Replace OP amps. Completely pointless, or not?

[antcollinet]

Maybe it could be as you say but this is what an EE who manufactures amplifiers said about the measurement Stereophile performed on the NAD C 298 amp , so I don't know what to believe:

Well he seems to be talking about THD there, not IMD.

What he is saying is that since, harmonic products are always a multiple of the "generating" frequency.the lowest frequency harmonic distortion product is 2x the frequency it is generated from. So if you limit measurement bandwidth to 25kHz, the highest frequency you will get a harmonic of is 12.5kHz.

THD is supposedly a measure of the "total" harmonic distortion but you are not measuring distortion of signals AT ALL above 12.5kHz, and are cutting off a lot of the harmonics of lower frequencies.

In reality it doesn't matter much - because the harmonic products that are ultansonic (that you are not measuring) are not audible.

However, this is only fair if you are comparing like with like. If you compare one amp measured with a wide bandwitdth, with another amp measured with a lower bandwidth then the higher bandwidth measurement will always give a higher THD figure, even if the actual distortion is the same.

So to compare the THD of two amps, make sure the measurement bandwidth is the same. If it is always 25kHz, then fine.

 

[SIY]

Maybe it could be as you say but this is what an EE who manufactures amplifiers said about the measurement Stereophile performed on the NAD C 298 amp , so I don't know what to believe:

Morello:

Note that the measurements on this class-D device are carried out with the low-pass filter (f=25 kHz) Audio Precision AUX-0025 connected between the output of the power amp and the input of the analyzer, which is why all THD measurements are irrelevant above about 6 kHz. I'm surprised Atkinson doesn't limit the graphs to just this.
....
The reason for the measurement procedure is that the analyzer from Audio Precision that Atkinson uses does not handle copious amounts of high-frequency garbage typical class-D spews out. What I object to is that THD data is still reported up to 20 kHz, even though the data is not relevant above 6-7 kHz. It is also clear that many readers of testers do not reflect on this, which is, of course, completely understandable.(because they don't understand how this thing with measurements should be done, if I interpret what he says)
.....
THD is a measure of the step nonlinearities - quadratic, cubic and higher order terms in the transfer characteristic. The same terms entail intermodulation distortion. If you want a correct picture of the amp's distortion, the bandwidth when measuring THD up to 20 kHz must be at least 80 kHz, which means that harmonics up to and including order four are included.

The Nad C298 shows high THD at treble frequencies above 10 kHz, despite the fact that the numbers are heavily embellished due to the low-pass filter used in the THD measurement.
(with reservation for inaccuracies via google translate)
Note I'm just the messenger so don't shoot me down.

Edit:
I'll translate, or let google do it, a little more from that thread what Morello says:

...measuring THD up to 20 kHz with 25 kHz bandwidth is nothing to have opinions about - if you understand the implications, namely that the harmonics in the top octave are not included, it is easily realized that it is de facto an inappropriate procedure as the result is nonsense. ..

..I don't think you understood what I wrote. If an amplifier exhibits THD of 1% at 20 kHz with a dominant third tone, it means that the third tone has a frequency of 60 kHz. If you measure with 25 kHz bandwidth, the third tone will be attenuated by, for example, 40 dB (depending on the filter's flank attenuation) and the measurement will show a THD of 0.01%. The measurement thus gives a false result. ..

I-or in that thread:
What Morello has tried to convey above (and in many older threads) is that the nonlinearities that give rise to inaudible spectral components above 20kHz when measuring THD also give rise to audible intermodulation distortion (in the audible range). If you measure the IMD instead, this will be clear.

faktiskt.io • Visa tråd - NAD C 298

NAD C 298 power amplifier Measurements

Sidebar 4: Measurements

www.stereophile.com

I don't know who "Morello" is, but there's a lot of half-true and thus misleading stuff in there.

 

[DanielT]

Well he seems to be talking about THD there, not IMD.

What he is saying is that since, harmonic products are always a multiple of the "generating" frequency.the lowest frequency harmonic distortion product is 2x the frequency it is generated from. So if you limit measurement bandwidth to 25kHz, the highest frequency you will get a harmonic of is 12.5kHz.

THD is supposedly a measure of the "total" harmonic distortion but you are not measuring distortion of signals AT ALL above 12.5kHz, and are cutting off a lot of the harmonics of lower frequencies.

