On Class D Amplifiers Measurements

[DDF]

Thank you, @DDF , to turn my attention to Jon Risch. I have googled some of his papers and also his posts on audioreview forum. He knows what he speaks about and he is one of those from whom I can still learn something. Thanks again!

Glad to help @pma. I was one of those corporate engineers Jon mentions in audioreview, that adopted his Phi signal. He was kind enough to discuss it at length with me 20 yrs ago and sent me a spreadsheet used in its calculation (iirc correctly, maybe for his first variant). PM me if you'd like a copy.

I tried to persuade @amirm to try it but no luck. Understandably, as the 31 tone AP test signal is very convenient for him and he aims for high throughput

 

[pma]

Glad to help @pma

I tried to persuade @amirm to try it but no luck. Understandably, as the 31 tone AP test signal is very convenient for him and he aims for high throughput

Yes it is completely understandable .

Just sent a PM, thanks!

 

[pma]

As for power into purely resistive loads being pointless, I disagree, but certainly that is only the first step and does not tell the whole story.

I was provoking and exaggerating to initiate feedback .

However, it is my deep confidence that measurements into resistive load must be accompanied by measurements into complex load and if possible into speakers used. The drawback is to listen to that roar .

 

[pma]

I admit I have re-considered my view of multitones, especially if properly chosen. Thanks to all contributors, especially @DDF .
Again 4ohm x JBL Control1, measured at amplifier binding posts, only 3V peak .. (please note mains frequency here is 50Hz and plots are completely free of mains hum lines, due to proper test setup).

 

[howard416]

Yikes, that is certainly something. I wonder how other amps (and specifically other TPA325x implementations) compare.

 

[Head_Unit]

All it needs is a catchy name. Audiophiles will lap them up by the thousand, others will deconstruct and make their own, but it will become an essential add-on. "Do you want a filter with that Class D amplifier, sir?"

"D-Noise"!!!! copyright anno domini 666

Introduce doubt as to whether instruments can measure something the golden-ears claim to be able to hear, and then present the solution.

I hereby grant you an M.B.A.! Old Pink University, class of 2019-3/4

 

[Head_Unit]

8. Power does not double when halving the load impedance.

Is that a Class-D accusation? I'm not being facetious, I'm asking, I wasn't aware of that being a Class D-specific criticism.

NO amps* truly double power at the clipping point; they are all under-rated at the higher impedance to give the appearance of doubling. It works-I persuaded my gigantic employer to "join 'em" and amp sales went up several hundred percent over the next years.

(*Well OK there are 3 exceptions. One Stereophile measured a few months ago-CH Precision? Massive white thing. Didn't *quite* double BUT John Atkinson doesn't hold the wall voltage constant either. Another "doubler" was a late 1980s Mark Levinson IIRC. The other I can't remember. As against dozens and dozens of amps I or my colleagues measured that did NOT truly double power. Which is not necessarily a problem.)

 

[Head_Unit]

So specifically what would you want the changes to be?

My main comments to the FTC were
(1) ENFORCE the bloody guidelines or all is useless
(2) Allow ONLY continuous power ratings with distortion <=1%. There is really no significant reason for anything else.* All else illegal. "Peak power" in particular illegal. Tired of that abused nonsense.

*I did not note to the FTC that yes there are some amps disserviced by this. Maybe a few current fleawatt designs, and ancient Proton & NAD integrateds. Oh well too bad so sad. I am not aware of anyone making designs focused on true big bursts of peak power-are there any? (And not Behringer or Crown cheapos that rate continuously but really only tests in bursts or whatever that controversy is).

 

[pedrob]

I'm not sure you should be type casting all audiophiles as idiots.

 

[Head_Unit]

I'm not sure you should be type casting all audiophiles as idiots.

If you're referring to my comments, that's not what I'm getting at at all. I'd posit the vast majority of consumers are ignorant about audio-because why shouldn't they be? Therefore it is good to have tight standards so consumers can compare apples to apples, and bad players can't snag them with lies. That's not just it audio but in many areas.

 

[Head_Unit]

I admit I have re-considered my view of multitones, especially if properly chosen. Thanks to all contributors, especially @DDF .
Again 4ohm x JBL Control1, measured at amplifier binding posts, only 3V peak .. (please note mains frequency here is 50Hz and plots are completely free of mains hum lines, due to proper test setup).

View attachment 108072

View attachment 108073

I'm confused-there are two different looking plots, but the labeling is the same. (I presume the tiny difference in dBFS level is not significant). What is going on here?

