Balanced Audio Technology VK-600M SE Monoblock Power Amplifier
A fundamental principle of high-end audio design holds that the signal path should be as short and simple as possible, and the power supply as elaborate and massive as practical. The VK-600M SE solid-state monoblock power amplifier from Balanced Audio Technology (BAT) takes this idea to the extreme; this amplifier has the signal-path simplicity of a low-power single-ended amp, coupled with a power supply that looks as though it could light and heat a small city. (See the accompanying interview with designer Victor Khomenko for details.)
We tend to think of an amplifier’s power supply as outside the audio signal path. After all, its job is merely to supply direct current to the tubes or transistors that actually do the work of amplifying the music signal. Looking at a schematic reinforces this view; we follow the audio signal from input to output, with the power supply represented as an adjunct to the signal-amplifying electronics.
A more accurate way of thinking of a power amplifier, however, is of a device that pulls 120V/60Hz alternating current from your wall outlet through the amplifier’s power transformer, converts the AC into direct current (DC), stores that energy in large capacitors, and then allows the tiny audio signal at the amplifier’s input to modulate the stored energy as electrical current that is driven through the loudspeakers by the power amplifier’s output transistors. This way of thinking of a power amplifier leads to the realization that an amplifier’s power supply is actually in the audio signal path. The current that ultimately drives the cones in your loudspeakers back and forth comes from the wall outlet via the amplifier’s power supply.
BAT’s emphasis on the supply’s importance is reflected both in the standard VK-600’s substantial power supply and in the upgrade path BAT makes available. The amplifier’s basic configuration is a stereo unit at $7995. Two levels of power-supply upgrade are available: the BAT PAK at $995 and the SUPER PAK at $3000. Both are boards containing rows of capacitors that beef up the power supply by adding additional energy storage. When fitted with both upgrade options, the amplifier becomes the SE version. (The SE is $11,500 when purchased initially, which saves you $500 over starting with the basic amp and upgrading.)
The next step up is the VK-600M, a monoblock version that combines the stereo amplifier’s two output channels into a more powerful single channel. BAT PAK and SUPER PAK upgrades are also available for the mono version. The ultimate realization is the fully loaded VK-600M SE reviewed here ($23,000 per pair).
Power output is rated at 300W into 8 ohms, a figure that doubles as the load impedance is halved (600W into 4 ohms). This suggests that the VK-600 can deliver power to currenthungry loudspeakers that have low-impedance dips. The two channels are completely separate (including transformers) with each supplied by its own AC power cord. Inputs are balanced only, reflecting the amplifier’s architecture of fully-differential circuitry from input to output. If you want to drive the VK- 600 with an unbalanced signal, you’ll need RCA-to-XLR adaptors, available from BAT.
This amplifier is built like a tank, with a very nice, but not overly lavish, front panel. The money went into performance rather than cosmetics.
The VK-600M SEs were in some ways revelatory, particularly when partnered with BAT’s top-of-the-line VK-51SE preamp. For starters, these amplifiers exhibited iron-fisted control over the Wilson MAXX 2’s big woofers, without the slightest hint of strain at any listening level. The VK-600M SEs didn’t just go low and play loudly; they produced a rock-solid, tight, and visceral bottom end that served as a strong tonal and rhythmic foundation for the music. Their dynamic impact, explosive transients, and effortlessness in the bass were peerless, in my experience. I’ve never heard the dynamic envelope of kick drum or tympani reproduced with such depth and startling impact, coupled with equally sudden decay. For instance, the spectacularly recorded bass and kick drum on Travis Larson Band’s Suspension [Precision] were portrayed so vividly that this high-energy power trio seemed to have much of the life and drive it has in concerts. The VK-600M SEs (along with the Wilson MAXX 2) even resolved the individual strokes of two bass drums played quickly, rather than turning the instruments into an undifferentiated low-frequency blur. Taut, muscular, and authoritative are how I’d describe the VK-600M SE’s bass. (The MAXX 2 turned out to be an ideal match for the VK- 600M SEs, since the Wilson’s superb bass took full advantage of the BAT’s bottom-end impact and resolution.) The musical result was a visceral, whole-body involvement in the music (some music, at least) that smaller-scale hi-fi systems just don’t deliver. Although I can greatly enjoy a well-chosen and set-up system of modest proportions, a playback system’s ability to deliver the bottom two octaves with unfettered dynamic contrasts is an experience unlike any other.
