Vacuum Tube Logic TL-7.5 Reference line preamplifier VTL TL-7.5 Series II, May 2007

Brian Damkroger wrote about the TL-7.5 Series II in May 2007 (Vol.30 No.5):

When it was introduced in 2002, the VTL TL-7.5 Stereo Reference line stage sent a huge ripple through the high-end world. Paul Bolin and John Atkinson contributed to the commotion with raves in the October 2003 Stereophile, and I repeated their superlatives in a Follow-Up in December 2005. I also echoed PB's conclusion that it was nearly impossible to attach any specific sound to the TL-7.5. With 20/20 hindsight, it's now clear that the TL-7.5, along with VTL's Reference Series power amplifiers and Halcro's own stunning Reference Series electronics, were the first truly 21st-century high-end products, establishing a new standard with the transparency of their sound.

In the five years since introducing the TL-7.5, VTL has systematically applied the model's technologies to other products. Mostly they've trickled them down to update existing, lower-cost models, but VTL has also used the TL-7.5's basic design elements to underpin new models—such as the single-chassis TL-6.5 line stage and, more recently, the Signature Phono Stage. "With each application," said VTL principal Luke Manley, "we learn new things, partially because the constraints and goals are different, but partially because new technologies and parts are available. And after doing the Phono Stage, we felt that there were enough things to take advantage of that it merited looking at the 7.5 again."

The TL-7.5 Series II differs from the original in four principal ways. First, it costs $16,500, or $4000 more than the original. (A Series I TL-7.5 can be upgraded to Series II status for $4500.) The second is an accumulation of several modifications to the power supply, each minor in itself, but adding up to and necessitating a substantial upgrading. The third was the replacement of a number of components. In some cases these were simple swappings-in of superior parts; in others, the replacement also required changes in circuit parameters and topography, or perhaps added a bypass to an existing circuit branch. As part of this scrubbing, VTL ended up designing and had made a number of proprietary passive components when they were unable to find off-the-shelf items that met their needs.

It's the fourth element of the update that VTL notes first, however: the replacement of the TL-7.5's 12AX7 tube with the Series II's 12AU7, and the redesigning of the surrounding circuit to run the tube at a substantially higher current than before. The resulting lower plate impedance improves the drive to the following MOSFET, resulting in a wider frequency range. Indeed, testing of my '7.5 confirmed that where the original's output began to dip at about 20kHz, the Series II was less than 0.5dB down out to 200kHz. The result, according to VTL, is a faster, more open sound, particularly at the frequency extremes. The change has also resulted in the Series II having slightly lower maximum gain: 20dB vs 26dB in balanced mode, which tends to be a better match for the high outputs of CD players.

If the original was perfect, is the Series II more perfect? The best way to set the stage for the TL-7.5 Series II is to revisit Paul Bolin's comments on the original, from Vol.26 No.10: "It is no easy thing to attempt to describe the 'sound' of a component that had less intrinsic sonic character than anything else I have ever reviewed. The TL-7.5 had such an infinitesimal sound of its own, and did everything so supremely well, that I found nothing to rationally criticize. Sonically, it simply did not exist in the signal chain."

I agreed with Paul wholeheartedly. My time with the Series I convinced me that it combined the best attributes of everything I'd heard up to that point. The TL-7.5 had the transparency of the best passive units, but without their slight dulling of transients. It also had the dynamics and speed of the best active line stages I'd heard, while completely removing from the sound all traces of "electronic detritus," to borrow a phrase from Michael Fremer.

My first response to the TL-7.5 Series II was exactly what VTL would have predicted. I was impressed. The II's presentation was more dramatic, and the presentation of even old favorites, such as Beverly Sills, with Aldo Ceccato and the Royal Philharmonic, in Verdi's La Traviata (LP, Angel SCLX-3780), had an energy that immediately caught my attention—the recording space itself seemed more alive. On the wonderful Heifetz-Piatigorsky Concerts (LP, RCA LDS-6159), I felt more aware of the instruments' textural and dynamic subtleties, and the subtle nuances of those masters were bolder and more tangible with the Series II than I remembered hearing through the original TL-7.5.

When I switched gears to a series of classic rock albums, I was impressed anew by the Series II. There was no doubt that its dynamics and speed were improved at the frequency extremes. Bass guitars sounded both cleaner and more powerful, and I was again aware of increased detail. At the other end, everything from the upper midrange/lower treble on up was definitely more dynamic, and the spaces between notes and instruments were clearer and more starkly defined.

Throughout my audition, which spanned most of a year and several iterations of associated equipment, I was consistently aware of and impressed by the Series II's performance at the frequency extremes. It seemed to have a slightly cooler tonal balance than the Series I, though the difference wasn't big. And although I thought the original was right on the money, after hearing the Series II I'd be hard-pressed to label either as the "more correct." The II's dynamics also seemed to increase as frequencies rose across the upper midrange and lower treble, giving it a more forward presentation and a boldness that, with unfortunate choices of program material, bordered on hardness. Like the cooler tonal balance, however, this more forward feel was hard to brand as "wrong"—even though the Series I had seemed so very right.

Most noticeably, the Series II had, overall, a fast, open, powerful sound that the original hadn't. And therein lay the rub—the Series II had a sound, and I was constantly aware of it. The obvious possibilities: perhaps the Series II was revealing nuances elsewhere in the system; maybe I'd gotten so used to the original that, even a year later, I still hadn't absorbed the Series II's improvements into my frame of reference; maybe there were synergies between the original 7.5 and my other gear that the II's improvements did away with. I don't yet have an answer, just the seemingly contradictory observation that while the original TL-7.5 didn't sound at all, the Series II sounds better—and in some cases, significantly so.

VTL's original TL-7.5 line stage was a benchmark product when introduced, and remains one today. The Series II is another step in its evolution, though in exactly what direction, I'm not sure. The update unquestionably offer the sonic benefits of speed, openness, and improved dynamics at the frequency extremes. VTL's rethinking of the initial gain stage reveals a new path to improved performance, but I'm not sure that path has yet been followed to its end. While the TL-7.5 Series II is a fantastic audio component in and of itself, it also underscores the magnitude of VTL's achievement in the original TL-7.5, and suggests that that achievement will be hard to definitively eclipse—even by VTL itself.— Brian Damkroger

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