I never said I was immune to sighted bias. But there are often cases where the sound differences easily overcome this bias. In the case I mentioned, the differences were quite easy to hear by several listeners.
Purifi Class D: Junk?
So, from the previous thread about high-end class D the Purifi module was brought up. I decided to get a cheap example from VTV, a simple stereo unit with a single Purifi module and matching Hypex SMPS. Standard input buffer. I got it in yesterday. First impression wasn't what I was expecting: weak, congested dynamics is what stood out to me. I expected greater expression through my ProAc D30Rs. The other problems such as poor soundstage, thin / boring character, etc, I marked up to needing burn-in before evaluating. So it's been 24 hours, I would still expect to get at least the high control / damping of high end class D and dynamic power, but it's just not present.
Could it be an impedance mismatch? Other manufacturers selling the Purifi with their custom input buffers are reporting 47k Ohms. VTV doesn't say in the manual or on the site. I checked the Purifi data sheet which reports...2.2k Ohms on SE???? That can't be right?? That's absurdly low! Am I reading the right spec? My preamp has an output impedance of 230 Ohms. Can someone confirm that the stock Purifi has this ultra-low input impedance?
Could it be an impedance mismatch? Other manufacturers selling the Purifi with their custom input buffers are reporting 47k Ohms. VTV doesn't say in the manual or on the site. I checked the Purifi data sheet which reports...2.2k Ohms on SE???? That can't be right?? That's absurdly low! Am I reading the right spec? My preamp has an output impedance of 230 Ohms. Can someone confirm that the stock Purifi has this ultra-low input impedance?
253 responses Add your response
In my past experience with the Nord Hypex NC500 monoblocks using the Sparkos and Sonic Imagery opamps, the differences were pretty obvious.Best is no buffer at all, direct in, as I’ve done. (you don’t need a buffer driving another buffer) If the output buffer in your source (pre or dac) is less than <50ohm output impedance and has high enough voltage output gain to give the volume you need, you’ll have better sound than any additional buffer (regardless of how good) that’s in the signal path Cheers George |
I should also point out these were not proper controlled tests and the subtle differences could be attributed to other factors than the amps. Mood, change in sitting position, not all done with same speakers. I really have a hard time understanding how a huge difference in sound claimed by many in these similar designed electronic components can be discerned over room acoustics and speaker distortion which is a lot more than any amp unless it’s design is purposely introducing it. |
And thank you for playing. However, the only thing worthwhile are subjective listening tests.......and they don't need to be blind. But, I value your incorrect opinion. I can remember how something sounded 10 years ago.....really! Some do not believe/trust their ears and think that a blind test must be done. Most all audiophile tests done by most all magazines and most all audiophiles (99%) are done without blind testing. Maybe those that need blind testing are also deaf......he he. |
georgehifi8,489 posts02-17-2021 7:10pmIn my past experience with the Nord Hypex NC500 monoblocks using the Sparkos and Sonic Imagery opamps, the differences were pretty obvious.Best is no buffer at all, direct in, as I’ve done. (you don’t need a buffer driving another buffer) No, it is not, especially with the Hypex amps. The input is meant to be driven by a very low impedance source. The input of the unbuffered amp is essentially in the feedback path. Not having a local buffer is a recipe for doing it wrong. This is the difference between people who actually know what they are doing and those who do not. People who do not make statements like this above. |
I can remember how something sounded 10 years ago.....really! Righhhhttttttt Most people can't pick their own speakers out of a line up once they are gain matched and frequency compensated, and often not even with frequency compensation. You may remember a general impression, but from a comparative basis 10 years ago, sorry, that is just not true. You believing that does not make it right. Toole showed in the Harmon tests how flaky most audio reviewers are. Looking at the rooms many of them used, different speakers, etc. the odds of them effectively pulling the characteristics of a single unit out are slim (to none). |