In reality it doesn't matter much - because the harmonic products that are ultansonic (that you are not measuring) are not audible.

However, this is only fair if you are comparing like with like. If you compare one amp measured with a wide bandwitdth, with another amp measured with a lower bandwidth then the higher bandwidth measurement will always give a higher THD figure, even if the actual distortion is the same.

So to compare the THD of two amps, make sure the measurement bandwidth is the same. If it is always 25kHz, then fine.

I don't know who "Morello" is, but there's a lot of half-true and thus misleading stuff in there.

Interesting. It is fascinating that two technically driven forums can have such different views on class D amplifiers and measurements on them. Now, I'm not the first to note that. Even Belker, who writes here on ASR sometimes, has noted it in the thread I refer to earlier:

.... the level of knowledge here (on the forum Faktiskt ) has high peaks, but so does it on ASR. A significantly larger forum with many designers and educated people. That is why it is so strange that there is no consensus.

I'm just curious but other than that I have nothing to add on the subject.

faktiskt.io • Visa tråd - NAD C 298

 

[Matias]

More measurements.

Hypex NCx500 Class D Amplifier Review

https://audiosciencereview.com/forum/index.php?threads/the-state-of-the-art-class-d-audiophonics-hpa-s600nc-and-audiophonics-hpa-s400et-sound-different-though-there-is-a-caveat-here.47377/post-1696152 @Apollon Audio It's nice to have some insight about the differences in sound using these op...

www.audiosciencereview.com

 

[antcollinet]

Interesting. It is fascinating that two technically driven forums can have such different views on class D amplifiers and measurements on them. Now, I'm not the first to note that. Even Belker, who writes here on ASR sometimes, has noted it in the thread I refer to earlier:

.... the level of knowledge here (on the forum Faktiskt ) has high peaks, but so does it on ASR. A significantly larger forum with many designers and educated people. That is why it is so strange that there is no consensus.

I'm just curious but other than that I have nothing to add on the subject.

faktiskt.io • Visa tråd - NAD C 298

There is one point mentioned that I didn't address.

If you rely only on THD, then limiting the bandwidth ingnores distortion products that can then inter-modulate with other frequencies back into the audio band.

This might be a problem. I don't care about ultrasonic harmonic distortion products, but I do care if we get resultant audible IMD products.

Luckly we don't rely only on the THD measurement. We also have the multitone measurement - which is somewhat representative of a complex real music signal.

The multitones will not only generate Harmonc distortion products at all multiples of all the tones. But will also inter-modulate between each other AND between themselves and all the other harmonic and intermodulation distortion products. Even the ones we don't measure in the THD measurement.

The result of all that distortion going on is the "grass" at the base of that measurement. What we see with competent designs is that grass is well below the level of audiblity.

So - who cares if we miss some of the ultrasonic harmonics, if they don't create any audible problem?

 

[SIY]

Interesting. It is fascinating that two technically driven forums can have such different views on class D amplifiers and measurements on them. Now, I'm not the first to note that. Even Belker, who writes here on ASR sometimes, has noted it in the thread I refer to earlier:

.... the level of knowledge here (on the forum Faktiskt ) has high peaks, but so does it on ASR. A significantly larger forum with many designers and educated people. That is why it is so strange that there is no consensus.

I'm just curious but other than that I have nothing to add on the subject.

faktiskt.io • Visa tråd - NAD C 298

There's a vocal minority who seem to have philosophical objections to Class D. Maybe it's because it commodifies high power and high performance? Maybe because it obsoletes their expertise on linear A/AB amps? But I've certainly had problems verifying their dark views (and sometimes measurements) on my lab bench and in my listening room. I have yet another high power moderate cost Class D amp on my workbench right now which outperforms the AP analyzer regardless of the load I throw at it. And it's far from the first. My challenge is finding something new to say for yet another stellar performer- it seems to be the norm now, not the exception.

 

[DonR]

There's a vocal minority who seem to have philosophical objections to Class D. Maybe it's because it commodifies high power and high performance? Maybe because it obsoletes their expertise on linear A/AB amps? But I've certainly had problems verifying their dark views (and sometimes measurements) on my lab bench and in my listening room. I have yet another high power moderate cost Class D amp on my workbench right now which outperforms the AP analyzer regardless of the load I throw at it. And it's far from the first. My challenge is finding something new to say for yet another stellar performer- it seems to be the norm now, not the exception.

