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