VTL MB-450 Series III Signature monoblock power amplifier
The VTL MB-450 series began life in the late 1980s as the Deluxe 300, a pair of which I once owned. Over the years the basic design has been improved and modified, in the forms of the MB-450 (1996) and the MB-450 Series II (which I reviewed in January 2008). The tube complement remains the same: eight 6550s in the push-pull output stage, a 12AT7 input tube, and a 12BH7 driver. Into a 5 ohm load, the MB-450 III is claimed to produce 425W in tetrode mode or 225W in triode, from 20Hz to 20kHz.
The Series II added VTL's Smart Tube technology, also found in the company's flagship Siegfried line. This optimizes tube performance through the use of logic-controlled auto-biasing, which continually monitors each tube, and adjusts its bias on power-up as well as when the system is idling. It also includes a diagnostic system: If a tube malfunctions, the amp indicates that with an adjacent LED and shuts down if the tube draws excessive current.
VTL's goal for the Series III Signature revision was, first, to improve the MB-450's ability to better drive a wider range of loudspeakers. These changes include a redesigned, fully balanced differential input stage driving a differential phase splitter, and a lower-impedance push-pull output stage terminating in an output transformer that is now fully balanced, and claimed to be "dramatically improved." The Series III also has a shorter, faster, fully balanced negative-feedback loop. VTL claims that this circuit completely eliminates ringing and maintains phase integrity without using capacitor compensation.
Precision-regulated power supplies for the output tubes' bias supply and screen voltage maintain the tubes' operating-point consistency, even when the AC supply fluctuates. VTL claims that this produces tonal stability and "sonic integrity" in the reproduction of complex, dynamic signals.
A front-panel button allows the MB-450 to be switched between tetrode and triode operation for the output tubes. Although there is a Mute button, switching the operating mode can be performed without having to mute the amplifier. The four settings of the Damping Factor toggle switches behind the glass window on the front panel vary the MB-450's output impedance by changing the amount of negative feedback in the circuit. The Low setting minimizes damping and to produce the "most natural sound," per VTL, while Med (Medium) has a minor impact on the overall sound but produces somewhat better speaker control. The Hi setting further improves speaker control, but has a greater impact on the sound; and Max applies the iron fist of maximum feedback, but with a noticeable negative sonic impact, according to VTL.
VTL claims that the addition of pricey premium Mundorf silver-oil capacitors produces a sweeter, more extended top end, a more relaxed-sounding midband, and superior midbass control. An MB-450 Series II can be upgraded to Series III status.
The Series III looks very similar to the Series II, with one significant exception. Now, to access the tubes, instead of removing the entire cover, which was unwieldy, VTL has cut from it a pair of small L-shaped sections directly above the output tube sockets, making them far easier to remove and replace.
Otherwise, with its gracefully curved front panel of matte brushed aluminum and tinted glass, the MB-450 remains a substantial piece of kit built to an extremely high standardwhich is what you should expect to get when you plunk down $18,000 for a pair of them. The beefy speaker terminals can take a hard torqueing, and everything else on the rear panel, as well as what's inside, is industrial grade.
While VTL's instructions suggest that unpacking each MB-450 Series III Signature is a two-man job, I managed it myself (thanks to the gym). No doubt your dealer will do this for you, but if not, be careful. Not only does each amp weigh 93 lbs, but because its transformers concentrate most of its mass in the rear, lifting and carry can be awkward and tricky.
The manual is usefully detailed, helpful, and informative, but sometimes makes the simple seem complicated: "For the MB-450 amplifier, there should be a total of 8 sockets for the output tubes. . . ." Should be? Is someone at VTL worried that a tech might have omitted a few sockets?
The output-tube sockets are numbered 1 through 8, but all of the 6550C tubes, which were separately shipped in another box, were labeled "#2," while two spares packed with the amps were labeled "#7." This caused some confusion, particularly as the instructions tell you to "Insert the output tubes into each of the output tube sockets from tube #1 to #4 on the left side of the cover and 5 to 8 on the right side of the cover."