Balanced Audio Technology VK-655SE Power Amplifier

The big, black BAT sat staring back at me with the same undeniable presence as “The Object” on the cover of Led Zeppelin’s album of the same name. The Editor had just left, having indeed delivered, as he had promised, “the flagship solid state Balanced Audio Technology amplifier”. Heeding the Call Up, we continued to eyeball each other as I scrounged around for another power cord to plug into its hulking, monolithic, dual mono chassis. The whole planet’s on fire, not least our sunburnt country right now, so what use did I have for all this grid-draining, solid-state fire-power?

Start

Pandering to prejudice, I switched the beast on and let it get to know the other boys and girls for a few hours by feeding it a steady diet of staples like The Clash, Minor Threat and The Adolescents “Kids of the Black Hole”. Pretty soon I too was on, rocking and rolling in the muscular, wide-band sound. So I went at it with Big Black’s “Kerosene” and that proved incendiary not only in terms of the resolution and slam on offer but also for the heavy duty emotional depth charges the BAT VK-655SE was starting to shoot out. I threw it a piece of meat in the form of The Stranglers “Tank”. Sure enough, the driving bass of Jean Jacques Burnel came charging through with indomitable grip, authority and ‘sock it to me’ growl. I heard the kind of bass dynamics, pitch definition, and control you would expect to hear from a statement solid state amplifier of this calibre and pedigree. But I also heard an uncommon fullness of sound, a 3D solidity, with extraordinary inner harmonic detail and touch-perfect physicality and swell. Nice (…and sleazy).

With all this overt power on tap, I continued on my Stranglers kick, arriving finally at former member, Hugh Cornwell’s latest, Monster. Disc two features some stripped down acoustic re-recordings of old Stranglers classics. By the last track, “Always the Sun”, I was starting to get it. The BAT VK-655SE is not just your prototypical solid state bass monster, being all grunt and no grace. It is equally capable of uncommon subtlety and very powerful nuance. The almost-but-not-quite monotonic voice could only be Cornwell’s. His solo acoustic guitar had such lifelike body and colour. A delicate single line overdub really hit home emotionally with incredible projection and yes, palpable presence. I could feel the exceptional phase coherence to the sound; the ‘suck me in’ immediacy and solidity. I could sense no prototypical solid state signature or artefacts. The beast had started to tame me. Perhaps, as BAT assert, and I suspect The Stranglers would agree, “there’s still no substitute for cubic inches”.

Rewind

In 2019, Balanced Audio Technology (BAT) is celebrating its 25th anniversary as a company. This is no mean feat given the routinely high casualty count in this industry of ours. The two principals Victor Khomenko and Steve Bednarski first came together while working at Hewlett Packard in the 1980s. Recently, I was fortunate to be able to chat with the affable pair. Bednarski told me:

Victor and I met when we both worked at Hewlett Packard, in the great heyday of HP as a company.We both shared a passion for music as well as audio equipment. Victor’s strength in engineering is a fine complement to my strength in business.Thus, Balanced Audio Technology was formed to create a lasting impact within the audio world.

BAT hit the ground running with the tubed VK-5 preamplifier and the stunning VK-60 amplifier and introduced the Russian 6H30 ‘supertube’ to the Gates of the West. However, the always innovative company never planned to be wedded to any one form of technology. Says Khomenko:

I don't like dividing the designers into pigeon holes based on particular technology used for a particular application.So it was only natural for us, after putting on the market the successful tube VK-60, to turn our attention to the realm of solid state.

Here, too, we approached the task in a non-traditional way. For a powerful amplifier to only contain two gain stages, and no global feedback, was perhaps the first at its time. However, while that design was based on MOSFET's, our next solid state model, the VK-6200, used the bipolar technology.So in essence we have solid state offerings in two different technologies, in addition to vacuum tubes.And all of them had received enthusiastic reception.