Example, i had not listened to the stereophile first test disc in 10 years. However, my system 10 ears later was more revealing than that older system except for the fact that the original system used super highly modified original Quad electrostats.(modified by me....the most extensively modded Quads that i have ever heard of). When I played the disc 10 years later it did not sound right at all....the orchestra, the air, the bell ringing in the backround, the guy singing in Hebrew.....just not right. So, I removed the midrange from its box and put it on top of the speaker and propped it up with a paper back book underneath (no baffle) and changed the xover on the woofer so it now went to 300hz as i knew the midrange driver on an open baffle would not do anything below 300hz. After listening......most all of the difference from what I had remembered 10 years before came alive. Still not quite as good as the old super modified Quad....but now very close....the reverb was going on for ever......before it was all blunted with the midrange in the box. Yes, indeed you can remember what something sounded like 10 years before. Anyone can do this.....with an open mind and open heart. |
The VTV amps use switching supplies and idle at something like 30 watts. If you have reasonable efficiency in your speakers you will not even use 100 watts on peaks. However, all transformers sound different from each other and most isolation transformers sound like transformers.....slightly slow......even 3000 watt Topaz isolation transformers can mess with the sound in certain ways. Every thing messes with the sound. Right now I am running my super modded VTV right out of the wall (power cord hardwired into the wall....yup, no joke). I want to get a 2000 watt low distortion pure sign wave inverter to run the amp. I am currently using a 400 watt inverter on my digital front end....giving super sound......really nice to get off the grid! http://enjoythemusic.com/magazine/equipment/0119/Goal_Zero_Yeti_400_Lithium_Portable_Power_Station_R... The reviewer is now using a 1200 watt Yeti on his Pass X250.8 along with the 400 watter on his front end. |
No, it is not, especially with the Hypex amps. The input is meant to be driven by a very low impedance source.Once again, you have no idea! The input impedance of the NC500 Hypex is 2k across the band, and can be driven by any buffer from a source that has 50ohm or less output impedance and the voltage gain needed to give you the max level that you need listen to. Once again, you have no idea!! I'll try to get my friend who heard it yesterday to post on it. |
George asked me to comment here. Happy to do so as I have no bias either way. But having bought, sold, reviewed & listened to enough audio for 3 lifetimes, I do have an opinion. I had the pleasure of hearing George’s Hypex NC500 mono’s driven direct (no Buffer) on his secondary system yesterday. Driven from the MSB Discrete R2R dac, it would have to be the cleanest and most dynamic Class D reproduction I’ve heard to date. And I’ve probably heard 30+ different Class D Systems over the years. Extremely low, coherent & powerful bass that remained tuneful at all times. Musical structure & impact maintained across all frequencies. For Class D and in fact any amplifier technology, the sound was superb by any measure. |
georgehifi8,490 posts02-17-2021 10:28pmNo, it is not, especially with the Hypex amps. The input is meant to be driven by a very low impedance source.Once again, you have no idea! Well I have listened to enough audio equipment for 5 lifetimes, do I win a prize? I actually know how to design an amplifier too. People who actually know something about high performance analog design (and Class D), can reference the data sheet here on the HYPEX website. Now consider the transient performance with the source 2 meters away in a high speed feedback loop for Class-D. |
Like I said, you have no idea, even with proof from Scott. Also no idea either on streamers, all are still waiting for you to prove yourself right to everyone. We’re still waiting for your proof with links and CD cat. no. to prove you know what your talking about |
tradeontheweb, Yes, I noticed you have attempted to come to george's defence when he was making a fool of himself, like here: https://forum.audiogon.com/posts/678981 (where you both were wrong) and here: https://forum.audiogon.com/posts/1422053 where you help with his even at that time ignorant knowledge of Class-D (which has not improved in 4+ years I see. and here: https://forum.audiogon.com/posts/1455259 ... where you help with his ignorance on DAC technology. Strange, seems you often pop up when george is trying to make a point about something he is wrong about. What is up with that? |
ricevs I am thinking more about how much better my EVS1200 + Oppo 105 with PS mods and Audio Alchemy DDP-1 + PS 5 could sound off grid. Some days the sound is magical, other less so, probably fluctuations in the grid and/or varying humidity. 2 weeks ago low temp and humidity, this past week mid to upper 80s and high humidity resulting in sticky skin The amp is plugged into 20amp dedicated line, the other two into a CorePower 1800. What do you suggest? |
