Master Thread: Are measurements Everything or Nothing?

[pma]

Here are the plots for the offending amplifier (shown in green on all graphs). The second amplifier used in the ABX test was the AHB2 (shown in red on all graphs). These tests plots were made in 2016.

Thank you for the data provided, I appreciate it.

 

[MAB]

The audio was a pure tone for all the listening tests? Was this disclosed in the marketing blog? Any attempts made with music?

Pure tone, 1kHz only is mentioned:

With an audio analyzer we can extract the error waveform (lower trace) and amplify it so that we can see what our ears are hearing. In this case, the error signal has been amplified by 1024 (about 60 dB). At a 1 watt output, this class-AB amplifier produces a distortion waveform that measures 70 dB below the output level of the 1 watt test tone (see graph at the bottom of this application note). This means that the power produced by the THD+N is 70 dB below 1 watt. If we drop the level of the test tone by 20 dB, the output power is 0.01 W. At this output level the amplifier was still producing distortion at a level of 73 dB below 1 watt. At 0.01 watt the distortion waveforms look virtually identical to the 1 watt waveforms. This distortion was clearly audible when we used this amplifier to drive a speaker at 0.01 watt. For our tests we used a stereo pair of Benchmark SMS1 speakers with a sensitivity of 87 dB 1 watt, 1 meter. The tone was reproduced at a sound pressure level of about 67 dB (measured 67 to 68 dB SPL) at the listening position while the amplifier distortion was reproduced at a calculated sound pressure level of about 14 dB (87 dB - 73 dB = 14 dB). With a 0.01 watt 1 kHz test tone, the amplifier distortion was clearly audible through the loudspeaker.

Power Amplifiers - A "First Watt" ABX Test

In Benchmark's listening room we recently demonstrated the importance of the first watt using two 100 watt stereo power amplifiers. One amplifier was a traditional class-AB amplifier, the other was Benchmark's AHB2 power amplifier with feed-forward error correction. Using a double-blind ABX...

benchmarkmedia.com

It's a legit test result, even if the stats aren't presented. I now understand. And am slightly intrigued as to if I could identify, likely many of us would be able to unless new revelations occur.

So what we have here is a very specific case of one brand and model of class AB amplifier that easily drifted into an under biased condition. Once it warmed up and entered this under biased condition, THD could be heard on a 1 kHz tone at very low power levels (0.01 Watt) and a listening level of 67 dB SPL (amplitude of 1 kHz tone). The sum total of the harmonics would have reached about 15 dB SPL at the listening location.

This was an important revelation that gives context, and I now understand. I certainly didn't before.

FWIW, I posted some time ago a defense of good performance at low levels. I took microphone measurements of several amps from a vintage micro integrated, to a Hypex and a PuriFi, all hooked up to a high sensitivity compression driver. I ranked the amps by measured noise, after carefully controlling the measurement process, which I documented, including the condition and age of the amps. One was a 20+ year old Bryston 3B ST that I use daily. The point was to demonstrate amps do sound different, but I was also careful to point out the corner case aspect of my study, where it is applicable, and where it is not. I did note the old Bryston was fairly quiet but sounded harsh compared to the a couple less quiet amps. I didn't quantify that though. For sure you have made me wonder about the bias, and perhaps I need to do a thread about proper bias, and what happens when you have an amp that has been allowed to drift too low in bias. But who knows what I will really find in the old Bryston, it was directly hooked to a compression driver close mic'ed...

Thanks for the info, it does help my understanding of your test result, and the state of amplifier you used.

 

[Nuni]

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:


You think that's a scientific approach?

Hack? At least I can post in meaningful English which appears to be more than you can do.

 

[BDWoody]

Hack? At least I can post in meaningful English which appears to be more than you can do.

Ok, we've prolonged the agony enough...

You can move along now.

 

[Jim Taylor]

Hack? At least I can post in meaningful English which appears to be more than you can do.

A few pieces of advice:

1) Focus on the problem ... and the problem only. Discussions that get sidetracked tend to dilute the quality of information.

2) If someone posts something that irritates you, simply ignore it. Remember; " .... just because a dog barks at you doesn't mean that you have to bark back."

3) Emotion allows people to manipulate you. It is, therefore, a weakness. The more emotional your responses, the more obvious your weaknesses.

Jim

 

[AdamG]

Emotion allows people to manipulate you. It is, therefore, a weakness. The more emotional your responses, the more obvious your weaknesses.

I’m getting all emotional about the wisdom bomb you just dropped. Now my eyes might be leaking because this brilliant post will not get a response. A few clicks too late for the visiting Troll. But the rest of us can benefit.

