Review and Measurements of Benchmark AHB2 Amp

[DonH56]

I'll have to disagree with Kal here; I can usually find a logical reason when I hear a difference among amplifiers. And yes, I do believe, and have heard and measured, differences among amplifiers. Not normally as dramatic as some would have one believe, but real (and measurable).

 

[josh358]

The problem (or at least 'a' problem) with Stereophile (and almost all the subjectivist mags--print and on-line) is what you describe. There is no discipline, editorial control, and no method to the madness. [Unless the 'method' is opportunistic salesmanship, something I don't discount.] I remember an exchange I had with the late Peter Aczel. He admitted that the problem with the early Audio Critic was a lack of method. He admitted that he 'instinctively knew' that the Mark Levinson had to sound better than the Pioneer going in to the listening session. There was no control, and no critical analysis of what he was doing. Way back in 1978 or so, Mark Davis, then at MIT, tried to school him on psychoacoustics. At the time, Aczel laughed at him, but some years later wound up apologizing to him in print. He realized that without some control and methodology, it was all useless.

Remember Enid Lumely (sp?). At the time I thought it was Harry Pearson making fun of both the entire high end reviewing scene, and himself. It was only a little later that I realized Harry in fact had no sense of humor, and was serious having this lady write for his mag. I never read another issue, after that. Stereophile lost credibility with me after the way it treated Bob Carver. Now I look at it, on line, simply for laughs.

I think you have to calibrate the reviewers. After a time, you find which reviewers have tastes similar to your own.

It would be great if we could just go with specs and measurements, but, as I like to say, where is the spec for depth? Many of these phenomena are unexplained, or don't correlate with the usual measurements, or are so subtle they aren't easy to interpret. For example, IIRC, third harmonic that is out of phase increases the sense of depth. And studies have found that the audibility of harmonic distortion is proportional to the harmonic. It changes the character of the sound as well, for the simple reason that we determine timbre largely on the basis of the harmonic spectrum.

Ultimately, we still need subjective reviews because despite some good work on the audibility of distortion, measurements still only tell us so much about the sound, and the suite of measurements in a publication like Stereophile is necessarily limited, as well.

Agree about Enid Lumley, though.

 

[Kal Rubinson]

I'll have to disagree with Kal here; I can usually find a logical reason when I hear a difference among amplifiers.

There is no disagreement. I am confident that there is a logical, scientific and real-world reason for what I have heard and reported. However, so far, I do not know what it is. This is one reason why I participate in forums such as this and appreciate posts such as the one above from josh358.

 

[PierreV]

I'll have to disagree with Kal here; I can usually find a logical reason when I hear a difference among amplifiers. And yes, I do believe, and have heard and measured, differences among amplifiers. Not normally as dramatic as some would have one believe, but real (and measurable).

Maybe this is simply because you are way more technically competent than the rest of us.

I've also heard clear differences between amps that I can't explain and some I could (impedance drop in a certain range, ability to drive that load). Now, the fact that I can't explain them means that either I have access to the measurements that would explain the difference but am just not competent enough or that I don't have access to the measurements showing the difference.

That's why this place is lovely btw: the expertise of some of the contributors is just amazing and I have had many "haha, this is how it really works!" moments here.

 

[Soniclife]

There is no disagreement. I am confident that there is a logical, scientific and real-world reason for what I have heard and reported. However, so far, I do not know what it is. This is one reason why I participate in forums such as this and appreciate posts such as the one above from josh358.

Measurements for the a31 are at the following.
A31 measurements.
Anyone have any ideas?

 

[josh358]

I'm reminded of an anecdote where Peter Aczel and (I think it was) David Rich heard a Max Wilcox DAT copy of one of his masters, and thought it was great. When the CD came out both were chagrined, accusing DGG of mucking up the original mix. Aczel called up Wilcox to complain. Wilcox said that it was impossible that DGG would screw up his recording. The two then found the DAT that Max had given them from his master, did a controlled listening, and found that they were indeed the same.

How can a DAC 'strip away huge amounts' of what is on a recording? Does that even make sense to anyone? How could that even happen?

