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

amirm

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The issue found by the OP has nothing to do with simple voltage divider math at the output of the amp - nothing at all.
We are not discussing OP. Conversation has frequently turned into complex load emulating speakers and someone post graphs from stereophile claiming that is the justification for doing those kind of tests as well. I and SIY have explained why the data is already contained in my measurements.

The issue of large capacitor across a resistor is different. I have said and repeat that it doesn't represent any loads. Nor to your argument, is it the hardest load. Pretty sure a 1 ohm load would kill many amplifiers regardless of topology. Should we adopt that too?

You speak of RF. It is easy to blow my many RF amps/output stages with too much SWR. I know, I put myself through college by repairing tons of CB and VHF radios. You are supposed to have matched impedance and no manufacture goes out of its way to make a consumer product be immune to poor loads. Maybe for some industrial applications where you don't care about the cost and worry about misconfiguration you handle difficult loads. But not in consumer products. I certainly have not seen any protection circuits in consumer high power RF gear whereas that is standard practice for audio.
 

dc655321

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Friends, there is no "problem" with the Hypex NC252MP. ZERO.

The OP postulated some hypothetical and pathalogical test load that drops the impedance below 2 Ohms at 20kHz AND keeps dropping it to ZERO Ohms above that point. As far as I can tell there has been exactly ONE speaker that has such a pathalogical load, some electrostatic. The solution is that the designer of that speaker should be SHOT, and NOT that the world needs to design audio amplifers that can operate into a dead short above 20kHz, where there is no audible output anyway.

Well said.

@pma created this load for (presumably) testing the amp beyond its limits.
Congrats. Done.

Interesting lessons should not be delivered from a soapbox, however.
 

amirm

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On a related note to @amirm - the people who are asking for amplifier performance measured into a dummy loudspeaker test load aren't crazy. While it is nice to see testing into various resistive loads, the sometimes widely varying phase angles of a loudspeaker with a passive crossover can also deteriorate the perfomance of an amplifier when it drives a predominantly capacitive or inductive load.
Again, I am investing in an expensive load to do this test and have included it in my last couple of amplifier tests. But I worry that we are testing for a spec that is not matching reality (music is impulsive whereas sine waves are not). I also worry about blowing up more amplifiers than I already have. Dealing with this is non-trivial.
 

Hapo

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61APV77VOCL._AC_SL1200_.jpg


About this item​

  • RMS Output Power: 40W Max Power Handling: 350W
  • Impedance: 8 Ohms Nominal, 2.5 Ohm Minimum
  • Sensitivity: 92.5dB
  • Frequency Response: 37Hz - 28kHz
 

jimk1963

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Again, I am investing in an expensive load to do this test and have included it in my last couple of amplifier tests. But I worry that we are testing for a spec that is not matching reality (music is impulsive whereas sine waves are not). I also worry about blowing up more amplifiers than I already have. Dealing with this is non-trivial.
Understood. I’ll repeat, your testing is invaluable to this community. That includes the considerable investments you’ve made in equipment, not to mention your time and energy. It’s greatly appreciated. As are your insights. Engineer in me wants to question everything, hazard of the trade. Now, where can I find more in-wall speaker reviews…. I’m tired of these B&W’s.
 

Rottmannash

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It is the opposite than @SIY is saying. RF susceptibility, input stage rectification, demodulation very often directly translate to audio. 20Hz - 20kHz view is short-eyed. Unfortunately, this is not a forum with many knowledgeable and experienced circuit designers.
Oh snap!
 

valerianf

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Thank you Amir for investing in a complex load simulator.
The difficulty will be how to set the test parameters.
May be it would be nice to measure the characteristics of a well known tower loudspeaker (i.e. the Focal, JBL, Klipsch, B&W...) that has 2 large woofers (the inductance/capacitance of the woofers may bring some strange amplifier behavior).
These values will be a good starting point.

About protecting the amp, ramping up the power while monitoring the distortion may be sufficient to stop the test if there is any abnormal increase.
Regarding the simulation of an electrostatic panel behavior, a new speaker electrical model needs to be used: I remember seeing an electrostatic panel placed above a large woofer. It may not be an easy task with the mixing of the two technologies.
 

