VTL S-400 Reference power amplifier Page 2

"But the biggest thing is the precision regulated power supply. Along with maintaining correct bias, the way to keep tubes at their optimum operating point is by regulating the high tension power supply. Normally, the voltage begins to sag as power is pulled out of the supply, and B+ voltage might drop by as much as 10%. The S-400's supply never varies by more than 0.1% or 0.2%.

"The S-400's supply is essentially a power amp in itself. The regulation requires six 3300µF capacitors and five 800V, 30A MOSFETs. They've got tremendous transconductance, so when the caps dump, the regulation keeps the voltage propped up. The screen grids are particularly sensitive, so they're actually double-regulated. Their supply is taken off the regulated plate supply, stepped down, and further regulated at the lower level."

1205vtlpic3.jpgWhen I asked him to contrast this with the supplies in older VTL amps, such as my own Ichibans, Luke said, "With the older amps, the power supply floated with AC voltage—in addition to the sagging. There was no way to keep the tubes at their optimum point. We always knew that precisely regulating the supply was better, but we didn't have the technology to manage the regulation."

Engineers love gadgets, but the proof is in the pudding
I spent the better part of a year with the S-400, during which time it shared power-amp duties with the Simaudio Moon Rocks, Mark Levinson No.20.6s, and my much-updated VTL Ichibans. I mated the S-400 with several different preamps and CD players and a wide range of cables. Two constants throughout were my Thiel CS6 loudspeakers and my analog front end: a VPI HR-X turntable and tonearm and a Lyra Titan cartridge.

First, I can attest that VTL has succeeded in their goal of "making the tubes invisible." After the novelty wore off of watching the blinking biasing LEDs and accessing the diagnostic displays, I never gave the S-400 and its wizardry a second thought. I'd push the Power button, watch "240" appear on the front-panel display and start counting down the seconds, and go pick out a record. Based on my year or so with the amp, I'd pronounce it both plug'n'play and absolutely bulletproof.

In fact, the only thing that reminded me the VTL was in the system was its no-extra-charge space-heater function, a result of its dissipating 480W at idle and a whopping 2500W at full power. To put this in perspective, the Sears Deluxe Digital Vortex Heater I was looking at last winter has settings of 800W and 1500W, and "efficiently heats a 16' x 16' room." When Trish and Luke Manley were discussing my then-new listening room over Christmas break, Luke asked if I'd installed any sort of heater. "Just your amp, Luke," she responded, "but that seems to be enough."

I'm teasing, but it's true—the heat it threw off was the only thing that consistently reminded me of the S-400's presence. I tried in vain to isolate its sonic thumbprint by installing it in familiar setups. Each time I replaced another amp with the VTL, a distinct coloration was removed from the system's sound, but I could never definitively identify what, if anything, the S-400 had added.

What was the VTL's character? In some cases of recordings or partnering gear I noted a faint coolness suggesting a slight frequency-response dip or reduced dynamics in the upper bass, and perhaps the opposite in the upper midrange—but that's an awful lot like the character of my Thiel CS6 speakers. At other times I detected a politeness, less-than-explosive dynamics, and a very minor softening of transients—which reminded me a lot of how Michael Fremer described his impressions of the latest Lyra Titan cartridge in one of our e-mail exchanges. I'd never heard these subtleties before, but does that link them with the S-400, or merely suggest that removing the other amps' colorations allowed the speakers' and cartridge's to be heard? I suspect the latter, but if that's true, what was associated with the VTL? Neutrality? Clarity? Transparency?

In the case of neutrality, the answer is yes, absolutely. I've heard a lot of amps, including a few really superb ones, but none that sounded as tonally neutral as the S-400 in tetrode mode. There was a simple, almost stark honesty to its portrayal of instruments' tonal structures that, in comparison, laid bare the artifacts that the other amps contributed to instruments and voices. In fact, merely switching the S-400 between its tetrode and triode modes provided a wonderful example of this.

Switching to the S-400's triode mode—or to the Simaudio Moon Rocks that I reviewed in September, for that matter—added a touch of warmth and sweetness to the sound and seemed to gently roll off the highest frequencies. Ramsey Lewis' piano on his trio's 1965 live recording, Hang On Ramsey (LP, Cadet LP-761), was a good example. It sounded pretty good with the VTL in triode, but transients were slightly dulled, giving me a sense that the top end wasn't as open and extended as it could be. Plus, the piano's notes sounded homogenized and slightly thick, with no obvious inner structure or temporal evolution. And although I was never aware of a distinct change in amplitude as Lewis played down into the instrument's lower ranges, there was a richness to the notes that seemed to increase as the pitch dropped.

In tetrode mode, on the other hand, Lewis' piano was dead-on. Each treble note was clearly a changing mix of harmonics, that went through a distinct evolution: from a sharp, cutting transient through a quick bloom and decay. Similarly, Red Holt's cymbals were slightly muted, with a thick, metallic hiss in triode mode but a startlingly clear mix of components in tetrode mode. The differences were even more obvious moving down through the midrange and bass. Going back to Lewis' piano, running the S-400 in tetrode mode resulted in an even more complex evolution of the sound after the initial transient, an obvious progression of string vibrations building in complexity and increasingly exciting the instrument's soundboard. Ray Brown's bass on the Pure Audiophile reissue of Soular Energy (LP, PA 002) was another great example: warm and full to the brink of being overstated through the Moon Rocks or my Ichibans, still highlighted but more clearly detailed and realistically balanced through the S-400.

