TAS 196: VTL MB-450 Series II Signature monoblock amplifier & TL6.5 Signature preamplifier

REVIEW
by Paul Seydor
Oct 14, 2009

It is impossible to review Vacuum Tube Logic’s MB?450 Series II amplifier as if it were a single amplifier because it is at least two quite different sounding amplifiers in one chassis. And depending on the speaker system it’s driving, each of these personalities can develop any of several sub?personalities. How can this be? One reason is that the amplifier is designed to operate in either tetrode or triode mode. Another perhaps the sparing use of negative feedback. Still a third the intrinsic characteristics of tube amplifiers in general and a specific design decision in the case of this amplifier. But that gets us ahead of ourselves—first, let’s get introduced.

The MB?450 II heads VTL’s Signature Series of products, just below its top?tiered Reference Series. A monoblock unit with eight 6550C output?tubes, it retails for $15,000 a pair and is rated at 425 watts tetrode/225 triode. The Mk II designation refers to improvements in parts, circuitry, features, and cosmetics, not least the company’s SmartTube technology that ensures automatic bias and fault-sensing. While amplifiers this powerful are usually talked about in terms of brawn, the 450 has a formidable brain in an impressive microprocessor that among many features allows for convenient front?panel switching from tetrode to triode mode in mere seconds (a -2dB level difference favoring the former) and that tracks tube life and bias. Should a tube go bad, ingeniously implemented LEDs immediately identify the culprit, replacement easily performed with the processor automatically biasing the replacement. Two additional levels of protection circuitry help make this one of the safest, most reliable tube amplifiers ever made.

Assuming no more than a day or so passes between one listening session and the next, the main power switch on the rear panel should be left on all the time and the front?panel button be used to take the amplifier into and out of standby. Operationally and ergonomically, this is the most intelligently designed and convenient?to?use tube amplifier in my experience. Build, quality of parts, and fit and finish are first-class in every particular.

One design element of the 450 is highly unusual: No choice of output taps for matching to 4 or 8 ohms is offered (16 has almost disappeared industry-wide). Instead, there is only one pair of speaker terminals set at five ohms. According to VTL’s owner Luke Manley, this is so that “the whole of the output transformer is utilized, for optimal power transfer.”

He believes that 5 ohms is a good value for most of the high?performance loudspeakers the amp is likely to be used with. I’m not technically knowledgeable enough to know if this contributes to the 450’s high output impedance, which is over 1 ohm. But whatever the reason, the amplifier does have a highly individual sonic character that must owe in some part to the inevitable deviations from flat frequency?response that such high output impedances typically result in when driving real?world loudspeakerimpedances.¹

Given its high power?output, the 93-pound 450 generates a lot of heat. Inside?cabinet or shelf-mounting is out of the question for these handsome beasts—you want a sturdy stand or table with plenty of open air around them. I am notexaggerating when I report that a pair of 450s gave off enough heat in cold weather to make us close the heating vents in our 2600+ cubic?foot listening room and on some unseasonably warm days this past spring to make the room too warm for comfortable listening, at least without air?conditioning.

In common with many amplifier manufacturers, Manley believes the more power the better. He has a point. My reference these last few years has been McIntosh’s monster MC402. From time to time when I use other, lower-powered amplifiers, I am reminded, whatever their virtues, how easy it is to take for granted the freedom from stress and strain, the confidence and composure that come from having huge reserves of power. Such was the initial impression of the 450s. Even with my allegedly dynamically challenged but quite inefficient Quad 2805s, which cannot come close to absorbing the full output of these amps, large symphonic ensembles and choruses, operas, and big bands were situated in a vast panorama across the front of my room with a scale and a dimensionality that only come from a large reservoir of clean, stable power. Whether it’s the comings and goings in the Bernstein Carmen, the organ pedal point and full-orchestra chords at the beginning of Also Sprach Zarathustra (Mehta/London on vinyl—eat your heart out), or the craggy power of Klemperer’s last Beethoven cycle, the VTLs’ strength and truly majestic authority remained a constant source of amazement and even awe.

This impression of size, scale, and dynamics carried over to other kinds of music too. Alfred Brendel’s piano in his Beethoven Op. 111 (Philips, Artist’s Choice album) was beautifully placed in the room, between yet free from the speakers, slightly set back behind them, in a perspective very close to life-size. So too my complement of favorite singers. In sum, all pretty much what you would expect from a high?performance tube amplifier.

