Benchmark AHB2 Power Amplifier Review: The First Commercial Power Amp with THX AAA Technology

Arrival and Features

When I received my review sample of Benchmark’s AHB2 power amplifier, I could only think that Rick Moranis’ shrinking ray gun from the movie, Honey, I Shrunk the Kids, had been unleashed upon a beautiful, five-figure audiophile amplifier.

Audiophiles have come to expect that high performance comes in a sizable form-factor. After all, many high performance audiophile amps are large. Yet right from the start, the AHB2 lets you know that it defies conventional wisdom.

Every aspect of the AHB2 is simply first-rate. As I noted in my AHB2 unboxing article (click to read), the quality, craftsmanship, and materials will remind you of some of the best-built amplifiers on the market. Even though the AHB2 is about 1/3 the size of a Class A monster, it simply screams “Made in America” pride.

But if we go down that traditional path, we’ll miss the main point of the AHB2. You cannot understand Benchmark’s AHB2 properly when you look at the amp in isolation, as just a single product. You can only understand the AHB2 properly through the lens of today’s high-resolution music.

Scott Wilkinson, host of the popular Home Theater Geeks podcast and editor at AVS Forums, teamed up with  Dr. Mark Waldrep, founder of AIX Records, to put together a test to see if people could distinguish between CD quality music and high-resolution 24/96 digital files (if you’d like to take the test yourself and read up on the results, click here). While the test was not strictly scientific, it did produced some very strong anecdotal results.

One’s ability to tell the difference between high-res and CD-quality music was predicated on the quality of their setup playback chain (the high-res file, DAC/preamp, power amplifier, and speakers). People with higher performance systems could tell the difference more easily between CD quality and high-res files than lower end setups. In other words, statistically speaking, mediocre systems couldn’t reproduce the audible benefits of high-resolution music files so that listeners could discern a difference!

For years, audio critics have been touting the superiority of Benchmark’s acclaimed DAC2 models. Nevertheless, regardless of all the accolades heaped upon the DAC2, did you know that audio reviewers’ setups were actually unable to bring the DAC2 models to their full potential? The bottleneck was the amplifier. From this quest, to bring out the DAC2’s potential performance, the AHB2 was born.

Audiophiles are very slow to think of their amplifiers as the weak link in their audio system. Nevertheless, one’s power amplifier can add audible distortion to the audio chain—especially as the impedance drops—to the point where the amplifier will no longer cleanly deliver the power and current required by the loudspeaker. Benchmark was determined to solve this problem and create an amplifier that could bring out the full performance of the DAC2 without all of the problems associated with traditional amp designs.

The feed-forward error correction of THX’s AAA™ technology keeps the AHB2 nearly distortion free when driving a heavy load. As noted in our interview with audio legend Laurie Fincham, THX’s Senior Vice President of Audio Research, the AHB2 is the first commercial amplifier in the world to use THX’s patented AAA™ technology to eliminate virtually all forms of distortion.

Benchmark set specific performance goals for distortion, frequency response, drive current, damping factor, and output noise. Benchmark designed an FPGA-based protection system to protect the amplifier from damage without interfering with the audio performance. The AHB2 is a high-efficiency design, but THX’s AAA technology can be pushed to deliver higher efficiencies at the cost of some increase in distortion. Overall, the engineering is something to marvel at. The AHB2 measures at -118db THD.

The AHB2’s THD vs. Power into a 4 Ohm Load.  This plot shows the THD performance of the AHB2. The feed-forward error correction circuits keep the output nearly distortion free.  The magenta curve is the output power. Power is shown on the right-hand vertical scale.  The green trace is the THD measured at the output of the left channel when loaded with 4-Ohms. The red trace is the THD measured at the output of the right channel when loaded with 4-Ohms. THD is shown on the left-hand scale.  The stair steps are caused by the THD within the AP2722. This stair stepping demonstrates that the AHB2 is at or below the measurement limits of the AP2722 test set, even when the AHB2 is driving a heavy load.  Measurements courtesy of Benchmark

The AHB2’s design wasn’t optimized for just vanishingly-low distortion, it was also optimized for low noise. I spoke with John Siau, Benchmark’s Vice President of Operations and Director of Engineering, about this. He shared with me that an amplifier with low THD could still have a noise problem. That’s not the case with the AHB2, it has both low distortion and low noise.

