VTL TL-6.5 Signature line preamplifier
By Larry Greenhill   •   June, 2007 I've long admired Vacuum Tube Logic's line of amplifiers and preamplifiers. Owners Luke Manley and his wife Bea Lam routinely appear at the Consumer Electronics and Home Entertainment shows with luxurious, microprocessor-controlled tube gear, soothing new music, good-sounding rooms, and a friendly, unhurried manner. Their show setups are dialed in so well that I often find myself taking refuge there, sitting and listening for hours with other Stereophile writers.

I first heard VTL's TL-6.5 Signature line-stage preamplifier at HE2005, in New York City, playing Lyle Lovett's cover of "Friend of the Devil," from the Grateful Dead tribute album, Deadicated. After the show, Luke Manley contacted me, we agreed that I would review the preamp, and he sent me the unit I'd heard at the show. For this review I listened to that unit, as well as a newer production version now available at all VTL dealers.

Design
The TL-6.5 Signature ($9500) is derived from VTL's cost-no-object flagship preamplifier, the hybrid, two-chassis TL-7.5 Reference ($16,500). In his review in the October 2003 Stereophile (Vol.26 No.10), Paul Bolin praised the TL-7.5. John Atkinson wholeheartedly agreed; his measurements found that the TL-7.5 "demonstrates virtually bombproof measured performance, with no clue—other than its sound quality—that it is a tubed preamplifier."

The TL-6.5 and TL-7.5 share the same control section and remote control, and have similar audio circuitry. The differences are that the TL-6.5 is housed in a single chassis, has only two power transformers instead of the TL-7.5's three, uses 24 unshielded relays instead of shielded ones, has three balanced inputs to the TL-7.5's four, and has smaller output capacitors and a higher output impedance. The TL-6.5 also has something the TL-7.5 does not: a bidirectional RS-232 interface that allows the preamp to communicate with centralized control software, such as that made by Crestron, and turn power amplifiers on and off. The TL-6.5's sophisticated control electronics allow it to be configured for many tasks, including turning on separate power amplifiers remotely and in sequence, to avoid drawing down the line current.

As with VTL's S-400 amplifier, the TL-6.5 owner's manual is a spiral-bound booklet that covers all possible installations and configurations, and includes a primer on the commands for its RS-232 interface.

Flexible, adaptable controls
The TL-6.5's faceplate is dominated by a large knob on the extreme right that controls volume level, balance, and input-level offset. Volume level and source selection are managed via line-level reed relays of instrumentation quality. The control varies the level of the audio signal in 95 steps of 0.7dB each that, at any setting, pass through only one relay.

The TL-6.5's tube circuit elements are carefully controlled by a microprocessor to preserve tube life. This programs tube turn-on over a 90-second period to delay the application of high plate voltage until the tubes' filaments have warmed up. They also allow any of the preamplifier's inputs to be set for unity gain or pass-through, or to adjust its gain level to produce identical volumes from every source feeding the system. The owner can control all of these functions, as well as mute, fade, and absolute phase, from both the front panel and the remote control.

Quality under the hood
The quality of the TL-6.5's circuit components, assembly, and circuit board are topnotch. The power supply's capacitors are bypassed with small polypropylene caps. The audio circuits and their related power supplies use precision low-inductance, laser-trimmed metal-film resistors. The same resistors are used in the level control; the level is set by selecting combinations among an array of these resistors.

Moving from front to rear, the internal components include a shift register board, a huge relay PCB, two 12AU7 tubes, four MOSFET modules, four metalized polypropylene capacitors, and the rear-panel input boards. The two channels are kept separate by a prominent vertical shield that runs down the center from front to rear. Two flat, 24-conductor cables run from the shift register board to each channel's relay board.

The TL-6.5 is run as a fully differential circuit that has "sufficient common-mode rejection ratio while using tubes," according to VTL. The TL-6.5 provides gains of 14dB (single-ended) or 20dB (balanced). Its rated input impedance is 45k ohms, its output impedance a low 25 ohms, except at very low frequencies.

Gain in the single-chassis TL-6.5 is provided by one tube per channel, run in a differential circuit with only 2dB of negative feedback. While the original version had 12AX7s dual triodes, the production unit had 12AU7 tubes because the latter's slightly lower gain—20dB rather than 26dB—results in a lower noise floor. The plate of the 12AU7 drives the gate of two high-voltage MOSFETs per channel. These low-impedance, high-current devices allow the TL-6.5 to drive any load, even as low as 600 ohms, to its full rated output of 30V.

The TL-6.5 includes full power-supply regulation for each stage of amplification. Extensive filtering has been applied to the AC mains input to block radio-frequency interference and other types of spurious signals.

Setup
Installation required lifting the remarkably beefy TL-6.5 up onto an equipment shelf and plugging its AC cord into the rear panel. I connected my Krell KRC-28 CD player's balanced outputs to the TL-6.5's CD1 balanced inputs. Other connections, including the outputs of my FM tuner, Bryston B100-DA preamplifier-DAC, and Margolis phono section, claimed three of the TL-6.5's six single-ended inputs. The TL-6.5's balanced outputs were connected, at various times, to the inputs of a tubed VTL S-400 or a solid-state Mark Levinson No.334 power amplifier. Later, I put the TL-6.5 to work driving a Bryston 4B-SST power amp and the amplifier inputs of a Bryston B100-DA integrated.