 

[pma]

Distortion into 4 ohm resistor x speaker, as stated. To demonstrate necessity of tests with complex or better real speaker load.

 

[Head_Unit]

Distortion into 4 ohm resistor x speaker, as stated. To demonstrate necessity of tests with complex or better real speaker load.

Oh NOW I see it! My eyes were were playing playing TrIcKs on me me me

Now, into real speakers can be a noisy pain. I wonder what happens into a simulated speaker load, if it is like the actual speaker or (perhaps due to woofer cone movement and back EMF and blah blah) different?

OR if you put a compensating parallel network to flatten the impedance does it make the amp "twice as powerful"???
(That was a KEF ad with Richard Small in it, talking about KEFs such networks which I forget the marketing name of. He told me later it came off a bit...I forget the word he used, not "silly" but something like that and not really what he meant. However for weaker amps it could literally have been true.)

 

[pma]

class D filter for noise and distortion measurements

Though I do not have special problems with soundcard saturation from class D amplifiers during measurements, I have decided to make a simpler version of something like AP AUX-0025 filter. I made an RLC filter that is possible to use with both single ended and BTL output class D power amplifiers, in 20Hz - 20kHz band.

It looks like this, and it has air-core inductors and MKP capacitors:

and this is the frequency response, measured with a balanced divider that is used to attenuate power amplifier output to the soundcard input level.

Not perfect, but usable.

 

[mocenigo]

What do you expect to happen to frequency response at higher levels?

If slew rate is (significantly) limited, then at higher output levels the response in the treble may dip.

 

[pma]

If slew rate is (significantly) limited, then at higher output levels the response in the treble may dip.

Read this, please

https://www.hypex.nl/img/upload/doc/an_wp/AN_UcD_and_IMD_DIM_slewrate.pdf

Conventional look at slew rate based on linear opamps investigations is pointless in class D topology.

 

[mocenigo]

Read this, please

https://www.hypex.nl/img/upload/doc/an_wp/AN_UcD_and_IMD_DIM_slewrate.pdf

Conventional look at slew rate based on linear opamps investigations is pointless in class D topology.

I have read this, and in fact I have used the same paper in an Italian forum to *defend* Class D designs by Putzeys with proper PFFB. And Bruno does mention the fact that the overall slew rate of the device (it does not matter much that he put it between quotation marks) depends on the output filter. Of course what I wrote (i.e. "If slew rate is (significantly) limited" applies to bad or older designs, or non-hifi designs (there is quite a bit of bad Class D around).

 

[pma]

Sorry I am back again with this thread . It was the Amir's review

Audiophonics HPA-S400ET Review (Stereo Amplifier)

This is a review and detailed measurements of the Audiophonics HPA-S400ET. It was sent to me by the company after member request and costs 1 490,00 € (US $1,638). I must say this is one of the best packaging of class D amplifiers (Purifi in this case) that I have seen! It is slick and has...

www.audiosciencereview.com

and especially this measurement

that has shown that even the best current class D module, Purifi 1ET400, suffers from quite severe high frequency non-linearity. Thank you @amirm for performing this kind of @45kHz BW distortion vs. power test as a rule. I have decided to accept it as my standard test method as well and I have tested 3 of my amplifiers under same test conditions, the comparative result I have posted here in this thread:

45kHz BW distortion study of 3 amplifier topologies (2xAB, 1xD) + A/B test

Following the discussion in the Audiophonics HPA-S400ET thread on Purifi HF non-linearity, I have decided to make a comparative measurement of 3 of my amplifiers. THD vs. power into 4ohm with @45kHz bandwidth. The amplifiers under test are: 1) A250W4R, 250W/4ohm class AB amplifier based on...

www.audiosciencereview.com

The only difference from Amir's test is that I plot THD (without N noise component) vs. power so I can better see distortions at low level which would be masked by noise in the THD+N test.

The result with Hypex NC252MP module is this>

It is a bit worse than the Purifi result however similar behaviour. And the result is worse than the class AB amplifiers that I have measured and posted in the thread linked here above.

It is an ultrasonic distortion, you will say. Correct, The poor behaviour results from distortion components above 20kHz. Does it matter, or is it OK? Though 1kHz spectrum from NC252MP is nice, the 5kHz and higher are NOT nice. There are not only ultrasonic harmonics, but also non-harmonic distortion products, like

This cannot be compared to a very good class AB amplifier with clean spectra free of non-harmonic products. So, I am taking up the glove and walking against mainstream again . At least raising an unpopular question. And asking the best class D designers, when are you going to get rid of such circuit behaviour???

