Benchmark Media Systems AHB2 power amplifier Page 2

These surprising and delicious details were evident from the bass up through the midrange and extreme treble, which nonetheless remained silky and diaphanous. One example was Hans Theessink's voice at the end of the phrase "as I lay sleeping," in "Late Last Night," from his album Call Me (CD, Blue Groove BG-4020): I could now distinctly hear how his gravelly tone faded off well before the fade of its ambient resonance. Yes, this was now audible through my other amps as well, when I returned to them—but until the AHB2, I hadn't heard it—or, at least, hadn't noticed it.

Similarly, the AHB2 uncovered so many of the little niceties peppering "Ride Across the River," from Dire Straits' Brothers in Arms: 20th Anniversary Edition (SACD/CD, Vertigo 9871498), that it was like savoring the spicy bits in a Xi'an broth for the first time. Classical music, of course, benefited from this newfound richness of information, as was clearly demonstrated by the last minute or so of Adám Fischer and the Budapest Festival Orchestra's recording of the Adagio of Bruckner's Symphony 7 (SACD/CD, Channel Classics CCS SA 33714). As the horns enter, the fading strings diminish but remain audible as they continue to provide the tonal foundation. The resulting impression was one of transparency, neutrality, and liveliness. I could hear into the ensemble and aurally navigate the soundstage. Or, better, I could just surrender to the warmly touching denouement.

In addition to quantity of soundstage detail, the AHB2 was also "large" in power output. It was clear that this 100Wpc (into 8 ohms) stereo amp had all the power needed to drive with aplomb my Bowers & Wilkins 800 Diamond speakers to any reasonable level. At an SPL of 90dB, the B&Ws are of neither particularly high nor low efficiency, but JA has told us what makes them difficult to drive:

"There are minima of 3.15 ohms at 91Hz, 3.1 ohms at 620Hz, and 3.67 ohms at 21kHz. Given that there are current-hungry combinations of 4 ohms and –52° electrical phase angle at 62Hz and 5.3 ohms and –39° phase angle at 8.6kHz, I believe this speaker should be rated as a 4 ohm load rather than the specified 8 ohms."

That seemed to present no problem for the AHB2, which, Benchmark claims, can pump out nearly 200Wpc into 4 ohms or 240Wpc into 3 ohms. In practice, I had to push the AHB2 to unreasonable and personally uncomfortable volume levels to get the least blink from one of its Clip LEDs. For this I used "Jazz Variants," from the O-Zone Percussion Group's La Bamba (CD, Klavier KD 77017), and the glorious conclusion of Saint-Saâns's Symphony 3, with organist Olivier Latry, Christoph Eschenbach, and the Philadelphia Orchestra (SACD/CD, Ondine ODE-1094-5). Both recordings are remarkably clean and spacious, with powerful bass; the AHB2 rose to the occasion, driving the two 800 Diamonds (sans subwoofer) to volumes louder than I ever hope to hear from them again (though it's nice to know I can). I also, very briefly, ran the AHB2 as a bridged monoblock into a single loudspeaker; it sounded the same, but I thought it would be pointless to try to get it to clip if I wanted to avoid annoying my neighbors. Just to let the power-hungry know. . .

Tonal balance is a curious audio parameter. Careful listeners often hear what sounds like an uneven or tilted frequency balance, even when measurements taken under lab conditions show it to be flat. Lots of real-world factors might account for this, including interaction of an amplifier's output stage with the complex input impedance of a loudspeaker. At first, I thought the AHB2 sounded less bright, and less full in the bass, than the other amps I had on hand, but after only a few days of listening I began to accept that it sounded more "right" than any of them. Now the treble wasn't so much un-bright as smoother and unaccented. The bass had all requisite extension and detail (see above), but seemed to be more naturally balanced with the rest of the audio band.

As time passed and I listened more, I grew to love the AHB2's sound, and undertook the rewarding effort of listening through it to as many different recordings as I could. It was a great pleasure, as old friends, some already mentioned, smiled anew. Yes, I could hear the differences between the different amplifiers, and that's the point. This compact, lightweight, cool-running, class-AB power amp is a real breakthrough in every way. I definitely did not look forward to giving it back to JA for testing.

Listening In the Country
I took the AHB2 along with me for a weekend at our country place, in Connecticut, to see if it would be as successful with my more modest system there. That's where I discovered that my new hero amp wasn't quite perfect. Oh, its power and resolution were still not to be faulted, but through a pair of Monitor Audio Silver 8 speakers, the sound was somewhat hard and thin. Could it have had something to do with the Silver 8s themselves? While that speaker's minimum impedance is 3.5 ohms at 165Hz, but with a benign phase angle, JA points out that "there is a combination of –34° and 4.5 ohms at 100Hz, a frequency where music can have high energy." Still, he says, "A 4 ohm–rated amplifier or receiver will have no problems driving this speaker to high levels." If so, I would expect the Benchmark AHB2 to be more than up to the task.