In John’s own words, “The low noise of the AHB2 is due to careful engineering. It is not related to the use of feed-forward error correction or to the use of the THX-AAA technology. The power supply on the AHB2 is exceptionally clean, and all circuits are designed with low impedances and high signal levels in order to reduce the effects of thermally-induced ‘Johnson noise.’” The results for such meticulous attention speak for themselves. On paper alone, the signal-to-noise ratio of the AHB2 is nothing short of astonishing, boasting a 132db SNR.

You may be wondering how this performance impacts heat dissipation. Well, the AHB2 runs relatively cool, though not as cool as most Class D amps. Therefore, it has prominent heat sinks to help dissipate heat in case the amp is driven very hard. In such cases where it is driven hard, it will get very warm to the touch. In my experience with the amp, that was certainly the case.

While the AHB2 is a Class AB amplifier, in many ways it is a radical departure from conventional class AB amplifier designs. I spoke with John Siau about some of these differences and he highlighted the following:

1. The amplifier’s low gain (9 dB) allows +22 dBu input at amplifier clip, which is essential for low noise.
2. The AHB2’s patented feed-forward error correction virtually eliminates crossover distortion and improves the damping factor. The feed-forward design makes bias currents unnecessary, and non-critical. This would make Class B operation possible with very low distortion, but we keep the output stage in Class AB operation. The feed-forward design also makes 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).
3. The AHB2 is much more efficient than a conventional class AB design. Peak power does not vary with AC line voltage (due to the amp’s regulated supply). Power drawn on one channel does not influence the power available on the other channel.
4. The AHB2 is a linear amplifier. It is not a switching amplifier. For this reason, it produces very little out-of-band noise. A-weighted noise is only 2 dB less than noise measured over an 80 kHz bandwidth. For Benchmark, this was an important design goal because ultrasonic noise can be folded into the audio band by non-linearities in speaker transducers.
5. On the AHB2, multiple output stages run in parallel to eliminate crossover distortion. One output stage is active while another is in the crossover region.
6. The class AB output stages use very low bias currents and due to low bias currents, idle power consumption is only 20 W
7. The high-bandwidth control loop on the switch-mode power supply responds to amplifier loading over the entire audio band and at ultrasonic frequencies
8. The AHB2 does not rely on capacitive energy storage.
9. The switch-mode power supply reduces AC line magnetic interference to levels that would not be possible with a linear power supply.
10. >200 kHz bandwidth to achieve excellent inter-channel phase matching at 20 kHz.
11. < 0.1 Hz low frequency cutoff to minimize low-frequency phase shift.

(You can read the in-depth interview with John Siau here for even more technical insights)

Only when you understand that the AHB2 was envisioned as the amplification for a high-resolution audio chain can you fully come to understand the brilliance of the product’s design. That’s where its hyper-focus on eliminating noise and distortion stem from.

For example, unlike most amplifiers, there is a gain switch that you use to match the amplifier to your preamplifier’s output. If you’re using a DAC2 or professional line-level equipment, you simply set it to the lowest position. If you’re using a preamplifier with balanced connections, you set it to either low-gain or mid-gain position (as you will see below your ears will know right away which is the right position), and if you’re using an unbalanced connection, you set it to the top position. Simple, right? The purpose of the gain switch is to make sure that the amplifier isn’t over-amplifying the noise from your system’s signal chain to the loudspeakers and to allow the amp to operate at maximum output without clipping.

The real question is, “Did Benchmark succeed?” I took the AHB2 for an extended review, using it as my reference amplifier for nearly three months, to find out.

Setup

My initial setup from Benchmark consisted of a single AHB2 running in stereo with Benchmark’s DAC2 HGC digital to analog converter. Throughout my review period with the AHB2 I swapped between the Benchmark DAC2 and my Anthem AVM50v 3D.

Although I initially connected the AHB2 to my Revel Ultima2 Salons using my existing speaker cables, I became so enamored with the SpeakON connectors on the AHB2 that I discarded my cables in favor of the SpeakON cables Benchmark provided. I never looked back. Believe me, once you have the option to use SpeakON connections between your speakers and a power amplifier, you’ll eschew your current cables just as I did.

I made sure to follow Benchmark’s recommended setup for optimal digital audio and high-resolution playback, which is:

Two-channel digital source —> DAC2 —> AHB2 

By connecting your two-channel digital sources directly to the DAC2 you will have a true high-resolution playback system that is capable of 127 dB system SNR with bandwidth that is, in Benchmark’s own words, “limited by the digital format.”

During one of my email exchanges with John Siau about optimal high-resolution playback and multichannel high-resolution playback, he shared with me that Benchmark, Revel, and AIX Records were going to have a room at AXPONA in Chicago to showcase the performance of high-resolution audio.

