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NAD M28 7 Channel Purifi Amplifier Teardown

SIY

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Wait. What? Doesn’t that mean that the different channels would have different phase?

No, things are wired and set for all the I/O polarities to be correct.
 

restorer-john

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Ah, that makes sense. Thanks.

Depending on how the AP handles the crosstalk testing, the result may be artificially high due to cancellation (from opposite polarity), like the previous NAD tested.
 
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restorer-john

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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

NAD have inverted one channel with respect to the other from the 1980s in certain power amplifier designs (starting with the 2200 IIRC). At the speaker terminals, the hot and cold are transposed at the end, so you don't notice any difference. It was to relieve the PSU from both channels pulling from the same polarity rail at the same time.

It has other effects such as we are seeing in the channel separation/crosstalk results.

Nothing to worry about, just pay attention if you use NAD amplifiers in bridged mode, in a multi amp situation, or with speaker switching boxes. Sparks will ensue if you are too casual.

Based on this amplifier's design, you could just for fun (and if you were brave) use the already opposite polarity channels, drive them with a common signal (Y splitter) and make at least 3 high powered BTL Purifi amps out of this thing and have a single channel spare. It could be quite a ball-tearer.

@amirm Give it a go. Do you feel lucky? (I would totally do it, each amp has comprehensive protection. I wonder what two Purifis bridged would hit into 8/4R?)
 
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Trouble Maker

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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.

Note: I'm just some engineer who now makes software that rewired his whole 1925 house a few years ago. So, take this with a big ole grain of salt since I am not an electrician or NEC expert.

The NEC has an '80%' rule, though I don't know how strict it is, or if it would even apply here.
https://www.electriciantalk.com/f2/80-rule-67638/

I think the general idea is that you can use up to 80% of the circuit/breaker in practice e.g. 15 amp breaker/circuit can use 1,440 watts (15*120*0.8).

In your 5 channels load test in the original review you were drawing around 1,455 watts (262*5/0.9, assuming 90% efficiency)!

The purpose of the 80% rule seems to be for long term continuous loads, like hours. So it's unlikely to cause an issue here,you can likely get more out of this unit than that in practice. It does make me think that a dedicated circuit would be a good idea.
 
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PeteL

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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
No, they are in phase, they reverse it again at the speaker terminal
 

restorer-john

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No, they are in phase, they reverse it again at the speaker terminal

The phase of the signal is unchanged, the polarity is inverted. There is a distinct difference.
 

PeteL

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Note: I'm just some engineer who now makes software that rewired his whole 1925 house a few years ago. So, take this with a big ole grain of salt since I am not an electrician or NEC expert.

The NEC has an '80%' rule, though I don't know how strict it is, or if it would even apply here.
https://www.electriciantalk.com/f2/80-rule-67638/

I think the general idea is that you can use up to 80% of the circuit/breaker in practice e.g. 15 amp breaker/circuit can use 1,440 watts (15*120*0.8).

In your 5 channels load test in the original review you were drawing around 1,455 watts (262*5/0.9, assuming 90% efficiency)!
I'm not an expert in ac power distribution neither, but I think that 20 percent is for continuous use, but the breaker would trip at 1800. Yes in the test it could be called "continuous" but we can still assume it was for a short time. In real life, this doesn't happens this power available is for peaks in the content but the RMS power drawn continuously will never be 1455 W. It's much less. I still like you was surprised by this test, it's a lot of power. I can't read the power rating at the back of the amp, and not sure how it's determined, but it will be less than 1440W
 

Timbo2

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Note: I'm just some engineer who now makes software that rewired his whole 1925 house a few years ago. So, take this with a big ole grain of salt since I am not an electrician or NEC expert.

The NEC has an '80%' rule, though I don't know how strict it is, or if it would even apply here.
https://www.electriciantalk.com/f2/80-rule-67638/

I think the general idea is that you can use up to 80% of the circuit/breaker in practice e.g. 15 amp breaker/circuit can use 1,440 watts (15*120*0.8).

