Benchmark Media AHB2 Stereo Power Amplifier
first taste of this remarkable new amplifier came earlier this year in April at
Chicago's AXPONA where I discovered it driving a newly redesigned Studio
Electric Pasadena loudspeaker, also bearing the Benchmark trademark. Now called
the SMS1, the Pasadena's crossover had been reworked by Benchmark, upgraded with
very low tolerance components, and the new look, more in line with the look of
the rest of the Benchmark product line up, had been adopted. The subsequent
sound in this room was one of the most musically engaging I encountered at this
year's event, a remarkable result considering its modest overall system price of
just over $7000, sans source.
The only word to describe this amplifier is tiny! Measuring just 11.04" wide by 3.88" tall and 9.34" deep, including feet, heat sinks, and binding posts, it weighs in at only 12.5 pounds! The actual amplifier circuit itself fits in the chassis space of only 8.32" by 3.88" by 7.88"! That is slightly less than one quarter of one cubic foot! Do you realize how small that is! So, just how good can such a miniscule Class D amplifier sound? That was my first completely erroneous thought too. Folks, this isn't a digital amplifier at all; it is a linear amplifier built on technology developed and patented by THX Limited, of San Francisco, California. Yes, that THX...
I had the good fortune of being able to speak with both Laurie Fincham, Senior Vice President, Audio Research and Development, and Jayant Datta, Assistant Vice President, Audio Research and Development, of THX. What I learned through several conversations with both gentlemen was fascinating, and led me to a clearer understanding of just what an achievement this new amp truly is. THX had an original design goal of building an amplifier that could afford Class A sonic performance, yet be able to provide that sound quality with ultra-low dissipation; in essence, to create spectacular sound without the consequential downside of all the heat and waste from the amplifier. The kernel of the idea was hatched back about 2006 or so, with a THX engineer toying with and developing the idea until somewhere about 2012. At that point, two THX engineers focused on its execution for the better part of another year. The collaboration with Benchmark began (it was Benchmark who really set the specification and performance goals for the AHB2), in early 2013, and they had created their working prototype by CES 2014.
The AHB2 (named in tribute to Allen H. Burdick,
who founded Benchmark Media Systems in 1983) uses a switching power supply with
rails regulated at +/- 45 Volts. It is a hybrid amplifier, using a Class A-B
design that also employs signal tracking to modulate the voltage on the rails
according to the amplifier's actual demand at any given moment. This next part
is where the real magic seems to take place, and is the one that Jayant prefaced
in one of our phone conversations with, "You're not going to like this..." The
next few sentences he spoke to me contained the word "feedback," several times!
THX has created and patented an approach for
taming switching distortions normally found in Class A-B amps by employing a
complex circuit that relies on multiple paths of both local and global feedback and
feed forward. Laurie is quick to point out that Bell Labs first developed feed
forward in the early 1920s. However, this circuit is trademarked by THX as THX
AAA, or THX Achromatic Audio Amplifier. A Merriam-Webster
search finds Achromatic defined as:
1. - refracting light without dispersing it into
its constituent colors : giving images practically
free from extraneous colors <an achromatic telescope>
Overall, given the sonic results afforded by this
circuit in this application, I'd have to say that I find that word to be a
fittingly descriptive adjective for this circuit. Here is a link
to Laurie discussing the amplifier with Enjoy the Music.TV's Steven R. Rochlin
at CES 2014.
The results afford lower dissipation by offering low standing bias, a lower fixed rail voltage to reduce dissipation for low level signals, and a tracking voltage rail that affords the same advantage to high amplitude signals. The feed forward error correction of the THX AAA technology ensures that distortion remains extremely low from the lowest amplitude signals through the full output of the power amplifier.
Audio output is realized with four pair of
complementary bipolar transistors (8 devices, in 4 pairs) per channel, with the
devices chosen for their ability to meet the desired current capabilities under
all drive conditions and to be able to account for any realistic load impedance.
