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NAD M28 7 Channel Purifi Amplifier Teardown

amirm

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#1
I recently reviewed the NAD M28 multichannel amplifier based on Purifi modules. As promised, this is a tear down of the unit with kind permission of the owner.

The amp was a delight to open with beautiful screws holding the attractive top down. Once removed, we are greeted to a nice and modular layout:

NAD M28 teardown purifi amplifier power supply.jpg


The right side is a unified power supply for all 7 channels. I searched quickly but could not find a matching Hypex power supply. The only markings on it are:

NAD M28 teardown purifi amplifier power supply marking.jpg


As you will see later, NAD marks their boards with their name so I suspect this was custom designed by someone and not NAD.

There was a question about the fuse size. The fuse is rated at 15 amps and 250 volts. Total volt-amps available then is 1800 assuming 120 volt mains in US. Assuming there is power factor correction, this translates into equivalent wattage minus some loss. Further assuming 90% efficiency we have 1600 watts available to all 7 channel amps. This is likely the upper bound as your 15 amp circuit won't tolerate that for log so maybe it is in the range of 1,200 to 1,500 watts of power supply juice.

The capacitor brand choice is disappointing in the power supply as it is the low tier Lelon brand and only rated at 85 degrees C despite being very close to the heatsink:

NAD M28 teardown purifi amplifier power supply capacitor.jpg


Moving on to the amplifier module, I was very relieved that it could be removed with just 5 screws:

NAD M28 teardown purifi amplifier Module overhead shot.jpg


The green board is the Purifi 1ET400A module. The capacitors have been replaced from Rubycon in the original reference design to United-Chemi-Con which is fine.

The Purifi is surrounded by the blue buffer board designed by NAD. The two op-amps are:

National/TI LM49720 "Dual High Performance, High Fidelity Audio Operational Amplifier " : https://www.ti.com/lit/ds/symlink/lme49720.pdf?ts=1599982501986&ref_url=https%3A%2F%2Fwww.google.com%2F

Microchip MCP6002 is a dual general purpose op amp: https://www.microchip.com/wwwproducts/en/MCP6002

Lower rate capacitors are used here from Samwha (old Samsung capacitor factory that was sold years ago):

NAD M28 teardown purifi amplifier Module Samwha capacitors.jpg


Back tot he board, note the two tables above the text "channel differences." There are configuration differences for channels 1,2,3,4,5 and 5,6,7! The board I pulled out is channel 6 and is marked that way with a red dot. The channels I tested in the review were 6 and 7. What possible reason would there be for the right three channels to have different configuration? When I put it back together I will do a spot check of other channels to see if the performance is different.

Conclusions
Mechanically the NAD M28 is beautifully put together. Should any of the amplifiers fail, you can swap parts to find out if it is the buffer board or the Purifi module. And possibly source Purifi modules in the open market to fix.

The power supply is a different matter as it seems to be a custom product. And sadly it uses less than ideal parts for long term longevity. Someone was pinching pennies for no reason here. What is another $50 in material cost in the power supply with better branded/temperature rated capacitors?

------------
As always, questions, comments, recommendations, etc. are welcome.

Getting serious cabin fever with smoke filled air from forest fires keeping me from going out. Once it clears -- which it is supposed to do by tomorrow -- I am going to go wild and go buy something expensive to make myself feel better. Problem is money as always and I need good bit of it. So please donate what you can using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 
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Vasr

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#2
So much easier on the eyes than their own AVR and integrated amps wiring and organization. Clean-slate design for the new technology helps.
 

Vasr

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#3
What is that solid core copper jumper doing on their pre-amp board running all the way?
 
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amirm

amirm

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Thread Starter #4
What is that solid core copper jumper doing on their pre-amp board running all the way?
It is carrying power from the connector to where it is needed on the input connector of Purifi board. It has lower resistance than a PCB trace.
 
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amirm

amirm

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Thread Starter #5
Put the unit back together and tested channels 1 and 3 and results are almost the same:

NAD M28 Multichannel Purifi Class D Amplifier Channels 1 and 3 Audio Measurements.png


SINAD remains the same so noise is the same. There is a bit less 2nd harmonic distortion (almost not seen above) but as I was preparing to show that comparison, it started to rise so this may be temperature dependent.
 
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#6
It's interesting they decided to use Lelon brand low cost capacitors in a $5000 amplifier instead of spending a few more $ and get Nichicon or some other respected Japanese brand of capacitors. Especially considering it's a switchmode power supply with high ripple current where the capacitors get quite a workout.
 
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#9
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amirm

amirm

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Thread Starter #10
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amirm

amirm

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Thread Starter #11
@amirm
It's not the first time that nad does that, the channel difference is that 3 of them has absolute polarity inverted from what I understand.. It reduce the strain on the power supply.
And then invert the speakers connectors obviously.
Ah, that makes sense. Thanks.
 

PeteL

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#12
@amirm
out of curiosity, did you identify the large silver rectangle up the rca plug? Looks like a shield. Transformer?
 

PeteL

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#15
According to the UL (E305905) the PCBs (also the buffer pcb) are made by SHENZHEN SHENZE ELECTRONIC CO LTD
http://www.shenzeelectronic.com

the logo matches
So PCB designed by somebody and manufactured in China (as 99% of the PCBs in the world)
I'm a bit surprised still that they didn't silkscreen the NAD logo anywhere tough. The shop you link are a pcb fab shop, not a board design shop. Nothing wrong with outsourcing but I'm wondering if they still have actual electronic designers or if it's all third party
 
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amirm

amirm

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Thread Starter #16
@amirm, Given all of the information you’ve gathered, it would be interesting to see if you could establish a communication channel with NAD to discuss your ... less than stellar ... findings ...

Casey
Sure. I tried to find an email address but it was hard. I finally found a support line and sent them a message. Hopefully it gets to the right person and they respond.
 

MakeMineVinyl

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#17
This is a bit geeky, but can you determine what PIC (20 pin) is on the daughter card off the Purifi board? The Hypex use a 16F690. I notice the firmware is V0.52 which looks kinda downright scary :eek:
 
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#20
@amirm
It's not the first time that nad does that, the channel difference is that 3 of them has absolute polarity inverted from what I understand.. It reduce the strain on the power supply.
And then invert the speakers connectors obviously.
Wait. What? Doesn’t that mean that the different channels would have different phase? That could be critical if you got different phase for the Left/Right Channels ... and even the Center Channel With respect to the Left/Right. I don’t think it would make any real difference for the various surround channels ... but I’m not sure.

This is seeming more and more like a hodge-pudge of bad engineering decisions ...

Casey
 
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