Purifi Class D

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Actually, Purifi’s own standard input boards (Eval 1 & Eval 2) sound great with the Purifi modules, but these are much different than the Hypex input board.  In my DIY Purifi amp I am running the Eval 1, powered by a custom linear power supply of my own design, and the sound quality is fantastic.

The standard Hypex demo input board is just not really suitable for final products (and was never meant for such).

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To add to my above comment:  The VTV standard Purifi amplifier build uses the Hypex demo input board.  This input stage is flawed in at least two ways:

 

1.  It has no input filtering for RFI.  All amplifiers should have RFI filtering on their inputs, and this is especially important for class D amplifiers, where RF getting into the inputs can cause serious, audible, consequences.

2.  It uses ancient, 78xx and 79xx voltage regulators with very poor noise performance and high frequency noise rejection ratios.  The opamp used in the input stage is a high speed precision audio opamp, which requires a very good low noise power supply to perform its best.

 

In comparison, the Purifi Eval 1 and 2 input boards have the appropriate RFI filter circuits at their inputs, and they have onboard very sophisticated discrete voltage regulators designed specifically to provide ultra clean low noise power to the input stage opamps.  These discrete regulators also feature very high power supply rejection at high frequencies, which is critical for a class D amplifier running on SMPS to keep high frequency noise out of the opamps.

 

It is an entirely unfair to evaluate the performance of the Purifi modules based on using them with the Hypex demo input board, and this is why VTV, and other amp builders, offer more sophisticated input board options, with proper input filtering and discrete voltage regulation.

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3 minutes ago, GUTB said:

Wait, why would Purifi release a compromised product to the public to represent their brand? Are they deaf? No one can listen to this thing and think it's fine, unless you've never listened to a high end amp before.

Purifi has not released any compromised product, please read closely!  The input buffer board used in the basic model of the VTV amplifier you have is made by Hypex, not Purifi.  Additionally, that board was never meant to be used in finished products sold to the public, VTV only does so in order to produce a very affordable amplifier model.

There is a reason VTV offers other input buffers for their amplifiers, it is because those other buffers offer better sound quality.

The standard input buffer boards offered by Purifi: the Eval 1, and the Eval 2 are both excellent, but your amplifier is not equipped with those.

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38 minutes ago, GUTB said:

BTW I went ahead and ordered a set of input boards with Sparkos opamps. I don't want to drill the case, I'll have to figure out an adhesive solution for the standoffs...

Good move, give at least 100 hours playing time before auditioning, and then give it a listen.  BTW is your source component balanced output?

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16 minutes ago, GUTB said:

Currently a tube pre single-ended only. I do have an active SE-balanced transformer unit which is currently in use with the DAC (Yggdrasil). I also have a Freya which is fully balanced but not in system. Does the Purifi really like to have balanced in?

It will perform a bit better with a balanced input if you can provide one.  In the DIY world, there is the option to use the input buffers made by Neurochrome, which are sophisticated enough to convert single ended inputs to balanced before they go to the Purifi module itself, but the buffers provided by VTV, etc, do not do single ended to balanced conversion. How have you connected the single ended source to the balanced input?  Best option is to use a single ended to balanced cable (no adapter) with proper differential wiring: The cable should be a balanced type cable, with a twisted pair inside a shield.  On the RCA end, the center pin should be connected to the hot wire of the pair, the shield should be connected to the shell of the RCA, and the cold wire should be connected also to the shell of the RCA.  On the XLR end, the hot wire should connect to pin 2, the cold wire should connect to pin 3, and the shield should connect to pin 1. By proper wiring like this, the amp will perform pretty well, but will be even better with a balanced source.

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1 hour ago, GUTB said:

So, the Purifi is going back.

 

After giving the new input boards and Sparkos opamps several days of burn-in after which it doesn't appear to improve any further I gave it a listen. I tried it with both the Yaqin B-2T all-tube SE preamp and my Freya using all-balanced connections. Digital is from my reference digital source (custom audio PC) to a Yggdrasil Analog 2. Also tried by vinyl system with it.

• Ugly midrange problem: appears to have been solved by the new input boards and opamps. Midrange quality significantly improved and is now almost as good as the reference (Odyssey Kismet / Stratos). Still a little less resolving / muddled compared to the reference.
• Dynamics seems to have improved, but not by a huge amount. It might also be in my head but my perception is that dynamics have improved somewhat.
• No other changes particular stood out to me.

