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Get The Noise Out Of Your Cables!

Wednesday, October 8, 2014

Audiophile Review!
Benchmark AHB2 Stereo Amplifier,
Exclusive Benchtest!


Exclusive First Benchtest!
Click Here

Brevis...
Price: $2,995
Likes: super smooth and open
Dislikes: not big and heavy
Wow Factor! 130+ dB SNR
More info: Benchmark AHB2


 by John Gatski
  Following my initial first listen to groundbreaking Benchmark’s AHB2 amplifier late last year, I finally got production samples in July and got a chance to do more in-depth listening in stereo — and in a multichannel setup. EAN also performed a series of lab measurements to verify the amazing numbers claimed by the manufacturer — especially signal to noise.
Features
  The $2,995 AHB2 stereo amplifier was designed in conjunction with THX. It features a bipolar-output power section, 180 WPC output RMS, (350W bridged mono), balanced XLR input, three-way input sensitivity switch and dual speaker input options: binding posts and Speakon, a professional termination that uses a twist-on connecter. The AHB2 is housed in a compact enclosure that measures only two times a DAC2 (11" W x 9"D to back of connector x 3 7/8"H). This is not your typical hi-fi sized amplifier.
  The AHB2‘s key highlight is the extremely low noise and distortion. Distortion at .0001% and signal-to-noise/dynamic range measured at more than 130 dB!!!! That is digital-like specs from an analog amplifier, and a source of pride for co-designer and Benchmark VP John Siau.

The AHB2 design
“It was my goal to create a power amplifier that matched the performance of the DAC2 D/A converter,” Siau explained. “After all, the performance of the D/A converter is only useful — if it can be delivered by the downstream power amplifier.”


Note the addition of the input sensitivity switch


  Siau said that the amp has exceeded his expectations — in terms of measured performance. The SNR of the AHB2 was spec'd at 130 dB (A-weighted) and THD is -115 dB, just about as good as the DAC2’s measurements. “The AHB2 power amplifier was designed from the ground up to deliver the full performance of the DAC2 HGC," he emphasized.
  In order to achieve the new amp specs, Siau utilized THX amplifier design concepts, but he also designed his own signal path design in order to get these impressive numbers and audiophile-accurate audio signatures. Siau said the following design highlights enabled Benchmark to create the AHB2’s “fantastic” performance signature: 
 •Low gain (9 dB). This allows +22 dBu input at amplifier clip, which is essential for low noise;
•Patented feed-forward error correction, which virtually eliminates crossover distortion, a problem, Siau says, is still common among typical amplifier designs;
•The feed-forward design also makes bias currents unnecessary, and non-critical. Thus, Class B operation is possible with very low distortion;
•The multiple bipolar output stages are run in parallel to eliminate crossover distortion; one output stage is active, while another is in the crossover region;
•The Class AB output stage uses very low bias current. (Due to low bias currents, idle power consumption is only 20 watts.);

 The Warren Bernhardt recording’s dynamic range is vast, and the AHB2 showcased its dynamics — with a live, accurate, musical portrait of the album, wrapped in a stunning smoothness. The transients were dead-on accurate — without the shrillness I often hear in bipolar amps.

