Purifi class-D amplifiers

[dwk]

What would be a good place to order these in US (seems they are not available in big ones like Mouser and Digikey and no longer in PartsExpress). I'm coming over to US in end of February and it would be right time to stock up. I suspect these will be harder to get in Europe.

quick hit: https://www.markertek.com/product/jt...r-4-1-stepdown

actually more reasonable than I was expecting.

[ergo]

What would be a good place to order these in US (seems they are not available in big ones like Mouser and Digikey and no longer in PartsExpress). I'm coming over to US in end of February and it would be right time to stock up. I suspect these will be harder to get in Europe.

**
Altium is the 'real tool' for sure. I'm getting by with a 50EUR tool called SprintLayout for these small projects where a tie to circuit diagram is not a must. I did buy a perpetual license to Eagle also before they went all subscription, but I find the SprintLayout to be what the name says - so easy and fast that I typically have a board done in same time it would take to figure out the circuit and part match in Eagle.

[Evil Twin]

The transformer I selected for this application, and in fact sent a set to Steve Manning a few years ago for a Hypex NC400 build, is the Jensen JT-6110K-B. Unfortunately, it is not available in a PCB mount version, to my knowledge.
It is the highest level handling input transformer that Jensen makes, with a bandwidth of -3dB from 0.15 Hz to 120kHz. Yes, that is 0.15 Hz, as in 1/6 of a Hertz.

Low frequency fanatics need not fear the impact... It is designed for levels up to +26dBu at 20Hz.

In the recommended configuration, it presents an input impedance of 37.5kOhms to the driving source; this load configuration provides optimum damping of the step response of the transformer, and a relatively ideal input resistance to the active stage.

Common-mode rejection is 130 dB at 60Hz, though it obviously drops at higher frequencies due to winding coupling capacitance. It is a 4:1 input ratio, so using some gain after the transformer or having a high output capability preamp is preferred. The circuit above has the necessary gain, and due to the low "ideal" terminating impedance of the transformer, runs with a very low noise figure for the LME49724.


Projects like this were a key impetus for purchasing the APx555. Other issues have prevented finding the time to implement a PCB and testing, but now with this new opportunity with PuriFi I think steps must be taken once the immediate speaker project is completed. Well, possibly the next project, too... In my work for the Imperium I have further whetted my skills with Altium Designer, and having my own license for Altium Designer 20, I have no excuses with regards to personal productivity...

[ergo]

This looks cool and elegant indeed. Which Jensen did you have in mind? How big is it physically? It seems that most of them have a PCB mounted variant also. The coolest would be to redo the whole buffer board PCB of course instead of augmenting it with another board ... I might take that up at some point.

****
We did a brief listening of the WHAMMY circuit as the buffer yesterday. I only had OPA2134 as the dual opamp in parts bin for now, but it sounded rather good. On OLA speakers it seemed to add a bit of sparkle to top end and consequently more feel of depth. Bass balance and attack was also good.
I also made an option on a PCB to feed right into final MOSFET stage and skip the opamp. We did try that too. My friend preferred the version where opamp was in loop also. I was more unsure - both sounded slight different, but I was unable to tell with so short listening if I really prefer one of the other.



One thing is sure - Purifi power module is very revealing and it is very easy to hear differences upstream from it. So lets see where our searches lead.... it would be kinda cool to try out your above circuit too as it is probably technically closest to 'blameless'.

Other equipment up stream of WHAMMY buffer was Soekris multibit dac and BPBP - Bruno Putzeys Balanced Preamp for volume control.

[Evil Twin]

From 4 years ago...

This is the input buffer proposal I was verifying in 2016 for a possible build configuration for both Hypex modules and my own Class D module.

A specific concept of this input buffer is using a high performance high signal level capable Jensen line transformer to achieve very high CMRR between the inputs and outputs; not necessary to protect the Class D circuitry, but to provide isolation for the preamplifier from any switching noise contamination.


This is the simulation circuit in TINA Industrial; Texas Instruments provides an encrypted model of the LME4972 compatible with TINA Industrial for local simulation, which they use for online simulation work. The Jensen transformer is modeled in detail for key parameters.