But here's how it played out. A 16/44.1 PCM file ripped from the above-mentioned Theessink CD seemed robbed of some of its warmth and resonance. Familiar recordings of women's voices, such as Marianne Beate Kielland singing Finzi's "Come Away, Death" (with pianist Sergei Osadchuk; 24-bit/192kHz PCM download from SACD/CD, 2L 2L-064-SACD), and Sara K.'s cover of "Can't Stand the Rain" from her Hell or High Water (CD, Stockfisch SFR 357.4039.2), sounded strange. Both voices were higher, not in pitch but in tonal range, as if they'd been transformed from mezzo-sopranos (which Kielland is) to sopranos. Again, I would describe it as a loss of warmth and resonance in the fundamentals of their voices. Unfortunately for the AHB2, this loss pervaded the sound of whatever recording I played. I wrote it off as an example of an amp-speaker mismatch.

But the seed had been planted in my mind. Back in Manhattan, all seemed better again with the AHB2—except for one troubling event. I've been ripping my CD collection to my 16TB NAS for a couple weeks (already it seems like months), and have become randomly reacquainted with some old favorites—such as the marvelous Emma Kirkby singing Mozart's "Exsultate, Jubilate" (with Christopher Hogwood and the Academy of Ancient Music; CD, L'Oiseau-Lyre 168055), which I hadn't played in ages. Sadly, the divine Dame Emma sounded to have aged badly on this 1984 CD. Akin to what I'd experienced in Connecticut, her voice was robbed of its bell-like richness by the AHB2, only to have it restored when switching to my other amps. Despite this, I'm still addicted to the Benchmark's sound—and I'm still unhappy about shipping it off to JA.

Just because I found Benchmark Media Systems' AHB2 not to be absolutely perfect under all conditions—after all, what is?—doesn't mean that I want to represent it as anything less than a marvelous-sounding amplifier. The AHB2 was capable of drawing more music from my B&W 800 Diamonds than I'd anticipated, and, mostly, sounded better than the other amplifiers I've used to drive those speakers. Combine the AHB2's superb sound quality with its low weight, low noise, low heat, and low cost, and it's clearly an amplifier that must be heard—even by those who can afford amps costing as much as 10 times more. That makes the AHB2 more than a Benchmark. That makes it a bargain.

Benchmark Media Systems, Inc.
203 E. Hampton Place, Suite 2
Syracuse, NY 13206-1633
(800) 262-4675

eriks's picture

Nice review, but I'm still trying to get my head around a single person liking both Monitor Audio Silver series speakers as well as B&W 800 series speakers. Maybe B&W has changed much in the last 5 years. Also might be nice to add the MA Silvers to the "Associated Equipment" list. Now that I think about it, I have only heard B&W's high end played by Krell equipment when my buddy Dan was still in charge. I may need to reset my expectations of the brand now.

John Atkinson's picture
eriks wrote:
Also might be nice to add the MA Silvers to the "Associated Equipment" list.

It's always been there: look under the "Connecticut System" subhead.

John Atkinson
Editor, Stereophile

eriks's picture

Didn't realize the two were separated! Makes more sense though.

Kal Rubinson's picture

This helps to minimize any corrupting interactions between them.

HammerSandwich's picture

This is one heck of an amplifier!

"However, I was surprised to find that the S/N decreased slightly when the amplifier was very hot."

JA, I believe you have (re-)discovered Johnson noise.

John Atkinson's picture
HammerSandwich wrote:
JA, I believe you have (re-)discovered Johnson noise.


Seriously, the rise in the noisefloor, though small, was a little greater than I was expecting.

John Atkinson
Editor, Stereophile

billyb's picture

Great article! Thank you for reviewing this amplifier and testing it so thoroughly. I wish more equipment could spec out so well. As this highlights the issue of measurements vs subjective listening, would you consider dispatching this amplifier for a second opinion? Would one of the other writers at Stereophile care to listen? Perhaps Mr. Dudley or Reichert could give their impressions since they have different equipment preferences? It would be fascinating to juxtapose viewpoints on this amplifier.

dce22's picture

This amp is designed to show good THD+N on the AP test you can't correlate this measurements to the other amps tests in stereophile because the amp has no gain it puts AP generator/analyser in the low distortion field if you loopback AP2 you will see that from 10-500mvRMS has 0.015-0.0004 THD+N if you add 23db gain to that line the amp that has no distorion has to have THDvsPower graph line from 0.003-0.0003 downward slope strait line from 100mW to 30Watts then flatten's out toward 100 Watt on 80kHz bandwidth, if put the analyser in 20kHz bandwidth the line will be lower but still the same shape like this

as is measured on a class a amp with lower distortion then AHB2 and when you crank the volume control on the pre-amp to compensate for the low gain you will amplify the noise of the DAC or Phono-pre up to levels of the other amps so low noise in the AHB2 is just red herring if you lower the designed gain of any amp the noise gets lower as well.