•The feed-forward design even makes the more efficient Class H or G operation possible — with no rise in distortion at class H or G switch point (Class H, or G, rail switching at a 1/3 power threshold);
•A tightly regulated power supply — with high-bandwidth control loop on the switch-mode power supply — responds to amplifier loading over the entire audio band, and at ultrasonic frequencies;
•The amplifier does not rely on capacitive energy storage, and the switch-mode power supply eliminates AC line magnetic interference to levels not possible with a linear power supply;
•The AHB2’s greater than 200 kHz bandwidth allows it to achieve excellent inter-channel phase at 20 kHz and greater-than-0.1 Hz low-frequency cutoff is said to minimize low-frequency phase shift;
•The feed-forward design also improves damping factor for improved bass response, and the PCB uses balanced star-quad signal and power supply distribution to minimize magnetic interference.
•Siau said that certain THX design elements allow him to make the amp as efficient as possible, but the AHB2 design was optimized for low distortion and low noise.
  “It was not optimized for the highest possible efficiency that could be achieved with the new THX topology,” Siau explained. “Nevertheless the AHB2 is much more efficient than a conventional Class AB design. Peak power does not vary with AC line voltage (due to the regulated supply). Likewise, power drawn on one channel does not influence the power available from the other channel.”
  Though the AHB2 utilizes a switching power supply, Siau explained that the overall amp design is not a “switcher.” “The AHB2 is a linear amplifier, it is not a switcher,” he noted. “For this reason, it produces very little out-of-band noise. The A-weighted noise is only 2 dB less than noise measured over an 80 kHz bandwidth. This was an important design goal because ultrasonic noise can be folded into the audio band by the non-linearities in speaker transducers.”


Premium parts, laid out nice and tidy.
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  The omission of the unbalanced input was intentional, according to Siau. You can use unbalanced sources with the AHB2 (the amp’s 2V and 4V sensitivity settings provide full compatibility with unbalanced sources), but you need adapter cables so that the balanced receiver can connect differentially to the RCA jack at the source device. Pin 2 is wired to the RCA hat, pin 3 is wired to the RCA ground.
  This back-referencing is essential for establishing a low-noise connection over a 2V interface. “The omission of the RCA jack is intentional. It forces the user to use a back-referenced connector cable to establish a balanced connection between the RCA output and the XLR input,” Siau said.
  He noted that the 2V RCA unbalanced output utilized by countless hi-fi equipment impedes high resolution playback performance. Siau claims that most unbalanced 2V outputs struggle to exceed an SNR that is better than 100 dB, and very few unbalanced outputs will come anywhere close to reaching 130 dB SNR, He noted that the Benchmark DAC2 series approaches 130 dB SNR via its unbalanced analog output, due to use of a low-impedance voltage divider (pad).

Like the Benchmark D/As and A/D, the company believes that great measurements correlate to great sound. In the case of the AHB2 I totally agree.

  Though he insists the balanced output/input path is the way to maximize noise performance from a hi-fi component, Siau said that Benchmark has “put considerable effort into overcoming the limitations of 2V unbalanced inputs. The input amplifier on the AHB2 has very low equivalent input noise. We can deliver the full performance of the AHB2 over a 2V unbalanced interface, but only if the source impedance is low. Johnson noise will degrade the performance if the source impedance is too high. The special unbalanced-to-balanced input cable rejects hum that can be introduced by traditional unbalanced connections.”

Production sample review
  For a more thorough sonic and user impression, I spent several weeks with the production version AHB2s. Since my late 2013 audition of the prototype, Siau had tweaked the performance even more before sending me the review samples. He was able to get even better specs in the final version, with S/N exceeding -130 dB. He also assured me that the slightly forward midbass that I heard during the prototype listening sessions is no longer there; the prototype had an out-of-spec part, according to Siau. And that statement indeed is true; the bass on the production versions was tight and clean.

The set up
  For my stereo testing, I auditioned a single AHB2 in a number of audio scenarios, including a two-channel audiophile amplifier, and as a multichannel hi-res audio system at a trade show demo (after receiving two additional amps from Benchmark). I also put the amp into my home recording system, using the AHB2 to power a pair of Bryston Mini-Ts and Legacy Studio speakers.
  In the audiophile system, I mated the AHB2 with several speaker sets including my reference MartinLogan Montis electrostatics, Pass Labs SR-1 three-way tower, the Bryston Mini-Ts, and Legacy Expressions, a two-way tower (review upcoming). Associated equipment included Coda line-stage preamp, Oppo BDP-105 universal player, Benchmark DAC2-D D/A, Mytek Stereo192-DSD D/A, Oppo HA-1 discrete HP amp-D/A. A Dell Venue 8 tablet also was used as a hi-res digital player, via USB Audio Player Pro software — up to 24/384.