The output filter displays the required bandwidth and bandwidth limiting, with excellent amplitude and phase response to 10Hz...

[Evil Twin]

In other words, something like this https://neurochrome.com/products/universal-buffer . Lots of folks over on diyaudio already scheming on the best way to use this as a front-end to the Purifi. Tom seems to know his stuff, so this seems like a rather promising option.

Exactly. He seems to have done his homework, and his testing seems reasonably rigorous. My investigations date from 2016, and were interrupted by a variety of personal and imperial issues... this included the development of a complete switching amplifier design using fully discrete circuitry, roughly based on Hypex concepts, with some of the newest MOSFETs available at that time from Seinar Industries.

I believe the LME49724 is a well engineered piece of silicon for many applications - it’s balanced outputs can drive a 600 ohm balanced load to up to 52V P-P.

[dwk]

I would not reasonably fault the design choices, such as the buffer amplifier, but it is interesting the trade offs involved. In my investigation of proposals for Class D amplifier front ends, I was favoring a solution developed with the LME49724. This part is not quite as quiet as the OPA1612, but the distortion floor is very similar, and it has a brutally capable output stage with nearly three times the current drive capability of the OPA1612. Oh, and did I mention it is a fully balanced amplifier with dual differential outputs by intrinsic design, not a combination of dual opamps operating independently?

I greatly appreciate the additional documentation, including the linear regulators and other schematics.

In other words, something like this https://neurochrome.com/products/universal-buffer . Lots of folks over on diyaudio already scheming on the best way to use this as a front-end to the Purifi. Tom seems to know his stuff, so this seems like a rather promising option.

[Evil Twin]

I would not reasonably fault the design choices, such as the buffer amplifier, but it is interesting the trade offs involved. In my investigation of proposals for Class D amplifier front ends, I was favoring a solution developed with the LME49724. This part is not quite as quiet as the OPA1612, but the distortion floor is very similar, and it has a brutally capable output stage with nearly three times the current drive capability of the OPA1612. Oh, and did I mention it is a fully balanced amplifier with dual differential outputs by intrinsic design, not a combination of dual opamps operating independently?

I greatly appreciate the additional documentation, including the linear regulators and other schematics.

[Evil Twin]

Thank you for the update, Ergo!

[ergo]

First buffer candidate working and did produce sine sines into 300 ohm load for test today.

My Purifi amp is still at friends place, so hopefully we get to listen to this one tomorrow and then at some point I'll measure it a bit more properly also. Did not want to create myself any measurement bias beyond just verifying that all is as expected and working fine.



****

BTW - Purifi folk have updated the "EVAL1 – User’s Guide". More info including a excerpt of opamp buffer and voltage stabilizer schematics.

[Evil Twin]

No, that is single ended supply suitable for lower power IR and TI bridge type class D amplifier chips that use single supplies and bridge mode to control supply rail pumping, as this way the pumping with inductive loads always goes back into the single supply.

[fish fingers]

Is the 3e Audio supply suitable?

[Evil Twin]

One might say it depends on the duty cycle and power delivery expected with real world loads and program material.

Adding some additional storage capacitance, it would not be unreasonable to be able to power 4 of the modules from a single SMPS1200A, say, for fronts and rears. Or three across for the L, R, and center with three channels and one SMPS1200A, and for a variety of other surrounds, four or more channels per power supply. Keep in mind the efficiency of the Class D amplifier itself is about 85-90%. So, for 150W delivered power, allow 200W of supply power.

For subs and centers, the power demands will be more substantial.

[theSven]

As I keep reading and trying to comprehend more about this class D amp, what power supply would you recommend if you wanted to make an amp that had 5 or 7 of these boards? I see that there have been two channels made, but I'm curious about making something with these that could be used to replace an older 5 channel amp.

[Bear]

I got an email back from Claus regarding group buys. The most relevant part:

Warranty-wise you will be covered, however communication and shipping should got through the person on the original invoice.

[Evil Twin]

Ergo,



One has to wonder at the human/business story of Hypex and Purifi. Launching a new company with evaluation boards that use a previous employer's and intended competitor's power supply module has to indicate the most amicable split imaginable OR a metric ton of nerve.

Yes, Bruno may be strong in the Dark Side, too...