Rottmannash

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Thank you Amir for investing in a complex load simulator.
The difficulty will be how to set the test parameters.
May be it would be nice to measure the characteristics of a well known tower loudspeaker (i.e. the Focal, JBL, Klipsch, B&W...) that has 2 large woofers (the inductance/capacitance of the woofers may bring some strange amplifier behavior).
These values will be a good starting point.

About protecting the amp, ramping up the power while monitoring the distortion may be sufficient to stop the test if there is any abnormal increase.
Regarding the simulation of an electrostatic panel behavior, a new speaker electrical model needs to be used: I remember seeing an electrostatic panel placed above a large woofer. It may not be an easy task with the mixing of the two technologies.
When I first received my Buckeye 252 amp I cranked the volume to ear splitting levels through F208's w/ nary a hiccup or sign of distortion or shutdown. Actually I couldn't discern a difference between it and the Purifi, truth be known. Anecdotal evidence doesn't mean much here but the F208's have 2-8 inch woofers so would be a good candidate for testing.
 

JktHifi

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NC252MP Power, all channels driven:

  • 180w per channel @ 2ohms
  • 250w per channel @ 4ohms
  • 150w per channel @ 8ohms
vs

A250W4R DIY 250W/4ohm

  • output power ….. 2x250W/4ohm for THD < 0.1% at 1kHz
where’s the 8ohm?
 
OP
pma

pma

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  • output power ….. 2x250W/4ohm for THD < 0.1% at 1kHz
where’s the 8ohm?
It is 2 x 150W/8ohm at same conditions. Absolutely unimportant point regarding thread topic.
More - there is no speaker that would have 8 ohm purely resistive impedance. The reason why distortion measurements were done into 8 ohm resistor is that it makes light load to the amplifier and distortion numbers looked better.
 

JktHifi

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It is 2 x 150W/8ohm at same conditions. Absolutely unimportant point regarding thread topic.
More - there is no speaker that would have 8 ohm purely resistive impedance. The reason why distortion measurements were done into 8 ohm resistor is that it makes light load to the amplifier and distortion numbers looked better.
Sorry to disturb. It’s from consumer perspective only.
After looking at the inside of A250W4R. It’s almost same construction to this $11,295 amp: (6 capacitors for each transformer)

PNG image.png


I think we shouldn’t compare $500 vs $11,295 amp. Again, I’m only a consumer.
 
Last edited:

Sokel

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[off-topic]


It seems that something went wrong with your analysis. If one takes your most extreme example, Pan Sonic-Suhteellinen, the sound pressure spectrum for the entire track looks like this:
View attachment 277129
I found what looks wrong in REW's analysis,I quote from their manual:

The difference between spectrum and RTA modes is how the information is presented. In spectrum mode the frequency content of the signal is split up into bins that are all the same width in Hz. For example, with a 64k FFT length and 48 kHz sample rate the bins are 0.732 Hz wide. The plot shows the energy in each of those bins. In RTA mode the bin widths are an octave fraction, so their width in Hz varies with the frequency. For example, a 1 octave RTA plot has bins that are 70.7 Hz wide at 100 Hz (from 70.7 Hz to 141.4 Hz) and 707 Hz wide at 1 kHz (from 707 Hz to 1.414 kHz). The plot shows the combined energy at each frequency within each bin. This is closer to how our ears perceive sound. The different presentations mean signals with a spread of frequency content will look different on the plot.
The best known examples are white noise and pink noise. White noise has the same energy at each frequency. On a spectrum plot, which shows the energy at each frequency, the white noise plots as a horizontal line. On an RTA plot it appears as a line that rises with increasing frequency, as each RTA bin gets wider it covers more frequencies and so has more energy. The bin widths double with each doubling of frequency so the energy also doubles, which adds 3 dB on the logarithmic plots we use to show level. White noise sounds quite 'hissy', we perceive it as having more energy at higher frequencies.