Note the size compared to a DAC2


  All connecting cables were furnished by Wireworld, and I connected all three-prong power cord components to the AC with Essential Sound Products (ESP) Essence II power cords and their Essence power strip.
  After a few days of break in, I employed the AHB2 into my audiophile system with the PASS SR-1 speakers. The AHB2 is a simple amp to operate. It contains a simple on/off front-panel switch, and rear-panel sensitivity selector switch and bridge mono switch.
  As mentioned, the production version is equipped with the pro-spec Speakon, twist-locking speaker cable connecters, as well as normal binding posts. The Speakon option is due to Benchmark’s new speaker, SMS1, that comes with that connector. According to Benchmark, the Speakon connector option ensures the best connection for a speaker/amp union and is more resistant to terminal oxidation, which degrades amp performance to speakers over time. (Siau said the Speakon also has lower distortion, according to his measurements.
  However, the Benchmark speaker is the only audiophile speaker that I know of with the Speakon connector, thus, this amp’s connecter may not get much use with most other hi-fi speakers. Benchmark sells a Speakon-to-spade cable to at least give half of the Speakon equation. (I did have a chance to use the new Benchmark speaker with the AHB2 for a full Speakon connection, but only for a few days before I posted the amp review to EAN. The speaker will get its own review).

The audition
  The first thing I noticed with the AHB2 is the lack of idle noise. Put your ear to the tweeter, and none of the low-level hiss you hear with conventional, high-gain amps can be heard. With no signal, nary a whisper from the Benchmark — even when adjusting the sensitivity switch to the higher gain positions.
  As with the prototype listening sessions, I first played Warren BernhardtSo Real SACD, an album that has a full dynamic range and spacious soundstage — piano, drums and bass recorded direct-to-stereo. I connected the Oppo BDP-105 XLR balanced output to my Coda preamp then the pre was linked to the AHB2 and the amp to the Pass speakers.
  Versus the prototype, the AHB2 has switchable input-gain settings, which allows the end user to tailor the amp to various preamps. The Coda had no problem driving the Benchmark on any of the settings, but it took a few more twists of the knob to get a loud volume in the low-gain setting. I used the middle gain setting.

 According to Benchmark, the Speakon connector option ensures the best connection for a speaker/amp union and is more resistant to terminal oxidation, which degrades amp performance to speakers over time. 

  The Warren Bernhardt recording’s dynamic range is vast, and the AHB2 showcased its dynamics — with a live, accurate, musical portrait of the album, wrapped in a stunning smoothness. The transients were dead-on accurate — without the shrillness I often hear in bipolar amps. The AHB2’s sonic character is dynamic, open, with quick, taut bass — yet with the silky ease of the best tube amp. But tube amps don’t have the energy and speed in the transients, nor bass, that the Benchmark possesses.
  On the Gene Bertoncini - Body and Soul SACD, the production AHB2 relayed the same character — that warm, percussive guitar tone that I had heard on the prototype. This is one of favorite acoustic DSD recordings — with an expansive stereo image; Mr. Bertoncini’s expert, plucky dynamics were reproduced with precision and the album’s imaging is wide with oodles of depth between the layers.
  No matter what music I played through the Benchmark, the sound was always first rate in the midrange and treble — and with bass authority and smooth delivery at any level. And it did not matter which speakers were connected. Within each speaker’s character, the AHB2’s attributes shone through. Heavy metal. No problem. The fizz and thunderous volume of the Thin LizzyJailbreak CD was a bit easier to listen to with the Benchmark’s smooth (but not soft) delivery. Yet, this amp also handles the delicacy of a wide range of classical instruments, such as Janos Starker - Bach Complete Cello Suites on SACD.
  As a jazz guitar fan, I appreciated the AHB2‘s ability to convey the Kenny Burrell - Midnight Blue SACD, circa 1963, with that warm analog tape sound of Mr. Burrell’s Gibson hollow-body electric guitar. Ditto, for my audiophile LP version of Wes Montgomery - Full House and the Grant Green - Green Streets SACD.