[Norm]

Ergo, thanks for the quick answer and information confirming the applicability of SMPS1200A.

FYI it appears NAD's first use of the Purifi amplifier tech was just announced at CES. The Master series M33 looks to be quite the 21st product.

https://darko.audio/2020/01/future-fi-at-ces-2020-the-nad-masters-m33/

[ergo]

Yes it's the SMPS1200A

It took a bit of reading and pondering of both datasheets as for example Purifi one has -VP power while Hypex module output VDR-. Also the 7pin strip connector has a different sequence of powers/signal. This is why I added many photos of different angles... I was assuming that they will revise the EVK datasheet to make it easier for self builders as matching these two should be how most would use it. But this far that has not happened it seems.

***
As an update on my tweaking -

I have replaced the small caps on EVK module. Interestingly the originals were 22uF/63V type... but they are all in aux supply lines that come in at about 20V and then get stabilized to +/-12V. So I replaced all with Panasonic Fine Gold 220uF/25V

I also added the big 8200uF/80V ones for the main +/-60V supply. The SMPS has absoluttely no trouble starting up and there are no ill effects. Starts just as well as before.

I'm bringing the amp to my friends place tomorrow for A/B listening. He's Purifi + EVK is fully in stock form still. So I'll post listening impressions after that.

[Norm]

Ergo,

Is that the Hypex SMPS1200 Power Supply seen in your test rig above?

If the answer is 'yes' is it the model SMPS1200A100 or one of the higher voltage 'A180, 'A400 or 'A700? A few lunch hour minutes comparing Purifi vs. Hypex documentation it appears Purifi may have had this in mind as EVAL1 and Hypex SMPS1200, the connectors 'J1' on each look compatible. And for the lower voltage rails EVAL1's J3 pins 5 (+Vunreg), 6 (GND), and 7 (-Vunreg) getting power and ground from SMPS1200's J5 pins 3 (+Vaux), 5 (GND), and 7 (-Vaux).

One has to wonder at the human/business story of Hypex and Purifi. Launching a new company with evaluation boards that use a previous employer's and intended competitor's power supply module has to indicate the most amicable split imaginable OR a metric ton of nerve.

[ergo]

Fully understandable and a very good focus point that is too
In my neighborhood the temps start to drop below freezing and we had a few days of snow after Christmas and more to come for sure. So a cold garage is not the most inviting for woodworking at moment to a hot soldering iron with some class-A circuits and indoor activity is more inviting
Hopefully we'll make some progress that leads to someplace in end.

[Evil Twin]

Interesting experiments, but I do see some likelihood of over current protection at startup unless the existing SMPS you’re using has a well managed soft start. There is only one reliable way to find out- on the bench

Your thinking about HV preamp drive is appropriate- I have pondered the idea of direct preamp output from my Benchmark HPA4 or even using the headphone drive...

But for now, my focus is elsewhere...

[ergo]

While others are making sawdust I've switched to different mode

Plan is to see if providing a lot of drive current to Purifi modules will make them better and Passlabs design are on table as they have always known the benefits of lots of current... luckily here we only talk about pre amp class-A that will not fully kill the nice efficiency of the class-D power modules.

First one is an old design from Nelson Pass from 1997 - Balanzed line stage or Bride of the Son of Zen. I've always wanted to try a transistor pre with crazy high rail voltages and this one will be happy with the same +/-64V as the Purifi main supply. This should be closest sound wise to tube stage without actually using tubes


WHAMMY is from Wayne Colburn from Pass and is directed more as headphone amp, but it will sure have enough current. This one will allow playing with various dual op-amps available...


Last one is again from Nelson Pass but from 2019. Basically a JFET/MOSFET discrete gains stage with mostly 2nd harmonic distortion.


I also got 4x 6800uF caps to try out on main supply rails... not sure if the switching power supply will be happy to start with that 'powerbank' as a load. But I will try and see... perhaps it takes few attempts before it fully powers up but then it should run.


