Review and Measurements of Hypex NC400 DIY Amp

[March Audio]

That makes no sense. You always vary the input level in mV to obtain whatever nominal output voltage you want for a given wattage over a specific load with 1KHz as the reference point.

Well, that's what I do. I confirm gain in dB and sensitivity for rated power that way.

So we have 0.09% THD at 20KHz at 157W@4ohms? Bit disappointing wouldn't you say?

Whats our frame of reference for "disappointing"? We need to test lots of amps !

 

[amirm]

So looking at a time waveform (guessing 96kHz bw?) you might see some of the high frequency noise that isnt apparent in the sub 20kHz audible band?

Yes, except that I usually use 22.4 kHz bandwidth for measurements (e.g. dashboard) unless otherwise noted. This is on top of AES-17 filter.

In other graphs where for example I plot distortion versus frequency the bandwidth is indeed 90 kHz and the noise there is taken into account in addition to distortion.

 

[March Audio]

Yes, except that I usually use 22.4 kHz bandwidth for measurements (e.g. dashboard) unless otherwise noted. This is on top of AES-17 filter.

In other graphs where for example I plot distortion versus frequency the bandwidth is indeed 90 kHz and the noise there is taken into account in addition to distortion.

Can you do an FFT of 20kHz to 96kHz to see what garbage is there?

 

[restorer-john]

Whats our frame of reference for "disappointing"? We need to test lots of amps !

My random example above is 0.0015% at 150+ watts, 8ohms at 20KHz, that's some 60 times better...

 

[March Audio]

My random example above is 0.0015% at 150+ watts, 8ohms at 20KHz, that's some 60 times better...

Got it, yep thats very good

Mind you Im not sure what my tweeters would make of 150w at 20kHz.....fizzle pop...

 

[amirm]

So we have 0.09% THD at 20KHz at 157W@4ohms? Bit disappointing wouldn't you say?

These are with 90 kHz bandwidth so the harmonics are all ultrasonic. Here is the FFT spectrum at 100 watts:

The distortion is dominated by third harmonic at 60 kHz so really not an audible concern.

 

[March Audio]

Thats a good point, what bandwidth are Denon/FTC specifying to look over for calculating THD at 20 kHz? The result entirely depends on how many harmonics are included.

 

[restorer-john]

Thats a good point, what bandwidth is FTC specifying to look over for calculating THD at 20 kHz?

That is a good point. Most analyzers back in the day were at least 100KHz IIRC, mine is 200KHz, but we have no idea what filters they were using to get their numbers back then. All I know is even my analog THD meter doesn't like Class D and it only has a HPF not a switchable LPF. We never had to be concerned about spurious out of band carrier signals with analog amplifiers.

 

[amirm]

Can you do an FFT of 20kHz to 96kHz to see what garbage is there?

At what power? Here is 5 watts as the dashboard:

And around 95 watts:

And 150 watts:

 

[Sal1950]

No, it's just typical BS marketing that is the default position these days. I for one, have no time for it.

Agreed on the issue of manufacturers playing games to inflate numbers, and a need to have a standard for comparison, but a more representative comparison criteria should be used.

I have to agree with RJ on this. I've been around to watch as the manufacturers have played the power games with us since the beginnings of hi fi. The FTC spec is a solid one that shows which amps have real muscle combined with long term reliability when run under hard conditions. It shows off the amps that will still look good after being ridden hard and put away wet.
Stick to it and insist all use it in official ratings. Otherwise they just make it up as they go along.

 

[March Audio]

At what power? Here is 5 watts as the dashboard:

View attachment 19502

And around 95 watts:

View attachment 19503

And 150 watts:

View attachment 19504

Thanks, its just to get a feeling of causes for the visible noise in the time waveform plot from earlier.

Is that modulation around the fundamental or mains harmonics?

 

[March Audio]

I have to agree with RJ on this. I've been around to watch as the manufacturers have played the power games with us since the beginnings of hi fi. The FTC spec is a solid one that shows which amps have real muscle combined with long term reliability when run under hard conditions. It shows off the amps that will still look good after being ridden hard and put away wet.
Stick to it and insist all use it in official ratings. Otherwise they just make it up as they go along.

Well thats the point, real music, even highly compressed stuff just doesnt have anywhere near the RMS level of playing pure sines. You will never encounter that sort of signal in actual use. Not disputing testing, I will being doing the FTC test with my amps, but it does need to be put into context.

 

[amirm]

123dB S/N ratio and wait for it, 400V/uS slew rate. FR 1Hz-300,000Hz +0/-3dB.

It says it was a-weighted for SNR. And the THD graph is at 8 ohms whereas I am using 4 ohm. I have not yet wired up my 8 ohm dummy load but I guess I should do that at some point.

 

[March Audio]

It says it was a-weighted for SNR. And the THD graph is at 8 ohms whereas I am using 4 ohm. I have not yet wired up my 8 ohm dummy load but I guess I should do that at some point.

but if we dont know the bandwidth they measured the denon to, we cant be sure of the 20kHz THD number for comparison. Isnt 10kHz therefore the highest we can compare to?

 

[amirm]

Is that modulation around the fundamental or mains harmonics?

Looks like jitter caused by the rectified mains power at +- 120 Hz:

 

[March Audio]

Looks like jitter caused by the rectified mains power at +- 120 Hz:

View attachment 19505

Presumably in the dashboard in the OP at 5watts this was obscured in the noise floor because of minimal averaging/lower FFT resolution?

 

[levimax]

I can only vary the input level so the output is whatever it falls at. Here are the results:

View attachment 19494

The graphs are not in order of input level. They fall on the graph based on noise level for the most part so I tried to label some and add the interpretation.

Perceptually our hearing is most sensitive between 1 and 3 kHz so it is nice to see distortion not falling in that region until way into power band.

Thank you very much for posting this information. Rising distortion at higher frequencies is a very common issue with power amps because of issues with negative feedback and stability. It is difficult to keep distortion low and consistent over the entire audio band .... how much you can hear this is of course another question. I would call 0.09% distortion at 20 KHz disappointing considering the performance at 1 KHz.

 

[restorer-john]

but if we dont know the bandwidth they measured the denon to, we cant be sure of the 20kHz THD number for comparison. Isnt 10kHz therefore the highest we can compare to?

We don't know the bandwidth. I have reviews of it someplace with FFTs, albeit CRT based ones with stated graticules, but until I dig out he reviews, I'm not sure how high they went above 20KHz.

My example was one I put up as it had a low THD, high power and tested very well back in the day and I have a few of them here I could run some tests on. Trouble is, confirming those numbers is a real challenge for me with my gear.

 

[March Audio]

We don't know the bandwidth. I have reviews of it someplace with FFTs, albeit CRT based ones with stated graticules, but until I dig out he reviews, I'm not sure how high they went above 20KHz.

My example was one I put up as it had a low THD, high power and tested very well back in the day and I have a few of them here I could run some tests on. Trouble is, confirming those numbers is a real challenge for me with my gear.

If you are willing I will pay for you to send it over here as it sounds like a good reference.. Wont be able to get as accurate as Amir, but for comparative purposes we can certainly see the differences.

 

[restorer-john]

I would call 0.09% distortion at 20 KHz disappointing considering the performance at 1 KHz.

As Amir pointed out, it's a 40KHz and 60KHz spur that is causing that number.