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

[valerianf]

@pma
Your last measurements confirm that with a complex load a class D amp has an increased distortion in the high frequencies when power level is going up.

It is easy to guess what would be the result with a real tweeter: mine has a capacitance of 3.0 uF @1khz and 735 nF at 10kHz.
Inductance is 166uH at 1 kHz and 47.6uH at 10 kHz.
It is a 8 Ohm tweeter.
My guess is that a class D amp driving this tweeter will provide distortion when cranking up the volume.
I will stay with class AB for my tower speakers, or bi-amp with class D for the woofers and class AB for the mid-range/tweeter.

 

[Rottmannash]

Bring the popcorn... Not sure what we are arguing about anymore.
There is nothing malicious with the 5W 1kHz SINAD list. Is it a tell all? No - it is too simplistic.
A composite score at different frequencies would be more relevant. One with different loads would be even better, etc.
I am not sure why a measurement showing a weakness in the class D is unwelcome here. I thought here we are interested in making gear better.

You assume those of us who own class D amps feel they need to be "better".

 

[Rottmannash]

No fight over the subject of class D progressing or over good engineering subject or Amir work. Not from me (I am not even a technical person).

But as long as class D discussion is directed to end consumer, like me, what matters is: audio sound quality, price and to some extent, convenience. For me, audio enthusiast, strictly speaking parameters like: consumption, heat, size, built in America etc have no impact. Consumption concerns are better suited for some 'ecology' chapter of the forum, not mixed with sound evaluation.

please send me your address so I can mail you my electric bill each month

 

[LTig]

@pma
Your last measurements confirm that with a complex load a class D amp has an increased distortion in the high frequencies when power level is going up.

You cannot generalize all class D amplifiers from just one measured model.

 

[atmasphere]

With the capacitive load (in parallel with 4.7ohm) in steps of 0nF, 47nF, 94nF, 141nF, 282nF there was no issue and no significant rise of distortion up to 47nF. The distortion started to rise from 94nF, as one can see in the cumulative set of plots below.

All amplifiers have load dependent distortion. Into a resistive load, any amplifier will have higher distortion as the load impedance is decreased. So there is nothing surprising here or unexpected.

A more useful measurement is distortion vs frequency. The ideal is a ruler flat line across the audio band.

I'm not used to seeing this sort of load in testing of class AB amplifiers but I suspect you'll find that they are not a lot different and may react worse due to less feedback to keep things under control.

If you want to keep the distortion of your amplifier down, one thing that is helpful is don't ask it to drive lower impedances. It does not matter if you can weld with your amplifier; the simple fact is if you want lower distortion the amplifier should not be working hard for a living.

To this end, its always been advised to have an amplifier that is more powerful for a given speaker and room combination as opposed to one that isn't powerful enough; the latter is classically associated with tweeter failure.

 

[LTig]

It is easy to guess what would be the result with a real tweeter: mine has a capacitance of 3.0 uF @1khz and 735 nF at 10kHz.
Inductance is 166uH at 1 kHz and 47.6uH at 10 kHz.
It is a 8 Ohm tweeter.

Is this tweeter fed directly by a power amp or through a passive crossover? If the latter is true you have to add the influence of the crossover.

 

[IPunchCholla]

Hello all, after some cooling down period. This issue of Ncore complex load dependent distortion is keeping my curiosity. I have made another sets of measurements, now with much lower value of parallel load capacitance, namely 141nF and 282nF. So the load is 4.7ohm//141nF and 4.7ohm//282nF. The results are much worse than I have expected.

Let's start with THD and THD vs. power plots, at 1 kHz.

View attachment 286265

View attachment 286266

View attachment 286267

We can see that:
- usual SINAD numbers defined at 5W/1kHz/4ohm would not tell any important difference,
- the distortion result with 141nF (3 x 47nF) parallel capacitor is worse than with the 282nF (6 x 47nF) capacitor,
- the distortion starts to rise above 5W with the added capacitive load and will strongly depend on capacitance value.
This all indicates to the fact, that the Ncore high-order feedback loop compensation has been designed taking into account pure resistive load only.

THD spectrum at 1kHz 76W

View attachment 286268

View attachment 286269

View attachment 286270

Again and consistently, the distortion with 141nF capacitor is much higher than with 282nF. Unfortunately, UcD180HG is much more consistent regarding load than the Ncore.

Is there a reason that the UCD180HG info isn’t included when you are drawing a direct comparison? The UCD180HG is more consistent. It is also consistently WORSE. It is adding 8-ish dB of distortion across the board (more in the mid-tones), isn’t it?