The VK-600M SEs weren’t just brawn with no finesse. The bass was highly detailed and nuanced, a quality I appreciated with acoustic bass playing. A good example is Eddie Gomez’ masterful work on Steps Ahead’s eponymous first album [Elektra Musician], particularly on the track “Pools.” The song starts with the bass playing the melody, and then Gomez and drummer Peter Erskine lock into an interesting rhythmic pulse that sets the foundation for the extended and inspired tenor and vibraphone solos from Michael Brecker (in top form here) and Mike Manieri, respectively. The VK-600M SEs beautifully expressed the intricate dynamic and rhythmic nuances of these great musicians.
The VK-600M’s bottom-end quickness extended to the rest of the spectrum; this amplifier is extremely “fast” sounding, reproducing transients with lightning-quick attack. Many amplifiers with high resolution of transient detail sound exciting for about five minutes, until the etch produces listening fatigue. The VK-600MSEs had a unique combination of transient zip and smoothness; the amplifier reproduced attacks without that little spike of high-frequency edge on transients.
Now we get to a characteristic of the VK-600M SEs that took me by surprise, minutes after connecting the amp for the first time and long before it had warmed up or broken in: a remarkable transparency and immediacy, particularly through the midband. Putting the amplifiers into my system rendered an instant jump in the sense of palpability and directness. Instruments and voices became more vivid and alive. This palpability stemmed from an overall impression that the VK- 600M SEs simply got out of the music’s way, imposing virtually no sound of their own. The VK-600M SEs had an almost SET-like immediacy, but without the lush romanticism of the 300B tube. It was as though the recording and playback signal paths were laid bare, the VK-600M SEs acting as a transparent window on the electronics chain and, ultimately, on the musicians’ expressiveness.
This sense of transparent palpability was accompanied by an overall presentation that was a bit on the forward and immediate side, with the soundstage projected just in front of the loudspeaker plane rather than behind it, reducing the sense of space between you and the music. These aren’t amplifiers that envelope you in a huge soundstage. The layers and layers of depth on Rutter’s Requiem [Reference Recordings], for example, sounded a bit foreshortened, as though the acoustic had become a little smaller. The voices didn’t quite float in air the way I’ve heard from other top-notch amplifiers.
Partially as a result of the VK-600M SE’s immediacy and quick reproduction of transient information, I heard a massive amount of recorded detail. This statement could sound like a warning, but I don’t mean it as such. Rather, the VK-600M SE artfully resolves every last iota of information—inner textural detail of instruments and voices, low-level instruments in the back of the soundstage, micro-dynamic nuances—without sounding overly analytical.
Another remarkable characteristic was the VK-600M SEs’ ability to maintain their composure at any listening level and through dense and complex musical passages. Orchestral climaxes were just as clean and resolved as low-level passages. This allowed higher listening levels without fatigue or irritation. You can often hear an amplifier running out of power on sustained loud passages as a congealing of individual instruments, both tonally and spatially. The VK-600M SEs were completely unfazed by any volume level or any music. (It should be noted that the Wilson MAXX 2 is a fairly challenging load for an amplifier.)
I was a little disappointed at first in the VK-600M SEs’ upper treble reproduction, which sounded as though it lacked extension at the extreme top end. I’m not talking about a softness that affects musical timbres, but rather the feeling of air and openness on which the music rides. Either the amplifier broke in and opened up, or I became used to this sound. Whatever the case, I came to appreciate the VK-600M SEs’ upper treble sweetness and lack of solid-state glaze. Part of my initial perception could have been caused by the VK-600M SEs’ extremely black background and lack of electronic haze.