Pink noise has energy that falls 3 dB with each doubling of frequency. On a spectrum plot it is a line that falls at that 3 dB per octave rate, on an RTA plot it is a horizontal line as the energy in the signal is falling at the same rate as the bins are widening. We perceive pink noise as having a uniform distribution of energy with frequency.

So I was clearly wrong presenting it in this mode for the actual energy even if absolutely justifies what sounds like.
 

MaxwellsEq

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You have it- if there's no difference in 8 and 4 traces, Zo = 0. If there is a difference that is frequency independent, the output Z is resistive. If there is a frequency dependent difference, then the output Z is reactive and again, that is the Thevenin equivalent source impedance from which interaction with loads is perfectly predictable.

The most common case is a Zo that is slightly inductive since the loop gain rolls off causing Zo to rise at high frequency.
Thanks, I've got it. In the real world, Zo won't be zero and won't be purely resistive (since that would imply a constant resistance to GHz frequencies and beyond). In general, Zo appears as inductive, because it trends from almost a short circuit at low frequencies to almost an open circuit at high frequencies.

Therefore,
  • No variation in 8 Ohm and 4 Ohm traces implies a perfect Zo of 0 which we know can't be true, so in the real world no variation in 8 Ohm and 4 Ohm traces implies Zo has almost no resistance and such a small level of inductance that is it is not significant in the audio band.
  • Also, in the real world frequency-independent variations in 8 Ohm and 4 Ohm traces implies Zo has a certain element of fixed resistance (relative to 4/8) and a very small amount of inductance, again not significant in the audio band
  • Finally, in the real world, frequency variations between 8 Ohm and 4 Ohm traces implies Zo has some (but possibly more) resistance and sufficient inductance to have an effect in the audio band.
 

IAtaman

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You have it- if there's no difference in 8 and 4 traces, Zo = 0. If there is a difference that is frequency independent, the output Z is resistive. If there is a frequency dependent difference, then the output Z is reactive and again, that is the Thevenin equivalent source impedance from which interaction with loads is perfectly predictable.

The most common case is a Zo that is slightly inductive since the loop gain rolls off causing Zo to rise at high frequency.
Thank you for this, it is such a straightforward and clear way of looking at it that only comes with loads of hands on experience I suppose.
What about the phase, can there be an interaction between the load reactance and the feedback network of the amp to cause the amp to oscillate that we can not estimate simply by calculating Zo?
 

SIY

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Thank you for this, it is such a straightforward and clear way of looking at it that only comes with loads of hands on experience I suppose.
What about the phase, can there be an interaction between the load reactance and the feedback network of the amp to cause the amp to oscillate that we can not estimate simply by calculating Zo?
Phase is accounted for, since electrically the load is minimum phase, i.e., the phase is equal to the derivative of the magnitude wrt frequency.
 

SIY

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Thanks, I've got it.
Yes, you do. :cool: But I'd caveat that in the audible range, the variation is not generally going to be high, that is, the inductance is not large enough to get you to "open circuit." But it will manifest itself in the resistive load curves as more rolloff at the top of the band. Of course, with the vast majority of speakers, the load is likewise inductive at the top of the audio band as well, which reduces the effect on frequency response in that region.
 

IAtaman

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Phase is accounted for, since electrically the load is minimum phase, i.e., the phase is equal to the derivative of the magnitude wrt frequency.
Sorry, not sure I got it right. I was asking interaction between the phase of the load and the amp, and if Iunderstand you corrrectly, you are saying that "yes, that is possible, but as we can calculate the phase of the amp's output impedance at any given frequency based on resistive load measurements, we can already predict how an amp will behave with any given load". Is this right?
 
OP
pma

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More facts, more measurements:

UcD180HG passed the test with 4.7ohm//(2.2uF+1.6uH+0.3ohm) load. Almost no difference with this complex load vs. resistor only. Also no additional noise spectrum problems.

UcD180_dummycapload_noise.png


UcD180_dummycapload_thdn.png


So far:

Amplifierclass of operationResult
Hypex NC252MPclass DFailed
Hypex UcD180HGclass DPassed
AIYIMA A07class D LC filter behind feedbackPassed
A250W4Rclass AB bipolarPassed
PM-AB2class AB MOSFETPassed
 
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