Master Thread: Are Measurements Everything or Nothing?

[Blumlein 88]

Given the measured 1W, 0.1W and 0.01W performance of the class AB amplifier under test, we could predict that the THD would exceed 0 dBSPL at all of these power levels. Given our speakers and listening position we predicted a 14 dB SPL level.

The ABX listening tests proved that a distortion playback level of just 14 dBSPL was easily detected in the presence of a 67 dBSPL 1 kHz sine tone.

At these low levels, the THD (measured in SPL) produced by the class AB amplifier probably exceeded the THD (measured in SPL) produced by the speakers, although we don't have any measurement data to confirm this. At the very least, the two sources of distortion were spectrally different.

The ABX listening test proved that the THD produced by the speakers was not high enough to mask the THD produced by the amplifier. It also proves that the 67 dBSPL tone did not mask the 14 dBSPL push-pull crossover distortion of the class AB amplifer.

The crossover distortion contains many high-order harmonics and I suspect that this makes it more audible than we might expect otherwise.

Quite a few, hard to say how many, good class AB amps operate class A into 1 or a few watts. Seems maybe that would fix lots of crossover distortion issues at 1 watt and lower.

 

[pma]

Quite a few, hard to say how many, good class AB amps operate class A into 1 or a few watts. Seems maybe that would fix lots of crossover distortion issues at 1 watt and lower.

That’s correct. There are in fact no special issues with crossover distortion in the optimally biased class AB amplifier with 25mV across Re emitter resistor, which is usually 0.1R to 0.22R. Then you have 100 - 250 mA quiescent current per pair of output devices. More pairs mean higher idle current and higher point of transition between class A and AB. Remaining crossover distortion is reduced to inaudible level by feedback. Frankly, I am getting tired of those pseudo technical discussions and disgusted of the evident marketing underlaying level.

 

[John_Siau]

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Just for reference, here's the residuals from the last two engineered AB amps I measured. Parasound is dominated by plain HD, the Cambridge by noise. No signs of crossover distortion because the bias was not misadjusted.

Thank you for these plots.

The top plot shows the 100W output of the JC5. The residual is amplified by 10,000 (or 80 dB). Notice that the displayed amplitude of the residual is about the same as the 100W tone implying that the THD is about -80 dB relative to 100W or -60 dBW (-60 dB relative to 1W). From this particular plot, we cannot assume that the THD decreases enough to be inaudible at a 1 W power level. The 100W THD will be reproduced at a level of about 30 dB SPL (given 90 dB speakers). We would need to look at the 0.01W output to see what is happening at low power.

In the second case, the Edge W is running at about 0.01W. Just the plot we need! In this case, the residual looks about 1/3000 the amplitude of the output (1/3 * 1/1000) which is about -70 dB relative to the 0.01W test tone or -100 dBW. This amplifier does not show a biasing problem and the amplifier THD+N will be about -10 dBSPL with typical speakers (10 dB below the threshold of hearing) at an output power of 0.01W. Of course, this plot does not show the THD at higher output powers, so it is just a part of the total picture.

The main take-away is that a few traditional specifications do not tell us if an amplifier has a biasing problem.

We need to examine the THD at output powers below 1 Watt. This THD should measure less than -90 dBW at all output powers below 1 Watt if we want to be sure that it is inaudible. This is 0.003% THD at 1 W, 0.01% at 0.1 W, and 0.03% at 0.01W. This can be done with a class AB amplifier, but more often than not, it is not achieved.

Benchmark's ABX listening tests detected audible distortion at a 0.01 W output in an amplifier that measured 0.2% THD at 0.01 W (-74 dBW), 0.06% THD at 0.1W (-74 dBW) and 0.02% THD at 1 W (also -74 dBW). We had two samples of the same stereo amplifier. All four channels measured about the same. Both amplifiers met or exceeded all of the published specifications. The 105 dB SNR of these amplifiers makes the biasing issue more visible in the residual. These biasing issues are not unusual.

At quick glance, 0.02% THD at 1 W seems adequate. But, if you do the math, you will find that it can deliver the residual at levels significantly above 0 dB SPL.

It is especially important to look at the 0.01W THD performance at all temperature and loading conditions. Unfortunately, this is rarely published by manufacturers.

 

[pma]

Lets examine the A250W4R.

Here is a more detailed THD curve for the A250W4R:

OK, but let's examine it properly, without masking by measuring system noise and without measurement misinterpretation. Below is the 335mW/4ohm. Can you see any hints of crossover distortion?? Audible crossover distortion?? All distortion components above H3 are below 0.0001%, thus below -120dB. Funny to speak about audibility.

 

[voodooless]

OK, but let's examine it properly, without masking by measuring system noise and without measurement misinterpretation. Below is the 335mW/4ohm. Can you see any hints of crossover distortion?? Audible crossover distortion?? All distortion components above H3 are below 0.0001%, thus below -120dB. Funny to speak about audibility.