The result is an output capability of just over 18 Amperes of current and 100
Wpc into 8 Ohms, both channels driven in stereo. In addition, this amp can be
bridged into mono with an output of 380 Watts into 8 Ohms! Honestly, that was
the configuration I had hoped to be able to implement and audition, two AHB2
amps bridged to mono, one each for left and right, but there just weren't enough
amps available to allow me that luxury – at least, not now.
Finally, because of the low dissipation this THX AAA circuit affords, cooling is more than adequately handled passively with relatively small heat sinks, two banks of 21 extruded fins (each fin measures approximately 1.5" by 3.5" by 3/16" thick) per side of the chassis. The resultant packaged amplifier is rather elegantly smallish, yet stylistically and functionally beautiful.
The brushed aluminum faceplate of this remarkably small amp has four hex bolts securing it to the chassis. The stylized Benchmark logo is engraved into the panel, just above center on the right side, while the model number is stenciled in red block letters to the left. Just below the model number is the smallish push button switch and its associated blue indicator LED. There are two banks of three red indicator LEDs, one each to indicate "CLIP," "TEMP," and "MUTE" for each channel, lined up and centered along the bottom of the faceplate.
The back panel is a very busy place, given the number of connections and switches that Benchmark has chosen to make available on a surface with an area just slightly larger than 30 square inches. Centered near the top of the back panel is the IEC power cord receptacle. Immediately to the left and right of that socket are WBT speaker binding posts for left and right channels. In line and immediately below that are two XLR connections, one each for left and right signal input, located on each end of a row, with three SpeakOn connections in line between them. SpeakOn connections are typically used to handle more complex pro wiring options and include the ability to manage stereo, bridged mono, bi-wireing and bi-amping situations. Those five jacks are labeled from left to right as 2 In, 2 OUT, BRIDGED MONO OUT, 1 & 2 OUT, and 1 IN/MONO IN.
Just to the outside of the two XLR sockets, to
the immediate right and left of that line up, are a matched set of 3.5mm jacks
set directly above a small micro toggle switch. The left bank jack is for 12V
TRIGGER IN/OUT and its corresponding toggle switch allows for MODE selection, up
for MONO, down for STEREO. The right side socket is also for 12V TRIGGER IN/OUT
with its corresponding toggle switch allowing for SENSITIVITY selection. Those
three switch options are labeled in both VRMS and dBu, with the settings being 2
VRMS or 8.2 dBu in the up position, 4 VRMS or 14.2 dBu in the middle position,
and 9.8 VRMS or 22 dBu in the down position. Counting the positive and negative
speaker connections as one each, or two per channel, that is a total of 14
connections and switches!
What's All This Then...
The sonic results of all the time and effort
spent refining this circuit and developing this design are nothing short of
extraordinary! Bass is rendered with exceptional detail and speed, and brother
can it go deep! In fact, I can't
think of another amplifier (or pair of monos) I've experienced for under about
$6000 that can do what the AHB2 can do with the bandwidth below 100 Hz. It has
jaw dropping, chest-thumping impact while still being able to render remarkably
accurate and refined pitch definition. In many ways, its bass performance
reminded me of my time with the $65,000 WAVAC MD-805 Mk II monos, which rendered
some of the most engaging bass I've yet heard.
Midrange is simply captivating, rich in both
musical resolve and harmonic texture. It affords an exceptionally well-balanced
mixture of both tonal truth and emotional expression. Instrumental voices are
provided the body and bloom needed to complete the "live" illusion, recording
permitting, and are related with the proper texture and tonal color, if ever so
slightly to the dryer side of natural.
Treble is extended, airy, and resolute. Once
again, I'm hard pressed to think of an amplifier for less than twice as much as
the AHB2 that can even begin to compare, let alone match, this amps performance
with the spectrum above 2.5 kHz. This extended and replete high frequency
performance bestows a sense of ease and relaxed enchantment to well-recorded
materials previously unheard of in this price point.