The VTV Purifi with the upgrades appears to be just one step below my Odessey Kismet and Stratos ($1500 used, later $800 used). With upgrades the Purifi is $1400. Essentially this is a budget-performance amp at a budget price. Mind you the Odysseys which are not high-end are still a great value in American hi-fi and if you get them used you can get a decent amp at a budget price. I would say the VTV is reasonably priced for the amount of performance it provides which amounts to "okay". This technical performance. There is one other issue that is absolutely killer:

 

The total lack of engagement. I tried listening to it, but from start to end, I felt like I was forcing myself. I was never involved or engaged in the music. At first I was willing to consider that it was the ugly midrange problem which was doing it. But after that was solved there appeared to be no other excuse. Dynamics, microdetails, etc, is just one step down from the reference. They're fairly close. I fixed the cables and tested various configurations, nothing helped. I just didn't enjoy the sound, and that reason had no particular reason related to sound technicalities. In the end -- it's the usual, traditional death blow to class D, fatigue. There's something it does which is murder on the ear-brain system. I used to associate that with the lack of bandwidth and destructive output filter. I was told that self-oscillating amps don't have this issue by doing the filtering by using feedback; I have no idea how that works but if it's true then there's something else doing it. But it's still there. Basically it's still unlistenable.

 

Those of you who think you like your class D, I recommend being honest with yourself. Do you REALLY enjoy listening to you music? When was the last time you turned on your class D and got sucked in? Has it ever happened? Is your system spending more and more time left alone and you tell yourself it needs this or that upgrade? Once again I'm forced to accept that class D has serious killer problems and I doubt it's a phenomena I'm alone in experiencing. I don't think it's matter of discernment or having experience with high-end audio.

Sounds like you prefer highly distorted and colored sound, there is nothing wrong with that, as long as you are willing to admit it.

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8 hours ago, GUTB said:

you honestly don't know what high-end sound is.

Well, we are of a different opinion here.  As one who has actually worked in the industry for over 20 years now, visited countless dealers all over the country, listened to their showroom systems, listened to the in house demo systems of manufacturers such as Ayre Acoustics, PS audio, and Avalon Acoustics, as well as attended many CES shows, and RMAF shows, I am pretty sure I have a very good idea of "High End Sound", of many different stripes even.

I am OK with you suggesting that you do not like the sound of this specific amplifier, you are entitled to your opinion.  But when you suggest that my experience is lacking, you are just plain wrong.

 

In my system, the first class D amplifier I heard which could play on a  level playing field with very good class A and A/B amplifiers was the Ncore, in fact, after comparing Ncore with my Pass Labs X.5 series amplifier the Ncore amp had the better sound overall, the Ncore amp also bested the highly regarded PS Audio BHK here.

More recently, even better class D became available from Purifi.  My Purifi amplifier beat out my Ncore amplifier quite easily, as it features slightly more refined high frequency reproduction, while retaining all the other qualities I liked so much with Ncore.  The only class A/B amplifier I have heard here which I consider a tiny bit better than the Purifi is the Bricasti M25, which retails for $18,000.  If I had the opportunity to own the M25 I would accept it. 

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51 minutes ago, RickyV said:

The second thing I am thinking about is if I am going to build a Purifi amp to build it with a transformer power supply instead of the smps. Could that be beneficial? I like the idea.

Any ideas on this?

There is no reason that a linear power supply would perform better powering the Purifi amp modules.  So far, the class D amps which have used linear power supplies for the output stages have proven to offer no better performance because of such, and in fact, the measured performance, especially noise, usually suffers due to magnetic coupling of the large power transformer required with the amplifiers output (see Stereophile measurements of the LKV Veros).  Note that Chord makes their top level class A/B amplifiers using SMPS for just this reason, they actually produce a lower noise floor because they eliminate this problem.  Good, well designed SMPS (not cheapo Chinese wall warts) are not nearly as "evil" as most audiophile mythology makes them out to be.  It makes me wonder if the audiophile desire for extremely large, heavy, wasteful, and antiquated designs is actually some sort of personal compensation for inadequacies in other areas.

