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Review and Measurements of Hypex NC400 DIY Amp

restorer-john

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Pretty sure my original list had ten things on it but I don't have the list anymore (at least anywhere I would be able to find it).

Probably tone control contours:

SA708.jpg


and low and high filters:

CA810.jpg


Probably also RIAA deviation and overload characteristics too.
 

6speed

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I also don't know if the interest in more tests is academic or needed for members. In this case the company clearly has documented the issues with long term performance. So me replicating them is not of a lot of value.

I think you have already conducted all the tests we need--we just seemed divided on whether we were going to accept the results. The only value I see in continued testing is to (at least somewhat) bridge the divide, or if required, standardize the conditions so that they are not further refined every time you review a new amp. For example, the growing SINAD chart for DACs is based on a discrete metric so it compares nicely, and presumably there was nothing about the manner in which the latest test was conducted that would disqualify it from comparison with the first. If you get a Benchmark amp to test, I would like to see it beat every other amp like we are expecting and not realize that we cannot overlay its graph with something else because a different filter or other variable was used.
 

restorer-john

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This is key. If you are a magazine publishing two to three reviews a month, then you can run extensive tests. For me, I have to be able to get in and out of a test including documentation in a couple of hours. Any more than that gets way too tedious and stops me from testing so many other products.

For occasional tests we could do more extensive testing but for routine tests, it has to be quick tests.

Therein lies the issue. Amplifier testing by its very nature cannot be done quickly if it is to provide comprehensive and meaningful performance data.

Amplifier testing also gets more time consuming, the more facilities and features that are on the unit. Properly testing takes most of a work day, even with good workflow and systems in place. And then there's the 'report' or the 'review', the pictures and editing and uploading it all here.

I agree with Thomas's suggestion, do comprehensive tests periodically, not superficial/incomplete tests regularly. Quality vs quantity.
 

restorer-john

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For example, the growing SINAD chart for DACs is based on a discrete metric so it compares nicely, and presumably there was nothing about the manner in which the latest test was conducted that would disqualify it from comparison with the first.

Apart from the fact we have D/A converters with <1.0V output being compared with D/A converters with >4.0V output in a bar chart based on absolute S/N numbers.

This has been raised many times on the forum. Hardly a remotely fair comparison is it?
 
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amirm

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Therein lies the issue. Amplifier testing by its very nature cannot be done quickly if it is to provide comprehensive and meaningful performance data.
Meaningful data instantly pops out of dashboard view so far. There is huge discrepancy in distortion metrics there. The other is maximum power while not clipping. Between these two we have tons of insight we did not have before and it takes very little work to perform.

Members do need to worry about reliability and safety of amplifiers. That won't come out of the measurements but as I did here, I will note them when I see them.
 

Bjorn

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The safe temperature values depend upon the components used; for example, you can get capacitors in 65, 85, 105, 125, and 140 degC versions (among others). There are some defined temperature qualification ranges we could utilize: 0 - 70 degC for consumer, -40 to 85 degC for industrial, and -55 to 125 degC for military spec components. There are variations on each theme, natch, and these are component, not external case ratings. If your amplifier's case is at 70 degC (about 160 degF, too hot to touch) then internal components are probably 40 degC or so warmer (110 degC), too hot for comfort.

I have noticed that during recent Stereophile testing there have been several amplifiers that shut down during preconditioning and/or would not reach their full power ratings at low impedance without protection circuitry being engaged. (I have about six months of back issues and am slowly catching up whilst I have some time off.)

Rather than music, how about using pink or other colored noise or some other signal with a defined crest factor for power tests? That has been proposed before but I think shot down because it was too stringent (too hard on amplifiers). Foggy memory again back to the 1970/1980 decades when power wars were on-going and various standards bodies were trying to develop something reasonable. These days, we could create all sorts of fancy test signals. How about a multitone test with 10 or more tones across the 20 - 20 kHz bandwidth? That could provide a good crest factor for testing. Or a number of tones weighted to follow the Fletcher-Munson loudness curve (but at what level)? For me the problem is more what test conditions are reasonable and realistic than our ability to create and run them.