Contact

The BAT VK-655SE (VK stands for Victor Khomenko, of course) currently sits at the pinnacle of the BAT power amplifier line-up. It is a high-current, solid-state stereo design that outputs a whopping 300 watts into 8 ohms and doubles its power into 4 ohms. I suspect there would be few loudspeakers these days that this mighty BAT could not drive, no matter how demanding the impedance plot. Built with Mil-spec precision, the VK-655SE is a true dual-mono component design featuring completely independent channel assemblies, power transformers, and separate power cords for the left and right channels. Only the blue power switch on the front panel is shared. There are two sets of high quality binding posts on the back of the VK-655SE (one for each channel) that, in my view, are just a little bit too close to each other for comfort.

BAT

What’s more, if you are truly power mad or simply feeling inadequate because your audio buddy just got a D’Agostino Relentless, the stereo unit can be factory reconfigured for “twice the current”, monoblock operation. (Yes, apparently it sounds even better this way). This unusual true dual-mono design is said to lead to maximal soundstage depth and width as well as improved reproduction of micro and macro dynamics. Weighing in at a mere 120 pounds, it certainly leads to a considerable physical presence on the listening room floor. I asked Khomenko what are the advantages of using a parallel mono channel architecture as compared to, say, bridging stereo amplifiers. He replied that:

These approaches are almost diametrically opposed.The traditional bridging approach increases the output power, on paper... while degrading the amp's ability to drive the load.Your bridged amp will play louder, but its drive will suffer. Paralleling the channels works very much in the same way as adding cylinders to your car's engine. You basically go from 4 cylinders to 8 and everyone knows the difference in car's behavior that such change produces.The car becomes more drivable, and the paralleled amplifier gains in its ability to drive the load... especially the demanding load.

The BAT VK-655SE also features a revised power supply, designated a ‘Second Generation’ SUPER-PAK, which incorporates the same custom, oil-filled signal capacitors as used in Balanced Audio’s impressive REX II preamplifier. The SUPER-PAK is said to increase the high quality filtering capacity of the amplifier by an order of magnitude over the first generation supply resulting in a further lowering of the VK-655SE’s noise floor and provides a more grain-free and refined portrayal of textures, harmonics and low-level detail. Why the oil caps? Because typical poly caps add “a ‘plastic’ colouration that made our components sound artificial and electronic”. According to Khomenko:

The power supply system in VK-655SE is fairly complicated. There’s a total of eight separate power supplies, four of which are floating.The total energy storage approaches 1000 joules. The power supply storage is distributed.Some of it is located right next to the output transistors, providing the quickest instantaneous current.Others are located in two BAT-PAK storage elements and yet more in the SUPER-PAK.The storage consists of two types.The high-value electrolytic capacitors supply bulk of it and give the amp the powerful punch answering the need for dynamic response. On top of that banks of oil capacitors work to improve the finesse of the sound.

Rush for a change of atmosphere

Despite the VK-665SE’s massive power reserves – the mono block version, in fact, contains 1800 joules of energy storage – and notwithstanding the need for those cubic inches, the amplifier circuit itself is a model of artful engineering simplicity more akin to a low-powered, single-ended amplifier circuit. The BAT power amplifier has three particularly unusual features which distinguish it from other high powered designs: no global feedback, having only two gain stages, and exclusive use of only N-channel MOSFET output transistors. Khomenko claims:

Back when we created the original predecessor to the VK-655SE, I had an encounter with Nelson Pass [of Pass Laboratories]…Where I basically told him we are working on an amplifier with only two gain stages, one at the input and one at the output with no additional buffers or gain stages. He replied with a statement that it couldn’t be done or something to that effect. Yet, we (BAT) had already done it!

Firstly then, the implementation of zero global negative feedback in the VK-655SE is said to result in an unconstricted, wide bandwidth amplifier where, according to Victor: “The gain stages are inherently linear”. Secondly, the output transistors are still all N-Channel MOSFETs “as these are superior in their characteristics”. Whereas most solid state amplifiers are typically push-pull with complementary pairs of P-type output devices for one side of the signal and N-types for the other (the + and - sides of the waveform), the VK-655SE uses only the inherently faster N-type MOSFETs for both sides of the signal making for a much faster and more dynamically responsive amplifier: “Pairing the faster N-channel devices with the slower P-channel devices is like pairing Ginger Rogers with her horse on the dancefloor instead of Fred Astaire”.