In your 5 channels load test in the original review you were drawing around 1,455 watts (262*5/0.9, assuming 90% efficiency)!

The purpose of the 80% rule seems to be for long term continuous loads, like hours. So it's unlikely to cause an issue here,you can likely get more out of this unit than that in practice. It does make me think that a dedicated circuit would be a good idea.

That 80% rule is for a continuous load. So for me when I charge my EV I can only pull 40A on my 50A 240VAC circuit. I think you can make the argument that this device doesn't present a continuous load.

That's the reason that most "1500W" hair dryers in the US actually only draw 1200W.
 

Trouble Maker

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That 80% rule is for a continuous load. So for me when I charge my EV I can only pull 40A on my 50A 240VAC circuit. I think you can make the argument that this device doesn't present a continuous load.

That's the reason that most "1500W" hair dryers in the US actually only draw 1200W.

100% agree that this would not be a continuous load. It was more just a point of reference that this thing can supply a ton of total power, at least in the realm of power amplifiers. For consumer devices, when we start talking about charging EVs, electric dryers, ovens, whole house heaters, electric instant water heaters those are in a different arena.
 

PeteL

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100% agree that this would not be a continuous load. It was more just a point of reference that this thing can supply a ton of total power, at least in the realm of power amplifiers. For consumer devices, when we start talking about charging EVs, electric dryers, ovens, whole house heaters, electric instant water heaters those are in a different arena.
I find a clearer picture on the internet, and it's rated 1200W, I would still like to know how they calculate this, but I never ran into a problem following these. So basically, a dedicated circuit, maybe, but it wouldn't be unsafe or problematic to add things on the same circuit that don't pull more than 240 extra Watts, unless you are running the same test as Amir for minutes.
 

restorer-john

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That's the reason that most "1500W" hair dryers in the US actually only draw 1200W.

1800W rated hairdryer here just tested at 1850 watts PF 0.99 (purely resistive).

You can pull 200% from a panel breaker for many minutes, maybe an hour before they trip. As for the 15A fuse of this amplifier, it will not blow at 15A. It will hold all day at 15A. To blow it, you may need >20A and it may take 30 minutes to do that.

At 22.5A through a 15A fuse it will take a minimum of 30 mins to blow. So you can pull 2.7kW (120V resistive) for half an hour at least through that 15A fuse.

Here is a typical ratings chart for Littlefuse's M205 fast blow fuses.

1600053611679.png
 
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amirm

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amirm

amirm

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Depending on how the AP handles the crosstalk testing, the result may be artificially high due to cancellation (form opposite polarity), like the previous NAD tested.
Oh, that is probably what happened as I changed the setting by chance here and it showed somewhat worse results.
 

Francis Vaughan

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The phase of the signal is unchanged, the polarity is inverted. There is a distinct difference
Well there is and there isn’t.
Sin(t) = -sin(t + Pi)
For any periodic signal this is useful.
It all goes downhill from here however, so the distinction has some merit.
Back in the days of Usenet this was the source of endless argument.
 

YSC

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really disappointed at the low quality caps in a product in this price range. Hell even normal computer motherboard costing $150 use much better caps for the goodness of not needing to pay huge repair cost. Audio manufacturers IMO should stop using such caps and get some reasonably good ones
 

restorer-john

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Here is a quick power consumption and efficiency test/result:
View attachment 82885

For a combined power supply and amplifier, this is darn good!

Amazing.

Bruno Putzeys, saving the earth from needless greenhouse gas emissions, one amplifier at a time.
 

restorer-john

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eally disappointed at the low quality caps in a product in this price range.

Samwha are pretty decent, but Lelon caps are just a cost saving decision and not really befitting of a product in the price range.
 

Vasr

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Bruno Putzeys, saving the earth from needless greenhouse gas emissions, one amplifier at a time.

While extracting a pretty penny in royalties from NAD who are forced to use their lunch money to pay for their caps. ;)
 

samsa

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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.

No. You just reverse the polarities on the speaker outputs for the channels that are "inverted". The speaker outputs then all have the correct relative polarity.
 
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