If people knew less about the Class D theory of operation they might not have so many objections (a little knowledge is a dangerous thing). Much like the advent of digital music which found many critics in the early days despite the fact that studios were using the technology widely up to a decade earlier.

 

[DanielT]

What we see with competnt designs is that grass is well below the level of audiblity.

So - who cares if we miss some of the ultrasonic harmonics, if they don't create any audible problem?

And to be able to sniff out amp distortion (if it is not driven into clipping) you need ultra-low distortion speakers and a lot of training in detecting distortion. Of which I have none, luckily for my wallet, I'm just saying.

There's a vocal minority who seem to have philosophical objections to Class D. Maybe it's because it commodifies high power and high performance? Maybe because it obsoletes their expertise on linear A/AB amps? But I've certainly had problems verifying their dark views (and sometimes measurements) on my lab bench and in my listening room. I have yet another high power moderate cost Class D amp on my workbench right now which outperforms the AP analyzer regardless of the load I throw at it. And it's far from the first. My challenge is finding something new to say for yet another stellar performer- it seems to be the norm now, not the exception.

Now that's more like it. It's just a matter of measuring at different bandwidths and then seeing how it looks with IMD, THD in the higher frequencies, within the audible range. Preferably comparative measurements for higher order harmonics.
When I say It's just a matter, I mean for those who have measuring equipment and the knowledge, which you seem to have.

If people knew less about the Class D theory of operation they might not have so many objections (a little knowledge is a dangerous thing). Much like the advent of digital music which found many critics in the early days despite the fact that studios were using the technology widely up to a decade earlier.

I like class D and so do a lot of people on that other forum. There may be a greater skepticism towards cheap load-dependent class D amps there than here at ASR, but good Hypex, for example, is appreciated and used by many on that forum.
There has been a development of class D amps in recent decades, which you are clearly aware of, but I still mention it for others who read this thread. Those who are notoriously skeptical of class D may have missed this development, what do I know?
Gone are the days when it was said that class D amps are only good for subwoofers. Well, maybe there are those who say that now, but missed that this is not the case nowadays.

Bruno Putzeys talks about the class D development in this interview:

Bruno Putzeys: Head of the Class (D, That is) Page 2

Class D is also thoroughly unforgiving. Every detail has to be right and there simply isn’t room to indulge scientifically unproven flights of fancy. Established audiophile habits become a blocking item. You can always construct a passable Class A amplifier that tiptoes around any conceivable...

www.soundandvision.com

 

[MaxwellsEq]

So to compare the THD of two amps, make sure the measurement bandwidth is the same. If it is always 25kHz, then fine

That's where I think the objections are. I've seen it expressed as "Class D has this high frequency problem and lab measurements are covering it up by including a filter that blocks this high frequency problem, but Class AB are not being filtered so are getting a tougher ride than Class D".

This is potentially valid. If all the historical reviews of Class AB amps have TOTAL harmonic distortion including harmonics we can't hear, then THD as a number is not a valid comparator if one is with a filter and one isn't.

 

[NTK]

That's where I think the objections are. I've seen it expressed as "Class D has this high frequency problem and lab measurements are covering it up by including a filter that blocks this high frequency problem, but Class AB are not being filtered so are getting a tougher ride than Class D".

This is potentially valid. If all the historical reviews of Class AB amps have TOTAL harmonic distortion including harmonics we can't hear, then THD as a number is not a valid comparator if one is with a filter and one isn't.

Amir's headline SINAD numbers are measured with a 1 kHz test signal. If the measurement bandwidth is limited to 20 kHz, that covers to the 20th harmonic. Do we expect the H21, H22, ... etc., harmonics to make any significant differences?

For THD frequency sweeps, Amir increases the measurement bandwidth to 45 kHz.

 

[DanielT]

Amir's headline SINAD numbers are measured with a 1 kHz test signal. If the measurement bandwidth is limited to 20 kHz, that covers to the 20th harmonic. Do we expect the H21, H22, ... etc., harmonics to make any significant differences?

For THD frequency sweeps, Amir increases the measurement bandwidth to 45 kHz.

Sure, but measuring a few watts at 1 kHz is one thing. Measuring half power or even closer to the clipping point at 6kHz and above in frequency is another matter.

