VTL MB-450 Series II Signature monoblock power amplifier Page 2

Pushing the front-panel Power button causes the Mute LED to blink red and the Power LED blue. The Mode light glows green for Tetrode mode, red for Triode; push the Mode button to switch between them, either in Standby mode or after power-up. The entire power-up cycle takes less than a minute, during which time, if you look down through the top cover, you can see the LEDs next to each output tube blink as they indicate that the auto-bias system is adjusting each tube's bias voltage. When every LED is emitting a steady green light, indicating that all output tubes have reached their operating points, the Mute relay clicks and the input signal can pass through the circuit..

Once the MB-450 is powered up, VTL suggests you leave it on and Muted overnight between listening sessions (assuming you'll be listening the next day). In Mute, the output tubes draw low current and output little heat. For longer breaks between sessions, VTL recommends you fully power down the amp.

Back to tubes!
Like me, VTL's Luke Manley and his wife and business partner, Bea Lam, own Wilson MAXX 2s. They drive the Wilsons with their biggest amps. (Wouldn't you, if you owned the company?) The Manleys' big concern when they visited to set-up the MB-450s in my room was whether the amps in Tetrode mode would drive the MAXX 2s to my and their satisfaction. When they left after setup, both seemed confident that the MB-450s were up to the task, especially after listening to side 1 of the astounding UK Decca vinyl edition of Gershwin's Porgy and Bess (3 LPs, Decca SET 609-11), with Lorin Maazel and the Cleveland Orchestra and Chorus. They were also disturbed and amazed by the sound produced by the Continuum Audio Labs Caliburn turntable, but then, most people are.

Once you've grown accustomed to powerful amplifiers, it's difficult to accept the dynamic limitations of lesser ones, assuming you want your system to create realistic sound-pressure levels. That said, the MB-450 Series II Signatures in Tetrode mode, with a claimed output of 425W each into 5 ohms, never seemed to run out of gas driving my 89.7dB-sensitive (footnote 2) Wilson Audio Specialties MAXX 2 loudspeakers regardless of program material, at moderate to relatively high SPLs. Even when pushed to reproduce full concert-hall climaxes; the MB-450s, while not quite as giving as much as the big solid-staters, never lost control or produced any noticeable hiccups.

According to JA's measurements of the MAXX 2, the speaker's impedance is between 4 and 6 ohms across most of the audioband, dropping to 2.25 ohms at 240Hz, making the load an effective match for the 5 ohm impedance of the MB-450's output transformer. His measurements also revealed a 3.8 ohm impedance combined with a relatively steep, 33.4° capacitive phase angle at 162Hz, a popular location on the musical dial. This means that the partnering amplifier will need to deliver plenty of current to drive the speaker properly. Overall, the MB-450s and the MAXX 2s were a good match, though more power would have made a better one.

Subjectively, at least, the MB-450's surprisingly deep, tight, robust bass performance in Tetrode mode indicated that it could supply the high current needed to drive and control the MAXX 2's dual-woofered bass bin. However, the same could not be said of the MB-450's 225W Triode mode, which was fine with chamber music, acoustic guitar, and the like. But with symphonic music or well-recorded rock, the amps just seemed to surrender their grip on the woofers.

The MB-450s produced gracefully rounded, effervescent, yet believably solid images that floated dramatically in three-dimensional space on an airy, generously proportioned soundstage. The opening number of the Decca Porgy and Bess includes a tack piano at stage left and the chorus, in a gradual crescendo that begins as a soft whisper, softly chants "Da-doo-da, Da-doo-da, Wa, wa, wa..." When Clara, sung by velvet-voiced soprano Barbara Hendricks, broke into "Summertime," she appeared as an eerily real, buttery-sounding, non-electronic apparition standing on the stage, the space between her and the rear wall more clearly delineated than I'd been used to hearing with my reference solid-state amps.

This enhanced holography remained a constant throughout the audition period. Eventually I could reliably extrapolate what the MB-450s would do, spatially, with familiar recordings, which was always to take them in the direction of slightly more space. Though the result was never bloated, soundstages were consistently somewhat wider, deeper, and more vivid than via my solid-state amps. While the images on those stages weren't quite as solid and compacted as their transistorized counterparts, they had greater transparency and delicacy, and floated eerily free of the speakers' confines to a degree that the solid-state amps couldn't match.

The biggest surprises were at the frequency extremes.

There's nothing like having a room and speakers capable of producing 25Hz bass without bloat or noticeable discontinuity. When I've got that, I'm not likely to let go of it for anything else. The MB-450's bass performance was absolutely solid, extended, and well-controlled, and free of midbass bloat, sogginess, and sluggishness. The amps reliably reproduced LPs and CDs containing deep, extended bass, and never left me with the feeling that more punch, more solidity, or greater extension were desirable, or even possible.

I rounded up the usual suspects: LPs and CDs of rock, jazz, electronic, and classical music that contain lots of deep bass. They revealed the MB-450's bottom end to be lithe, remarkably well-textured, and communicative. Acoustic bass had a woody warmth, and electric bass had a depth, speed, and solidity usually associated with solid-state amps.

The news was almost as good at the other extreme. High-frequency transients were fast, relatively clean, and not at all in keeping with the cliché of "tube sound." Overall, the MB-450's top end was airy, extended, and satisfying, even to someone accustomed to fast, powerful, well-focused solid-state amplifiers.

If there was anything not quite right with the VTL's overall sound, it was a slightly splashy quality to transients that gave cymbals and sibilants a more prominent role than I was used to hearing from familiar recordings. This wasn't brightness or grain, but it did give transients a slightly sloppy, less-well-organized quality than I've become accustomed to from these recordings. It was an issue either of break-in or of acclimation—it became less noticeable over time—but if the MB-450 had any sonic signature, it was that. But don't make too much of it; it was very minor.

During the review period, I received a copy of Cisco's absolutely stupendous reissue, on 180gm vinyl, of Nathan Milstein's performance of the Tchaikovsky Violin Concerto, with William Steinberg and the Pittsburgh Symphony (Capitol/Cisco). Milstein's old-school performance throws in all the schmaltzy, weepy bells and whistles (that's the best I can do; I don't know the musical terminology), and this supple recording captures them all with almost alarming transparency and textural believability.

Coincidentally, in early October 2007 I attended a performance of the same work at Avery Fisher Hall with Dutch violinist Janine Jensen (who is much prettier than Milstein), accompanied by Lorin Maazel and the New York Philharmonic. Jensen gave a stripped-down, low-fat, ultramodern reading that I suspect Cisco's Robert Pincus would have found appalling. (I could be wrong, but I don't think so.) For me, it breathed new life into the sappy old warhorse.

Footnote 2: JA's measurement; see his testing sidebar in Stereophile, August 2005.
VTL Amplifiers Inc.
4774 Murrieta Street, Suite 10
Chino, CA 91710
(909) 627-5944