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Monday, October 27, 2014

EAN Audiophile Review!
Legacy Expression Ribbon-Tweeter
2.5-Way Passive Tower Speaker





Brevis...
Price: $3,000
Likes: silky top end, good bas
Dislikes: no way, too good for the $
Wow Factor! a serious entry tower
More info: Legacy Expression


by John Gatski
  Since Bill Duddleston re-acquired his Legacy Audio speaker company a few years, ago, he has been kicking out a whole new generation of speakers that appeal to a wide range of audio enthusiasts. Although the big speakers (Whisper, Aeris and Focus) get all the press, the small speakers are genuine bargains. Check out my StudioHD review from 2009.
  Legacy’s entry-level tower, the Expression, is an example of super-sound delivery, yet reasonable price from a USA-made speaker. In fact, for smaller rooms that can fit a tower speaker, It is a genuine bargain

Features
  The Legacy Expression is a 2.5-way driver tower loudspeaker, utilizing a 1-inch neodymium-magnet, spiral ribbon tweeter, a 8-inch midbass/midrange woofer (silver/graphite woven diaphragm, cast basket, phase compensation plug) and a second 8-inch subwoofer/bass driver, augmented by a ported enclosure. The silky smooth highs and spot-on bass output down to the low 40-Hz region — with the extra throw of a small tower — makes these speakers ideal for small-to-medium listening rooms, or as part of a home cinema system with use of a Legacy center channel or the small StudioHD two-way that I am a big fan of.

“The Expression gives those with more limited budgets a chance to sample the renowned Legacy loudspeaker sound. Dollar for dollar in a typical small-to medium room, the Expression is hard to beat for stereo or multichannel duties. It gets a well-earned Everything Audio Network Stellar Sound Award.”

  The compact tower measures 38.5-inches tall 10.75-inches wide and 10.75-inches deep. Its solidly braced MDF cabinet helps contribute to its 70 pound heft. The 2.5-way crossover, as described in the marketing literature, refers to the traditional two-way crossover of the mid-woofer to the tweeter at 3 kHz, plus the subwoofer crossover at 500 Hz to the midbass/midrange woofer. Spec-wise, rated frequency response is 38 Hz to 22 kHz, plus, minus 2 dB. Impedance is 4 ohms, and the room sensitivity is listed at 94 dB, with a 2.83V signal input. Power handling is 250 watts RMS continuous.
  The speaker includes bi-wire binding posts with included jumpers for single-wire use. The Expression contains two tone-tailoring switches to allow a mild cut in the treble and bass: -2 dB at 10 kHz and -2 dB at 60 Hz. Up is flat; down is the cut for each switch. The treble cut can even the response in a bright room, while the bass trim can knock down some bloom when the speaker is too close to a boundary.
  The Expression is a great-looking speaker and comes in a variety of finishes including the test samples rosewood option. The grill is removable, and to my ears, is more present sounding without it attached. Perhaps in a more live sounding roof, you can get away with leaving it on.

The set up
  During the review process, I set up the Expressions for use as an audiophile stereo playback speaker system, and as the L-R in a 5.1 system. I also used the 5.1 system at the 2014 Capital Audiofest for hi-res surround and movie soundtrack playback.
  Stereo listening components included a Rogue Audio Medusa hybrid tube/class D amplifier, Pass Labs XA30.5 Class A MOSFET amplifier, and the new Benchmark AHB2 amplifier. For preamps, I used either a Coda or the discrete output of an Oppo HA-1 DAC/headphone amp, via 1/4-inch-to-RCA adapter. Other demo DACS included the Benchmark DAC2D, Mytek Stereo 192-DSD and a Resonessence Concero linked to a Dell tablet. All interconnects, including digital and speaker cables, were furnished by Wireworld. Power products including Essence Reference II cords and power strip, were from Essential Sound Products.
  The multichannel system was set up in my main home cinema listening room and at the 2014 Capital Audiofest. I used the Expressions for the L-R, and three Legacy StudioHDs, which are very close in audio character to the Expression, for the center and rear surrounds.
  In my main 5.1 room, I powered the speakers with the audiophile caliber AudioControl AVR-6 multichannel receiver. At CAF, the speakers were powered by three Benchmark AHB2 amplifiers.