As my friend has the other Purifi kit we will have one that can run in stock Hypex PS + Purifi EVK buffers while we tweak the other one. That should keep us honest if we truly get benefit or just fool ourselves into thinking that every self made mod is a good one

[Bear]

Does Purifi have anything on the site limiting resale (no warranty support etc)? If not, I have to imagine that there is going to be a very active group-buy/resale market for these - more than enough margin in the volume discount to cover costs and provide a substantial discount. I agree that at $250/driver they are significantly more attractive than at $400 - very attractive, actually.

I can tell a story, and one that I'd need to confirm with their sales team, that they are handling things direct, but just don't want to sell pairs (order handling can be a non-trivial amount of work for a small company). I could be in for 3 - 5 of them, but I'd want some assurance that I'm not taking all of the risk for a manufacturing process that may need a few more kinks worked out.

[dwk]

The web shop seems to be coming online, with the volume-sensitive pricing for the woofer shown. In quantities of 8+, that's ~$250/driver, which is pretty reasonable (4+ week lead time, though). So, when's the group buy?

Does Purifi have anything on the site limiting resale (no warranty support etc)? If not, I have to imagine that there is going to be a very active group-buy/resale market for these - more than enough margin in the volume discount to cover costs and provide a substantial discount. I agree that at $250/driver they are significantly more attractive than at $400 - very attractive, actually.

[Bear]

The web shop seems to be coming online, with the volume-sensitive pricing for the woofer shown. In quantities of 8+, that's ~$250/driver, which is pretty reasonable (4+ week lead time, though). So, when's the group buy?

[Evil Twin]

Received, decrypted and decoded.

Your assistance is fully appreciated. It seems that they are not going out of their way to make this easy for interested parties- reading between the lines, they seem to be interested in getting a lot of EVAL1 kits out for listening evaluations and online reviews producing favorable publicity, but are slow to release technical information that was readily forthcoming from Hypex.

Given the history involved, I would expect them to realize comparisons like this would be inevitable.

Regardless, my chassis parts have been shipped also, and should arrive before the end of the year.

Now, if only there were 8 days a week, like the old popular song by the Fab Four.

[ergo]

For EVK what is on the web seems all we get for now. I would also like to see the circuit diagram to perhaps try to mod it or at least be able to use the AUX stabilized supplies. I guess I can get them from opamp pins but would be better to find taps close to regulators instead.

For the 1ET400 I got an extended datasheet via e-mail after confirmed purchase. Not sure about the policy of publishing it, so I've sent a directed info beam towards "Location Coruscant" with it attached

[Evil Twin]

Have I qualified for the Purifi Club?

Documentary evidence-









and what I find to be incomplete documentation-

The EVAL1 manual provides the necessary information for connecting and controlling it.

But...


I have been expecting to see documentation about the 1ET400 pin interface and the application schematic- the component connectivity and a schematic for the Eval 1 would all be necessary to proceed with a proprietary development.

Is there something I have overlooked on the Purifi web site? Perhaps some one can point me in the right direction...

But so far I have seen no module data sheet and no schematics of the module board interface, not as provided commonly for Hypex Consumer or OEM module products- for which I have all information.

If some one could enlighten me, it would be most welcome... Google has not helped.

[wkhanna]

ergo,
the work you have done is very impressive.
alas, i do not deal in the world of milivolts or circuit design, i do know some folks who do.
Bill Hutchins is the designer for LKV Research.
i am certain he would be not only interested but actually excited to hear what you are doing, and lend some guidance based on what he has learned working with these types of amps.

if you decide to talk with Bill, let him know it was Billy Hanna, of Finley Audio, who referred you.
he will take great care with you if you let him know i sent you.

you can use the email or phone from the LKV site, or pm me for his private email.

[ergo]

A review of the Purifi amp + the kit speakers
https://audiophilestyle.com/ca/revie...y-review-r866/

There is also a link in the article to the speaker kit box and Xover details

https://purifi-audio.com/wp-content/uploads/2019/12/SPK5-App-Note.pdf

Lots of interesting reading and details. No details on what is inside the amp box though - meaning no info what power supply and buffer was used. The reviewer seems perfectly happy with the sound balance and does not mention thin or missing body. So either it's down to taste or the box hides something different inside.... or then the speaker kit has been tuned with relative high baffle step compensation that will probably fix it too.

[Evil Twin]

Thank you Ergo for sharing your experience and comments with the board.