It was in the area of tonal balance that some issues became apparent. I began my listening in tetrode, which is more or less the default, supposedly the more accurate mode, mightily impressed by how clean, clear, and apparently low?distortion the presentation was. But the longer I listened the more I became aware of a certain subtle character that I find difficult to define: a slight “whiteness” to the sound, akin, say, to a photograph fractionally overexposed. Hand in hand with this came a mild thinness or dryness, again subtle but present, a “clinical” impression—“silvery” might be a better word. Violins, for example, could sound midrangy yet ever so subtly bright at the same time.

At this point, I decided to switch to triode, almost as a lark—I put it that way because I’ve never found myself preferring triode to tetrode (or pentode) in any amplifier allowing this option. Wow, talk about yin and yang!—the light and the dark, the hard and the soft, the cool and the warm. Suddenly I heard a warmth and naturalness that for all the dynamic impressiveness of tetrode let me slip back into my chair with a newfound relaxation. Was the sound less dynamic? Yes. Was it less resolved and transparent? Yes. Was it less clean and clear? Yes. But it was also lovelier and more restful. Granted, it was a bit dark and dim way up top, plushier without ever being actually ill-defined at the bottom, with the mid? and upper?bass present in a way that removed the impression of slight leanness I heard in tetrode.

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SIDEBAR

I asked Luke Manley to comment on some aspects of the 450’s design that are unique or otherwise unusual:

On low negative?feedback and high output?impedance:“We have found that too little feedback yields insufficient control of the load and that too much feedback makes the amplifier sound flat and lifeless. Through listening evaluation we have chosen the best ratio between damping factor and harmonic resolution, with the result that the output impedance of the MB-450 is over one ohm. Despite this, the 450 has good bass control; and even though more feedback would improve the measured performance, its unique sonic performance would be negatively impacted.”

On transformers and multiple output?taps: “Multiple output?taps are inherently a serious compromise in performance. Inactive winding?segments are inevitable in a tapped transformer and result in additional leakage inductance that affects high?frequency response. Also, in VTL amplifiers the negative feedback loop is designed to encompass the whole transformer; tapping the secondary would upset the feedback ratio and cause only parts of the secondary to be used, which would result in sub?optimal power transfer. Speaker impedances vary with frequency; in order to have the optimal match it is best to choose a secondary impedance at the minimum impedance of the loudspeaker. We have found that setting the turns ratio for maximum power?dissipation into 5 ohms is generally optimal for most speakers and provides amplifier stability down to 2 ohms.”

On tetrode and triode: “Triode mode typically offers a deeper soundstage, but not as much width and impact as tetrode mode. Triode might suit solo vocal, jazz, or classical quartets, where a sweeter sound and depth of soundstage is prized; tetrode might be preferred for more demanding programs, in particular large?scale classical or rock pieces or big?band jazz, where the amplifier needs to be able to resolve a large soundstage. Power output is halved in triode and there is some high?frequency roll?off. Different bias points are required for the different modes, but the autobias system automatically adjusts for either operating point. And our convenient on?the?fly switching from one mode to the other makes matching to source material quick and easy.”

With respect to triode’s greater depth vis?à?vis tetrode’s greater width in the 450, I noticed this but interpreted it differently: To my ears triode mode tends to set the whole presentation back slightly and to shrink it a little while preserving its proportions—in other words, rather like looking at, say, the same photograph in two different sizes which nevertheless preserve the aspect ratio. These effects are withal quite subtle, mostly, I suspect, because the differences in tonal balance were so large. PS

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I should point out that differences such as these are more or less par for the course when comparing different modes in tube amplifiers. What startled me here is how pronounced they were in the same unit—almost as if, as I’ve suggested in my title, one chassis houses two completely different amplifiers designed and engineered according to antithetical sonic aesthetics.

Some wag once said that when two components sound different, probably neither one is right. This often seemed the case here, when familiar recordings were sometimes reproduced with curious effects. For example, a longstanding reference is the Boston Camerata’s Sing We Noel (Nonesuch), which ends with the chorus receding as it sings the “Gloucester Wassail.” The engineers, so far as I can tell, do not fade them down from the console; instead, as the singers move further upstage, the reverberant sound gradually dominates the direct. In either mode, the 450s let me hear plenty of side?wall reverberation but somewhat obscure that from the back wall. By contrast, the McIntosh MC402 lets me hear both, as I am accustomed to, that from the back wall subtly increasing as the singers move closer to it.