The audition
  I set up the speakers in the midfield — about eight feet away from the listening position with the grills off. The sound was immediately familiar upon first play of the Warren Bernhardt — So Real SACD. The similarity to my smaller Legacy Studios was undeniable. That silky smooth top end, generous helping of width in the imaging and very good, clean bass extension, plus more of it, were projected by the Expressions. But they filled up the room quite a bit more and offer increased oomph under 60 Hz than the Studios.
  The neo ribbon HF driver is key to the Expression’s, er, expressive top-end. This ribbon is accurate with zero harshness, making the Expression a speaker you can listen to all day long. The two-way crossover for the midwoofer/tweeter also keeps the audio focused in the more ear-sensitive frequencies.
  For a $3,000 speaker, the piano tone was very convincing when reproducing the a Steinway D from the Bernhardt recording. I heard much of the upper-register piano key percussion that I hear on much more expensive speakers. Perfect for that cocktail jazz sound.
  My jazz guitar SACDs sounded open and warm through the Expressions. Grant Green — Green Streets, for example, exhibited that warm, tube amp tone that I like. Anthony Wilson’s honey-hued Gibson guitar tone also was golden via the Expression when playing the Anthony Wilson TrioOur Gang SACD, Love that “Briitta’s Blue” track.


Expression (R) is a bit smaller than Legacy's ClassicHD

  On classical music, the ribbon tweeter is aces on string instruments. On a 2L, 24/352.8 Haydn cello duo. I could definitely hear the bit of extra depth of cello tone overtones that the best speakers pull out of this ultra-hi-res recording. Bigger, multi-driver speakers may give you more level in a bigger room, but the Expressions are remarkable in what they bring to the listening position. Lots of detail for a budget USA speaker.
  The Expressions could handle anything I put through them. Pop, Rock or Country? No problem. Daft PunkGet Lucky” disco/funk retro sound was presented in its full flair; nice bass, keyboard and guitar layers. David Bowie’s, hi-res version of “The Rise Fall of Ziggy Stardust and The Spiders From Mars” is a great transfer from the original master tapes with improved width and depth around the various guitar layers. All that extra sound comes through the Expressions. I especially liked the acoustic guitar intro on “Rock and Roll Suicide." Nice natural presence from the Expression’s ribbon tweeter.
  The Expressions vocal performance also is smooth — without excessive sibilance or peakiness. Frank SinatraNo One Cares album, now on SACD, features Sinatra at the peak of his phrasing and vocal power. The Expressions definitely do that recording justice. Just listen to the vocalist’s velvety tone on the “Cottage For Sale.”
  The Expression’s two-woofer system, augmented by the port, produce clean bass and were flat in my room down to 40 Hz, which is plenty low for most kinds of music. If you get closer to the back walls, the midbass may pump up a bit, but I kept my distance. You can engage the bass roll-off switch and plug the port if you can’t avoid closer wall placement. I tried it, and it definitely tightens up the bottom end — even at eight-inches from the wall.
  I did not have any speakers, sans the StudioHDs, in the Expression’s price range during the trial, but i did have some more expensive ones: my reference MartinLogan Montis electrostats ($10,000) plus and a pair of Pass Labs SR2  three way towers ($21,000). The Pass SR2’s had a bit more richness in the upper midrange and low treble, and the Montis projects a nearly unbeatable capability to separate instrument layers. But the Expression hangs right in there relaying plenty of hi-res music nuances and subtle resonance cues. It is definitely better than its price would suggest — in terms of audiophile sound.

  On a 2L, 24/352.8 Haydn cello duos. I could definitely hear the bit of extra depth of cello tone overtones that the best speakers pull out of this ultra-hi-res recording. Bigger, multi-driver speakers may give you more level in a bigger room, but the Expressions are remarkable in what they bring to the listening position.

  In the EAN surround system, I used two Expressions and three studios, one for the center channel and two for the rear surrounds. The Expressions integrated seamlessly with the Studio’s, producing a convincing dimensional sonic portrait for several of Tom Jung’s wonderful recorded DMP surround SACDs, such as Warren Bernhardt - So Real, and surround Blu-rays from 2L and AIX. I purposely ran them without a subwoofer, to see if Expression could kick out enough bass. Other than the lowest sub-35 Hz LFE sounds, the Legacys kicked out solid music bass.
  I took the same system to the Capital Audiofest, powered by the new Benchmark AHB2 amp, three to be exact. The DSD surround music, courtesy of Mytek Digital’s music server and Blu-ray movie soundtracks, played through an Oppo BDP-105, sounded great in my little demo room. Numerous show attendees commented they were impressed with the speakers — especially its clean bass output and easy top end.
  I had no complaints with the Expressions. They are well built, and were a cinch to to connect with my big Wireworld cables, and they are not that heavy to move around. And as a small tower, you can put them in about any room — except a really big one. The finishes are first rate, and this company really knows how to package a speaker for safe shipping, yet easy to remove from the box. 
  Legacy used to be a mail order/Internet order/phone order factory-direct seller, but now it has a dealer network of 22 dealers in the U.S.A. And it also has distribution in 21 countries, and now features a second line of speakers — for pro installation.