One of the areas where the AHB2 really shines is
in its ability to render the space between and the sense of both the space
occupied by, and the interrelationships of instrumental positions and locations.
Instruments and vocalists occupy an uncannily realistic sense of size and
locale. I was left speechless more often than I cared to count by this engaging
and enchanting ability.
While I wouldn't really call the AHB2 sensitive
to power cable choices, it does react to them nicely. Though I could easily
distinguish the differences between all the cords I chose to feed it with, those
audible results clearly corresponded to the reasons I have all these different
power cables on hand. I settled on the new Audience Au24-SE powerChord, as it
allowed all the AHB2's strengths to shine through at their best.
One caution. Many looking for an amplifier in
this price range will be pairing it with comparably priced equipment that simply
will not allow it to reveal all its abilities and strengths. I dropped this
unassuming little upstart into a system that has a retail value over 65 times
its own $3000 retail cost. Not only was it not embarrassed in that company, it
rose to the opportunity and shone spectacularly!
Think about that for a second. I'd challenge you to do that kind of substitution in any system in that price range and not find overall system performance to degrade rather than remain undiminished, or even arguably improve in some areas! So consider yourself warned; you honestly may not be able to appreciate just how extraordinary this little amp is, to hear all it is capable of delivering, until you hitch it to some considerably more expensive components than would normally be considered in its associated price range. No, really!
I was further frustrated by the use of what
appear to be the 0735 or 0765 safety series of WBT speaker binding posts. They
sport a clear Makrolon protective cover and guide to prevent accidental
shorting. However, this added safety measure affords only a very
limited area of engagement for spade-terminated cables. Only spades up to a
certain width will pass through the guide to actually engage the post, and then
only from one fixed and immovable insertion angle. On the AHB2, cables may only
be inserted up from the bottom of the posts. This prohibits the use of larger,
less flexible cables in general, and of those with large spades in particular,
like my Stealth Dream V14's. I had to use a banana converter (my trusty ol'
Monster Cable Xterminators) to make a connection.
While I completely understand these decisions were made to both promote safety (given the necessary proximity of all the connections on the very small rear chassis footprint) and because of the objectively higher performance that the SpeakOn connections can offer, I'm hopeful that future iterations might offer RCA inputs and similar quality, less restrictive binding posts for the home audio only customer, as the lack of SpeakOn connections in the high performance loudspeaker market makes their inclusion almost a moot point except for either Pro applications or an all-Benchmark system, the likes of which is soon to be realized. And, if it sounds this good when using an additional connection interface on both the signal inputs and the speaker outputs, imagine what it must really sound like!
Those few nits aside, the AHB2 is a notable exception in the world of high performance audio marketing today; it is a product whose performance actually exceeds the hype surrounding its innovative technology. In today's world, where the advertising and self-promotion of nearly every new product coming to market claims to employ some exemplary breakthrough, and the term innovative is bandied about so freely that it seems to have lost any real meaning, Benchmark's AHB2 amplifier is a truly breakthrough, innovative design.
Results: The Little Amp That
Can... And Does!
Does it sound the same as the more expensive pure
Class-A amplifiers I've made mention of or alluded to? Of course not. But just
as earnestly, it has no right to perform this bloody well while being this small
and this affordable. You simply
must hear this little overachiever for yourself, as it represents one of the
absolute best values in amplification in the industry today!
Don't make the mistake of assuming that because
of its realistic price, miniscule size, or its feather-like weight, that this is
not a serious amplifier. To dismiss it with such a foolish and inaccurate
assumption would be to your great loss! This little upstart will change the way
you look at, and listen to, amplifiers from the first time you hear it. And
while I don't really need another
amplifier, I'm going to find a way to both placate my wife and work with the
fine folks at Benchmark to give this little giant slayer a new home here in the
Midwest. Yes, it is that good! I can't imagine a higher recommendation, can you?
Voice: (800) 262-4675