It came as no surprise, then, to find the MB?450 dividing my informal group of listeners almost as much as speakers sometimes do. Their reactions ran the gamut from . . . well, again, the yin to the yang. One thought triode mode driving the Quads produced some of the most magical sounds he’s ever heard; another felt tetrode was so “white” and lacking in midbass punch he suspected an impedance mismatch. In between there was another who brought over the Korngold String Sextet on Kleos Classics, an almost voluptuously recorded disc. In triode he felt the sound entirely too saturated—albeit rather bewitching in its warmth and richness—so we switched to tetrode, which he (and I) liked much better. But the kicker is that on an almost everything else he preferred triode operation.

Concerned that there might be something to the impedance?mismatch question, I substituted a pair of Verity Leonore speakers, a three?way dynamic design quite different from my Quads. Essentially the same relative differences between the two modes and also the same intrinsic characteristics were observed, with one difference: the hints of whiteness, thinness, and dryness became more than hints with the Verities. This was not a good synergy.

It’s precisely this sort of thing that makes trying to get a fix on these amplifiers so frustrating. It’s almost impossible to predict how they’ll interact with any given speaker system. Having lived with a pair of MB?450s on and off for several months now, I remain impressed by their clarity, dimensionality, dynamic range, and ability to reproduce size and scale. But while I’ve greatly enjoyed them—especially in the seductively “romantic” triode mode with my Quads—I never felt I was hearing a sound that I could ever quite describe as truly neutral. Allow that this may owe in part to its interaction with the Quads and that, as I’ve already suggested, this is to some extent true of the vagaries of most tube amplifiers faced with real?world loudspeaker loads—and the conclusion is inescapable: If ever there were a classic instance of a component you should most definitely audition before buying, preferably with the speakers you plan on using, this amplifier is it.

TL6.5 Signature Linestage Preamp

Luke Manley suggested that I listen to the system with his TL6.5 Linestage Preamplifier, from the same Signature Series as the amp. I heard the same differences between tetrode and triode modes, as well as all the other tonal characteristics I’ve described—just as I did with the solid?state McIntosh C46, the preamp I used throughout most of the evaluation period. This suggests that the TL6.5 is a preamplifier of rare neutrality, as indeed it is. To get to the quick: This is as fine a control unit as I have ever had the pleasure of using, bettered by none in my experience. If it has a sound of its own as regards tonal balance as such, I have been unable to discern it over almost three months’ listening, its presence in my system more notable for its absence. Its subjective dynamic range is astonishing, its transparency state?of?the?art, and its tonal characteristics nonexistent as regards additions to or subtractions from the presentation. This is one of the rare tube preamplifiers I’ve heard of which I can make these statements.

The T6.5’s functional characteristics are everything I could ask for, with a microprocessor as effective as and considerably more sophisticated than the 450’s. Its line?level-only inputs (despite the phono label on one, there is no built?in phono preamp) are individually adjustable for level, it has more inputs and outputs (both balanced and unbalanced) than most people are likely to need, and for convenience it even trumps the C46 by offering polarity-switching, with both it and channel balance available via the remote. Used with VTL amplifiers’ 12?volt trigger, you can power up and down both preamp and amps in one simple operation. Yet for all its sophistication, the TL6.5 is so ergonomically intuitive that I must have used it for a month before I even bothered to crack the manual, which, like the amplifier’s, is extremely thorough and very nicely reproduced.

I know I should say more about the sound, but for the life of me I am brought up short. It’s super-clean without being clinical, extremely resolved without being analytical, very involving without having any particularly obvious—or subtle, for that matter—colorations. And it does its job with nary a tick, swish, or other transient noise except the—by design—quiet and satisfying “ticking” when a button is pushed or a control engaged (to let you know an operation is being performed). Not only was the T6.5 a pleasure to use from the moment it was plugged in, it did its job so thoroughly and unobtrusively that I quickly forgot it was there—which, after all, is the point, isn’t it? For those who value the unique life, body, and dimensionality of the best tube designs yet want the tonal neutrality that solid-state provides, few control units I’ve used quite equal the TL6.5 and none surpasses it.

SPECS & PRICING

VTL MB?450 Series II Signature Monoblock Amplifier

Power: 425 watts tetrode, 225 triode 20Hz?20kHz into 5?ohm load, <2.5% THD @ full power
S/N: -110dB, 120Hz
Dimensions: 18.5″ x 18″ x 9″
Weight: 93 lbs.
Warranty: 5 years parts and labor
Price: $15,000/pr.

VTL TL6.5 Signature Linestage Preamplifier

Frequency response: 1Hz?200kHz 0/-1dB
Distortion: <1% @30volts maximum output
Gain: 14dB single ended, 20dB balanced
Dimensions: 17.3″ x 16.75″ x 5.75″
Weight: 45 lbs.
Warranty: 5 years parts and labor
Price: $11,500