VTL MB750 monoblock power amplifier The Evolution of the MB-750

Sidebar 2: The Evolution of the MB-750

The MB-750's roots lie in two very different VTL models: the 500W Monoblock and the Ichiban, both introduced around 1988. The 500W Monoblock was a growth version of the DeLuxe 300, reported to have grown out of a conversation between David Manley and Harry Pearson of The Abso!ute Sound, who were independently wondering how an ultra-high-powered tube amplifier might affect music reproduction. With 12 KT88 output tubes per channel run in ultralinear configuration and biased to run in class-AB-1, the Monoblock's 500W of power were targeted directly at the American audio aesthetic of big rooms and inefficient speakers.

The Ichiban, on the other hand, arose from the Japanese and European fascination with triode amplifiers. The concept was an outlandishly high-powered triode amplifier, originally 100Wpc with a dozen 211 or 845 output tubes per channel, intended for the Japanese market. The 211/845 model never came to fruition, but a second version, a push-pull design based on either two (45W) or four (90W) 300B output tubes per channel, was built and sold in Japan and Asia. In the US, the Ichiban was introduced in 1989 as a 200W mono amp with 12 triode-wired EL34s per side, biased to run pure class-A. Later versions used triode-wired KT90s, bringing the power up to more like 275W—certainly fulfilling the original concept of a super-power triode amp.

Or, as David Manley referred to it in an earlier edition of The VTL Book, "...assumes the status of broadcasting and power-stations." Although the 500W Monoblock and Ichiban were very different in concept, they shared one distinctive characteristic: They were the first VTL amps to use the dramatic double-decker chassis configuration, essentially two standard VTL chassis supported by African redwood side panels, with the bottom chassis dedicated to the power supplies and the top to the signal-carrying circuitry.

In 1993, the Ichiban and 500W Monoblock merged to become the MB-600 Ichiban. Tetrode/triode-switchable, biased to run in class-AB1, and with KT88 output tubes, the Ichiban's power output rose to 300W in triode mode, 600 in tetrode. The double-decker chassis configuration was retained, but the redwood side panels were replaced with powder-coated steel rails. A faceplate was added as well, which housed the triode/tetrode switches and a meter and selector for reading the bias on the output tubes. Although the MB-600 was introduced after Luke Manley had taken over VTL, design was finalized under the old regime.

By 1996, however, the new era was in full swing, and all of VTL's amplifiers had been thoroughly updated again. The major changes to the Ichibans were the incorporation of the Signature output transformers and increases in rail voltage and power-transformer size, which raised the output to 350/750W per mono amp. Other changes included a redesign of the circuit boards to improve manufacturability, lower the noise floor, and optimize the driver and output stages for the new transformers. Additional chassis damping was incorporated and, externally, the front-panel biasing system was eliminated, the rear panel was redesigned, and the cosmetics were upgraded to the current style. The final upgrade, which was scheduled for October 1997, will be to make the (now optional) switchable balanced/single-ended input stage standard on all MB-750s.

Several of the upgrades are available for older models, and my Ichibans (the original US-spec, 12-tube, 200W, redwood-side-panel version) have been given the full treatment: Signature output transformers ($1000), MIT caps and upgraded input and output jacks ($1000), balanced input stage ($600) (footnote 1)—and the KT90s were replaced with a fresh set of Russian 6550Cs.

The upgrades resulted in a substantial change in the amps' basic character, making them sound much less like stereotypical triode amps or even tube amps, and far more neutral. Pre-upgrade Ichibans were notable for their power and dynamics, but even more for their spectacular mid- and upper-midrange bloom. Instruments seemed to expand effortlessly as their volume increased, projecting outward and standing out in stunning relief from the surrounding acoustic environment. The soundstage was huge, and the individual images were rich with harmonics and character—in a word, vivid. In comparison, the updated Ichibans seem more controlled in their dynamic contrasts, and to have less of the original's dramatic midrange bloom.

Unquestionably, the upgraded Ichibans are far better amps—faster, cleaner, more open, and with better detail resolution. Images are more sharply focused, in terms of both inner detail and specificity within the soundstage. The upgraded model sounds more controlled, but it's not the least bit constricted in terms of dynamics. Dynamic contrasts are just more accurately reproduced, and reproduced evenly across the frequency spectrum as opposed to favoring the midrange. It's much better at the frequency extremes as well. Whereas the original began sounding soft below about 120Hz, the upgraded version reaches down to the 40s with reasonable punch and control. On top, the upgraded version definitely has more air and extension, with cymbals having noticeably more shimmer and piccolos more presence than with the original.

Even fully upgraded, the Ichibans' hardware and specs differ substantially from the MB-750s'. The Ichibans have much smaller transformers, less capacitance than the MB-750s, a lower rail voltage, and are biased to run further in class-A. According to the test sheet that came back with my pair, their output between 20Hz and 10kHz is approximately 275W, or 75W less than the MB-750s' rated output in triode mode. There are sonic differences as well, though not quite as dramatic as those between the pre- and post-upgrade Ichibans. Most noticeably, the MB-750s (in triode mode) have a great deal more air from the midrange on up. There's a greater sense of space surrounding images, and less of the Ichibans' slightly liquid texture, which darkens instruments and thickens the ambient environment.

Other differences include the MB-750s' lower noise floor, better low-level detail resolution, and more precise reproduction of notes' initial transients, all of which have the effect of further sharpening the overall focus. The difference between the upgraded Ichibans and the MB-750s perhaps isn't as dramatic or as fundamental as that between the pre- and post-upgrade Ichibans, but it's certainly a major improvement, and another big step toward neutrality.

In some ways, the MB-750's evolution is a microcosm of the High End in general. The 500W Monoblocks were an extreme example of one school of thought, the pure class-A triode Ichibans examples of another. Both bloodlines were improved over the years and eventually merged, resulting in an amplifier that retains the favorable attributes of both concepts, and is far more neutral and free of obvious character than either. In its latest evolution, the MB-750 is solidly amid the amplifiers that represent the very state of the art.—Brian Damkroger

Footnote 1: If all three upgrades are done at once, elimination of repetitive tasks drops the total price from $2600 to $2000.
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