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I think John would argue that you’re still 33x too high in power Though I doubt it will make a massive difference, it won’t suddenly have more distortion, mostly worse SNR.

 

[SIY]

The main take-away is that a few traditional specifications do not tell us if an amplifier has a biasing problem.

But 30 seconds with a voltmeter and a screwdriver will.

 

[John_Siau]

OK, but let's examine it properly, without masking by measuring system noise and without measurement misinterpretation. Below is the 335mW/4ohm. Can you see any hints of crossover distortion?? Audible crossover distortion?? All distortion components above H3 are below 0.0001%, thus below -120dB. Funny to speak about audibility.

View attachment 384016

Very nice. The worst-case H3 is -98 dB relative to 335 mW and this would be reproduced at an SPL of about -10 dB SPL with 90 dB speakers (completely inaudible, even if H3 was played by itself). The FFT provides the necessary information that was difficult to extract from the THD sweeps you provided earlier. It almost looks like the THD sweep was really a THD+N sweep, or it was limited by the THD+N of the measurement equipment. Nice amplifier.

 

[pma]

I think John would argue that you’re still 33x too high in power Though I doubt it will make a massive difference, it won’t suddenly have more distortion, mostly worse SNR.

It all goes down below amplifier noise, then, when you reduce power. No mystical xover distortion components. With this amplifier, transition from class A to AB is at about 0.32W, so 335mW was a good choice. However, you will see higher order harmonics rising from noise floor no sooner than above 1W. Still, very low in level. And, remember that uncontrolled crossover distortion usually looks like a "grass", "comb" of high order harmonics above the 7th, often higher than the low order harmonics.

Any simplification and any generalizing is usually wrong. What John said about "class AB" distortion might have been correct in case of the amplifier he tested, however, he brought no proof, no plots. What I am saying applies again to one specific amplifier and is design related. So please let us stop saying that class AB amplifiers will have audible crossover distortion at low power, it is in general nonsense and may be specific for certain design.

 

[John_Siau]

What I am saying applies again to one specific amplifier and is design related. So please let us stop saying that class AB amplifiers will have audible crossover distortion at low power, it is in general nonsense and may be specific for certain design.

This interesting discussion was started by someone asking if anyone had proof that amplifiers could sound different.

The ABX test I presented shows that two amplifiers can sound different, even when the specifications for both look quite good. In this specific case, class AB amplifiers were compared to an AHB2 which is a class AB amplifier with a parallel, passively-summed, class A feed-forward correction amplifier.

Both samples of the specific class AB amplifier under test showed output biasing problems (or crossover distortion problems in prior stages). No matter the exact cause of this crossover distortion, these traditional class AB amplifiers produced audible distortion at 0.01W playback levels, while fully meeting or exceeding the manufacturer's published specifications.

This implies that we need to put more emphasis on the 0.01W performance as well as on the performance measurements at the class A AB transition region. This is more of an issue than we would guess when looking at THD or THD+N numbers that are expressed in %.

It is important to think of THD in terms of dBW so that we can directly translate this to the playback SPL of the THD. It is unfortunate that we traditionally use % units for THD.

 

[ahofer]

The "lenient" threshold for Amp/DAC transparency here is -90db, so it seems like this AB amp wouldn't pass, right?

 

[pma]

One single number says nothing, regardless it is expressed in % or dB (any technically competent person is able to transfer % to dB and vice versa).

I cannot imagine a competent class AB amplifier with high crossover distortion products at 10mW power. So I would respectfully request for @John_Siau to post a proof in a form of a distortion spectrum graph of the ill behaving amplifier. It is the only way to lead a scientific discussion based on facts and not on the stories based on one’s authority, especially if there is a business interest.

 

[Newman]

Here is the evidence:

At Benchmark we ran an ABX test using a 1 kHz low-level tone. We compared a class AB amplifier to the Benchmark AHB2. The test level was 0.01 watt, producing an SPL of 67 dB at the listening position. The measured distortion produced by the class AB amplifier was reproduced at a level of 14 dB SPL. In contrast, the measured distortion produced by the AHB2 was well below 0 dBSPL (actual level was calculated at -21 dBSPL). Input voltages to the speaker terminals were set to 0.2828 Vrms using a precision volt meter.

Given these well-controlled conditions it was very easy to hear the difference between the two amplifiers in a fully-blind random ABX test. I scored 100% correct on every ABX test sequence I ran. Other listeners achieved similar results.

You can read the entire writeup here: "Power Amplifiers - A "First Watt" ABX Test"

Given the results of this test, there is no question that power amplifiers can sound different even when they have respectable specifications at high output levels. This specific test shows that we should pay more attention to the 0.01 watt THD.

Thanks for this John.

