3rd hd.

I can't speak on the newer chip amps but I purchased a Hypex Nilai recently. To me it is a step above the Ucd & Ncore's I tried briefly in the past. I purchased the DIY model and feel it is a great amp for the money. Subjectively speaking of the class D modules i had tried in the past I found them to be a hair dry..touch analytical & 2 dimensional at times. For the low noise, transparency, power & size I still think the Ncore is a good amp.. especially when you consider the price. If I was still into Surround/Multi speaker movie systems I would be all about Hypex/Purifi modules. 

I tried an Elac Alchemy amplifier built around the older Ucd modules and felt that Class D amp was good for the money. Definitely better than just the basic older Ucd module but I still felt the top end wasn't as good as say a good class AB. Still a solid amp for the money in my opinion, I used a pair of them for almost a year. 

Anyway, the Nilai is definitely an improvement in my opinion. It's hard to describe an amplifier but I just find it smoother & groovier than D's I've tried in the past. I've only used it for a few weeks now but I can't find any faults in it. If Class D is able to improve upon this and keep it at these low costs I think that is a win for a lot of audiophiles. I definitely welcome more builders designing amplifiers around these new modules.. especially this Nilai.  

Let's get real. While the Fosi stuff is good, especially for the money, amps such as Technics SU-G700M2 at 10x the cost are 2-3x better.

I’ve really grown to like class d, and I’ve certainly noticed how different modules bring a differing palate of sound to your speakers. They can be neutral as heck, or, like the older ICEpower modules, have tons of slam. Having said that, has anyone tried to contact Red Dragon audio? I’ve sent the guy several emails, and no response. Are they going out of business? I was interested in one of their amps, because based on the reviews I’ve read, they seem to have the sound signature that I like from class d. 
Any info would be appreciated.

@tubeguy80 DIY Audio Forum, will offer up extended info from builders on these Amp Designs, Bruno Putzey is  a point of reference.

I agree that class D is well past the point of being "mature", by at least a few generations at this point. Class D amps can be amazing, or sometimes mediocre, just like tube designs or class A etc.

Nice to see quality become available at such a low price like this amp. I'll be looking into it for a small bedroom system.

@lanx0003 The differences you hear in the sound of amps, if FR isn't an issue (which most of the time it isn't) is the difference in distortion of whatever amps are being compared.

There are three things that define those differences. What frequency, if any, that distortion rises (IMO/IME this aspect of amplifier distortion is the most important), the distortion spectra and finally how much distortion. There's no filter that would make a solid state amp sound like tubes; just the distortion signature. So if a class D has the same distortion signature as a good tube amp, it will sound like a  good tube amp simply on that account. 

This is because our ears use harmonics to identify any sound. So you can see that if an amp is to sound musical, its distortion signature must be as innocuous as possible. To that end, the 2nd or 3rd must be significantly higher amplitude than succeeding orders, so as to mask them. That is literally what has kept tubes in business these last 60 years.

@atmasphere  I was there initially but just like asr said "... The third harmonic hovers around -130 dB which again, is almost at the limit of what we can measure..."   It is inaudible either at that noise floor.

I am sure that, with such a low noise floor the Purifi / Hypex class D modules provides, the sound from it is transparent bar none.  I also concur with you that the resultant sound traits hinges on how the modules is implemented.  But I don't think the sound will be sweetened approaching to tube amp unless specifically with a filter or other component.  Maybe that is why VTV adds the input tube buffer.  Just my two cents.

@lanx0003 

I was looking at a different article. In this one we see a dominant 3rd (my memory playing up; I still seem to recall seeing a dominant 2nd) which is treated by the ear in much the same way as the second and is the only odd ordered harmonic for which this is so.

A dominant 3rd suggests cubic non-linearity; if so the circuit is inherently lower distortion than when a 2nd is dominant (which suggests a quadratic non-linearity). Our OTLs, being fully differential and balanced, have a dominant 3rd.

@atmasphere Just look at the harmonic spectra of the Purifi module. You’ll see a dominant second.