As great as the VK-600M SEs are—and I believe they are in many ways one of the world’s great solid-state amplifiers—they won’t be the amplifier for all people. They lack the lush romanticism and slight sweetening of timbre that makes many tubed amplifiers so seductive. They are also better at dynamics and resolution than at presenting a feeling of air around instruments and a sense of bloom that expands with an instrument’s dynamic envelope. Lush, forgiving, expansive, and enveloping are not adjectives that describe the VK-600M SEs. These amplifiers are at the other end of a continuum that may have at one end, for example, the Audio Research Reference 600s—amplifiers with gorgeous rendering of timbre and a huge spatial presentation, but lacking the bottom-end authority, control, dynamics, and palpability of the VK-600M SEs. Finally, you should judge the VK-600M SEs only after they have been warmed up for at least one hour, preferably two. These amps take longer to warm up and to sound their best than any others in recent memory. The BAT VK-600M SEs delivers an astonishing combination of sheer brute-force power with the midrange immediacy and palpability of low-powered single-ended amplifiers. They have a stunning sense of transparency, among the best I’ve heard from any amplifier, tubed or solid-state. They also possess great finesse and resolution, qualities not often associated with high-power solid-state amplifiers that can also exert ironfisted control in the bottom end and express seemingly unlimited dynamic contrasts.
All these audiophile descriptors aside, what really counts is how readily and deeply I become involved in the listening experience. Judged by that criterion alone, the VK-600M SEs are worthy of my highest recommendation.
Victor Khomenko Talks with Robert Harley about Founding Balanced Audio Technology, Designing Audio Equipment, and the VK-600
Robert Harley: Tell me about your background in electronic design.
Victor Khomenko: My basic education was in electronics and physics from the Polytechnical Institute in St. Petersburg, Russia. I graduated in 1973 and worked in the Russian military industry before emigrating to America in 1979.
I’ve been tinkering with electronics (especially audio) since I was eight years old. My electronic education was simply icing on the cake because by that point I was already a fairly experienced home tinkerer. I’d done a lot of do-it-yourself projects just for my own system. There was no way to obtain common highend products in Russia at the time— amplifiers, preamps, phonostages, tape recorders, turntables. I had to build them all myself.
I came to America in 1979 and worked at Hewlett-Packard developing analog and digital instrumentation. At that time my audio interest moved to the back burner, because there were so many more serious things to be addressed. It wasn’t until the late ’80s that I came in contact with American audiophiles, and my first acquaintance in this area was with my now-partner, Steve Bednarski.We worked together at Hewlett-Packard and he started talking about his audio system. I made comments about how the products could have been better designed. Steve replied: “If you are so smart that you know how it could have been done better, why don’t you try it?” So I just had to try it. I revisited my audio-design hobby, built some products, and the results were so good that we decided it was worth doing commercially. We started Balanced Audio Technology in 1994. About a year later we were joined by Geoffrey Poor, our Director of Sales.
RH: And more than ten years later you’re still going.
VK: We’re still going, yes.We just passed our tenth anniversary, and the company is doing fine.We started with one or two products in 1994, and today we have, I believe, twenty models on our price list. Our product line is unusual because it presents a cross-section of technologies. We don’t just make tubed or solidstate equipment—we use whatever is right for the particular application.
RH: The VK-600 has three unusual design elements: 1) no global feedback; 2) a single stage that serves as both an input and driver stage; and 3) the output transistors are all N-channel MOSFETs rather the Nchannel and P-channel complementary pairs.Why did you choose these approaches? [N-channel and P-channel transistors are the FET equivalents of PNP and NPN bi-polar transistors. —RH]
VK: We started with the idea of simplicity of the audio circuit.When people talk about why some small, single-ended amplifiers sound so good, they always mention simplicity as one of the reasons.Well, if you look at the schematic of the VK-600, you can actually make the case that it is even simpler than your typical seven-watt, single-ended amp, because those amplifiers have output transformers and the VK-600 doesn’t.