On a side note. We just cracked 57,000 Members!

 

[SIY]

We just cracked 57,000 Members!

Do you need some Krazy Glue?

 

[Jim Taylor]

... the wisdom bomb you just dropped.

Ain't no wisdom bomb. Everything I learned, I learned the hard way, not from reading Plato. If I hit my head on the beam enough times, I learned to duck.

Jim

 

[Blumlein 88]

Ain't no wisdom bomb. Everything I learned, I learned the hard way, not from reading Plato. If I hit my head on the beam enough times, I learned to duck.

Jim

Sadly I see way too many who flunk out of the school of hard Knox these days.

 

[DonH56]

On a side note. We just cracked 57,000 Members!

I'm pretty sure a lot of us were already cracked...

 

[Newman]

2) If someone posts something that irritates you, simply ignore it. Remember; " .... just because a dog barks at you doesn't mean that you have to bark back."

Clearly, Jim, you are not a dog living near to me!

 

[pma]

Here are the plots for the offending amplifier (shown in green on all graphs). The second amplifier used in the ABX test was the AHB2 (shown in red on all graphs). These tests plots were made in 2016. The offending amplifier was an RA-500 "studio reference" amplifier. We had two samples that were both purchased new at the time of the test. Both behaved identically. This amplifier was passively cooled and was designed to be rack mounted. I suspect the amplifiers may have been under biased because of thermal issues related to the passive cooling and the need to run in a studio equipment rack. It is also possible that there was some problem at the factory and these units got shipped with incorrect bias settings. I searched our facility and we no longer have these two samples. In any even, the RA-500 is no longer available.

I have tried to replicate your test, with 2 class AB amplifiers, A250W and OPA549. The later has quite horrible crossover distortion, the former is free of low power crossover distortion. The THD vs. power plots, scaled in dBW as you did, look like this:

For those less familiar with dBW scale I am also posting the more usual dBr scale related to base frequency amplitude:


10mW spectra (2kHz tone) look like this:


Now, for those interested in possible sound difference with the 1kHz sine signal recorded from both amplifiers at 10mW/4ohm, below you will find the zip file with 2 flac files. Please feel free to make a foobar ABX test and tell me if you can hear the difference, and possibly post a test report.

Note: test files (1kHz) are reduced to 16bits/44.1kHz, thus lower "ENOB"


Edit #2: generated 1kHz sine tone added for possible ABX comparison

 

[restorer-john]

Alesis RA500.

Schematics attached...

Built by Homni : https://www.homni-elec.com/facility/16.htm

 

[John_Siau]

The audio was a pure tone for all the listening tests? Was this disclosed in the marketing blog? Any attempts made with music?

Yes, here is the description from our blog:

At a 1 watt output, this class-AB amplifier produces a distortion waveform that measures 70 dB below the output level of the 1 watt test tone (see graph at the bottom of this application note). This means that the power produced by the THD+N is 70 dB below 1 watt. If we drop the level of the test tone by 20 dB, the output power is 0.01 W. At this output level the amplifier was still producing distortion at a level of 73 dB below 1 watt. At 0.01 watt the distortion waveforms look virtually identical to the 1 watt waveforms. This distortion was clearly audible when we used this amplifier to drive a speaker at 0.01 watt. For our tests we used a stereo pair of Benchmark SMS1 speakers with a sensitivity of 87 dB 1 watt, 1 meter. The tone was reproduced at a sound pressure level of about 67 dB (measured 67 to 68 dB SPL) at the listening position while the amplifier distortion was reproduced at a calculated sound pressure level of about 14 dB (87 dB - 73 dB = 14 dB). With a 0.01 watt 1 kHz test tone, the amplifier distortion was clearly audible through the loudspeaker.

 

[John_Siau]

I have tried to replicate your test, with 2 class AB amplifiers, A250W and OPA549. The later has quite horrible crossover distortion, the former is free of low power crossover distortion. The THD vs. power plots, scaled in dBW as you did, look like this:

View attachment 384210

On the basis of the above graph, it looks like the 0.01W THD of the OPA594 is about 9 to 10 dB lower than our RA-500 test amplifiers. Given this 10 dB reduction, it may be much more difficult to hear the distortion. At this level, the THD would only be 3 to 4 dB SPL (assuming 87 dB speakers) and this would make it very hard to hear (perhaps impossible).

Any files provided for ABX testing should be acquired directly from the speaker terminals and not using a microphone.

Obviously, the files will allow playback at higher levels. With the recorded levels boosted on playback, it may be possible to hear the effect with the OPA594 recording (assuming the noise is low).