No, it is not, especially with the Hypex amps. The input is meant to be driven by a very low impedance source. In order to make the comparitor work properly (no offsets, no oscillation) with all the comparitors we tried we did wind up with an input impedance of about 2K. Through a lot of our prototype process, we simply drove this with our MP-1 (or MP-3) preamp, which has no problem at all driving a load like this. Of course, it was obvious that the circuit needed more gain and an easier load to work with most preamps. But it worked fine since we were not including the input buffer in the feedback loop anyway. FWIW our MP-1 can drive 32 ohm headphones- its got a miniature tube power amp (with direct-coupled output) as its line stage. So it is possible that George was telling the truth, but that would mean that he had a pretty gutzy preamp on hand. Most preamps I can think of wouldn't drive a load like that, so I do think his comments should be held in abeyance until he can explain how he did it. Since you really don't need much gain from the input buffer, almost any opamp that can drive 600 ohms (and most can these days) can do the job since your gain only need be a value of 2 or 3. So it would not surprise me in the slightest that the kind of opamp used has an inaudible effect. A proper buffer will have more than just one opamp of course, especially if you are executing a balanced input. But they would not need much gain so the feedback on the opamp would be very high! IME its when you are asking more than about 20dB of gain that you start hearing differences between opamps. |
@unsound Thanks. There's still a bit of missing information, but you might be able to 4-5 volts out of one of those DACs (balanced), which would certainly be enough to make our module play although you wouldn't be able to drive it to full power. I can't speak to the Hypex but based on the idea that its input is about 2K (and thus similar to our module in that regard) I would not be surprised that George did this. |
although you wouldn't be able to drive it to full powerNo need to, I'm still only at 3/4 volume on the MSB for ear spitting level, I think I'd blow the 90db speakers before the amp. And so it doesn't matter if I can reach full level or not on the NC500's, think of it as a safe guard against blowing them up. So it is possible that George was telling the truth, but that would mean that he had a pretty gutzy preamp on hand. Most preamps I can think of wouldn't drive a load like thatNo not really, the output buffers have .7ohm output impedance, but use a 50ohm series resistor for protection https://www.analog.com/media/en/technical-documentation/data-sheets/13645fa.pdf Cheers George |
As far as I know the only company not using a dedicated buffer for the input of the Purifi module is Mivera....they made a streamer last year that used just one gain stage between the DAC chip (1 volt out per phase AKM DAC chip) and the Purifi module. Normally the DAC has a gain stage and so does the Purifi.....by removing one gain stage and having it all in one box you get some serious transparency. Most Purifi amp users will want a built in gain/buffer stage. You will not know if your high gain preamp/DAC can drive the 2.2K input impedance very well, unless you listen. The only way to listen is to buy the VTV amp with the Purifi input board that has the jumper on it so you can use the built in OPA1612 op amp or run it without. However, if you like it with the 1612 buffer (read Sarajen’s review in 6 moons......he liked the buffer over his preamps direct).....then you have no choice for better op amps. The VTV amp with the Purifi buffer cannot be changed to the VTV version with the discrete buffer board.....they won’t fit....its a smaller chassis. Here is an interesting factoid. The IceEdge amp I made last year had 47K input impedance when I was first modding it. I raised the input impedance to 150K and it sounded noticeably better. I am using 150k on my mods to the VTV Purifi amps. My source was a discrete output stage with a 50 ohm output impedance......The cable in between was low capacitance and two feet long. So, if you have 500 ohms output impedance and you are driving 2.2K.....then maybe you should try lowering that 500 ohm resistor to say 10 ohms......I could sound much better. You do not need a 500 ohm resistor in series with anything unless it needs it for stability (which might indicate a bad design). Most high end solid state preamps have an output impedance around 100 ohm........The Gryphon Pandora is 7 ohm. |