I have done brassboard experiments with different buffer circuits for the NC400 and NC500 several years ago, but due to various exigencies have not been able to follow up with fabrication.

The front end buffer seems to have impacts that are not easily revealed on the test bench- do not underestimate the difficulty of many preamps driving a 2k ohm load- while 600 ohm capability is often claimed, the listening experience that way can be most unsatisfactory. My own experience is that if one designs to drive a 75 ohm load robustly, then 600 ohm will sound quite "adequate". If one "designs" for 600 ohms, the results may not please...

Steve Manning is looking at a similar situation as he hopes to get an NCORE solution running in the not distant future. He has some options, with his Auralic Taurus Pre.

Just to whet your Dark Side Appetite, here is an interior picture of that preamp...




As the Dark Side encourages self torment as much as for others, I have ordered an Evaluation kit and cable sets, and compatible SMPS1200A400 power supply, along with some chassis components. Given the low likelihood of spare time for this in the next 3-6 months, this is more a down payment on the future and to have hardware on the shelf locally reproaching me for not being able to invest time.

But I did order some low dollar stand ins for my Halcro's, after finding some favorable pricing on a pair of Cambridge Edge W's.

[MikePM]

Have you considered obtaining the Nord buffer boards that include two of either the Sparkos or Sonic Imagery Op Amps? I know Nord sells the boards as an upgrade for its NC500 based amp, and appears to be using them without modification on their purifi amp. The Nord One REV D Input Buffer Board is listed for 274.50 pounds sterling, so two of them would not be cheap, assuming they are compatible.

Also, has anyone used any of the Chinese amp chassis from Aliexpress. I would love to buy local, but there is limited selection, at least until someone builds a kit for these boards.

[ergo]

I've been holding off posting the listening expressions as I wanted to be more sure if the first impressions stay and if it changes with some burn in.

As a comparison base I have a B&O ICEpower 200ASC based power amp and two NCore NC500MP based monoblocks
(NC500MP are the NCores with power supply and buffer integrated onto module - not the NC500OEM that would be a direct competitor)

Source has bee the Cambridge Audio Azur 851D and speakers the Wavecor Ardents EE edition

The first impression was that the Purifi sounded better in mids and highs. Even bit more clarity and precision in upper end. But at the same time the subjective feel was that the body of voices and music instruments seemed a bit thin. Like a speaker with baffle step compensation not quite there yet.

We listened with my friend and tried it also by bypassing the buffer and having 851D directly drive the Purifi. That had clear signs of trouble and the thinness remained while some more graininess emerged. Plus there is not enough gain in system anymore to play loud enough with some songs.

Anyhow - the solution I have for now is a Burson Soloist Headphone amp / preamp. As this one can drive the cans
Output power: 4W at 16 Ohms
Output impedance (Headphone Amp): 3 Ohm
Output impedance (Line Level): 30 Ohm
it was a good candidate to be a 'better pre stage' and the hope was that if it is the pre stage that matters for NC500OEM so much it can probably help here too.

And what it does is precisely what is needed in my system. The feel of 'missing body' and low end punch is now there and I would say now beats both the ICEpower and Ncore listed above. The mids and highs are also very much to my liking and there is more feel of 'darkness in between instruments' in lack of a better description.

My friend tried the Burson at his place with the OLA speakers we recently finished (he got the other Purifi EVK and built a replica just in a different enclosure). He had the same outcome for body and bass - meaning that the buffer in EVK kit lacks in these while running Burson with Purifi buffer bypassed. In his setup though he felt that Burson took away some of the top frequency sparkle. A bit too much for his liking... So the plan is to trial some pre/head amp DIY solutions. Nelson Pass's ACP+ preamp and WHAMMY headamp are two that we are thinking of trying.

I did not necessarily believe all the vendors selling NC500OEM amps that the buffer stage tweaking can have a huge effect. But I must say I do believe that now. I do find it interesting though as by datasheet the input impedance of the Purifi power modules is about 2.2kOhm. Which does not seem super challenging as even the opamps in the EVK should drive down to 600ohm ok probably.