BAT

Thirdly, whereas most high power amplifier designs have three or more gain stages including, for example, a driver (buffer) stage between the input and output sections, the BAT has only two gain stages, an input stage and an output stage. In the VK-655SE, the input stage also serves as the driver for the output stage.

The VK-655SE is the continuation of our successful line of MOSFET-based solid state amps.The first model was the VK-500, then followed by the VK-600 and VK-650, each one incorporating various improvements to the already solid foundation of the VK-500.The design goals for the VK-655SE are to offer a perfectly symmetrical design with as simple a signal path as possible, yet featuring a massive power supply.Thus, the VK-655SE is a high power amplifier that features only two gain blocks with zero global feedback.

Bednarski continued:

Having two gain blocks simply means that the signal is only amplified twice.Imagine the game of tell a joke and pass it on.Pass that joke on too many times and you won’t recognize the joke when it is retold.Pass the signal on too many times and you won’t recognize the beauty of the recorded signal when it is reproduced.

With Khomenko clarifying:

Just to clarify Steve’s point: Two gain blocks is the best that has been achieved in an amplifier with this level of power output.

The trade-off is the need for serious AC current draw and the generation of serious heat energy. The input stage in the VK-655SE has to be run at a very high Class-AB bias and places a very high demand on the SUPER-PAK power supply leading to the emission of mucho heat. Sprung from cages on Highway 9, this amplifier gets really warm in use and the additional cost in my home was literally a trebling of our power bill for the two quarters the amp was in residence. Fortunately, one was winter. It also sounds considerably better when left on all the time, taking days to get up to speed from cold. Given the sweet sound quality on offer on this last chance power drive (oh!), it was an added cost I could rationalise. Self-conscious about your carbon footprint? Ditch the fridge instead, eat cleaner, and enjoy even higher quality low noise sound!

E=MC2

I’m not sure that any of us would ultimately be happy musically with the sound of just the theoretical ideal straight wire with gain. Nonetheless, the ultimate objective of any amplifier is to amplify the puniest of input signals while doing the least amount of sonic harm and while achieving the greatest transparency and fidelity to that signal. For BAT, from the very beginning, this means the superiority of balanced operation is fundamental: “In our opinion, balanced topology simply provides a complete signal representation. Something magical happens when you free yourself from the limitations of the single-ended structure with its half-signal processing.If a one-handed craftsman can be very good, imagine what he could do with two hands”.

But for BAT also, “balanced” has a far deeper meaning and praxis. This fundamental philosophy informs the true dual mono construction of the VK-655SE. It informs the unique implementation of those two only gain stages. It informs the use of those only N-channel output transistors thus ensuring that both sides of the waveform are handled by identical devices in identical circuit configuration “assuring ultimate symmetry of the resulting signal”. The amplifier is, of course, fully balanced from input to output so that the signal never connects with the ground for the least amount of added noise and distortion. However, digging deeper, the VK-655SE is not your average balanced audio design. Nor is it incompatible with other single ended equipment. Like the flagship REX II tube amplifier, the internal circuit of the VK-655SE is neither a traditional single-ended design nor a push-pull design.

We refer to [these designs] as single-ended bridge designs. This fundamental circuit dates back to the early 1950's and was invented by Henry Wiggins while working for the Electro-Voice Company. The original name for this type of output topology was Circlotron. The single-ended bridge design relies upon two single-ended amplifiers arranged in a floating bridge formation. This differentiates it from other forms of bridge topologies, most of which effectively incorporate two push-pull sides.

BAT

The output circuit is inherently balanced and is driven differentially from a preceding stage. The two DC bias currents that exist in each half of the output stage cancel each other in the load. It is a more expensive way of building an amplifier as it requires multiple floating power supplies, as opposed to the more traditional ground-referenced supplies. The benefits of this approach, however, include enhanced symmetry when compared to push-pull designs and lack of the DC saturation problem inherent to single-ended designs.