Try oversampling with different reconstruction filters in HQAudio Player. You'll be surprised at how some reconstruction filters bring out small details, and others seem to suppress them. I certainly was. And you'll really hear this difference between DAC's as well, as opposed to just the chip. Forex, closed form filters as in the Schiit DAC's or in HQAudio player seem to accentuate small details. I believe Schiit attributes this on their site to the fact that time domain performance of their closed-form filter isn't stochastic.

This stuff is endlessly fascinating. DACs sound far more different than they should, given their specs. In some cases, it's easy to see why -- poor jitter rejection, for example, gives some DAC's a "gray" coloration, or you have issues like departure from monotonicity, or they intentionally use underdamped filters with overshoot (ESS in at least some of their chips). In other cases, it isn't so easy. But while the difference between good amplifiers is subtle, I've found that DAC's can sound dramatically different. I mean, tone-control-level different, despite the fact that they're flat within a fraction of a dB. Comparing them has been a real ear opener.

 

[JohnPM]

* H2 and H4… are good armonics.

* H3 and H5… are BAD armonics.

I wouldn't be so quick to label any harmonic distortion as good. The same nonlinearity that generates the harmonics also generates intermodulation components which bear no harmonic relationship to the signal. Here is the difference between a two tone loopback test with no distortion and with 0.1% H2 (no other harmonics). The added content isn't a good thing.

 

[Shadrach]

You are implying that Stereophile is a monolithic entity whereas, in reality, it is an aggregation of individual reviewers. I think most of us are relatively consistent even though our perspectives may be quite different. Can you ascribe the behavior above to any individual at Stereophile?

I am yes. Like most publications it has editorial guidelines. You can of course argue that this isn't the case with Stereophile from your experience and I can't prove otherwise.
It's the consistency that is concerning.
I'm not here to argue with you. I don't like Stereophile. I don't like what it has imo done to audio over the years and I have zero respect for those who write subjective reviews of audio equipment.

 

[Shadrach]

Maybe Benchmark are just content to sit back and be comfortable in the fact they have produced a product which is essentially without fault, at a price which is unfathomable, in a package which suits both crazy-ass audiophiles and professionals alike, all the while without feeling the need to buy favor with HiFi magazines or their scribes.

Color me impressed.

Colour me annoyed that I can't afford one, annoyed that they didn't make this when I could afford one and a bit green of those who can afford one.

 

[Shadrach]

In one of their other reviews, in comparison of the Benchmark DAC with another DAC, the reviewer said the Benchmark amp got rid of the sound of church walls in the recording...

I wonder what those walls sounded like with the other amp then.

 

[SEKLEM]

All with SMPS too.

I don't believe SMPS is inherently inferior. It's probably just an audiophile misnomer, and an outdated way of thinking at this point. Implementation is key.

 

[Shadrach]

not a better buy? i have an 6 channel NC500-based amp that cost the same and is equally as transparent.

That depends on what you include in your purchasing assessment.
I agree that there are many amplifiers that perform to a standard that means that in normal operation any performance differences will be unnoticed by the listener.
Despite my objectivist leanings I would like to be able to afford the Benchmark for a number of reasons that have little to do with objective sonic performance. It's a world class product.

 

[Shadrach]

I have never seen an amp that measures this way. I don't know that anybody has. It isn't the first amplifier to use feedforward -- indeed, John Siau points out that feedforward is actually an older technology than negative feedback -- but it's rare in doing so.

It's great to see someone breaking all the "rules" of amplifier design, because it's been in a rut for some years now, with the best designers squeezing pretty much everything that could be squeezed out of available devices using conventional topologies.

Anyone who thinks all amps sound the same will be in for a rude but happy awakening when they compare an inexpensive amp to an amp like the Benchmark.

That said, anyone who thinks that the Benchmark is a *perfect* amp because of its steady state measurements will learn that that doesn't yet exist.

I've owned an AHB2 for several years now, and it's the cleanest amplifier I've ever heard. I was listening to it last night and I can only describe the sound as ravishing. Like silk. Or like Class A without the absurd size, weight, and heat. (If you've ever heard crossover notch distortion you know why Class A amps sound better than A/B ones, it's incredibly audible and annoying in small amounts.) And the low weight, tiny form factor, and efficiency of the AHB are really welcome. It's a save-the-planet kind of amplifier, without the sonic compromises of Class D.

