VTL MB300 & MB450 Signature monoblock amplifiers Page 2

The output transformer is claimed to be an Ultralinear type, à la Hafler and Keroes, but it isn't really. The original patent for the Ultralinear circuit specified that the screen tap be at a point on the transformer primary 43% of the distance from the center tap to the anode end. VTL found that a somewhat different tap-off point worked better with their tubes under their operating conditions, and used that (proprietary) value instead, but in view of the change, the transformer should perhaps be called, simply, a tapped-primary type. Nit-picking, to be sure, but that's what high-end audio is all about.

The output transformer uses bi-filar winding: the turns of the secondary are interleaved with the turns of the primary (fig.1), which may need some explaining. Many audiophiles, having seen pictures and schematics of transformers like those below, assume the primary and secondary windings are always wound on separate legs of the core, and that coupling takes place through varying magnetization of the core. Transformers usually are made this way, but it is not the best way to do it, for at least two reasons. First, even the best core materials exhibit hysteresis effects, which distort the signal waveform. And second, the more physical distance between the windings, the more HF losses occur as a result of leakage inductance. Interleaving the windings—alternating several primary turns with a secondary turn—minimizes both problems. VTL designs their transformers to maintain flat power response down to a remarkable 5Hz. (No upper limit is specified.)

Fig.1 Transformers (from left to right): typical construction; electrical symbol; bifilar winding.

Unlike other tubed amps, which typically offer a choice of two or three different output impedances (usually 8 and 4, and sometimes 16), VTL's amps are internally strapped for a single impedance range (say, 5 to 7 ohms) and are not user-changeable—not because it couldn't be done, but because no instructions are provided for doing it. (VTL will supply custom output versions.) The reason for this is that Manley prefers to take the loop negative feedback directly from the positive output tap feeding the speaker, rather than tying it to the highest-impedance tap, which will then be slightly decoupled from the speaker line when connection is made to a lower tap and will thus be less able to deal with back-EMF signals generated by the speaker.

All VTL amplifiers are shipped with two paralleled 274k ohm input load resistors, totaling about 137k. This will match any preamp outputs, but many passive electronic crossover networks require a specific value of amplifier load in order to work properly, and this can be obtained by clipping out one of the resistors supplied and/or by bridging other resistors across them, as needed. Any value under 275k ohms can be obtained; a higher value should never be needed.

You may have noticed from the description above that the 300's sensitivity is quite a bit higher than usual: 775mV, against the typical 1.25 to 1.5V. VTL did this because 775mV is the standard professional audio line level, which means the 300s can be dropped right into a pro system, where they will deliver predictable output levels. For the audiophile, this high sensitivity means the associated preamplifier can be operated at 'way below its overload point, and allows the use of a lower-output cartridge than would otherwise be practical. It also permits the amplifiers to be driven to full output, with gain to spare, by any CD player and a passive volume control such as the PAS-01 described in Stereophile Vol.11 No.2. But there are also two potential drawbacks with this unusually high sensitivity.

If the preamp generates any noise in the stage(s) following its volume control, this will be heard 4 to 6dB more loudly than it would be from most other amplifiers. And if your loudspeakers are very efficient, you may find yourself running the main volume control at such a low setting that it will be in the range where channel-balance tracking is at its worst. For this reason, the 300s should never be used for driving horn loudspeakers in the home. (VTL's lower-powered stereo amplifiers, including the 75/75 reviewed in Vol.11 No.1, have more than enough power to drive a horn system, and are available on special order fitted with input level-set controls that will eliminate both problems.)

VTL publishes no specifications for frequency response and HF power bandwidth, preferring to let the sounds of their products speak for themselves. Since I am no longer equipped to measure amplifiers, having retired my test equipment years ago when everything started measuring the same on it, I was unable to measure those performance parameters. But since VTL doesn't make any claims for them anyway, I can hardly say "I'll take their word for it." There's no "word" to take.

The styling is spartan but by no means unattractive. The rectangular chassis is finished in black all over except for a separate natural-color aluminum "deck." And even though the 17"-wide chassis has a standard rack-width 19" front panel, there are no rack-mount slots in it. Neither is there any protective perforated cover (footnote 1), as it is designer David Manley's observation that people who like to listen to tubes like to look at them too. Their only physical protection is a pair of heavy metal strips that fold up and over the front and back edges of the chassis. Appearance notwithstanding, Manley insists (rather vehemently) that these metal strips are not handles, and indeed, they will bend if you pick up the amplifier by their midpoints. But if you grasp them at diagonally opposite ends, where they rise from the chassis, you can safely hoist the amp without bending the strips, and with much less difficulty than trying to get your fingers under the chassis.

System
Equipment used for my tests included the Ortofon MC-3000 cartridge in the Well-Tempered Arm and the SOTA Star Sapphire turntable (the controller for my Versa Dynamics 2.0 turntable/arm got zapped by lightning and is away for repair), a Stax Quattro CD player, a Sony PCM-F1/SL-2000 digital tape system, the Vendetta Research MC-to-line preamp/equalizer, Threshold's FET-10 line controller, and Infinity IRS Beta and Sound Lab A-3 loudspeakers. Audio interconnects were Monster 1000s, speaker cables were Audioquest Greens, and the listening room is extensively treated with ASC Tube Traps, which are still the only retrofit devices available that will tame LF standing-wave problems. Program material was some of my own and others' original tapes, plus CDs and analog discs from Sheffield, EMI, Opus 3, Telarc, and Reference Recordings. The VTL amps were given a 12-hour warmup before listening.



Footnote 1: One is available as a $60 extra.—J. Gordon Holt
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