Data sheet: Each output drives a 150Ω load to ±7.5V with ±15V supplies No they are the output buffers in the rear of the MSB discrete R2R dac, which can swing 7.5v and do it with ease into 2kohm, with no temp rise or voltage drop or heat sink needed into the raw NC500 module And the proof is how it sounds, far better than having "any additional buffers" at the input of the NC500 module/s. And everything is direct coupled with no DC offset from dac outputs right through to speakers. ("small very fast settling" switch on low level bump from the dac) so amps go on last how it should be anyway. Cheers George |
Datasheet: The output drives a 100Ω load
Sorry, just looked at my notes, the MSB’s output buffer could be the LT1819 https://www.analog.com/media/en/technical-documentation/data-sheets/18189fb.pdf Cheers George |
atmasphere, The issue is performance, and without a proper low impedance buffer, at the input, not 6 feet of cable, with capacitance and inductance, the performance suffers in my testing (with actual test equipment). I don't doubt he did it, just pointing out that it is not an optimum implementation. |
No they are the output buffers in the rear of the MSB discrete R2R dac, which can swing 7.5v and do it with ease into 2kohm, with no temp rise or voltage drop or heat sink needed into the raw NC500 moduleDid you test to see if they actually do that in this application? There are a lot of variables here, not the least of which is how surprising 1 watt of power actually is and how distortion can affect your perception of sound pressure, to say the least. The issue is performance, and without a proper low impedance buffer, at the input, not 6 feet of cable, with capacitance and inductance, the performance suffers in my testing (with actual test equipment).As you might know our MP-1 is a tube preamp but can drive this through 30 feet of balanced cable with no worries- but that is our module, not a Hypex or the like to which I have no exposure. I would think that due to the low impedances involved the interconnect issues wouldn't be an issue. But there is a lot to unpack here; IMO the whole thing can be taken with a grain of salt without more information. |
Did you test to see if they actually do that in this application?Yes, that why I said it. I would think that due to the low impedances involved the interconnect issues wouldn’t be an issue.And to have the MSB buffer at the Hypex input or 1mt away with quality interconnect, is no difference at all and you know that. But what is worse, is having one buffer in series with another buffer. When if one "can" if loud enough do the job perfectly and yield even better sonic results. You have probably already seen this, but again for the masses, from the Hypex NC500 data sheet. Audio input"good way of tuning in a “house sound”" Take that as I would as colouration distortions, countering what they say earlier in the same sentence as "The NC500 is practically devoid of any sonic signature ” And this is what you hear by not! having an extra buffer in series that’s not needed. |
From Nord Acoustics; All Purifi 1ET400A are modular, PSU, AMP, and buffer. Making them easily upgradable in future developments of our Input Buffer. They are also easy and open to maintain. The REV D Buffer board uses modern versions of an old Op-Amp used primarily in Studio mastering and mixing equipment. Their development was focused solely on Audio performance and has been bought up to SOTA design and performance. Rev D will also accept the previous 8 pin Sparkos and Sonic Imagery Op Amps. Details of Op Amps Click Regards |
The VTV input buffer board is pretty much the same as Nord’s. However, VTV offers more op amp options and the amps cost less <<<<<<<<<<<<<<<<<<<<<,>>>>>>>>>>>>>>>>>>>>>>>>> Actually I did a visual side by side. The Nord is a much better made board, by no small margin either, NO copper foil resistors, and SLOPPY, solder joints. Second the Nord Rev. D has the greater options. You can not use the Sparco 2590s, or other pro op amp options. Either buffer can use the smaller op amps, SI, or Sparco, a few of the Class A op amps.. The Wiess op amp is HUGE and has a proprietary mount, that only it can use. The cost of that buffer and opamp is the most expensive you can buy. The funny part the guy that sells the VTV boards admits the Nord with 2590s sound better and cost LESS. 1/3 less. 274.xx (Euro) 323.xx usd He took back the VTV board, I got from him. I thought the option for Wiess was for TWO. It is for one board 442.xx usd. Over 100.00 usd difference AND the guy that makes the VTV likes the Sparco better, ASK HIM... I think both use Sparco Voltage regulators. Nord does.. Regards |
