ATI Battle Class AB vs Class D Amplifier Shootout

[PENG]

How many times on this forum have folks complained that manufactures, especially cable, won't do blind tests.

I might have read just above everything I could find about the AHB2 amps since it was mentioned on AH. Since you asked, I read it again and still I think I understood correctly that they did the "blind test" using a single tone, at 0.01 Watt output to a 87 dB/1W/1M speaker.

"Here is an application note about a double-blind ABX listening test that we did between the AHB2 and a typical class-AB amplifier with decent specifications. This test was examining the audibility of zero-crossing distortion when playing a single tone through loudspeakers at a low level (0.01 watt producing 67 dB SPL at the listening position). It was very easy to hear the difference between the two amplifiers. I scored 25 correct out of 25 trials on my first attempt."

So there was no masking effects, as a pure tone was used, the class AB amp was not named, so it could be one that happened to have significant crossover distortions at 0.01 W output. That seems consistent with my belief, that amps could sound different depending on the amps being compared and the conditions.

Here we are talking about comparing amps with less than 0.03% THD and IMD when listening to music under controlled conditions. For THD to be below 0.03% full bandwidth and at levels 0 to rated output, IMD is not going to be an issue either. I have no idea why John Siau and his Benchmark team even bother with that ABX tests, what would be his score if he do an DB ABX between the AHB2 and the AT522NC at the more realistic 0.2W to 2W average using real music, classical, jazz, and pops. My bet is, about 50/50.., but we would never know wouldn't we?

 

[PENG]

There is no convincing anyone of anything in audio, I think that is irrefutable.

- Rich

Haha, we are in agreement on this one..

 

[Out-Of-Phase]

"I am not saying everyone will like them but they are different."

I would like them. They're smaller, lighter and greener at idle. And I'm sure the music would sound just fine.

But I've already got that for far less money.

 

[Irvrobinson]

At 0.01 watts into 4 ohms, it would be a rather cheaply-designed audiophile-level Class AB amplifier that wasn't biased high enough to be running in Class A mode. So I don't know where this crossover distortion is coming from, especially since BM never mentioned which amp they were comparing theirs to. I don't know if it's still true, but several Class AB amps used to deploy a variable output transistor bias level determined by some operational factor, like temperature, which means that some amps will perform differently depending on the environment and how they were exercised prior to the crossover distortion test.

I use and recommend BM products (I'm on my second preamp-DAC from them),but sometimes John Siau's marketing material raises my eyebrows. I ignore it and just look at measurements. It would be interesting to ask John, "How did you end up with an amplifier of only 100w/ch?" Now that would be an interesting white paper.

 

[Out-Of-Phase]

Good grief. That review thread on that other website has turned into nothing but a large ad.

 

[Irvrobinson]

Good grief. That review thread on that other website has turned into nothing but a large ad.

And - bonus - our old pal GrimSurfer is posting there.

 

[PENG]

At 0.01 watts into 4 ohms, it would be a rather cheaply-designed audiophile-level Class AB amplifier that wasn't biased high enough to be running in Class A mode. So I don't know where this crossover distortion is coming from, especially since BM never mentioned which amp they were comparing theirs to. I don't know if it's still true, but several Class AB amps used to deploy a variable output transistor bias level determined by some operational factor, like temperature, which means that some amps will perform differently depending on the environment and how they were exercised prior to the crossover distortion test.

I use and recommend BM products (I'm on my second preamp-DAC from them),but sometimes John Siau's marketing material raises my eyebrows. I ignore it and just look at measurements. It would be interesting to ask John, "How did you end up with an amplifier of only 140w/ch?" Now that would be an interesting white paper.

If I understand correctly, many class AB amps are not biased to run in class A mode even at that low output level. If John Siau picked something like a $1000 Parasound Halo A23, he wouldn't have to worry about crossover distortion, but it seemed their goal was to hear it so they had to pick the right amp, I guess.

 

[PENG]

"I am not saying everyone will like them but they are different."

I would like them. They're smaller, lighter and greener at idle. And I'm sure the music would sound just fine.

But I've already got that for far less money.

Me too, in fact if waf was not a factor, I would have pulled the trigger just for the sake of owning one amp that has seemingly the lowest distortions money can buy.

 

[RichB]

Me too, in fact if waf was not a factor, I would have pulled the trigger just for the sake of owning one amp that has seemingly the lowest distortions money can buy.

It's small enough to hide under another component.
My WAF was pleased with the change, after a while. AHB2's look like a miniaturized Parasound, without the gold lettering, heat, weight, and power

- Rich

 

[Out-Of-Phase]

"And - bonus - our old pal GrimSurfer is posting there."