 

[JP]

Yes, here is the description from our blog:

At a 1 watt output, this class-AB amplifier produces a distortion waveform that measures 70 dB below the output level of the 1 watt test tone (see graph at the bottom of this application note). This means that the power produced by the THD+N is 70 dB below 1 watt. If we drop the level of the test tone by 20 dB, the output power is 0.01 W. At this output level the amplifier was still producing distortion at a level of 73 dB below 1 watt. At 0.01 watt the distortion waveforms look virtually identical to the 1 watt waveforms. This distortion was clearly audible when we used this amplifier to drive a speaker at 0.01 watt. For our tests we used a stereo pair of Benchmark SMS1 speakers with a sensitivity of 87 dB 1 watt, 1 meter. The tone was reproduced at a sound pressure level of about 67 dB (measured 67 to 68 dB SPL) at the listening position while the amplifier distortion was reproduced at a calculated sound pressure level of about 14 dB (87 dB - 73 dB = 14 dB). With a 0.01 watt 1 kHz test tone, the amplifier distortion was clearly audible through the loudspeaker.

And any attempts with music?

 

[John_Siau]

And any attempts with music?

No. At Benchmark we are not interested in determining how bad a defect needs to be before it can be heard in music. Instead, we aim to reduce noise, THD, phase errors, and frequency response errors to levels that are inaudible (even when using test tones).

Music can hide many defects, but these defects may be revealed by the right piece of music. Running these tests on music would be a long and arduous task. Inaudibility on a few music test tracks does not prove inaudibility. Instead, you need to find a music track that proves audibility.

You can never prove inaudibility using music tracks. You can only prove audibility.

The fact that audibility with a pure tone was confirmed is enough to suggest that we should avoid this particular distortion problem. If it can be heard with a pure tone, there is probably some piece of music somewhere that will reveal the same defect. After all, a musical recording could be a series of pure tones.

 

[DonH56]

^^^ +1

Using single, or perhaps two-three (for IMD), sine waves for testing is much more revealing than musical signals that tend to mask distortion. The problem is compounded because music generally has much less dynamic range than modern electronics, and it is much more difficult to establish a reference when using music compared to a pure sine wave. There are a myriad of studies showing we tolerate much higher distortion in music than when tested with a pure sine wave. The level of distortion detectable (JND, just noticeable difference) in music is thus much higher than when using a pure tone. As @John_Siau said, sine wave testing will more readily expose distortion levels well below what is detectable with (most, likely the vast majority of) music. It is much easier to hear distortion on a single isolated tone than buried in a random sample of many other tones.

 

[krabapple]

Now, for those interested in possible sound difference with the 1kHz sine signal recorded from both amplifiers at 10mW/4ohm, below you will find the zip file with 2 flac files. Please feel free to make a foobar ABX test and tell me if you can hear the difference, and possibly post a test report.

Note: test files (1kHz) are reduced to 16bits/44.1kHz, thus lower "ENOB"
View attachment 384218 View attachment 384219

From playing A and B in foo_abx I knew very quickly that I wouldn't be able to tell them apart. And 16 trials of this tone would trigger my tinnitus to an unhealthy degree so I stopped there.

(fwiw, replaygain scan shows a 0.06 dB difference in required gain to level match them)

 

[Titurel]

No. At Benchmark we are not interested in determining how bad a defect needs to be before it can be heard in music. Instead, we aim to reduce noise, THD, phase errors, and frequency response errors to levels that are inaudible (even when using test tones).

Music can hide many defects, but these defects may be revealed by the right piece of music. Running these tests on music would be a long and arduous task. Inaudibility on a few music test tracks does not prove inaudibility. Instead, you need to find a music track that proves audibility.

You can never prove inaudibility using music tracks. You can only prove audibility.

The fact that audibility with a pure tone was confirmed is enough to suggest that we should avoid this particular distortion problem. If it can be heard with a pure tone, there is probably some piece of music somewhere that will reveal the same defect. After all, a musical recording could be a series of pure tones.

I agree. The problems using natural sound as test signal are manifold, not the least of them being the qualification of a test instrument, i.e., a listener. For example, if listeners aren't familiar enough with violin sound to readily tell one fiddle (or one player) from another, what chance would they have recognizing the addition of small amounts of distortion. Real world instruments (non-electronic) produce a mixture of well correlated sounds (e.g., from the vibrating string) and poorly correlated sounds (the scraping of the bow). The presence of the latter could easily defeat reliable detection of amplifier generated distortion. Add one more fiddle, a viola, and a cello to your test signal and your listeners are pretty much SOL A possible alternative might be a synthesized instrument (not sampled!), one that produces the overtone structure of real world instruments, but omits the poorly correlated components, like bowing, reed noise, etc.