 

I do run a custom linear power supply to power the input stage of my Purifi build though.  I did this because it was relatively easy to do (I design high performance low level linear supplies in my work), and there was no disadvantage in doing so, as the small transformer and currents required would not create a noise problem like powering the output stage does, and because a totally separate power supply for the input stage might make an audible improvement vs. using the aux supplies of the main SMPS for this.  My custom supply is a fully linear, +/- 18 VDC supply using ultra low noise discrete regulators.  But I have not made any comparisons, the amp sounds fantastic though, using the Purifi Eval module for the input.

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And also worth considering, two stages of regulation, assuming decent regulators, will almost always be better than just one.  Consider that a regulator is asked to do three things: reject incoming noise, reduce the voltage to a the target value and output precisely that voltage, and keep the output voltage precise and constant regardless of load conditions-in other words, be able to respond very quickly to current demands without output voltage drop.

By having two steps of regulation in series, the noise level going into the second regulator is much lower, which allows for better regulator performance.

In my set up, i use a linear power supply for the input to the Eval 1 board, with +/- 18 VDC, regulated by very good Belleson regulators.  Then that supply is regulated again by the discrete regulators on the Eval 1 board.

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8 hours ago, hopkins said:

The audiophonic Purifi model has 3 gain settings on its interface card: https://www.audiophonics.fr/en/blog-diy-audio/36-set-gain-bypass-on-purifi-amplifier-module-ampli.html

If you can live with only 12db gain from the amplifier, would it not make sense to use the "bypass" mode which avoids the use of opamps entirely ?

This can be problematic sometimes, the only thing one can do is try it and listen.  Sometimes the sound will lack "body" and "force" without the active input gain stage, and this will depend on the output impedance/current capability of the driving component.

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1 hour ago, mocenigo said:

 

I can understand that moving from a crappy fuse to a well engineered one may make sense. After all it is just a wire in a little glass tube, and engineering that well should not be an overtly complicated matter.

 

But then I read that some fuses have "silver salts" inside, that have been "tuned" according to "quantum resonance" and that apparently they also have gold, plutonium, unobtanium, and whatever. Then I know they are in the realm of pure fraud.

All of the the power going to one's component passes through the very small AC line fuse.  Considering this, it is not hard to believe that the fuse it self will influence the final sound of the component;  when things like power cables and transformers used make significant differences in sonics.  I would suggest you audition a Synergistic Research Orange fuse as an AC line fuse in one of your components, they are returnable for a full refund, to at least have some experience with fuses, allow it 100 hours of break in, and then listen compared to a stock OE fuse.  See what you think, you might be surprised.  I know the results with aftermarket fuses have consistently surprised me, when my expectation was to hear no difference.

My Purifi amp build uses an Orange fuse for the linear supply feeding the input stage, the fuse for the SMPS 1200 feeding the modules is soldered in, so I have yet bothered to change it, but I will get around to that someday.

My experience is that incoming AC power responds to "treatments" (various power conditioning approaches) quite notably, and in fashions which seem to defy normal engineering concepts.  I suspect that that all of these results can be properly explained technically, but I certainly get that because they are not explained technically (purveyors of AC line treatments, including fuses, often appear to use obfuscating concepts in their marketing explanations, apparently in an effort to protect their actual tech) many believe them to be "snake oil".

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18 minutes ago, R1200CL said:

Hi Barrows. Nice post. 
I would like to think a properly designed power supply ought to be immune to fuse changes. 
Any thoughts ?

You may post answer in the fuse tread I made. 

No link for the fuse thread?  Anyway, a quick answer, then back to the Purifi stuff!

My experience suggests that the better the power supply is, the less it may respond to upstream changes (power cables, power conditioners, fuses, etc).  But, all of these things still seem to matter some, it is just that with really really good power supply design they make less of a difference.  I find the same deltas in difference to apply to things like USB cables: they make less of a difference when the USB source and USB receiving designs are very, very good, but they still make some difference.

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6 minutes ago, Revelation said:

Well that's not a nice thing to say. 🤣

Not nice, and also not based in reality either.

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6 hours ago, Iving said:

This apart from the argument that you can build a musically satisfying analogue playback system on a budget - whereas making digits musically inoffensive demands money, blood, sweat and tears.

