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

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pma

pma

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I have contacted Bruno directly and asked him what he things about the issue with 4ohm//2.2uF. This is the reply I have received:

It’s actually not a problem with electrostatic speakers because the transformer has sufficient leakage inductance and series resistance to decouple the capacitive part. You only run into trouble with big capacitance directly across the output with no series impedance.

Frankly, I appreciate him to reply, but am not sure that it covers the issue. So I have added an inductor of 330uH (quite high inductance) to the 2.2uF capacitor and got this impedance:
(The inductor used has 0.8 ohm ohmic resistance)

impedance_4R+(2.2uF-330uH)Rs.png


Though the NC 252MP now passes the usual test of THD+N vs. level at 1kHz, it is still unacceptable at higher frequencies, namely I tested 5kHz, THD+N vs. output voltage with BW22kHz.

NC252MP_4R_4R+(2.2uF-330uH)_THD+N_5kHz.png


NC252MP_4R+(2.2uF-330uH)_THDN_freq.png


So, the distortion of this amplifier (and probably any other Ncore) is not "load invariant", if we have complex load instead of pure resistors.
 
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fpitas

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Yeah I understand that, I wonder though what magnitude of effect having two crossovers in parallel would have in some of these speakers. Some crossovers have upwards of 10 elements affecting phase and impedance, I would’ve thought 2 in parallel would make for a more difficult load, only measurement would tell though.

Cheers, Andrew
They do interact, sometimes quite badly so there is too low impedance. It's all part of designing a passive crossover.
 
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pma

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Any specific reason why no series resistance as suggested by Bruno?

Thank you, it is a good question. In fact, there is a series resistance of the inductor and it was measured as 0.8 ohm. I have corrected the previous post. And I will make impedance measurement - you will see that the load will be "lighter" than that simulated.

Bruno may be suggesting, but I have quite a lot of my own experience.
 

boXem

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I have contacted Bruno directly and asked him what he things about the issue with 4ohm//2.2uF. This is the reply I have received:



Frankly, I appreciate him to reply, but am not sure that it covers the issue. So I have added an inductor of 330uH (quite high inductance) to the 2.2uF capacitor and got this impedance:

View attachment 275491

Though the NC 252MP now passes the usual test of THD+N vs. level at 1kHz, it is still unacceptable at higher frequencies, namely I tested 5kHz, THD+N vs. output voltage with BW22kHz.
I am not sure that you got what BP said. Transformer, not inductance
Es_spk.gif


Only the speaker cables cause a purely capacitive load to the outputs of an amplifier. And it is far below the uF.
 
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pma

pma

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Frankly, I appreciate him to reply, but am not sure that it covers the issue. So I have added an inductor of 330uH (quite high inductance) to the 2.2uF capacitor and got this impedance:
(The inductor used has 0.8 ohm ohmic resistance)
Thank you, it is a good question. In fact, there is a series resistance of the inductor and it was measured as 0.8 ohm. I have corrected the previous post. And I will make impedance measurement - you will see that the load will be "lighter" than that simulated.

This is the measured impedance. The power resistor was 4R7/200W this time, thus higher resistance

impedance_4R7+(2.2uF-330uH)Rs_measured.png
 
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pma

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I am not sure that you got what BP said. Transformer, not inductance
Es_spk.gif


Only the speaker cables cause a purely capacitive load to the outputs of an amplifier. And it is far below the uF.

Old story repeated, some electrostats are difficult and you have to look at real impedance measurements. I understand the wishes of class D designers and OEM assemblers, but I am showing facts. And my load was this time light. With 0.8 ohm resistor in series with 330uH inductor and 2.2uF capacitor.

IMG_2053.JPG


IMG_2054.JPG



Martin Logan

martinlogan.png
 

boXem

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Old story repeated, some electrostats are difficult and you have to look at real impedance measurements. I understand the wishes of class D designers and OEM assemblers, but I am showing facts.
Sorry Pavel but you are not showing facts. Just spreading FUD, I hope more due to incompetence than anything else.

You take one sample (measurement point) and find a solution for a circuit of your liking. This supposes that there is a single circuit producing the (Re, Im) couple you measured. This is obviously plain wrong for anybody having attended to electricity 101.

This circuit has nothing to do with what a real speaker is at any other frequency than the one of the sample. But you submit this circuit to a wide frequency range. This is again plain wrong.

To quote your signature "oversimplification always leads to false conclusions".

You will always find a load for which a control system will oscillate at a certain frequency. There is nothing new here.
When facing oscillations of a control system, two questions need to be asked:
- is my load representative of real world? Answer is no.
- was the system able to survive? Answer is yes, which is pretty good news.
 