****
Anyhow - what I have in plans and I've ordered part for is adding a 6800u Nichicon Audio series electrolytics close to the Purifi power module pins and also extra caps for the buffer stage pre and post the regulators. My thinking is that perhaps the power supply being still some 20+cm away (via wires) it might help create a better 'local energy reserve'. It's easy enough to try out and would be interesting.

I've been also thinking about Dynafrips Hestia or the Auralic Taurus Pre that is available as used.... but it seems too much cheating to take the easy way. More fun would be to experiment and do the buffers as DIY.
The Burson Soloist I have is not self build but is also modified - I replaced the power section with Jan Didden's SilentSwitcher, so it is half cheating

[Evil Twin]

Interesting... most interesting... well worth additional investigation.

But given that time is the most precious resource, why can I never find a reliable Sith Apprentice when I need one?

[dwk]

Looks like pricing is up on the site, finally. I'm pleasantly surprised - a bit of a premium over NC400, but in the same ballpark. I was honestly expecting something closer to 2x on initial rollout. I'm not sure I really have a use for one, but it's rather tempting.

[ergo]

As I struggled with figuring out what exact wiring is needed to be done between Purifi EVK and the Hypex SMPS1200A400 I'll inlcude few photos that hopefully show enough.
(Note how 7 pin connector has last 3 connected on Purifi EVK end but a different pinout on SMPS end!

[ergo]

Purifi amps initial phase complete and the modules are singing. Too early to commend on sound yet.

I created XLR socket holes for the potential upgrade with BPPBP - Bruno Putzey's Purist Balanced Preamp. That module has two XLR inputs and output.

I bought an extra 4mm front panel from HiFi 2000 and cut it down on band-saw to use as a separating wall between the power supply and amp.

The case is extra big to allow potential upgrade with "Integrated amp functionality and different buffer stage etc.

[ergo]

The rest of bits and pieces have arrived also. Will be seen if I can get them singing by Sunday evening. Some CNC work to be done to get all the holes done for the connectors and then the wiring etc.

[ergo]

Yes - e-mailed the sales e-mail and got a price quote back in matter of hours. The modules shipped from Denmark.

PS. There is a good discount if buying 4 or 6 or ... If buying only 2 then getting the Eval kit makes sense at it is relative low addition to the full sum. I might attempt a custom buffer stage in future but the eval buffer will sure help get it up and running faster.

[Evil Twin]

Similar progress... got the Eval kits and enclosure from Italy is waiting at office. Power supplies and connectors etc. should arrive next week as well.

ILCCC, what type of pre-stage/buffer you have for the module?

[ATTACH=CONFIG]29781[/ATTACH]

Interesting... most interesting. I do not see that an online shop is open yet, so I would speculate that you arranged these through interaction with the sales email...

I have an urgent task still underway, but in a couple of weeks I may have some free time...

[ergo]

Similar progress... got the Eval kits and enclosure from Italy is waiting at office. Power supplies and connectors etc. should arrive next week as well.

ILCCC, what type of pre-stage/buffer you have for the module?

[ILCCC]

Get one piece from a friend and will measure it by AP.

[ergo]

I have a bit more time, but less funds atm. So a problem is the other way around

We are pondering with my friend (who we built the OLA speaker project for) if we would buy Purifi modules for both of us or only for him for now.
The other point is a decision if to go for the EVM kit which includes "stereo front-end board (which has connectors and a configurable pre-gain stage)" or only the modules themselves. Latter is of course cheaper but would need some version of buffer still. I have balanced DAC-pre, but my friend has a single ended out on pre-amp. So balancing that signal would be needed. So thinking through the options there - even transformers as Jon considered in past, but the other way around - ie for balancing the SE signal coming in

[Evil Twin]

What's one more TIE fighter in the hangar bay? They can't possibly be stolen by rebel infiltrators and used to blow-up a Star Destroyer or anything, right?

This one is strong in the Force, too... your logic is impeccable! It is merely the emotional pain of knowing there is so much unrealized potential awaiting future efforts...

That, plus some possible tool purchases lining up.

[Bear]

What this means, is that I must resist... must not give in further to the Dark Side... no more parts for new projects I cannot work on!

What's one more TIE fighter in the hangar bay? They can't possibly be stolen by rebel infiltrators and used to blow-up a Star Destroyer or anything, right?