But perfect amplifier? No. It excels in steady state measurements, but it isn't as dynamic sounding as other good amplifiers with which I've compared it. Again, this isn't a question of steady state measurements or amplifier power -- I'm not clipping it. But listen to a piano on the Benchmark and another good amp and you'll hear what I mean: it rounds off the attack.

Amplifiers actually differ surprisingly in the degree to which they do that. My Parasound A21, for example, is a less refined amplifier than the Benchmark, but it happens to be one of the best amps I've heard in this particular regard. There are those who say that the Benchmark robs music of excitement, and after several years of listening experience, I tend to agree. It's strength is in beauty of sound. The highs in particular are to die for, like gossamer.

Another issue is, paradoxically, lack of distortion masking. Lower order harmonic distortion is known to render higher order harmonic distortion euphonic. This is because of the mechanism by which we detect the timbre of sounds. And higher order distortion is unfortunately quite common on poorly made recordings and badly-designed equipment. What this means in practice is that the AHB2 will sound great on clean recordings, but will pass the harshness through on more distorted ones, while some more colored amplifiers with lower order harmonics will tend to mask that harshness.

So the Benchmark isn't a perfect amplifier, but anyone who has never heard a really good amplifier or thinks that all amps sound the same is going to be blown away by the sound. It's a bargain, too, by the standards of high end amplifiers, and with its sophisticated protection circuitry, quite bulletproof!

I would be interested to read what other amplifiers you compared it to (apart from the Parasound) and how you determined any differences.

 

[maty]

I wouldn't be so quick to label any harmonic distortion as good. The same nonlinearity that generates the harmonics also generates intermodulation components which bear no harmonic relationship to the signal...

You are right. The key is the magnitude and that are monotonously decreasing.

Probably, H2 should be at 70-75 dB, H3 10 dB below or not exist, and H4 another 10 dB more below, more or less. And without H5, H6... clogged by background noise. Recordings with few acoustic instruments.

The problem comes when you want to play complex music, for example an orchestral mass, as Nelson Pass used to remember a long time ago. Then H2 is more convenient at 90 dB or more.

The ideal, as I already proposed in diyaudio, would be an amplifier with a selector that would allow those decibels to be lowered to a decreasing harmonic profile to adjust it to the type of music to be played.

The other day, I discovered that Katz Corner had already thought about it and built a more complex device.

https://www.innerfidelity.com/content/katzs-corner-episode-25-adventures-distortion

If we want technical perfection, then the harmonic profile of the Benchmark is unbeatable. Now, will it be able to transmit emotion or will it be limited, that is not little, to a spectacular sound but without soul?

 

[Shadrach]

a spectacular sound but without soul?

 

[maty]

An example is the same record on vinyl and on SACD. These days I have had two examples, with Bob Dylan and Cannonball Adderley. In both the vinyl transmitted me emotion, and that the SACD of CA had a slightly better DR!

I want to get excited when listening to music and not only appreciate how well it sounds.

 

[josh358]

I am yes. Like most publications it has editorial guidelines. You can of course argue that this isn't the case with Stereophile from your experience and I can't prove otherwise.
It's the consistency that is concerning.
I'm not here to argue with you. I don't like Stereophile. I don't like what it has imo done to audio over the years and I have zero respect for those who write subjective reviews of audio equipment.

A manufacturer once told me that when reviewers review his equipment, they hear most of what he did. What I think differs is priorities -- the kind of music someone prefers, what they value most about sound. And also to some extent perspective, since no one can hear every piece of equipment under ideal conditions.

I've found personally that when it comes to subjective reviews, I have to calibrate the critics, and find those who share my tastes and priorities. But like the manufacturer I mentioned, I've found that they typically hear what I hear -- which is a good confirmation that I'm not imagining things if I read the review after I've listened myself -- but that they may have different sonic priorities or listen through very different systems. Once I'm familiar with a reviewer's preferences, I find the reviews helpful, albeit there's ultimately no substitute for hearing a piece of gear in good conditions.

I'd add that as an engineer, I've gone down the "objective" route several times over the years and been burned each time. The measurement suite in a magazine is too limited to fully encompass the sound of a product, the interpretation of the measurements too difficult. Just try listening to crossover notch distortion sometime, and compare its audibility to harmonic distortion. There's even a formula for the audibility of harmonic distortion -- as you might expect, the higher the harmonic the more audible it is -- making a single harmonic distortion figure next to meaningless.