 

[valerianf]

"Is this tweeter fed directly by a power amp or through a passive crossover?"
I measured the tweeter alone to be able to design a new passive crossover.

 

[SIY]

@pma
Your last measurements confirm that with a complex load a class D amp has an increased distortion in the high frequencies when power level is going up.

It is easy to guess what would be the result with a real tweeter: mine has a capacitance of 3.0 uF @1khz and 735 nF at 10kHz.
Inductance is 166uH at 1 kHz and 47.6uH at 10 kHz.
It is a 8 Ohm tweeter.
My guess is that a class D amp driving this tweeter will provide distortion when cranking up the volume.
I will stay with class AB for my tower speakers, or bi-amp with class D for the woofers and class AB for the mid-range/tweeter.

They do no such thing. They show that this specific amp measured with this specific gear gives a particular set of results. That cannot be generalized. I have measured quite a few Class D amps and do not see anything untoward with capacitive loads.

I seriously doubt that your tweeter has anything like those numbers- this sounds like using the wrong tool for a measurement and believing the nonsense numbers.

 

[amirm]

Adding to SIY's post above, I want to make sure everyone can follow what is going on here. A capacitor is an open load at low frequencies. So putting one in parallel with the resistor does nothing with respect to changing the impedance. However, as frequencies increase, it gradually presents a smaller and smaller load. Raise the frequencies high enough and it will look like a dead short! Then the resistor doesn't matter. This is NOT the behavior of almost any speaker. See this latest 2-way speaker measurement:

The lowest impedance point is at only 200 Hz at 3.5 ohm (note essentially zero phase angle as well). The next minimum is at 6 kHz with roughly 5 ohm or so. In that regard, impedance has gone up, not down from 200 Hz. It continues to rise up to 20 kHz limit of the measurement. This is the opposite of the load being used here in PMA's tests.

Here is another recent test:

Again we see rising minimum high frequency impedance, not lowering as this dummy load attempts to do.

 

[ctrl]

I have made another sets of measurements, now with much lower value of parallel load capacitance, namely 141nF and 282nF. So the load is 4.7ohm//141nF and 4.7ohm//282nF. The results are much worse than I have expected.

This all indicates to the fact, that the Ncore high-order feedback loop compensation has been designed taking into account pure resistive load only.

Please make clear for the inexperienced forum readers that your thoughts and measurements are purely academic and have virtually nothing to do with the reality of speaker or tweeter.
It is really disappointing how you try to construct a problem by extreme, unrealistic boundary conditions without pointing it out. With every post you succeed in misleading less experienced readers - this is the opposite of what ASR actually stands for.
Don't get me wrong, I have no problem with your measurements, just with your intention that this is a real problem with real speakers - it's not.

First of all, your examples do not specify the sample rate at which you performed the measurements. In 99% of the cases the power amplifier does not "see" more than CD quality sample rate.

Secondly, the dummy load you used (4.7Ohm resistor with || 142nF capacitor) has nothing at all to do with the behavior of real tweeters.

Below the impedance measurement of your dummy load with display of resistive and imaginary part at 10kHz and 40kHz.
With increasing frequency your dummy load always shows a capacitive behavior.

This is opposite to the behavior of normally functioning tweeters, which always show inductive behavior with increasing frequency.


Here are impedance measurements of various tweeters from well-known manufacturers with resistive and imaginary part at 10kHz:

SBAcoustics_SB21RDC-C000-4 (silk dome tweeter)


ScanSpeak_D2010-851100 (silk dome tweeter)


Seas_27TAFC-G (metal dome tweeter)


Fountek_NeoCD1.0 (ribbon tweeter)


EighteenSound_ND1018BT (compression driver)


Not a single tweeter shows capacitive behavior towards high frequencies!


Now one could object that with passive speakers a high-pass filter comes into play with the tweeter and thus changes the overall behavior.
A filter of first or second order is significantly determined by the series capacitor. A small capacitance for a high crossover frequency and a large capacitance for a low crossover frequency.

Let's just play through two cases of 3.3µF and 10µF capacitors in series with the SBAcoustics SB21RDC-C000-4 tweeter:

In the case of the high crossover frequency, the tweeter again shows an inductive behavior in the imaginary part above 34kHz.
In the case of a low crossover frequency, the inductive behavior already starts at 18kHz.