The planned AXPONA setup would consist of five Revel Ultima2 Salons. The five Salon2 speakers would be driven by five AHB2 power amplifiers running in bridged mono. A modified Oppo Blu-ray player was equipped with dedicated S/PDIF digital outputs which fed 3 DAC2 converters. The converters would also be connected to AES digital outputs on a PC running JRiver. In either mode of operation all six audio channels provided bit transparent digital outputs from the AIX demo disks. Talk about a system to die for!

Given that I too would be testing the Benchmark AHB2 power amplifier with a pair of Revel Ultima2 Salons, John sent me a second AHB2 so that I could mimic the amplification setup of the AXPONA show. However, instead of a multichannel setup consisting of five Salon2s, I would be running the full Benchmark setup in traditional two-channel.

For the next several months, I ran each AHB2 in bridged mono. I feel it is important to note that when an amplifier has the option of running in bridged mono, it usually cannot drive lower-impedance loads. The Ulitma2 Salons dip down into a 3 Ohm load. John assured me that the AHB2 would have no problem handling the Salon2s in bridged mono. The only potential scenario that would send the amps into thermal protection would be if I drove the speakers very hard near ear-bleed levels. Regular listening would be no problem. John was spot on with his assessment.

Rear panel detail of the Benchmark AHB2 Power Amplifier

The option to drive lower-impedance speakers in bridged mono was a key point of interest for me. Why? Well, that means if budget is a limiting factor, you could purchase a single AHB2 for your speakers and run it in stereo. Then, as budget allows, you could add a second AHB2 and run both in bridged mono, essentially doubling your power output. (For comparison, the AHB2 will deliver 240 Watts into 3 Ohms stereo and about 486 watts into 3 Ohms bridged mono). To me, that represented a potentially important option for many audiophiles or home theater aficionados building their systems over time.

As I mentioned earlier, in addition to the Benchmark DAC2, I also used my Anthem AVM50v 3D preamp-processor to test the AHB2 for movies and home theater. When I connected the AHB2 to the Anthem and then ran ARC’s (Anthem Room Correction) calibration, I had an interesting anomaly.

When the ARC calibration started, the Ultima2 Salons sounded like the volume had been turned way down. The volume difference between the Salons and the rest of the setup was startling. In fact, ARC yielded an error because the volume output on the Salon2s was so low. ARC was unable to compensate. Unless I lowered the volume of my dual SVS SB13-Ultra subwoofers by a full -8db, ARC would not complete an audio sweep. When I lowered the dual subwoofers’ volume levels, ARC finished its calibration without issue. The final reference target volume was only about 68-69db instead of the reference 75db. I re-ran the calibration with the AHB2’s gain setting in the middle position, raised the subwoofer volume back up by 8db and bingo the results were back at reference—75db.

If you read the user manual, the AHB2 states that either the mid- or low-gain setting should be used when using balanced connections (which I was). I used the low-gain setting based on the user manual’s guidance and that was what I was using with the DAC2. In case you are wondering, the low-gain setting is the correct setting when using a DAC2.

I asked John Siau about this peculiar experience and he explained that the low-gain setting is intended for use with professional audio sources (such as the Benchmark D/A converters). Most consumer and audiophile products will require the use of the higher gain settings. Audiophile balanced connections usually require the mid-gain setting. Consumer-grade unbalanced sources will require the high-gain setting.

I related to John that I perceived there being a difference between the low-gain and mid-gain setting. When I put the Anthem to the mid-gain setting, it sounded… well… noisier. John told me that products that cannot deliver high signal levels usually cannot deliver high signal to noise ratios. The noise I was hearing in the mid-gain position was the Anthem/ARC combination and not the AHB2.

To put it into raw numbers, the Anthem AVM50v 3D has a SNR of 104 db. The AHB2 by contrast has a SNR of 135db in bridged mono. The Anthem is 31 db noisier than the AHB2 and and 23 dB noisier than a DAC2, a potentially audible difference.  For this reason, John told me, “This is why we always recommend driving the AHB2 directly from the Benchmark DAC2. Do not insert a preamplifier or processor between the DAC2 and the AHB2. Almost any inserted preamplifier will raise the system noise by about 25 dB. The DAC2 is much cleaner and quieter than other preamplifiers. The comparison is not even close.”

If you have a multichannel pre-pro or receiver and want to get optimal performance out of the AHB2, Benchmark recommends that you use a DAC2 for two-channel listening and run the DAC2 in Home Theater (HT) Bypass mode. Using such a setup, which is very common with high end two channel preamps, you run unbalanced cables from your multichannel pre-pro to the DAC2 and then select the analog input on the DAC2. The HT Bypass mode will automatically activate and it will ramp the DAC2 volume control up to maximum. In multi-channel mode, your system volume will be controlled by your pre-pro or receiver in your home theater system. In other words, in such an architecture, you get the best of both worlds.