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@pma I don't get it. You are clearly skilled enough to do both circuits and the measurements and the math. -Why is it that you don't try to prove your point with a real equivalent circuit test? -Based on measurements from a real electrostat or another speaker with troublesome characteristics. This way we can actually investigate amplifier limits proper. I would really love to see tests like this but they should be an the basis of real loads (and within spec. of the tested amp) so we perhaps can learn something from them.
 

LTig

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The ESLs I've measured (admittedly a limited sample) show a capacitance in series with a resistance. So a more realistic load, even for an edge case, is the 2u2 in series with (say) 1R.
That's the resistance of the primary coil of the transformer, I'd say, or am I wrong?
 

SIY

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That's the resistance of the primary coil of the transformer, I'd say, or am I wrong?
That plus the reflected series resistance of the secondary. Setting up a series resonant circuit as a dummy load makes less sense to me.
 

SIY

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@pma Based on measurements from a real electrostat or another speaker with troublesome characteristics.
Problem is, of course, that an accurate model for an ESL will show sparks and smoke with full power signals at the top of the audio band.:D
 

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Just a note,coming back after listening to Martin Logans (ESL15).
Normally driven by Lamm M1.2 which the owner says can go down to 1 Ohm,had a look on net is probably is true.

Tested it with the known Audiophonics Purifi Sparkos edition and no apparent problem occurred.
No hiss or anything weird when close,only thing is that got very hot when we pushed matters a little so we dialed a little lower.

Tested with my Ice Power 1200as2,the same,only it didn't got so hot,good thing is no indicator lit and no protection kicked in.

Wish we had some Ncore at hand to see.

Maybe some other ELS can get more difficult.I don't know.
 

HarmonicTHD

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Seems with enough (unrealistic?) capacitative load one gets every amp to ring.

Here one well measuring A/B class example (see at the very bottom of performance graphs - time domain scope shots). BTW @tomchr deems 100nF already rather large.

 
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MaxwellsEq

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Seems with enough (unrealistic?) capacitative load one gets every amp to ring.

Here one get well measuring A/B class example (see at the very bottom of performance graphs - time domain scope shots). BTW @tomchr deems 100nF already rather large.

The designer points out that the ringing dies out quickly, which he considers to be a positive feature.
 

antcollinet

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Perfect, thanks.

Honestly, if it turns out to be the speakers, I'll move them elsewhere in the house where they're less used and stick something else at the end of the NP***MP based setup.

I'm not particularly constrained by budget and feel I have simple requirements, roon server -> (anything) -> my ears - for 8-12 hours a day - constrained listening environment and speaker placement. The audio world is a minefield to navigate, wherever I turn, and the more I research and read the more of a minefield and rabbit hole it becomes.
Seems a fair approach. The ncore amp is excellent, and if purchased will not be the bottle neck.
 

sonder

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Seems a fair approach. The ncore amp is excellent, and if purchased will not be the bottle neck.
Tony, in other fields I work in, we have an expression "angels on pinheads", in a thread full of - I'm sure we'll intentioned - pedantry, you've provided a moment of simple clarity, that will make a real world difference, and my ears thank you in advance. Cheers for taking the moment out of your day to share your insight.
 
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pma

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I have contacted Bruno directly and asked him what he things about the issue with 4ohm//2.2uF. This is the reply I have received:
It’s actually not a problem with electrostatic speakers because the transformer has sufficient leakage inductance and series resistance to decouple the capacitive part. You only run into trouble with big capacitance directly across the output with no series impedance.

Back to the topic, load 4R//2.2uF. Unfortunately, the advice above does not work. I have added a small air-core inductor in series with the 2.2uF capacitor to get this measured impedance:

impedance_4R+(2.2uF+Ls).png

Please note the inductive magnitude rise above 75kHz and phase rising above 45kHz. Also note there is a minimum resistance of about 0.3 ohm.

The NC252MP does not like this load. It is even less stable than it was with 4R//2.2uF load. The quasi-periodic oscillations are moving through audio band above 1kHz.

NC252MP_noisespectrum_impedance_4R+(2.2uF+Ls)_90k.png

In-band noise (BW22kHz) is 19mV.

Though the class AB amplifiers I have here work normally with this load and even with 4R//2.2uF load, NC252MP cannot be used with it. Below the comparison of THD+N vs. frequency into 4R7 // (2.2uF + Ls) at 7V output voltage, compared to small class AB amplifier PM-AB2.

NC252MP_PM-AB2_THDNfreq_7V_4R7+(2.2uF+Ls).png


Again Ncore is excellent with pure resistors, good with "usual" speaker load but hardly usable with speakers that have high capacitive impedance, even if in series with small inductance and resistance.
 

boXem

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Back to the topic, load 4R//2.2uF. Unfortunately, the advice above does not work. I have added a small air-core inductor in series with the 2.2uF capacitor to get this measured impedance:
...
I am starting to wonder if you do not understand the advises on purpose. "the transformer has sufficient leakage inductance and series resistance"
 
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