The verdict
  If you get the impressions that I liked the Legacy Expression, you are correct. With the expanding line of Legacy offerings on the top end in recent years, it is good to see that the company has not neglected its entry points. Like the compact StudioHD that I reviewed in 2009, the Expression gives those with more limited budgets a chance to sample the renowned Legacy loudspeaker sound. Dollar for dollar, in a typical small-to-medium room, the Expression is hard to beat for stereo or multichannel duties. It gets a well-earned 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.



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.
Click here  for full benchtest


  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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Benchmark AHB2
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Bench Measurement and Commentary
Benchmark AHB2 Power Amplifier

  The new Benchmark AHB2 is the result of considerable research and patents on its resultant circuitry. The intent was to make it an ultra low distortion design. I would have to say they succeeded as it is the lowest noise and distortion power amp I have measured and I have measured a LOT of power amps in my career.
  Measurements were made at the 2V sensitivity and in stereo mode unless noted. I did spot check amounts of distortion at the lower sensitivities and they were little different than at the 2V sensitivity.


The S/N ratio was about 131.6 dB. Pretty impressive, and this is the lowest output noise level I have seen in any power amplifier I have measured!

  Frequency response as a function of loading, not surprisingly, is mostly independent in the audio range with a typical effect of more roll-off with lower loading in the ultrasonic range. Overall HF bandwidth is quite high with a 3 dB down point of 200 kHz or more.  This is shown plotted in Figure 1.

Figure 1 - Frequency Response: Open, 4 and 8 Ohm Loads

  What is a good measure of success in lowered distortion is a plot of THD+N of a 1 kHz and SMPTE IM distortion as a function of power output for 4 & 8 Ohm loading.  This is shown in Figure 2. The THD+N is pretty much noise dominated up to clipping whereas the IM distortion does show above the noise above 10W of output.

Figure 2A - IM Distortion 4/8 ohm Loading


  Clipping with the design is very abrupt and my usual attempts to put extra points in the clipping curve to get the highest power before distortion rises was an exercise in futility. When connected in bridge mode, power was close to 400W into an 8 ohm load.  1 kHz THD+N and SMPTE IM distortion are plotted in Figure 2B.

Figure 2B - 1 kHz  THD+N/SMPTE IM Distortion



  In Figure 3 THD+N is plotted vs. frequency and power.  Here, a 4 ohm load is used and distortion is down in the noise at all power levels up to 2-3 kHz where is start to show as distortion.  The amount of rise with frequency is very low and stays below 0.01% at 20 kHz.  Amazing!

Figure 3 - THD+N Re: Frequency/Power
Red=2W, Magenta=20W, Blue=60W, Cyan=120W, Green=180W


  Damping factor vs. frequency is plotted in Figure 4 and is quite typical of solid stage amplifiers being high at low frequencies and falling above a few hundred Hertz. Damping factor in mono mode, while not measured, would be expected to be about half that each channel in stereo mode as the outputs of the two channels are in series with the load in mono mode. 

Figure 4  - Damping Factor Vs. Frequency


  Another benchmark – pun intended – measurement is a spectrum of 10W 1 kHz THD+N arranged logarithmically along the horizontal axis to show line frequency harmonics as well as the signal harmonics.  I have not seen one of these with such low numbers.  I couldn’t increase the sensitivity any more to show the noise floor.  But look at that third harmonics component at 0.0001%!  That is a low distortion number for sure.  A bit of 5th harmonics also shows with the even harmonics down in the noise.

Figure 5 - THD+N Re:Frequency/Signal Harmonics


  Of interest is how low the noise level is in this amplifier. I measured noise in three bandwidths, wideband (WB), 10Hz – 20 kHz (10-22K), and with A weighting (A wtd) for all three input sensitivities. The inputs were terminated in 300 Ohm per phase.  Results in uV are shown in Figure 6 for both channels. Taking the number of uV of noise and relating them to the reference power output voltage for 100W/8 Ohm, the A wtd. noise for the 2 V sensitivity is a little shy of the claimed 130 dB S/N ratio at 128.1 dB. But at the 4 V sensitivity, the S/N is 130.4 dB and at the 9.8 V sensitivity, the S/N ratio was about 131.6 dB. Pretty impressive and this is the lowest output noise level I have seen in any power amplifier I have measured! (Editor's note: Benchmark measured 3 dB better when input was terminated with 60 Ohm impedance input.)


Figure 6 - A-Weighted Noise (in uV) Re: Wideband, 10 Hz- 22 kHz

 A few final observations: AC line draw at idle was 20W with a power factor of 0.46 and 0.37A. Input impedance was an easy to drive 47K at 1 kHz.


—Bascom H. King


  Bascom King is owner and chief technician for BHK Labs in Santa Barbara, Ca. and a regular contributor to the Everything Audio Network.