 

[RichB]

Sorry I am back again with this thread . It was the Amir's review

Audiophonics HPA-S400ET Review (Stereo Amplifier)

This is a review and detailed measurements of the Audiophonics HPA-S400ET. It was sent to me by the company after member request and costs 1 490,00 € (US $1,638). I must say this is one of the best packaging of class D amplifiers (Purifi in this case) that I have seen! It is slick and has...

www.audiosciencereview.com

and especially this measurement

that has shown that even the best current class D module, Purifi 1ET400, suffers from quite severe high frequency non-linearity. Thank you @amirm for performing this kind of @45kHz BW distortion vs. power test as a rule. I have decided to accept it as my standard test method as well and I have tested 3 of my amplifiers under same test conditions, the comparative result I have posted here in this thread:

45kHz BW distortion study of 3 amplifier topologies (2xAB, 1xD) + A/B test

Following the discussion in the Audiophonics HPA-S400ET thread on Purifi HF non-linearity, I have decided to make a comparative measurement of 3 of my amplifiers. THD vs. power into 4ohm with @45kHz bandwidth. The amplifiers under test are: 1) A250W4R, 250W/4ohm class AB amplifier based on...

www.audiosciencereview.com

The only difference from Amir's test is that I plot THD (without N noise component) vs. power so I can better see distortions at low level which would be masked by noise in the THD+N test.

The result with Hypex NC252MP module is this>
View attachment 196111
It is a bit worse than the Purifi result however similar behaviour. And the result is worse than the class AB amplifiers that I have measured and posted in the thread linked here above.

It is an ultrasonic distortion, you will say. Correct, The poor behaviour results from distortion components above 20kHz. Does it matter, or is it OK? Though 1kHz spectrum from NC252MP is nice, the 5kHz and higher are NOT nice. There are not only ultrasonic harmonics, but also non-harmonic distortion products, like

View attachment 196114

This cannot be compared to a very good class AB amplifier with clean spectra free of non-harmonic products. So, I am taking up the glove and walking against mainstream again . At least raising an unpopular question. And asking the best class D designers, when are you going to get rid of such circuit behaviour???

That would be nice. I predict the first Class-D to reduce high-frequency noise and distortion will market it as superior to all other designs.

- Rich

 

[ahofer]

Sorry I am back again with this thread . It was the Amir's review

Audiophonics HPA-S400ET Review (Stereo Amplifier)

This is a review and detailed measurements of the Audiophonics HPA-S400ET. It was sent to me by the company after member request and costs 1 490,00 € (US $1,638). I must say this is one of the best packaging of class D amplifiers (Purifi in this case) that I have seen! It is slick and has...

www.audiosciencereview.com

and especially this measurement

that has shown that even the best current class D module, Purifi 1ET400, suffers from quite severe high frequency non-linearity. Thank you @amirm for performing this kind of @45kHz BW distortion vs. power test as a rule. I have decided to accept it as my standard test method as well and I have tested 3 of my amplifiers under same test conditions, the comparative result I have posted here in this thread:

45kHz BW distortion study of 3 amplifier topologies (2xAB, 1xD) + A/B test

Following the discussion in the Audiophonics HPA-S400ET thread on Purifi HF non-linearity, I have decided to make a comparative measurement of 3 of my amplifiers. THD vs. power into 4ohm with @45kHz bandwidth. The amplifiers under test are: 1) A250W4R, 250W/4ohm class AB amplifier based on...

www.audiosciencereview.com

The only difference from Amir's test is that I plot THD (without N noise component) vs. power so I can better see distortions at low level which would be masked by noise in the THD+N test.

The result with Hypex NC252MP module is this>
View attachment 196111
It is a bit worse than the Purifi result however similar behaviour. And the result is worse than the class AB amplifiers that I have measured and posted in the thread linked here above.

It is an ultrasonic distortion, you will say. Correct, The poor behaviour results from distortion components above 20kHz. Does it matter, or is it OK? Though 1kHz spectrum from NC252MP is nice, the 5kHz and higher are NOT nice. There are not only ultrasonic harmonics, but also non-harmonic distortion products, like

View attachment 196114

This cannot be compared to a very good class AB amplifier with clean spectra free of non-harmonic products. So, I am taking up the glove and walking against mainstream again . At least raising an unpopular question. And asking the best class D designers, when are you going to get rid of such circuit behaviour???

Not quite following this. Are you saying that these artifacts, all under -100db are audible? 5Khz iz the fundamental here, right?