What I have done in the primordial past back when I worked "in the biz" and had audio test equipment and all the gear to play with:
  1. Standard frequency response at 1 W into resistive dummy load;
  2. SINAD (THD+N) sweeps to full power (typically low duty cycle);
  3. IMD sweeps or spot checks (e.g. 100 Hz, 1 kHz, 10 kHz);
  4. A few square-wave signals;
  5. Full-power test (FTC, including pre-conditioning);
  6. IHF burst test;
  7. Overload test -- pulsed signal at 3 dB over max output to assess clipping and recovery behavior; and,
  8. Pink noise at -20 dB and -10 dB (latter would usually cause some clipping).
Pretty sure my original list had ten things on it but I don't have the list anymore (at least anywhere I would be able to find it). I did run the slew/TIM test that was (I think) a 100 Hz square wave with a 10 kHz sine wave riding on it, or something like that, and some other burst tests like five full-power cycles at some frequency so I could see how quickly the amplifier recovered. At lower power (usually) it was interesting to see what happend with such a test signal when driving speakers. I did make up some speaker dummy loads with a couple of resonators to add a peak and valley at LF (100 Hz) and HF (10 kHz). Not a Power Cube, just a home-brew reactance network I added to my dummy loads that emulated some of the gnarlier speaker impedance plots I measured at the time. And I have said before my dummy loads were big gold-finned power resistors stuck in gallon paint cans filled with transformer oil. I had 4- and 8-ohm and could create other values by combining those.

Steady-state responses were (and are) typically well-defined and well-measured by manufacturers so, while I measured those when repairing or checking amplifiers, I got caught up in transient time-domain response of amplifiers to various "burst" conditions in my long-ago search for what differentiated amplifiers. Sometimes it was very revealing, sometimes not. Remember back then there were no, or at least I did not have, all the nice digital analysis gear we have now. (I had Nak and HP audio analyzers plus all the usual test gear like analog 'scopes and meters, including my big HP rms voltmeter, and a bunch of commercial and DIY filters for testing.) I could grab frequency sweeps on a spectrum analyzer and take a screen shot using a Polaroid camera pack, hand-record the THD numbers and graphs, etc. (HP made a chart recorder but it was pricey and the places I worked did not have one, though the local college did so I got to play with it some.) Burst response and recovery I could see on a 'scope with persistence (phosphor, not digital memory like today) and again grabbed the Polaroid to capture what I saw.

I could easily spend a day or two testing an amplifier, and had a blast doing it. Sometimes literally... :)

And at the end of the day, I found that pretty high levels of distortion (1% ~ 10% or more depending upon frequency and the type of distortion) was essentially undetectable when music was playing. I added distortion before the amplifier to emulate high nonlinearity and in blind tests it was rarely detected. Music is often complex and includes so many harmonic- and non-harmonic signal components that distortion added by the amps was in the mud.

Oops, this got long, sorry! This is why I should not take days off.

FWIWFM - Don
Have you read the article below?
http://www.aes.org/e-lib/browse.cfm?elib=19371

Send me a pm with email address if you don't have access to it.
 

DonH56

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Have you read the article below?
http://www.aes.org/e-lib/browse.cfm?elib=19371

Send me a pm with email address if you don't have access to it.

No, I dropped my AES membership many years ago and have not really kept up with audio (career took a different path). Looks interesting; I keep thinking about renewing my membership but then other things come up.
 
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amirm

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OK, here are the results of thermal stress test.

I hooked up input to one channel and let it the unit warm up driving that one channel at 40 watts. I logged THD+N during that time and got this:

Hypex NC400 Amplifier One Channel Driven Warm Up Distortion Measurements.png


There is some equilibrium that occurs after 3 to 5 minutes but then thermal effects start to set in and distortion increases a bit (1 dB or so). An arrow to the heart of people who think they should leave the gear on 24/7. :D

I then jacked up the power to 330 watts or so and started to cook the unit. Here is the above graph again:

Hypex NC400 Amplifier One Channel Driven Thermal Stress Distortion Measurements.png


Here is the power measurement showing the shut down with power dropping to zero:

Hypex NC400 Amplifier One Channel Driven Thermal Stress Power Measurements.png


I had hooked up a 4-channel themorcouple and here are the results at the end of warm-up (left) and max power (right):