OK, I agree that if one listens to a significant amount of music which is not available in a decent digital version an analog rig is needed.  To be sure, I have a certain affection for vinyl discs, and have been buying them again (although I have currently have no analog 'table or phono stage, perhaps in retirement I will add such, otherwise the discs are collectables). But looking at it from the other side: there is much more music these days which is very good, and is often only available in digital versions, and often when vinyl is available the pressings are questionable.  Which means that if one loves music, one is going to need a well sorted out digital playback system, regardless of how much money, blood, sweat, and tears is involved.  Actually, a good digital set up is not too hard to achieve, or too expensive in audiophile terms (Holo Audio Spring as a start, oversampling via HQPlayer) these days.  Doing so does not require a dCS or MSB investment level.

But to suggest that one does not have an adequate reference to evaluate amplifiers unless they are spinning plastic discs is just in error.

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I would propose, that most folks who prefer vinyl playback, tend to do so because of, rather than in spite of vinyl's rather significant shortcomings/artifacts.  Whereas digital artifacts, even at magnitudes of lower levels, are often quite objectionable.  I have a witnessed a couple of times, dedicated vinyl listeners, unable to distinguish between the digital playback of a needle drop (using a moderately priced ADC, like an Apogee element) and the vinyl record playing on the same system.

And, these days, we have affordable digital systems which are entirely listenable, true to timbre, highly detailed, and without objectionable artifacts (I am not suggesting that all current digital systems are this way though, just that one need not spend 5 figures for an excellent DAC).

 

 

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6 hours ago, John Hughes said:

Yes, I have hundreds of irreplaceable records pressed in that era.  Ribbon mics, judicious use of tube compressors, tube recording gear, and engineers that actually understood music :)  Those are the gems

So here, what you are actually saying is that you like the sound of lots of distortion/noise added to the music: tube gear of that era certainly added lots of distortion, same with ribbon mics, etc.  There is nothing wrong with that per se, but it is not actually high fidelity to the music.  And yes, I have heard many vinyl records from the late 50s and 60s.  In fact it is very likely that a clever digital recording engineer, today, could emulate that sound for you in their recordings using DSP.

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6 minutes ago, PYP said:

Is the Purifi 1ET7040SA ( High 40A Current Output) for "professionals" only?  I don't see it on the Purify website, only the 1ET400A.   

Yeah, no DIY option with the higher power modules it seems.  I might just purchase an affordable build from a manufacturer and then DIY it to be better.

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1 hour ago, Allan F said:

 

Apart from the higher current, do you know if the sound quality of the Purifi 1ET7040SA module is superior to that of the 1ET400A? As you note, there are manufacturers offering Class D amplifiers using this new module.

Bruno Putzeys is on record as saying that when one increases available power/current, there will always be a slight rise in distortion, and that this is unavoidable, but considering that the distortion of all of these amplifiers is at levels generally considered to be inaudible, I suspect that is merely a technical concern.  My own experience suggests that a more powerful amplifier of the same general design quality and implementation, always seems to sound better in my system, even if the loudspeakers do not technically "require" the additional power.  This may be because the higher power amplifier is operating in its more linear range when being asked to deliver a lower percentage of its overall power (look at distortion curves of amplifiers), or it may be because musical dynamics and transients, require gobs of current delivery to be rendered realistically in the very short term.  I do not know the answer here for sure, but have heard, for instance, the NC-400 vs. NC-1200 based amplifiers in my system, and the NC-1200, despite nominally having higher distortion, sounded better (enough that I wished I could get my hands on a Mola Mola set of mono blocks), eventhough my speakers do not technically "require" the increased current/power.

There are plenty of reports out there in audiophile land of a high degree of amplifier current/power headroom leading to better sound quality, even at moderate volumes where one is mostly operating in the first few watts (long term) of amplifier power.  My own experience leads me to conclude that I would like to have the higher power Purifi option here.

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3 hours ago, John Hughes said:

@Barrows. This is a smart answer!  My speakers are super efficient so these differences won't be audible.  But I'd bet the higher power modules would make a difference in speakers at 4ohms and less than 88db efficiency.