The one thing I did dig out of the thread is, I need to pick up a pair of Nord built Purifi 1ET400A, with Nord buffers and Sparco 2590. He'll give me a deal. I own 9 of his NC500. I bought 5 from him. 3 pairs of MBs and one X3 NC500 with trigger and 3 x PS (no sharing). ALL use 1200/700 PS. All but 2 have rev. d boards (rev c). Let's say we have a rapport. My 3X was the first to roll off the line with rev d boards and multi Mono rail within a single enclosures, withOUT shared (or compromised) PS 1200/700. Serious piece of gear.. Now to modify the fuse circuit. It needs to be inline and remove the onboard 15 and add a 20 amp iec copper/silver non fuse female. These are great sounding units with small planars MUCH better than any of the earlier 400/500/700. I really didn't like that sound as ALL. The buffer made all the difference in the world for me.. BUT then I like tone control, Mcintosh and VMPS speakers. I'm one of "those guys". To top it all off, kenjit, kicked me off his forum, I'm heart broken.. (took a while to work). Ok I all better now.. I'll quit yipin', I mean typing. :-) Regards |
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The Nord board is not better made and the VTV board has the exact same options as the Nord and VTV sells more op amp options. You can use any of the the op amps that Nord sells and more. There are no copper foil resistors on the VTV board......what the heck is a "copper foil resistor"? He uses standard metal film resistors that were tested by Andrew Sparks as having the lowest noise. |
Here is a comment from someone who bought the VTV buffer boards and installed them in his NC500 amp.....this is from the VTV website: "I purchased a pair of the VTV input buffer boards for my NC500 amps using the Sparkos SS2590 opamps and WOW. I must add that I have been using the Nord RevD buffers and before that the Bellissimo Audio buffers. I had tried the SS3602 and preferred the SS2590 in each case. After installing the VTV buffers in low output configuration I was blown away. The amp was more cohesive with smoother overall performance and finer details also better bass." |
Ok I looked at my Nords and my phone pics. I deleted the pic of the boards after I returned them.. Now I remember the whole deal. I bought 2 buffer boards and 2 Wiess opamps. They arrived but it took 4 opamps to do 2 boards. It was worded weird. I looked the boards over, and he (whoever built the boards), didn't use copper foil resistors and the work on the boards was not as nice. Plane and simple. I didn't get to listen to the Weiss, as I recall. I was ticked that I need two more. 950.00 for a pair.. 700.00 for the Nords.. The boards side by side, LOL yea, about that... VW vs Porsche. The boards I looked over.. Even Rev C had copper foil resistors vs Weiss with cheapos. They can take the 2590s I was mistaken, I was ticked the it took 4 Wiess Op Amps and he sent me 1/2 what I needed. Didn't make any sense. The guy couldn't read my mind.. Go figure.. My mistake..:-) They were sent back. Customer service was great though.. No problems at all.. Good guy to deal with.. I'll stick with Nord.. only problem it's in England.. Turns out it's not an issue so far, 5 years now with class ds Hypex NC500, no issues.. Sure like them better than the ICE products.. I have Wyred4sound, too mercy.. Where is the tone control.. Regards |
There are no such things as "copper resistors"....generally speaking. Both Nord and VTV use metal film resistors. Maybe you think the 3 polystyrene capacitors on the Nord board are "copper resistors"? You are once again mistaken. I wish people were more focused on what they write. Every word has power and meaning. Did you read above what the guy posted about the VTV board versus the Nord Rev D board? He said the VTV board sounded better. I tend to trust what people say when they do listening tests. When they just talk about something and even some of their info is wrong.....well, not a reliable source to believe. |
As Hypex say, "The NC500 poweramp module is devoid of any sonic signature" this is "without the buffer". And it will give the best "sonic transparency/dynamics" if doable, as I do without any compromises, just better sound and a purer signal path from input to output. No additional "house sounding" opamp buffers with colourations/distortions in the signal path. |
This is what it says, "Furthermore,the NC500 OEM is an unbuffered amplifier leaving the implementation of an input buffer up to the manufacturer. The NC500 is practically devoid of any sonic signature so this external buffer is a good way of tuning in a “house sound”." It does not say any of the other things you are implying, and there is no truth that this is true. We have noted the performance THD+D and IMD is impacted by the output impedance of the buffer when we did testing .... you know on actual test equipment. |
What you are hearing is what class d amps do they produce high wattage with low current to save money on energy and expense of manufacture to make more money for the manufacturer and they will not have the required current to dive the proac speakers you have they like a good high current ab type amplifier i am glad you did not spend much money on the class d amp you dodged the bullet with that decision. |