The VK-600 essentially accomplishes everything that you need to accomplish in a high-power, solidstate amplifier with just two gain blocks. As you mentioned, the first block is the input stage/buffer and after that is just an output stage, and that’s it. That was unheard of at the time we first introduced this type of product in the VK-500, because most power amps at that time followed a multi-stage approach to design. In a typical high-power solidstate amp, you see dozens of amplification devices. When you put a VK- 600 schematic next to one of those amps, it’s almost as though there’s nothing in the VK-600.
You have to work hard at achieving simplicity; it’s more difficult to design a simple circuit than a complex one. Our single gain stage does everything, with low distortion and large voltage swings, and it drives the output stage without a buffer and with no loss of bandwidth.
RH: Stravinsky said,“I compose with the eraser.”
VK: [chuckles] Yes, that’s a very, very good statement. As far as no global feedback,we belong to the school of thought that feedback should be used only in moderation and only when absolutely needed. The designer’s job is to develop circuits that don’t need feedback in the first place, and then perhaps add a little feedback as a final touch, rather than rely on it to make the circuit work. Because the VK-600 is a zero-feedback design, it also allows some very interesting possibilities that other typical designs simply do not have. For example, we can parallel any number of channels for more power, without any problems of stability or conflict. This is what allows the stereo VK-600 to become a monoblock, and it doesn’t achieve this through conventional bridging of channels.
RH: What about using a single-polarity device throughout the circuit— the N-channel MOSFET?
VK: That’s an interesting question. It’s been known for a long time that P-channel MOSFETs are always inferior in terms of their bandwidth, speed, and other characteristics. If you design a circuit using complement- ary devices, you must use those infinitely inferior P-channel devices. By dispensing with the Pchannel devices, the circuit opened up with wider bandwidth, and that also made the circuits much more stable.
RH: What else is interesting about the design?
VK: You can start with power supply. Each channel comes with its own dedicated,massive power transformer. Good amplifier design always begins with the power transformer and power supply—it is the foundation of good sound.We use a 1kVA toroidal transformer in each channel, and the two channels are completely separate.We also use vast energy storage in the power supply. The power supply is in fact so big that when you turn the amplifier on, each channel powers-up sequentially so as not to trip the circuit breaker in your breaker panel.
You can further increase energy storage with the optional BAT PAK. It triples the electrolytic capacitor bank in the power supply. The increase in energy storage is especially noticeable when the amplifier is driving difficult speakers. With some easy-to-drive speakers, the effect may be smaller. There is also another type of energy storage option—what we call the SUPER PAK. You can see this as a large circuit board filled with special top-quality paper-in-oil capacitors that are made for us in Europe.With the SUPER-PAK you’ll immediately notice additional liquidity to the sound; it becomes more open, transparent, and far more fluid. The SUPER PAK is largely responsible for the amplifier’s beautiful finesse.
RH: You mentioned earlier that the VK-600’s two channels are converted to one channel in the monoblock version without bridging.
VK: That is correct. When people talk about converting a stereo unit into a monoblock, they immediately use the word “bridging.” Bridging is connecting two channels in series, which is commonly done to achieve higher power rating.
We decided to go with parallel channels instead of bridging, because although you get more output power “on paper” with bridging, you sacrifice drive capability due to an increase in the output impedance. A bridged circuit also doesn’t work as well driving the load with two channels driven in series.
When you think about power, you have to think about the difference between maximum power, which is academic in many cases, and the ability to drive and control the speaker. Maximum output power is akin to horsepower in a car; it’s responsible for the maximum attainable speed. Drive is like torque, which is much more meaningful to what the driver feels in the seat of the pants. By running the channels in parallel, the amplifier feels substantially more powerful even though on paper the maximum output rating doesn’t go up as much as if you had bridged the channel.Very few architectures will allow you to parallel channels as we do in the VK-600.
By Robert Harley
My older brother Stephen introduced me to music when I was about 12 years old. Stephen was a prodigious musical talent (he went on to get a degree in Composition) who generously shared his records and passion for music with his little brother.More articles from this editor