I wonder how long he'll last on that forum. If I was a betting man I'd say.....

 

[PENG]

It's small enough to hide under another component.
My WAF was pleased with the change, after a while. AHB2's look like a miniaturized Parasound, without the gold lettering, heat, weight, and power

- Rich

I have no way of hiding C$5000 (shipped and taxed). It's a shame, I know..

 

[Irvrobinson]

If I understand correctly, many class AB amps are not biased to run in class A mode even at that low output level. If John Siau picked something like a $1000 Parasound Halo A23, he wouldn't have to worry about crossover distortion, but it seemed their goal was to hear it so they had to pick the right amp, I guess.

The definition of Class AB means that the positive and negative transistors (or tubes) are both powered to some non-zero level continuously. For most solid-state amplifiers the bias voltage will result in a power level into 4 ohms of less than one watt, but 0.01 watts is a lot less than one watt, and I can't imagine a legitimate Class AB design that doesn't have enough bias voltage for that.

I've always liked the really open way Nelson Pass discusses technical issues, even if I choose not to spend enough to buy his products. This is a very good discussion of the topic:

https://www.passlabs.com/press/leaving-class

IMO, Benchmark could learn a lot from Nelson about technical marketing.

 

[Irvrobinson]

It's small enough to hide under another component.

And it's probably best to avoid Canadian federal prison for tax avoidance...

 

[RichB]

If he doesn't get you, the beaver will.

- Rich

 

[PENG]

The definition of Class AB means that the positive and negative transistors (or tubes) are both powered to some non-zero level continuously. For most solid-state amplifiers the bias voltage will result in a power level into 4 ohms of less than one watt, but 0.01 watts is a lot less than one watt, and I can't imagine a legitimate Class AB design that doesn't have enough bias voltage for that.

I've always liked the really open way Nelson Pass discusses technical issues, even if I choose not to spend enough to buy his products. This is a very good discussion of the topic:

https://www.passlabs.com/press/leaving-class

IMO, Benchmark could learn a lot from Nelson about technical marketing.

I read that a few times, in fact I saved the downloadable version in my archive. What he said "Class AB push-pull amplifi ers are hybrids between Class B and amplifiers are hybrids between Class B and Class A. Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias. " bothers me. It is one contributing factor why so many repeat the same about class AB amp runs in A at lower output and in class B at higher outputs. I take it that he, and others who made similar statements, mean to emphasize the "determined by the bias" part, that is, in class B like if the bias is low enough. The thing is, would you buy a 200 W rated class AB amp that would operate in class A up to 5 W and in class B above 5W? I won't for sure. I may email him about that for further clarity.

A definition such as this is less confusing imo.
https://www.powersoft-audio.com/en/support/faq-s/113-rack-amps-faqs/1122-what-is-the-difference-between-class-ab-and-class-d-technology

Class AB refers to the amount of idle current flowing in the outputs at zero output. Amplifiers have positive and negative output transistors which handle their respective halves of the output signal. They must "hand off" the output current to each other as the signal passes through zero. A "Class A" output stage begins to transfer current well above its "cutoff point", resulting in much current overlap. This eliminates any chance of "crossover distortion" but generates tremendous waste heat at idle, limiting the possible power of the amp. A "Class B" output stage attempts to make the transfer at exactly zero current, which is impossible to maintain perfectly and leads to "zero crossing distortion" (more commonly called "crossover distortion", a buzzy form of distortion most audible at very low levels). Class AB is the practical compromise--just enough idle current to ensure a smooth transfer between the positive and negative output transistors, without a wastefully high idle current.

Or AH's own:
https://www.audioholics.com/audio-amplifier/amplifier-classes

Class A/B, as one might deduce, combines the best of Class A and Class B in order to create an amplifier without the drawbacks of either. Thanks to this combination of strengths, Class A/B amplifiers largely dominate the consumer market. So how did they do it? The solution is actually fairly simple in concept: where Class B utilizes a push/pull arrangement with each half of the output stage conducting for 180 degrees, Class A/B amplifiers bump that up to ~181-200 degrees. By doing this, there is far less potential for a “gap” in the cycle to occur, and consequently, crossover distortion is pushed down to the point where it’s of no consequence.

As described by AH's, for argument sake, you can have a class AB amp biased to have each half conduct 181 degrees so there is an overlap of 1 degree, and chances are that even at 0.01 W, depending on the load current, you may end up with some crossover distortions.