Because it can be confusing, here is what Benchmark says your optimal setup should look like:

Multichannel Playback:

• Multichannel Source > Multichannel Pre-pro/Receiver > DAC2 > AHB2

Stereo Playback:

• Stereo Source –> DAC2 –> AHB2

Never do this:

• Stereo Source –> DAC2 –> Multichannel Pre-Pro/Receiver –> AHB2

And, NEVER DO THIS:

• Stereo Source –> Multichannel Pre-Pro/Receiver –> DAC2 –> AHB2

With everything ready-to-go, I was anxious to put the AHB2 through an extended trial.

Listening Tests:

One of my first critical listening sessions was with Sade’s Soldier of Love. Sade’s husky yet smooth contralto vocals can serve as an audiophile’s aural panacea. The DAC2 and AHB2 combo reproduced this album with an intoxicating quality. The opening track, “The Moon and the Sky” evoked palpable, audible images of a “you are there” experience. Sade’s vocals streamed forth as though emanating from a pitch black, quiet night on a desert island. Bass lines on every cut were clean and taut. They also had great punch.

Even though I was initially playing the AHB2 in stereo and it was rated at only 100W, I never felt like I was lacking power and performance. Not once did I sense that the amp was gasping for breath. Soldier of Love was like a clinic in speaker control. Instruments started, stopped, and snapped with precision; and the black, quiet backdrop of Benchmark’s dynamic duo allowed me to hear deeper into the music, picking up delicate nuances and details that ordinarily would be overshadowed or muddied by a lesser signal chain.

I had the same experience with Thao Nuguyen. Instruments from Thao Nguyen’s alternative rock cut, “Bag of Hammers” popped from a dead black background. Thao’s vocals were exceptionally well rounded. There’s a lot acoustically going on in this song and the Benchmark combo held it all together in a wonderfully coherent soundstage that preserved just about every detail imaginable.

As I played album after album and track after track, I became increasingly sensitized to the absence of noise and distortion and the beautiful cloak of silence. It is so incredibly hard to put into words the experience that the DAC2 —> AHB2 —> Revel Ultima2 Salon setup produced. There was simply no distortion, no noise, just a blank stage. Regardless of genre, vocals and instruments popped to life and they did so with a almost true-to-life “you are there” feel to them.

To make an analogy, the noiseless, distortion-free background that the Benchmark produced was similar to the effect you get listening to live orchestra where the instruments simply spring to life from a space in time. To put it another way, for those of you who remember the old cassette tapes, you will easily recall the noise they produced. When you were listening, you didn’t always readily realize the noise and distortion that was there. However, once you turned on Dolby B, C, or S noise reduction, not only was the noise gone, but you then realized that the noise was part of what you were listening to. You had simply become accustomed to it. Unlike Dolby noise reduction, however, the AHB2 didn’t kill the purity and clarity of the highs. The full audio range remained dazzlingly clean and open.

That Dolby-style noise reduction was in full display on Fairground Attraction’s album, Aye Fond Kiss. I’ve played the opening track, “Jock O’Hazeldean” dozens and dozens of times. The difference playing this song through the Benchmark Combo was immediately tell-tale. Vocals and instruments were hung on a velvety wall of silence. Bass lines had life and intricate nuances to them. The texture of the instruments was more readily palpable and discernible.

Transitioning to the AHB2 to bridged mono was like loading the Salon2s with bazookas. There’s bass and then there’s bass. The Salons are some of the most capable full-range speakers on the planet. When paired with the AHB2s in bridged mono, bass notes were the cleanest, most detailed, and most controlled I’ve ever heard out of the Salon2s. What I appreciated most of all was how increasingly detailed the bass became. It’s as though the noise floor was lowered to such a degree that you no longer had something fighting for attention. I was able to hear more and discern more.

Because the AHB2 was born and bred for high-res playback, it was time to let the amp flex its stuff. I played many high-res audio tracks through the signal chain. However, allow me to highlight the Naxos 2xHD 192kHz/24bit recording of Saint Saëns Symphony No. 3 “Organ” by the Malmo Symphony Orchestra with Marc Soustrot conducting. I downloaded the high res album from Acoustic Sounds.

