NC252MP (class D) vs. A250W4R (classAB) burst measurements into 4ohm//2.2uF load

[pma]

Thank you @pma for this very interesting measurements.
As I see that you are using a Picoscope, there is an interesting measurement that could be added. i hope that the Picoscope is able to do a FFT.
With the same test conditions as before, it it possible to inject a 6khz signal and make the FFT?
I would be interested to see what happens at 12 khz and 18 khz with the class D amp.
These are not ultrasonic frequencies!

Picoscope is able to do FFT but at low resolution, due to 8bit ADC. So you cannot use it to measure low distortion in audio band. It can measure harmonic distortion of about -50 dB or worse. On the other hand, one can catch transient spectrum, with the mentioned limit, which is still fine in case of flawed amplifier behaviour. For audio band distortion measurements, you will use AP or a good DAC/ADC combination. Picoscope is fast and good to capture transient signals like the bursts used, with high sampling rate and wide BW. Such task is impossible to do properly with slower audio analyzer.

 

[JSmith]

This information is valuable for the potential customers

Do you have any recent examples of actual customers that have experienced any problems driving the type of load you refer to, like a high capacitance ESL, with a class d ncore specifically? I wouldn't normally quote this, but even our ol' mate Paul McGowan thinks this is a non-issue (maybe just trying to protect his class d sales though).

Surely a high current amp that is stable below 2ohms is fine to drive ESL's?

In relation to the oscillation you noted, are you saying the feedback loop is no longer stable?

AFAIK Ncore use the feedback at the output filter, so doesn't that keep the capacitance of the output filter from interacting with the speaker load?


JSmith

 

[pma]

Such tests are part of my amplifier tests. See for example:

Please honestly add that your test is with resistive load only!!

NC252MP is unable to work with 4ohm//2.2uF load, under any conditions, below is the proof

The test is made at 1kHz, with 22kHz BW (this is to prevent the objections of those like @Julf , @amirm and others who keep saying that the behaviour above 20kHz is unimportant). The spectrum is shown up to 85kHz, but only 22kHz used for distortion calculation.

1kHz, 4ohm resistor load, 59W/4R

THD+N (SINAD) = -101.4dB. Excellent!


1kHz, 4ohm//2.2uF load, 59W/4R

THD+N (SINAD) = -59.2dB, severe oscillations

At 1kHz, the load is still resistive. But, the capacitance 2.2uF completely destabilizes the Ncore!

 

[pma]

Surely a high current amp that is stable below 2ohms is fine to drive ESL's?

You only know in 2 cases:
1) You are a designer of the product
2) You made intensive testing with such test conditions

If you stick with resistor load and 5W/1kHz/4ohm SINAD results, your results are useless in case of more difficult complex impedance.

 

[pma]

NC252MP comparison of THD+N vs. power at 1kHz into 4ohm resistor load and 4ohm resistor in parallel with 2.2uF capacitor load

The "ASR chart parameter" at 1kHz/5W falls from -91.4dB to -51.0dB, 100 x higher THD+N with 4R//2.2uF load at 1kHz. Still "fine"?

 

[Sokel]

NC252MP comparison of THD+N vs. power at 1kHz into 4ohm resistor load and 4ohm resistor in parallel with 2.2uF capacitor load

View attachment 273978

The "ASR chart parameter" at 1kHz/5W falls from -91.4dB to -51.0dB, 100 x higher THD+N with 4R//2.2uF load at 1kHz. Still "fine"?

Are there any other effects (apart from SINAD massive degradation) that we should pay attention to such a match?
I mean any physical danger,like the amp impose some danger for speakers,etc?

 

[pma]

Are there any other effects (apart from SINAD massive degradation) that we should pay attention to such a match?
I mean any physical danger,like the amp impose some danger for speakers,etc?

Good question. I would call it dangerous, from my point of view, and unpredictable. The phenomenon is also accompanied by DC at the output. I have made a risk and connected the 3-way speaker in parallel. It is not only the distortion, but the squeeze noise is audible from tweeter and midrange. At the bottom of the post you will find recorded sound from speaker.

Noise spectrum looks like this

You can see it is 20mV in audio band, and it is a lot.

Spectrum recorded from the speaker

Below the microphone sound record (limited to 24kHz BW)

 

[MaxwellsEq]

Are there any other effects (apart from SINAD massive degradation) that we should pay attention to such a match?
I mean any physical danger,like the amp impose some danger for speakers,etc?