Enough on the definition and back on the very topic, if you look at fig.3, Distortions vs watts for 7 bias currents in the article you linked, with the output stage biased at 0.08 amps, the distortion is 0.67% at 1 watt. The graph does not show output below 0.1W but by calculation, you can see that if the bias current drops to 0.008A or even higher, you are going to have significant distortions at 0.01 W. I realize I am taking a short cut, but if I were to come up with a more vigorous argument or example, I would have to spend more time revisiting and digesting part of my text books.

 

[Irvrobinson]

Class A/B, as one might deduce, combines the best of Class A and Class B in order to create an amplifier without the drawbacks of either. Thanks to this combination of strengths, Class A/B amplifiers largely dominate the consumer market. So how did they do it? The solution is actually fairly simple in concept: where Class B utilizes a push/pull arrangement with each half of the output stage conducting for 180 degrees, Class A/B amplifiers bump that up to ~181-200 degrees. By doing this, there is far less potential for a “gap” in the cycle to occur, and consequently, crossover distortion is pushed down to the point where it’s of no consequence.

As described by AH's, for argument sake, you can have a class AB amp biased to have each half conduct 181 degrees so there is an overlap of 1 degree, and chances are that even at 0.01 W, depending on the load current, you may end up with some crossover distortions.

Enough on the definition and back on the very topic, if you look at fig.3, Distortions vs watts for 7 bias currents in the article you linked, with the output stage biased at 0.08 amps, the distortion is 0.67% at 1 watt. The graph does not show output below 0.1W but by calculation, you can see that if the bias current drops to 0.008A or even higher, you are going to have significant distortions at 0.01 W. I realize I am taking a short cut, but if I were to come up with a more vigorous argument or example, I would have to spend more time revisiting and digesting part of my text books.

What AH wrote is consistent with what Nelson wrote. The bias current essentially "lifts" the waveforms conduction duration above 180 degrees for those waveforms with a peak power requirement greater than the bias current. So that means that another way of describing the effect is that the time that both transistors are conducting for a given waveform at a given power level will effectively increased as the bias current increases.

 

[<eargiant]

The definition of Class AB means that the positive and negative transistors (or tubes) are both powered to some non-zero level continuously. For most solid-state amplifiers the bias voltage will result in a power level into 4 ohms of less than one watt, but 0.01 watts is a lot less than one watt, and I can't imagine a legitimate Class AB design that doesn't have enough bias voltage for that.

I've always liked the really open way Nelson Pass discusses technical issues, even if I choose not to spend enough to buy his products. This is a very good discussion of the topic:

https://www.passlabs.com/press/leaving-class

IMO, Benchmark could learn a lot from Nelson about technical marketing.

So I guess you'd like the updated version...

https://positive-feedback.com/audio-discourse/leaving-class-a-2019-redux-a-biased-opinion-from-nelson-pass/

 

[Verdinut]

Here a text basically explaining construction of Class AB power amps:

http://www.learnabout-electronics.org/Amplifiers/amplifiers55.php

They never mention of the Class AB amplifier starting to operate in Class A at low levels and switching to Class B at higher output.

As it ever been clearly proven with measurements that some Class AB power amps operate in class A at lower volumes? Unless electronically proven by an independent reliable source, I would be inclined to think that all Class AB amps operate constantly biased in a class AB topology.

 

[Irvrobinson]

Here a text basically explaining construction of Class AB power amps:

http://www.learnabout-electronics.org/Amplifiers/amplifiers55.php

They never mention of the Class AB amplifier starting to operate in Class A at low levels and switching to Class B at higher output.

As it ever been clearly proven with measurements that some Class AB power amps operate in class A at lower volumes? Unless electronically proven by an independent reliable source, I would be inclined to think that all Class AB amps operate constantly biased in a class AB topology.

There isn't any "switching" from one topology to another. There's no switching from one class of operation to another. This is why Nelson's explanation is so good. It gets to the crux of the matter.

If you have a positive standing bias voltage on both transistors in a push-pull configuration, there must be a power level at which the configuration operates in Class A, at which both transistors can satisfy the waveform with the bias voltage. It may be a very low power level, but it has to exist. The only complicating factor I can think of is that some modern amplifiers support a variable bias voltage. Some manufacturers claim this adaptability is based on the input signal, some on heatsink temperatures, some on "steps" based on input signal amplitudes, but in the end it is just changing the bias level.

 

[lovinthehd]

So I guess you'd like the updated version...

https://positive-feedback.com/audio-discourse/leaving-class-a-2019-redux-a-biased-opinion-from-nelson-pass/

Interesting he writes for that blog and all the phoolishness it represents.