Vacuum Tube Logic TL-5.5 line preamplifier
By Chip Stern   •   November, 2002 Over the course of several months, during which time I auditioned the Vacuum Tube Logic TL-5.5 tubed line-stage preamp with a variety of power amps and loudspeakers, I began to reassess many long-held notions about the "characters" of solid-state and tube components. Sometimes the TL-5.5 revealed its musical pedigree with all the midrange juiciness and sublime textural detail that one traditionally associates with a triode front-end, while at others it evinced a level of focus, transparency, and frequency extension I more readily associate with solid-state purity—all in a stylish package featuring a remote volume control and a full range of performance enhancements that belied its affordable price.

Audio Verity
The VTL TL-5.5 owes much of its performance/value pedigree to developmental work done on the VTL TL-2.5 line stage, from which it descends. The two triode line stages share a number of features: toroidal power transformers, robust power supplies for each gain stage, gold-plated RCA jacks, a glass-fiber circuit board with heavy copper traces, chassis of 16-gauge stainless steel, relay switching, and a laser-trimmed Alps remote volume control. In other ways, the 5.5 represents a significant advance over the 2.5.

Luke Manley and the VTL developmental team took a decidedly practical approach to high-end criteria in designing the TL-5.5—the bottom line was to accentuate performance value. So while they've observed many audio verities, they've disdained others. For instance, while VTL eschews the use of cathode followers in the circuit path, feeling that they rob the signal of brilliance, speed, and dynamic vitality while unduly darkening the sound and adversely affecting soundstaging, they do employ a buffer in the output stage.

"Lowering impedance and delivering current is the primary function of a buffer," Manley explained. "Some preamp designers would rather not have a buffer in the circuit, but as far as we're concerned, the output impedance would be unacceptably high. The buffer we employ on the output stage of the 5.5 (and on the optional MM/MC phono stage) is called a totem-pole buffer, and it's sonically superior to a straight cathode follower because it works actively on both phases of the signal, where a cathode follower works actively on one phase while using a resistor pull-up—which works passively—on the other phase. A follower that is active in both phases of the signal swing offers symmetrical clipping at full output, while a straight cathode follower will clip on one phase before the other."

Likewise, while VTL claims to employ the shortest possible circuit path to maximize sonic purity, in other areas they're less fussy and doctrinaire than some other high-end designers. As a result, the 5.5 doesn't invert absolute phase, but does include a high-quality Noble balance control (when aligned at its center detent, it is not in the circuit), as well as phase flip and Mute switches, both of which can be activated manually or by remote control. Housing a separate power supply for each channel in a single chassis is a prudent approach at this price point, VTL putting the money saved toward audiophile-grade Multicaps in the signal path.

System & Cables
It took 50-100 hours for the VTL TL-5.5 to open up and breathe enough to resemble what I ultimately determined to be its true sound signature. During this burn-in phase I was reviewing a pair of Meadowlark HotRod Shearwater loudspeakers—see September 2002, p.103—and I became more conscious of the 5.5's noise floor than I was with the Joseph RM33si Signatures I also used for my listening. The left channel was more noisy than the right, although neither could be heard from more than a few feet away. I opened the chassis to ensure that all tubes were firmly seated in their sockets and after the VTL had been powered up again, I wasn't disturbed by background noise any longer.

The TL-5.5 line stage offers two sets of single-ended outputs as well as a fully balanced out, which proved useful in running long lengths of Acoustic Zen Silver Reference interconnects to the power amps on my double-decker PolyCrystal amp stand: a 15' single-ended run to the Musical Fidelity Nu-Vista 300 and Rogue Audio Magnum M-120 monoblocks (review to come), and a 20' balanced run to the VTL MB-450 monoblocks on a sturdy timber table to the rear. Most of my critical listening was done on the Nu-Vista 300.

After returning to the familiar sound of the tube/solid-state Nu-Vista preamp and leaving it on more or less continuously, I alternated between the tubed VTL TL-5.5 and the Blue Circle Galatea until I'd gotten a sense of their distinguishing characteristics. Switching from a more or less solid-state line stage to a fully triode front-end, I was expecting the hybrid Nu-Vista preamp to be, if not significantly brighter, then much more open and extended in the top end. But instead of a slightly diffused, velvety top end, the 5.5 had a profoundly crystalline sound—remarkably clear, sweet, and extended.