Yeah, I suspect there is some level of efficiency where more headroom will not bring improvements, but without trying one never knows for sure.  My own speakers are 4 ohm, and rated 90 dB for 1 watt, but I suspect that rating is a bit generous, as the speakers do appear to like some more power.  Theoretically, the 400 Watts at 4 ohms should be much more than enough, but the amp with even more headroom has sounded better in the past.

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7 hours ago, mocenigo said:

“high quality capacitors and resistances” may make a difference, I.e. faith in snake oil,

Umm, dude  you are wrong if you think all capacitors and resistors are the same.  Noise and distortion of different capacitors and resistors are both measurable and audible.  While I agree that there are some audiophile "beliefs" which are not based in reality, quality and performance differences between passive parts such as these are not debatable.  Perhaps how large those differences are could be debatable...

Just a single example: all resistors add noise to the circuit path they are used in, some resistors add less noise than others.  This noise adds up, considering how many resistors are in many audio circuits, and given contemporary low noise circuit designs resistor noise is a significant part of the noise at the output of components.  Susumu makes some of the very best low noise thin film resistors available, and their company touts the low noise characteristics of their resistors-they do not do this for "audiophile" reasons, as that market is tiny, they do so for industrial use applications, such high precision measurement and med-tech uses.  There is nothing crazy about this, some resistors are better than others.

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15 minutes ago, mocenigo said:

Maybe we are so negatively spoiled by crappy and cheap components that what I consider decent engineering, a normal prerequisite, is now a luxury.

This could be the case.  A huge part of the training of engineers is that they are always trained to deliver a design which will "work" at the lowest possible cost.  While given design may "work", this approach might not be the best when sound quality is a design goal.  For example, the sound quality of a resistor used in a control circuit for a washing machine is of no consequence, and cost is likely the most important factor besides longevity/reliability.

In my work in audio, when hiring engineers for design help, it has often been hard to get those engineers to understand that cost may not be so important, and that it is OK to spec a resistor which might be 2x or even 4x the cost of a lessor part.

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3 hours ago, Jud said:

Sorry for helping to continue this tangent about parts. We were discussing possible sound comparisons between Purifi modules. Miska explained this may well depend on the complex impedance of one's speakers. Something else I found interesting along analogous lines is that in the video interview with Bruno Putzeys that Duck Toller posted, Putzeys says a TI op amp with higher power and higher distortion specs actually works better than the one with a lower distortion spec he put in the Purifi input buffer. He gives a brief explanation as to why.

Thanks @Judfor pointing this out, I had not listened to this interview.  I happened to have a pair of OPA 1656 left over from a  previous project, and just swapped out the 1612s on my Purifi build with the 1656s (on the Purifi EVAL 1)...  Warming up in the system now...  the 1656 has 2x the current of the 1612, and I always suspected that the current delivery of the 1612 was a bit on the low side for this purpose.  Very cold amp right now, but sounds a bit better with the 1656s...  I'll let it run overnight to warm up and listen more tomorrow.

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2 minutes ago, 57gold said:

Who is producing high powered/high current Purifi amps?

Appollon, VTV, and Nord do.  I suspect there are, or will be, others soon.  And yes, to achieve full output power a single Hypex SMPS1200 is inadequate, one would use two such power supplies, or equivalent alternatives.

 

Hypex already produces the higher power Ncore amplifier, are you suggesting they have newer model with an improved design?  Do you have any details?

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Yeah, really cool to see that Hypex has developed the new Nilai amp for DIY, with a matching improved SMPS to replace the NC400 and SMPS 600.  Also really cool that they incorporated discrete circuitry for the input buffer and onboard regulators, which is a similar approach to what the NC-400 used.  Keeping a single board design with the dedicated onboard input stage is really cool as well.  I suspect they will be able to sell a whole lot of these.  I am looking forward to hearing them!

[barrows]

The claims are a bit confusing.  The introduction of Ncorex and Nilai, at nearly the same time is also confusing.  According to Hypex, Ncorex is an improved version of NCore, and Nilai (for now DIY only) is something more, with a bit of a new approach and a larger improvement over NCore than Ncorex.

So far there appears to be no third party listening tests, or measurements of either Ncorex or Nilai, so we will have to be a little patient.

For me, I think I will get my hands on the Nilai stuff, and fire them into the chassis which now houses my NC400 amp, and then listen test vs my DIY Purifi amp (recently changed to OPA 1656 in the input stage).