I was initially suspicious of the S-400's apparent neutrality. Given its relatively high output impedance (1–1.5 ohms), I expected to hear at least some frequency-response anomalies due to the amp's interaction with the speaker load. Plus, John Atkinson's measurements always show this sort of behavior in VTL and other, similar amplifiers. I dug into this a bit, however, and discovered one possible explanation. Between 50Hz and 10kHz, JA's simulated speaker load fluctuates between 4 ohms and >20 ohms, with huge peaks around 70Hz and 1kHz In contrast, his measurements of the Thiel CS6 showed a much flatter impedance curve that varied by only about 1 ohm over the same range, and within an even narrower band for much of that span (see the March 1998 Stereophile, Vol.21 No.3, p.96).

Another component of the S-400's presentation and its lack of a sonic thumbprint was its clarity. In addition to the changes I describe above, I noted that switching the VTL to triode mode, or swapping in the Sims, Ichibans, or Levinsons, wove a liquid texture into the sound, and fine details—spatial, temporal, and tonal—weren't quite as clear. Going back to the S-400 or switching from triode mode to tetrode was akin to opening a window that is not quite optically perfect.

With the S-400 in tetrode mode, fine details were much easier to identify and follow, but a more significant effect was the realistic presence and immediacy that imbued every component of the performance. On Hang On Ramsey, it was there as much in the applause and bassist Eldee Young's singing along with his own solos as it was in Lewis' piano. I went through box after box of my favorite opera sets, just to hear them come alive with the S-400's clarity. I'm sure that there was fine detail in spades, the soundstage was appropriately large, and the image dimensionality and ambience cues were beautifully rendered, but I really didn't notice any of that. I was too busy sitting in the audience of a virtual opera house, alternating between quickly reading the libretto and closing my eyes to melt into the performance.

Perhaps my favorite example of the S-400's stunning clarity was Jimmy Smith's Hammond B-3 organ on his Who's Afraid of Virginia Woolf (LP, Verve SMAS 90577), a wonderful Rudy Van Gelder production from 1964. The organ's slightly buzzy, electronic texture was perfectly reproduced by the S-400. Combined with Smith's mesmerizing, expressive playing, the VTL's clarity seemed to erase the recording/playback chain altogether, plopping Smith, his organ, and the backing players right in front of me.

Most spectacularly, voices lit up and came alive with the S-400. I constantly found myself digging out older, more simply recorded live recordings, such as Johnny Rivers' Whisky-A-Go-Go LPs. "Brown-Eyed Handsome Man," from Johnny Rivers at the Whisky-A-Go-Go (Imperial LP-12264), is one of my favorite cuts; with any good system, it will transport me to a front-center seat at the club. But the first time I heard it through the S-400, I was stunned. I actually caught myself looking around to see which of my fellow audience members was talking and singing along. Incredible.

The third of the VTL's attributes, its transparency, is the easiest to describe in audiophile terms. I could simply hear much farther into the soundstage with the S-400 than with the Sims, Ichibans, or Levinsons. And with the S-400, the limit wasn't the system running out of resolution at the rear of the stage, or losing ambient detail into a background texture. Instead, I could either hear whatever surface bounded the recording space, or hear the microphones' sensitivities reach their limits. And up to either of these points, I heard nothing other than the recording venue.

This transparency favored live recordings—especially more simply recorded performances—where the miking patterns had been integrated into a seamless envelope. The Johnny Rivers and Ramsey Lewis albums sounded coherent, immediate, and alive—and these are standard, buy-at-the-corner-store LPs. Simple, well-done recordings given a modern audiophile makeover, such as AcousTech's reissue of Lightnin' Hopkins' Goin' Away (LP, Prestige/Bluesville/AcousTech 1073), were amazing, and closer to fulfilling the promise of recorded and reproduced performances than I'd ever heard before.

How do you put the puzzle together when you can't find the pieces?
After about a year of listening to the VTL S-400 in different configurations and comparing it with some very good amplifiers, I'm still at a loss to assign to it any specific, overt sound. It might sound a little polite, and it might lack a little weight in the upper bass and lower midrange—but then again, it might not. Its colorations were so minor that I have no frame of reference within which to isolate and identify them. All I can do is describe what the S-400 did not add to the sound, which is really just a description of what other amps I compared it with and the other components in my system. As superb as those other amplifiers and components are, the VTL's neutrality, clarity, and transparency set it apart.

The VTL S-400 may be expensive, but it is the product of an intense, five-year development effort and the marriage of several engineering disciplines. Whether you measure it against VTL's ambitious design goals or against its peers, the S-400 is a stunning achievement—a technological tour de force and a home run in terms of both usability and sonic performance. Congratulations to Luke Manley, Bea Lam, and the VTL design team. Twenty years from now, we'll all remember the S-400 as a benchmark in tube amplifier design, and the first of a new generation. Today, my recommendation is that you find one, see and hear what it can do, and get out your checkbook.

VTL Amplifiers Inc.
4774 Murrieta Street, Suite 10
Chino, CA 91710
(909) 627-5944