PS. To John Atkinson you cant measure AES48 compliant device's noise by shorting it to the GND you will pickup noise from the enclosure, you need to short out the signal wires or put a resistor between them.

Ishmael's picture

You might have a good point in there somewhere, but the use of punctuation would sure help us decipher it. Did you hammer out your comment on a vintage cell phone or something?

dce22's picture

My apologies im always not very good at this grammar... today :)

AHB2 is a excellent amp you just can't look at the THD+N graph and directly compare to the other amps with "normal" gain structure.

elvisizer's picture

the ahb2 is not "designed to show good THD+N on the AP test". It's designed to work with professional line levels, which allow lower noise and distortion because the amp has lower gain.
If your equipment doesn't support professional levels, you can switch the ahb into higher gain mode.
there's nothing shady going on here, you just don't understand the design.

eriks's picture

Hi Stereophile,

This isn't necessarily directed at Benchmark, but I would really like to see that Stereophile will be publishing THD+N at 1 watt instead of at full power. Of course, it IS included in the charts, but I think reporting this as it's own number would go a long way to improving the quality of amplifiers instead of the power of amplifiers. Even if we set a target for 2017, I think it would be a good improvement for the 21st century.



johnf's picture

Will M22 be reviewed soon?

dumbo's picture

As a current owner of a few DIY Hypex NC400 amplifiers I see some of the measurements of this Benchmark amp looking very similar to what has been seen in the Hypex amp/smps modules in terms of low distortion figures. Given that both units use an SMPS power supply I wonder how different the two units would sound from one another. Or more appropriately, how little sound signature each of the two amps impose on the original source material.

There are many folks who claim that Class D amps sound very flat and lifeless (I don’t agree) but is this claim due to the use of an SMPS or more to do with how a Class D amp works compared to their Class A/B brothers?

I dont think Stereophile would ever measure a Hypex NC400 amp given its DIY status but it would be very interesting to see a comparison of this benchmark unit which appears to be well liked by the reviewer up against the Hypex offering. Both units would fit within the criteria of being light weight, quite powerful and fairly inexpensive. Maybe the reviewer could consider one of the Hypex units in his search for a replacement amp which meets his needs.

If measuring a DIY Amp against an OEM would burn too many bridges for Stereophile then I think another alternative would be to put the Merrill Audio Veritas on the test bench instead as it should be just as impressive in terms of measured performance.

pablolie's picture

I had both poweramps in and compared them for over a month. In the end I picked the NAD M22. It just seemed more *fun*, seemed to add some sparkle. Mind you, it was of course extremely close, and sometimes I thought I was just imagining it all. But during extended listening I kept thinking I was having more fun listening to the M22, there seemed to be additional extension and a nano-ounce of extra clarity. BTW the NAD M22 has built-in Hypex Ncores, not a DIY, and supposedly NAD adds some tweaks to the design.

Associated equipment was
Source: Benchmark DAC2HGC (I feed it via Squeezebox Touch, library for more critical listening is FLAC)
Speakers: Totem Element Fire and also KEF LS50.

PS: It's also worth noting that the $12k Theta Digital Prometheus -which received much praise from Stereophile ("measured performance of Theta Digital's Prometheus is superb")- is also based on a Hypex Ncore design.

johnny p.'s picture

If Kal says he can 'hear new details in his old amp', he at least owes us an update. Say in a few months - to see if the details were retained.

TJ's picture

Kal, it's reasonable that the AHB2 could be a better match with one set of speakers than another. However it seems puzzling that once you heard a mismatch with the Monitors, you could then identify a sound problem with this amp on the B+Ws, but only on one recording. How would one know which playback is more faithful to the original recording, the AHB2 or collectively the other amps chosen for comparison? Is it possible that the nice "bell-like" quality of her voice on the other amps could reflect some coloration that's cancelled in the AHB2 circuitry?

RaimondAudio's picture

Measurements were made with what gain?

hifiluver's picture

astonishingly good measurements. good luck benchmark. real cutting edge stuff. you deserve all the success.

JRT's picture

On the measurements page, the caption under figure one mentions red colored curves twice, while there is only one red curve, and does not mention the green colored curve at all. I suspect that the green colored curve is the one with the 2_Ohm load.

Typos happen. Thank you for the measurements.

BTW, I am surprised that I have not noticed that before, because I have probably looked at this webpage more than a dozen times since it was first posted more than 5 years ago.