IMHO the article needs to include measured frequency responses of the specific sample amplifiers used for this test, when connected to the speaker used for the test, measured at the speaker terminals. This helps to eliminate the risk that there is an audible difference in the frequency responses when connected to the specific speakers and cables as a system. (Of course if your test, as apparent from your article, was only of a 1 kHz test tone, then the FR measurements I described are not needed. But to only test with a single test tone is not good enough - see below.)

The article also needs to include 'baseline' ABX tests with no signal present, demonstrating that the amplifiers cannot be detected from their idle noise output alone. Because you haven't eliminated the possibility that they can be told apart based on idle noise.

Finally and most importantly, I don't see any evidence that you passed an ABX test when playing music at 0.01 watts approximating 67 dB SPL. Audio research is littered with examples of artefacts that are audible with specific test tones, but inaudible when playing music. You have not conducted the 'test of truth' when the amp is being used for its intended purpose. This is especially disappointing because your article repeatedly describes in theory the effect when playing music, but neglects to 'put it to the test'. "Musical details", "between the transient peaks", "change the harmonic character of musical voices", "add a fatiguing harshness". Many ambit claims, none of them tested.

Even your tone test, to be consistent with your article's references to music-playing between peaks, should have the 0.01 watt test tone interspersed by kick drum samples at 25-65 watts (per your article) and say, 110 bpm. Let's see if the ABX is "easy to pass 25/25" now. In contradiction to your ambit claims, I suspect that the ear 'turns the gain down' in these conditions and is less discriminating of the 'musical detail' level between the peaks.

Even then, it still is not good enough, if you want to claim that the effects are audible (not only audible, but preferable...try that test), then you need to demonstrate it with typical music recordings....and backed up with measured frequency responses as per my first paragraph.

You need to re-test and re-write, if you want to do more than mislead (by drawing musical conclusions from test signal results).

cheers

 

[pma]

This implies that we need to put more emphasis on the 0.01W performance as well as on the performance measurements at the class A AB transition region. This is more of an issue than we would guess when looking at THD or THD+N numbers that are expressed in %.

It is important to think of THD in terms of dBW so that we can directly translate this to the playback SPL of the THD. It is unfortunate that we traditionally use % units for THD.

10mW power, dBW scale, class AB and ancient class D amplifier. I do not see any valuable hints to explain audible differences, if they would exist.

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Adding Purifi EVAL2 high gain setting to the list. As one can see the difference between the 3 amps at 10mW is only in noise level. In my opinion, the 10mW test is pointless unless the amplifier is totally incompetent design.

 

[Nuni]

It's laughable that you would think that's what I'm saying.


So you are saying you're clueless about science?

As you have no idea whatsoever about my academic qualifications I would say I laugh at your comment and treat it with the scorn it deserves. (You seriously have no clue.)

 

[pma]

Here is another challenge, prove that everything that can be heard is measured. do not just say it can.

I would rise the challenge if that "everything that can be heard" would be supported by properly made double blind test protocol. If not and if based on a mere subjective claim, then there is no reason to rise the challenge.

 

[voodooless]

While you're at it, please prove that there is no teapot floating between Earth and Mars!

 

[Jimbob54]

As you have no idea whatsoever about my academic qualifications I would say I laugh at your comment and treat it with the scorn it deserves. (You seriously have no clue.)

Probably time to park the dare/ double dare dialogue from the last few days please people. The far more interesting discussion in this thread currently is regarding actual measurements of amps vs blind test results- suggest we all see where that leads. Thanks

 

[ahofer]

As you have no idea whatsoever about my academic qualifications I would say I laugh at your comment and treat it with the scorn it deserves. (You seriously have no clue.)

Nobody cares about your credentials or lack thereof. You certainly aren’t doing them any credit with that sort of puffery, or by attempting to shift the burden of proof.

 

[krabapple]

As you have no idea whatsoever about my academic qualifications I would say I laugh at your comment and treat it with the scorn it deserves. (You seriously have no clue.)

I don't know what kind of hack you are, it's true, but also irrelevant and I don't care.

My question "So you are saying you're clueless about science?" was prompted by this gem of yours:

Here is another challenge, prove that everything that can be heard is measured. do not just say it can.

You think that's a scientific approach?

 

[krabapple]

I just don't want people coming here and coming to the conclusion that a Marantz AV 10 sounds better than an Anthem AVM 90 and many people here seem to think they essentially sound the same, that's simply not true. Anthem sound is far superior and anyone that has owned both of these units would understand that in the real world vs peaking at numbers and casual conversations with you experts here. I trust the numbers from Amir and his advice but let's not stretch the truth on Marantz and Denon as being right in line with the sound everyone hears out of these units not just me.

Ok, so after a detour to speakers , we're back again to AVRs. Or is it DACs in AVRs? The original subject was DACs. Then on that thread you started garbling about Marantz, Denon, and Anthem.

I'm thinking your serial vagueness is intentional.