Furthermore, the BAT VK-655SE sounds like neither a typical tube nor a solid state design. Indeed, this rare beast might just combine the most magical qualities of both approaches in a one, admittedly, XXL-sized package.

Just Play Music

For an amplifier this physically conspicuous, the BAT VK-655SE just gets out of the way and plays music. In use, it is one of the most transparent devices I have yet come across, especially in the wide open grain-free mid-band. The overall tonal balance from top to bottom is remarkably even-handed, if slightly warm and dark – but not too warm and dark – particularly in the upper reaches of the treble. The amplifier’s presentation is so seamless and organically complete it seems churlish to break it down artificially into its constituent elements.

The VK-655SE played any genre of music and drove every speaker I threw at it without any stress or strain. There was no sense of dynamic compression at any playback level. Expressiveness and articulation are of the highest order as is differentiation between different tracks and recordings. It makes the most of its seemingly boundless power reserves to impart very little character of its own on any program. In short, I could not conceive of a more flexible, stable and dependable all-rounder.

BAT

In operation, the big BAT is dead silent. Resolution of low level detail is extraordinary. Resolution, point blank, is extraordinary. There is an uncanny feeling of limitlessness to the sound attesting to an imperceptibly low noise floor. Again for a big amp, the VK -655SE is fast… very fast. Transient and dynamic responses leap out so quickly it is ghost-train-frightening. But there is no trace of MOSFET ghosting or smearing so the BAT brings you that much closer to the real event. The big amp is fluent and agile with a fine sense of timing leading to an incredibly nuanced and involving presentation. Unlike many exemplars of the breed, there is nothing flat or clinical or cardboard about its presentation. Tonal colour and density would be the envy of many a tube amp. For all of its explosive power and speed, layering perspectives are first rate. There is no sense of that typical solid state vivisection and evisceration of a recording. There is no trace of hardness or etch or silicon-based ‘uptightness’ as if the sound has got stuck up Ferris Bueller’s mate’s sphincter either. From the artful build-up of musical tension through the layering and looping of found and doctored sounds on Eno’s Ambient 4: On Land to the release of tension through dying fuzz box decays and feedback squalls on Ragged Glory by Neil Young + Crazy Horse, music just flows through the VK-655SE.

The bass is just about perfect. To hear deep bass reproduced through the VK-655SE is a mystical experience. No matter how deep it gets (or the woofer allows) there is never any loss of impact and definition. There’s always exceptional control and focus but the ride is smooth and never stiff. Check out the crucial bass line on “Johnny Was” by the Wailers or even its version by Stiff Little Fingers on Inflammable Material. I have been weaned on this “Bass Culture” since the 1970s and hearing “The Crooked Beat” through the BAT is simply awe-inspiring and very moving. Stringed, piped or electronic, bass is reproduced with a physical solidity and propulsive immediacy that eludes lesser amplifiers. The VK-655SE grips your speakers with authority but without distortion or blur or overhang. Consider Gary Peacock’s visceral playing on Spiritual Unity by the Albert Ayler Trio. Hell, listen to Gary Peacock’s playing on almost anything. Through the VK-655SE, Peacock’s instrument is rich, full-bodied and alive. It has realistic bass weight and punch but with the kind of textures and unfolding inner harmonic detail I’d never thought possible previously. It’s an exhilarating and positively addictive experience.

C’Mon Every Beatbox

As I hinted in the introductory paragraphs, these qualities are not limited to the amplifier’s prodigious nether regions. Everything seems effortlessly direct and immediate through the VK-655SE. No matter how busy or complex the program, the amplifier maintains a composure and ease that is truly beguiling. Indeed, complex musical strands are maintained with Joe Meek levels of separation. Take Big Audio Dynamite, for example, the pioneering musical diaspora that has lent this review its highly apposite sub-headings. Led by Mick Jones post The Clash, the ground-breaking group was a heady clash of reggae basslines and hip hop beats married to rock guitars and a most English of voices. I have rarely heard B.A.D. sound so potently organised and coherent on a high-end system.