Saint Saëns Symphony No. 3 is a well-known classical masterpiece and audiophile test track. With its beautiful, deep, resonating organ notes, this symphony is a rite of passage for any full-range audio system. What impressed me immensely was not only the AHB2’s command of the entire audio spectrum but also the pristine presentation from top to bottom. The organ notes were deep and rich. I cranked the volume. No noise. No distortion. No hint of strain.

Instruments never intruded on one another to blur the masterful orchestral illusion before me. From a backdrop of pure vacuous space, the symphonic elements danced gracefully across the soundstage. Every group of instruments was firmly entrenched in its own space in time. The presentation was nothing short of glorious.

The delicate notes of violins resonated precisely, perfectly laid out and you were able to discern the tonal character of the strings and the warmth of the violin’s body. The resonant timbre of brass notes was spot on. The entire orchestra came together in a coherent, dynamic whole. If this is high-res music, then I will take it with joy every time. There wasn’t a single note where the DAC2/AHB2 combination didn’t bring out the immaculate character of the Salon2s. Crescendo!

Moving back out the realm of high-res to rock, I popped in U2’s classic album, the Joshua Tree. The first notes of the synthesizer from the opening cut, “Where the Streets Have No Name” were deep, tight, and intense. My entire room pressurized without any bloat or distortion. The Edge’s characteristic guitar riffs had clinical precision; but it was Bono’s vocals that stole the show. Never—and I mean never—have I heard such a detailed, spot on vocal presentation through the Salon2s. Bono’s vocals came to life, not only through their timbre, but through the presence of the slightest audible cues that you can discern in an intimate audio session. I pursed my eyes closed and just blurted out an ecstatic “wow!” What I’m going to say isn’t a hyperbole. Although I’ve been playing the Joshua Tree for almost 29 years, this was the first time I ever heard the album in all its glory.

There are so many other musical sessions I could gush upon, but let me just highlight Elaine Paige. The First Lady of British Musical Theater’s vocals have set the standard for so many beloved musical pieces. Playing “Don’t Cry For Me Argentina” from Evita and “I Dreamed a Dream” from Lés Miserables through the Salon2s and AHB2s were like a personal concert in my living room. But it was Elaine Paige’s rendition of “Memory” from the musical Cats that literally sent every hair on my body standing on end. I cranked it up louder, and played it again. I cranked it up even louder, and played it yet again. Each time, the emotional response was the same. Elaine’s vocals were so clean, so detailed, so emotive, so free of any hint of distortion that it was uncanny.

It’s no surprise the movies through the Anthem AVM50v were a cake walk for the AHB2s and the Salon2s. I won’t belabor the the expected and the obvious. The audio experience with American Sniper, Big Hero 6, Kingsman: The Secret Service, Tron Legacy, and Gravity to name just a few that have outclassed many a commercial movie theater. I never clipped the AHB2 in stereo mode but I did trip one of the bridged mono AHB2 amplifiers into over current protection (at 486W) when I was playing Gravity at near ear-bleeding reference levels.

The AHB2 showed time and again, it could attain total mastery over any genre and elevate it to a blissful experience.

Conclusion

The Benchmark AHB2 Power Amplifier is one of the finest sonic experiences to ever come through my system. The AHB2 belongs on every audiophile’s short list. Yes, it’s that good.

Don’t judge this amp by its size. Judge it by its specs. Judge it by its emotional impact. The AHB2’s power, finesse, and measurable freedom from noise and distortion bring with it the potential to elevate every aspect of your listening experience. If you think you’ve heard your system free from noise and distortion, think again. The AHB2 will change your perception.

If you are determined to get the most out of high-resolution audio then you need a system whose optimal performance can bring out deliver the full potential of high-res audio. There is no more compelling case than to anchor a high-res setup with the Benchmark DAC2 and AHB2 combo.

The Benchmark AHB2 amplifier and DAC2 preamplifier took complete, unfettered control of the universally acclaimed Revel Ultima2 Salon speakers and never let go. In fact, the AHB2 allowed the Salon2s to achieve pinnacle performance. At $2,995 the AHB2 isn’t cheap; but its build quality, materials, and astonishing sonic performance are a marvel.

The AHB2 is an audiophile’s dream and a competitor’s nightmare. For my part, I don’t want my dream with the AHB2 to end. Not only does the AHB2 get my highest and strongest recommendation, but it also gets an enthusiastic nod as our 2015 Audiophile Amplifier of the Year.

Benchmark Media Systems, Inc.
203 East Hampton Place, Floor 2
Syracuse, NY 13206 USA

Price: $2,995

• Revel Ultima2 Salon Loudspeakers
• Anthem AVM50v 3D multichannel processor