Hypex NC400 Amplifier One Channel Max Power Temp Measurements.jpg


As you see, the large power supply heatsink was the hottest at 81 degrees. Strangely, shortly after I took the above right picture, two of the thermal probe started to glow red in the middle and completely melted the insulation!!! Cheap junk. :)

Thermal imaging showed I had probed the wrong things:

Hypex NC400 Amplifier One Channel Thermal Image at Max Power Measurements.jpg


As noted, the hottest components were actually the inductors/transformers! Both in the amp and power supply. Max temp for the large power supply transformer is about 101 degrees C. Wonder what their wire insulations are rated at.

After the stress test, I ran a power versus distortion test:
Hypex NC400 Amplifier One Channel Driven after Thermal Stress Measurements.png


Power increased to 315 watts which is not much more than when both channels were driven. Distortion performance is the same as when the unit was much cooler in the original review.

Now please me go and cry over my damaged thermocouple meter.....
 

restorer-john

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Well done Amir. :)

So is there a situation where the NC400 on its recommended power supply can deliver 400W@4ohms at 1% THD? From your tests, it appears not, as it took a mere <4 minutes to shut down at 84% of its rated peak power and there is no point on your AP plot where the Po hits the 400W graticule.

The 2ohm figure now also appears extremely suspect. I would expect a much shorter shutdown period.

The other point to note is the THD at 330W@4ohms hovers around 0.3%, not exactly a fabulous number.
 
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amirm

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So is there a situation where the NC400 on its recommended power supply can deliver 400W@4ohms at 1% THD?
I stopped at 2 volts into it which provides 365 watts at 1.2% distortion. So yes, it fails the spec if that is what they are rating it at.
 
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amirm

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The other point to note is the THD at 330W@4ohms hovers around 0.3%, not exactly a fabulous number.
At 315 watts it is 0.007% though so pretty good. After that it is clipping so things get bad in a hurry.
 

6speed

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Sorry about your probes Amir. :( That makes me feel a little better seeing normal operation resume after the thermal failsafe disengaged, although that was probably equivalent to 5 years of normal usage, so hopefully nothing was cooked too much.

That seems odd that part of the larger heatsink was white and the rest dark blue. I would have expected a gradient. The large caps peaking out the right side are red, but the small ones near the power connectors are cool.

Looking at SMPS datasheet again, I'm not sure why it should supply 2x200W for 5min but only 1x300W for an unspecified time period.
Capture.JPG
 

restorer-john

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The SMPS600 specs say the following (I presume you are on 120VAC):

1546471057693.png


So it hit the power supply specifications (for <4min), but not the rated specs if the amplifier module.

1546471288973.png


I expect it was the PSU over temp based on your IR camera images.
 

6speed

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The tape Amir was using was acting like a blanket.
The taped part was the hot part. I would have expected it to block the heat from the camera, not hold it in to make it appear hotter.
 

Sancus

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A bit disappointing that the SMPS600 doesn't meet their spec for 5 minutes of operation. Still seems like the amplifier itself provides great performance, but the power supply isn't great and is definitely the weak link.

Extra thanks to both 6speed(for allowing it) and amir for the test :)
 

restorer-john

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bit disappointing that the SMPS600 doesn't meet their spec for 5 minutes of operation. Still seems like the amplifier itself provides great performance, but the power supply isn't great and is definitely the weak link.

They need to be called on all their published specifications if they are found to be inflated. Hypex are clearly not expecting typical DIYers to have the gear or the knowledge to actually confirm their numbers, let alone challenge them.

I'm sorry, I see it as a bunch of followers of a 'guru' lapping up everything as gospel and never daring to question the general consensus for fear of being labelled a 'non-believer' and kicked out of the audiophile playground. Thank goodness none of us have that fear around here.

Based on what I see, the SMPS needs active cooling across the flag heatsinks and the PSU as a whole. ATX PSU style. Even that may not be enough for the main transformer. Those spot temps are ridiculous after <4mins, especially as the thing took another 2 mins+ to reset. Output devices on an adequate heatsink, no problem, they can sit on 110 degrees above ambient and not blow up, but none of those heatsinks are adequate for just one NC400 are they?
 
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