These are freq. spretrum FFT plots for THD and IMD from the published Purifi 1ET400A data sheet. For IMD, all spikes are beyond the audible freq. range so not a concern there. But, in the THD plot, where is the ’dominant’ 2nd HD? All odd- and even- order HD are almost inaudible.

@lanx0003 Sure! Just look at the harmonic spectra of the Purifi module. You'll see a dominant second.

Keep in mind that in a class D amp, the things that cause traditional solid state amps to sound harsh and bright might not exist. For example, in our class D the primary non-linearity is caused by the deadtime used in the output section (something that all class D amps have). In our circuit this results in lower ordered harmonics.

When feedback is applied to most tube and solid state A or AB designs, it is traditionally applied to a non-linear input to the amp (the cathode of an input tube or base/gate of a differential pair at the input of a solid state amp). The feedback signal is thus distorted and so does not do its job properly. This causes higher ordered harmonics and IMD. This has been documented by both Norman Crowhurst (technical writer for Sams publications) and 20 years later by Peter Baxandall.

In most self-oscillating class D amps this technique is not used. So they often lack the higher ordered harmonic generation common to 99% of traditional solid state designs.

Since the ear uses harmonics to tell the difference between sounds, you can see that class D amps are capable of very different distortion as opposed to traditional solid state. So I don't see it as counter-intuitive that a class D amp can have a lot more in common with the sound of a tube amp than regular solid state amps. 

Of course, class D designs vary a lot in their sound so YMMV. IME they can vary more than the sound of tube amps, which can very quite a lot! The point here is don't write off class D because you may have heard one that sucked. As with class A or AB, the sound quality is a function of design and execution rather than the class of operation. 

@atmasphere Thank you for the very helpful information. I recalled someone asking / commenting about the musicality and I have examined all the ASR measurements but could not find one appropriate to be an indicator.  After a brief search I was only able to identify two gears (see below) that ASR give the THD vs frequency chart. I am not sure what the thought process was behind making this chart obsolete.

The THD vs frequency chart was soon replaced by the following load-dependent THD charts by frequencies.  As you may know, the load-dependent THD is usually tested using 1kHz tone in the industry but was later criticized because it ignores the adverse effect of higher frequencies over 1kHz on the overall THD measure.  So, including Erin, the reviewers start tests using multi-tones.  As shown in the following chart, the 15kHz frequency has big jumps in THD levels, while the better class D module like Purifi demonstrates a much more controlled increase in THD at these high audible frequencies.   

The phenomenon of even-order harmonic distortion (HD) contributing to a "pleasant" or "musical" sound in tube amps, while odd-order HD in transistor designs may sound "edgier," is a well-known concept among audiophiles. However, the idea that modern class D modules can also produce a "sweetened" or "musical" sound despite lowering total harmonic distortion (THD) might seem contradictory.  Can you provide some actual test done for those class D circuits demonstrating how they accomplish what you have described in the last paragraph?

I added a THD (%) scale, which corresponds to the dB scale, into the graph for comparing more affordable NAD C268 ($1k) and the Bel Canto S300iu ($2.5k) with the Fosi V3. As shown, the NAD C268 is comparable to the Fosi V3, but the Bel Canto is far inferior. Again, measurements do not mean everything, but the THD measure does give an indication of the sound clarity. I just provided you with the data in one place for your convenience, and it is your discretion how you interpret the data.

@lanx0003 The THD really doesn't tell you how musical an amp might be. A better indication is distortion vs frequency. If it rises at too low a frequency, higher ordered harmonics might be unmasked and so contribute to brightness and harshness. Ideally (and a number of class D amps can do this) distortion vs frequency should be a ruler flat line across the audio band.

Zero feedback tube amps (like most SETs) have always been able to do this. But a good class D can do it better, with lower distortion (which obscures detail).

Once DvsF is sorted, the next hurdle is the actual distortion spectra. Ideally a 2nd harmonic should be the most powerful, followed by the 3rd with succeeding harmonics quite low in comparison. In this way the lower orders can mask the presence of the higher orders, allowing the amp to be as smooth as possible. Again, some class D circuits are very good at this, challenging SETs and other tube amps on their own turf.