BAT

Imaging is stable and focused but image outlines are slightly softer and more diffuse – significantly less razor sharp and etched – than some might be accustomed to from a high power solid state amplifier. To me it sounds more natural and leads to zero diminution of the BAT’s powers of musical involvement and analysis. It must also be pointed out that the slightly warm and dark upper treble which so aids extended fatigue-free listening sessions also results in a slight foreshortening of depth perspectives should that be the way you measure musical performance from your home audio system. The VK-655SE’s resolving powers remain first rate and always music first in priority. It is never over-analytical. The only things mechanical and electronic are the sounds being concocted by the group itself.

Conclusion: Big Audio Dynamite

To say that the flagship BAT VK-655SE power amplifier is just another high powered solid state amplifier from America is to say that Bob Dylan is just another song and dance man. Its outward appearance may be more utilitarian than many products these days but the serious engineering is where it matters, underneath the hood. The design is ingenious. With its zero feedback, two gain block design, it owes as much to simple and pure low power designs as it does to scorched-earth, high power types. To say the BAT gives you a warm tube sound without the tubes is not telling the whole story of the core sound signature of this fantastic amplifier either.

Its performance was not at all what I expected and it soon left no doubt that I was in the presence of something very special indeed from an audiophile perspective. In the six months that it took up residence in our system, it performed without a hiccup. But for the power bill, I would have forgotten it was on. The VK-655SE is a fantastic all-rounder that played with a myriad of different components, both balanced and single ended, and drove a variety of speakers, all without a glitch. In every instance, the big BAT was intensely musical with ample soul to burn.

When it came to effortless dynamics, ultimate bad-ass low bass performance and rhythmic drive and grip from the basement to the treble, the self- effacing BAT VK-655SE was unsurpassed in my experience. It otherwise delivered in most every sonic parameter you could ever want or think of. When you factor in the price, although not cheap, it represents massive value for money compared to many other “statement” pieces. And you can get it as a gleaming silver machine too. The BAT VK-655SE is big audio dynamite and is highly recommended.

…Peter Katsoolis
www.soundstageaustralia.com

Associated Equipment

  • Speakers - Shun Mook Bella Voce; Wilson Audio Sasha 2 and Sabrina; M8 Audio Sweet Maxwell
  • Amplifier - VTL MB 185 Signature Series iii; Wavelength Audio Napoleon 300B; First Watt F7 and SIT - 3
  • Preamplifier - Thrax Audio Dionysos; BAT VK-53SE (on loan)
  • Source - Aqua Acoustic Quality La Diva transport and Formula xHD DAC; Metrum Acoustics Hex DAC; Auralic Aries Streamer
  • Cables - Shun Mook RCA; Acoustic Revive XLR-1.OPA II balanced; Sablon Audio RJ45 CAT ethernet; Skogrand Wagner AC and Luna Cables Orange PC; EnKlein Titan speaker cable; Nordost Heimdall 2 and Vermouth Audio Black Pearl Mk. II looms
  • Equipment Support - SRA Scuttle Mk 2; Shun Mook; Mana Acoustics
  • Accessories - Les Davis Audio 3D - 2 Viscoelastic discs and mats; Shun Mook Mpingo discs and maple and ebony boards; Nordost QRT products and Sort Kones BC; Acoustic Revive RIQ- 5010 and 5010W resonators; SPEC Corp. RSP - AZ1 Real Sound Processor; Ikea Aptilig maple butcher’s blocks

Balanced Audio Technology VK-655SE Power Amplifier
Price: AU$26,100
Warranty: Five Years full parts and labour

Australian Distributor: Audio Heaven
+61 (0) 412 650 723
www.audioheaven.com.au

Balanced Audio Technology Inc.
Suite A, 1300 First State Blvd.
Wilmington DE 19804
United States of America
1 302.999.8855
www.balanced.com