Good question. An unplanned oscillation may damage tweeters if they actually are able to attempt to play the frequency. I think it's a long shot if it's way beyond their normal operating range.

however, it may increase the heat generated by the amplifier and may increase voice coil and crossover components.

 

[HarmonicTHD]

What capacitance do well engineered passive speakers such as the Revels or KEF exhibit?

 

[boXem]

Interesting test.
- did the amplifier loose a lot of it's performance? yes
- are the measurements plain ugly? yes
- did the amplifier become unable to play music? no
- did the amplifier stop working? no
- did the self oscillating amplifier stop oscillate? no
- did the self oscillating amplifier start oscillating in the audio range? no

One can see glass half full or half empty. What I see is that the system is robust enough to continue working.

It seems that this amplifier is not a good fit for estats. That would explain why estats owners have been avoiding Ncore since more than a decade .

 

[AdamG]

Surely a high current amp that is stable below 2ohms is fine to drive ESL's?

Going on 2 years + driving ESL’s on the nc502mp and they never sounded better. Have gone all in on 12 channels of Nc502mp’s and everything is just peachy. Small & light weight footprint, plus the much cooler operating temperatures are significant Bonuses.

 

[miero]

@pma Could you repeat some measurements using a capacitor with a smaller capacitance, that would be a representative for common well-designed loudspeakers?

 

[MaxwellsEq]

Interesting test.
- did the amplifier loose a lot of it's performance? yes
- did the amplifier become unable to play music? no

Well, it can but with considerable distortion and increased noise. It's certainly not a "HiFi" component when operated into this load, based on the graph in https://audiosciencereview.com/foru...ments-into-4ohm-2-2uf-load.43162/post-1529131

 

[Ruffy]

I am loving this thread, popcorn at the ready. People on ASR will hound down a DAC as not being SOTA even though it’s downfalls are inaudible. But come class D people will defend the hell out of class D for not being SOTA (even again if inaudible) because it’s class D and represents a huge paradigm shift (cheap, accessible watts) from traditional class AB amps and the patriarchy of expensive hifi.

I am remaining agnostic in regards to the audio gods, owning both AB and D amps, but as said earlier am enjoying this thread immensely. Play on!

Cheers,
Andrew

 

[JSmith]

the much cooler operating temperatures

Well if the amp isn't running hot... then all seems well in oscillation world, hence why everything is peachy I'd suggest.


JSmith

 

[fpitas]

I'm torn. The oscillation shouldn't be there, but perhaps it has no real world detriment. Be interesting to see what Bruno has to say. In any event it's nice to see some testing that might be pertinent. Good job, pma!

 

[raindance]

Perhaps a 60 kHz oscillation is interesting from a scientific or purist perspective. But it’s is irrelevant from an engineering (practical) standpoint. Since the typical speaker load cannot respond to a 60 kHz stimulus, it is not relevant to the analysis of an audio amplifier. So, why measure it and portray this as a “defect” compared to another amplifier topology?

So if a speaker cannot produce audible output at 60 kHz, should we ignore the heating effect of the signal on the tweeter voice coil? You're also wasting amplifier power by amplifying non audible signals and possibly "eating into" the reserves of the power supply. Tell me if I'm fundamentally wrong.

 

[pma]

@pma Could you repeat some measurements using a capacitor with a smaller capacitance, that would be a representative for common well-designed loudspeakers?

There will be no problem up to at least 200nF, as declared here.

 

[fpitas]

So if a speaker cannot produce audible output at 60 kHz, should we ignore the heating effect of the signal on the tweeter voice coil? You're also wasting amplifier power by amplifying non audible signals and possibly "eating into" the reserves of the power supply. Tell me if I'm fundamentally wrong.

Not fundamentally, but practically speaking. The tweeter voice coil most likely is too inductive to conduct much 60kHz.

 

[pma]

So if a speaker cannot produce audible output at 60 kHz, should we ignore the heating effect of the signal on the tweeter voice coil? You're also wasting amplifier power by amplifying non audible signals and possibly "eating into" the reserves of the power supply. Tell me if I'm fundamentally wrong.

You can ignore it, but the question is if you are willing to ignore distortion and unstable squeeze sound from speakers - the oscillations and audible whistle vary with time and signal amplitude.

I hope that everybody understands what this test is saying. Ncore is a very good amplifier with most of "usual" speakers. However, it is not an universal amplifier and its use with speakers with high capacitance, like some electrostats, should be avoided.
Note: regarding listening with a "usual" speaker, Ncore is indistinguishable from the class AB amp that it was compared with. With speakers with very high capacitive impedance, that class AB amp is better, without any doubts and questions.