Not that I can't tell something about the sound of equipment from the measurements -- but as I like to put it, where is the measurement for reproduction of depth? I actually know of many technical factors that influence that, including the timing of early reflections, the polar pattern, consistency of polar response ("power response"), baffle diffraction, group delay, stochastic timing in DAC's, amplitude response, even phase inverted third harmonic distortion. And there are no doubt other factors as well. So how are you going to infer that from measurements? You just can't, not completely, anyway.

Even where I know how something that is frequently measured, how do you translate the measurement into what you hear? Jitter, say -- jitter rejection is routinely measured, and I know what jitter sounds like. But I have no idea at what point I'd hear it. I'm sure that a DAC designer does, but I don't have that level of expertise.

I find measurements interesting because as an engineer I'm curious about how the correlate to what we hear and because they're important to design, and of course they can tell you something about the sound -- but ultimately, I know of no substitute for listening, despite all the vagaries and subjectivity.

 

[Shadrach]

A manufacturer once told me that when reviewers review his equipment, they hear most of what he did. What I think differs is priorities -- the kind of music someone prefers, what they value most about sound. And also to some extent perspective, since no one can hear every piece of equipment under ideal conditions.

I've found personally that when it comes to subjective reviews, I have to calibrate the critics, and find those who share my tastes and priorities. But like the manufacturer I mentioned, I've found that they typically hear what I hear -- which is a good confirmation that I'm not imagining things if I read the review after I've listened myself -- but that they may have different sonic priorities or listen through very different systems. Once I'm familiar with a reviewer's preferences, I find the reviews helpful, albeit there's ultimately no substitute for hearing a piece of gear in good conditions.

I'd add that as an engineer, I've gone down the "objective" route several times over the years and been burned each time. The measurement suite in a magazine is too limited to fully encompass the sound of a product, the interpretation of the measurements too difficult. Just try listening to crossover notch distortion sometime, and compare its audibility to harmonic distortion. There's even a formula for the audibility of harmonic distortion -- as you might expect, the higher the harmonic the more audible it is -- making a single harmonic distortion figure next to meaningless.

Not that I can't tell something about the sound of equipment from the measurements -- but as I like to put it, where is the measurement for reproduction of depth? I actually know of many technical factors that influence that, including the timing of early reflections, the polar pattern, consistency of polar response ("power response"), baffle diffraction, group delay, stochastic timing in DAC's, amplitude response, even phase inverted third harmonic distortion. And there are no doubt other factors as well. So how are you going to infer that from measurements? You just can't, not completely, anyway.

Even where I know how something that is frequently measured, how do you translate the measurement into what you hear? Jitter, say -- jitter rejection is routinely measured, and I know what jitter sounds like. But I have no idea at what point I'd hear it. I'm sure that a DAC designer does, but I don't have that level of expertise.

I find measurements interesting because as an engineer I'm curious about how the correlate to what we hear and because they're important to design, and of course they can tell you something about the sound -- but ultimately, I know of no substitute for listening, despite all the vagaries and subjectivity.

Oh, I wholeheartedly agree, there is no substitute for listening........................as long as you don't know what you are listening to.

 

[maty]

Another simile.

In a recording studio, to detect the quality of the audio we want a loudspeaker that gives the frequency response as flat as possible, and an amplifier that is very transparent, like the Benchmark.

But if we want to enjoy music at home, such a flat response can cause auditory fatigue in a short time. From there many search speakers with very flat response and few oscillations but with a negative slope.

In short, that a little imperfection can be the panacea. Another thing is what usually happens, with very imperfect sound systems but that enchants its owners for a while until they spend more money on changing components again and again, without rhyme or reason, ignoring the available specifications and measurements, only attending to subjective evaluations of other people.

 

[JohnPM]

If we want technical perfection, then the harmonic profile of the Benchmark is unbeatable. Now, will it be able to transmit emotion or will it be limited, that is not little, to a spectacular sound but without soul?

So if you sit in the studio while the musician plays with no equipment between you will the lack of added distortion strip away the emotion of the performance?