An amp for the masters
  For recording pros and home audiophiles, who know their way around today’s record/edit systems, the AHB2 amp is definitely an amp to consider for those who like their separate amp and passive speakers. A small foot print, and honest, accurate amplification — without noise in an easy-to-monitor/zero ear fatigue mode. I mated the AHB2 with two Bryston Mini-T passive speakers (review upcoming) and Legacy Studios in my edit suite.
  With either speaker, the playback of hi-res recorded tracks, as high as 24/352.8 and DSD2X, was open, detailed and ultra smooth. The Legacy delivered that exceptional smoothness and the new Bryston speaker beamed copious detail from work station monitoring rig. I spent hours mastering jazz guitar 24/352.8 cuts from recordings of my precious Gibson L5 and Fender Twin Reverb — without one ounce of ear strain from the aHB2 and speakers — especially those Legacy Studios woth ribbon tweeters.
  I also got a chance to utilize three AHB2’s at the 2014 Capital AudioFest in Silver Spring, Md this past July. In a 5.0 system, the Benchmark amps powered two Legacy Expressions and three Legacy Studio’s with various samples from a Windows server, which provided stereo and surround DSD hi-res music files, Blue-ray and DVD Audio surround music, as well as spirited movie soundtracks from an Oppo BDP-95 universal player.

 The AHB2 multi-amp system delivered all the music and film soundtracks, along with plenty of my favorite stereo hi-res, in this medium-sized hotel room; the amp performance was perfection: a wide multichannel spread of instrumental layers, rock-solid center vocals and, best of all, the amps delivered the dBs without strain and exhibited zero harshness.

  The system delivered all the music and film soundtracks, along with plenty of my favorite stereo hi-res, in this medium-sized hotel room; the amp performance was perfection: a wide multichannel spread of instrumental layers, rock-solid center vocals and, best of all, the amps delivered the dBs without strain and exhibited zero harshness.
  In fact, on movie soundtracks, such as Avatar and the first Captain America film, viewed on Blu-ray, I had inadvertently pushed the level to 96 dB, but it did not sound that loud; I, nor my guests, succumbed to any ear fatigue. This is one of the easiest-on-the-ear amps I have ever auditioned.
  BTW, that ease and ability to deliver the finesse and fine detail, does not go away with the bridge mono operation. You just get more oomph (over 300 watts of extra oomph) to fill up the bigger rooms. And the noise still stays low.

The verdict
  In talking to various audio engineers about Benchmark’s amazing signal-to-noise numbers, I heard some skepticism and a array of opinions that an ultra quiet amp does not matter on the real listening world. “Any noise is lost in the music anyway,” one engineer said. But, as I said in my original first look last December, other audio components, such as DACs, players, etc. have shown generational improvements in measured performance, and I believe these improvements have brought audible, if subtle, improvements as well.
  For those who listen with very accurate, discerning speakers, an extra 20 dB of lower noise could be heard — in terms of ultra detail that may be hiding in the noise of an average audiophile amp. The -130 dB noise/dynamic range just gets us that much closer to the live music. After years of amps languishing in the 100 dB SNR threshold, Benchmark has pushed the mark way higher.
  As for specs, I had no doubt that the AHB2 would measure well. In our EAN SpecCheck, Bascom King’s tests (Click here to go to EAN Spec-Check) showed that the amp may be the quietest analog amp ever built. As good as that accolade is, I don’t hear a sterile, thin amp — attributes sometimes equated to amps with stellar measurements. The numbers confirm its impressive audio delivery.
  Like the Benchmark D/As and A/D, the company believes that great measurements correlate to great sound. In the case of the AHB2 I totally agree. I can’t wait to get two more for a full-high-end home cinema amp test in my big A/V room. For now, based on mostly stereo listening, it receives, with certainty, an Everything Audio Network Stellar Sound Award.

John Gatski is publisher/owner of the Everything Audio NetworkArticles on this site are the copyright of the ©Everything Audio NetworkAny unauthorized use, via print or Internet, without written permission is prohibited.

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