So if you are really interested in a realistic dummy load, you would need to add a small air coil in series.
Here is an example 4.7Ohm and 3.3uF in parallel with 0.05mH air coil in series:

Unfortunately, I don't have any smaller air coils in stock, so the example has a minimum impedance of 2.7 ohms (and the overall behavior is more "extrem").
This corresponds approximately to the behavior of a tweeter in the high frequency range with first or second order high pass.

 

[Sokel]

I thought that the subject here was about electrostatics,wasn't it?
So,what's with real life electrostatic conditions?

Still academic,or...

 

[restorer-john]

The lowest impedance point is at only 200 Hz at 3.5 ohm (note essentially zero phase angle as well). The next minimum is at 6 kHz with roughly 5 ohm or so. In that regard, impedance has gone up, not down from 200 Hz. It continues to rise up to 20 kHz limit of the measurement. This is the opposite of the load being used here in PMA's tests.

Oh come on Amir. You know full well speakers are all over the shop in terms of impedance, especially at 20kHz.

Here's just the two speakers I have in my system at the moment:

Shall I add a few dozen more?

 

[SIY]

Please make clear for the inexperienced forum readers that your thoughts and measurements are purely academic and have virtually nothing to do with the reality of speaker or tweeter.
It is really disappointing how you try to construct a problem by extreme, unrealistic boundary conditions without pointing it out. With every post you succeed in misleading less experienced readers - this is the opposite of what ASR actually stands for.
Don't get me wrong, I have no problem with your measurements, just with your intention that this is a real problem with real speakers - it's not.

First of all, your examples do not specify the sample rate at which you performed the measurements. In 99% of the cases the power amplifier does not "see" more than CD quality sample rate.

Secondly, the dummy load you used (4.7Ohm resistor with || 142nF capacitor) has nothing at all to do with the behavior of real tweeters.

Third, potential sources of measurement error are not explored. To be fair, I don't have the exact same model amp, but I do have several other Hypex, including two different nCore, and using AP equipment for measurement, I cannot duplicate these results. Some examples of what I *do* see will be in an article appearing next month in AudioXpress.

 

[restorer-john]

Some examples of what I *do* see will be in an article appearing next month in AudioXpress.

Sponsored by Hypex or is it Purifi?

I can't wait...

And, at what point does shilling for your magazine put you into the desperate dealers section of ASR?

 

[SIY]

Sponsored by Hypex or is it Purifi?

I can't wait...

And, at what point does shilling for your magazine put you into the desperate dealers section of ASR?

I have no financial interest in the magazine other than getting paid by the page for articles. There are no "sponsors" and I have NEVER been asked to change anything in a review to satisfy an advertiser. And trust me, I have pissed off several of them!

 

[ctrl]

Oh come on Amir. You know full well speakers are all over the shop in terms of impedance, especially at 20kHz.

Here's just the two speakers I have in my system at the moment:

But if I interpret it correctly, your examples also show an inductive behavior towards high frequencies, in contrast to the dummy load used by pma, which always shows capacitive behavior towards high frequencies - which is a crucial difference.

 

[pma]

Frankly, I do not like excuses on technical parameters and circuit behaviour. Any good amplifier MUST handle capacitive load up to at least 200nF without worsening of parameters and dramatic rise of distortion, such as we can see with the Hypex. No excuses, guys, please. This is supposed to be the forum that shows facts, not feelings and excuses. And I can see the opposite, feelings, emotions and excuses drive the replies here.

For the moment, I am done with class D. After extensive testing, I am unsatisfied with both TPA based AIYIMA and with better, but still unsatisfactory Hypex. I hate the messy spectrum that the Hypex creates depending on load and that its behaviour is quite unpredictable. Cannot compete with a consistent behaviour of an average class AB design. Sorry. Send me a Purifi, if you like to know something more than 4/8 ohm resistor load results.

 

[SIY]

Frankly, I do not like excuses on technical parameters and circuit behaviour.

Or questionable measurements. Your data MAY be correct, but it also may be something in your test setup; I wish you'd poke into that a bit more and validate the results. Like I said, using reliable test gear (AP), I can't find any of this claimed behavior in the Hypex amps I've tested.

BTW, still waiting for details on your test setup a month or two ago where you claimed some oddly high current at ultrasonic frequencies through a tweeter. I asked at the time and was ignored.

 

[theREALdotnet]

Any good amplifier MUST handle capacitive load up to at least 200nF

Must they, though? I guess that relevance is the whole point of contention here. One might as well argue (or even demonstrate) that class AB amplifiers perform better under water, but who cares?