@audiophil88 I have heard the Aavik D it is good on Borreson but if you look on the Aavik website their flagship amps are pure class A. Why do you think that is?

I could not agree more, @pindac. Such a ’bybrid’ amplification could be even better on a active crossover system. A good (mini)DSP could easily lead us there, which will be more convenient than the passive crossover system. A lot of class A / tube amp or these class D module amp do not even have ’pre-out’ or ’line-out’ any more for bi-amping.

I have heard Active Driven OB's with a configuration for Amplification with similarities to the method suggested by yourself.

The OB's although with 4 Drivers and a Amp for each Driver with Class D for the Lower Frequencies, has been the best Speaker Array heard in a unusual venue.

I would strongly suspect if heard in the home set up, they may be the best Speakers experienced over multiple years.

I made it known recently, that very recently, a friend who is a EE, has bought into a DHT Pre Amp Design, and in using this with a Neurochrome Amp, it has blown them away with what is on offer. I am yet to receive an Demo', but some of the Local HiFi Group have, and they are thoroughly impressed with the experience had.

I myself am still remaining thoroughly impressed with the Pass Korg B1 design for a DHT Pre Amp, especially after it has a few well known circuit mod's added.   

@pennfootball71 Have you heard any of the Aavik amplifiers?  Word on the street is they are true A/B killers.  Aavik is one of the brands within Denmark Audio Group, Borressen Speakers and Ansuz cables being the other two.  

Has anyone heard the Aavik I180 or 280 Class D integrated amplifier?  I hear it is magnificent.  A true Class A/B killer.  Thoughts? 

@pindac Please forward the link once you have it. I also dived into the DIY project based on Purifi 1ET400A and Hypex NC400. I am particularly interested in the hybrid one with tube buffer made by VTV. Although its price tag is relatively pricier, marrying a tube buffer might be a rather good idea. The idea is not new and, the more I think about it, the more I feel it is doable. We could biamp a tube amp (or low-wattage class A) and these powerful class D modules, where the tube amp/class A is used for the tweeter/midrange and the class D is used for the bass driver.  What do you think?

Further miniaturization of everything everywhere is inevitable.

@lanx0003 In the not too distant, I will put up a link to a Build of the latest Purifi design I have linked previously within this thread.

There are already some build tweaks already known for this design and these are being incorporated into the build I am following.

When all is in use, I will ad the link to the build thread.

This as a design will be cheaper than the Audiophonics Option and be a few steps forward in the design choices. It may even be cheaper as a commission build, if the DIY idea is not appealing.

@pindac Truly amazing stuff that Audiophonics has accomplished. Built on Purifi/Hypex modules/ps, the HPA S400-ET has been highly praised by ASR based on its excellent measurements. The compact size version, LPA S400-ET, has almost identical specs and performance in a half-size chassis. Similar to the Fosi Mono, the S400-ET offers multiple gain settings, dual replaceable op-amps, XLR inputs, and quality parts inside. The upgraded binding posts in the back are attractive. The sound quality is also highly regarded, surpassing class A/B or even class A based on the limited reviews I could find. I really don’t need to repeat all the positive reviews, but without exception, let me include its performance curve in the chart for the last time before wrapping up. It is almost neck-and-neck there with the Benchmark. The Fosi Mono starts losing its pace at higher loads, but that’s understandable given its price disparity.

You know what? This beast only weighs 12 lbs, so I could easily carry it back on the next trip to France. I guess that is one of the advantages of class D.

There actually was an extensive discussion on Audiogon a year ago.

https://forum.audiogon.com/discussions/new-class-d-amplifiers/post?postid=2543479

Eddnog, that pretty much leaves Class A amps. Not many "Dyed In The Wool" audiophile is going to get rid of their expensive, heavy, Class A monoblocks and replace them with them light-weight, cheaper Class D amps even if they sound better!   Or will they?