 

[NTK]

Sure, but measuring a few watts at 1 kHz is one thing. Measuring half power or even closer to the clipping point at 6kHz and above in frequency is another matter.

You listen to music with half of its power at 6 kHz or above? Or you are just interested in measurement for measurement's sake?

 

[DanielT]

You listen to music with half of its power at 6 kHz or above? Or you are just interested in measurement for measurement's sake?

But it might be interesting to know what it looks like. Sooner or later, probably later, the high-frequency distortion becomes audible. Amir includes it in his measurements. An example:

Topping PA5 II Stereo Amplifier Review

This is a review and detailed measurements of the Topping PA5 II stereo balanced amplifier. It was sent to me by the company and costs US $219 (sale). There is a plus version with higher current power supply for $289.50. The fonts are hard to see but there is not much to read anyway. A power...

audiosciencereview.com

 

[MaxwellsEq]

Amir's headline SINAD numbers are measured with a 1 kHz test signal. If the measurement bandwidth is limited to 20 kHz, that covers to the 20th harmonic. Do we expect the H21, H22, ... etc., harmonics to make any significant differences?

I don't think anyone expects to hear variations in H21 etc.

 

[egellings]

That is not needed. If IM Distortion components appear in the audible frequency range, they will be there whether or not we measure higher (ultrasonic) frequencies.

To clarify, the high frequencies that inter-modulate and create audible band component are there in the signal whether we measure them or not. Those components will be visible in a measurement limited to 20kHz.

Agree!

 

[egellings]

There is one point mentioned that I didn't address.

If you rely only on THD, then limiting the bandwidth ingnores distortion products that can then inter-modulate with other frequencies back into the audio band.

This might be a problem. I don't care about ultrasonic harmonic distortion products, but I do care if we get resultant audible IMD products.

Luckly we don't rely only on the THD measurement. We also have the multitone measurement - which is somewhat representative of a complex real music signal.

The multitones will not only generate Harmonc distortion products at all multiples of all the tones. But will also inter-modulate between each other AND between themselves and all the other harmonic and intermodulation distortion products. Even the ones we don't measure in the THD measurement.

The result of all that distortion going on is the "grass" at the base of that measurement. What we see with competent designs is that grass is well below the level of audiblity.

So - who cares if we miss some of the ultrasonic harmonics, if they don't create any audible problem?

Agree; so long as the 'grass' does not cause amplifier misbehavior, then it does not matter sonically.

 

[spandy]

interesting video on subtractive testing op-amps:

 

[KSTR]

interesting video on subtractive testing op-amps:

Quite outdated procedure etc given that we now have DeltaWave which is the most sophisticated tool on the planet for that kind of stuff.

 

[DanielT]

Quite outdated procedure etc given that we now have DeltaWave which is the most sophisticated tool on the planet for that kind of stuff.

That's how it is but..

interesting video on subtractive testing op-amps:

Interesting though. No audible difference. Test via Youtube with its limitations, but the result would certainly, most likely, be the same if the test was carried out with a top notch audio reproduction solution.

 

[cgallery]

Guys, just leave tangband alone and have pity on him.
- He's wasting his time
- He would never pass a blind test
- Listening to 20 minutes of music in a row for comparison is nonsense
- Experienced listeners need 5-10 seconds, after 30 seconds it is no longer possible to verify differences
- The Aiyima a04 is completely unsuitable for OPAmp rolling
- All the circuitry around the OPAmp is cheap and buggy
- The SE to Balanced circuit with inverted OPAmps connected in series also generates time errors.
- The power supply with one-sided 12v is even worse
- 0-12v with artificial ground somewhere around 6v
- Cheap switching regulator to generate the 12v with bad filtering
- Extremely dirty 12v supply, just measure it

I even believe that the OPAmps sound different in the Aiyima a04. But that will rather be the effects of mismatches and wrong working environments for the rolled OPAmps. Just go with an oscilloscope.
The LM4562 in particular is known to oscillate in such circuits. This often leads to a preference.
There are reasons why the Topping PA5 sounds so much better and cleaner than all the cheap TPA235X units. All the shortcomings listed above do not exist.

Sorry for the bump, but I thought that it was already determined that the LM4562 (and OPA2134) are stable in the A07?