Tube Preamp Reviews

When I first met Luke Manley, proprietor of VTL, he and his father, David, with whom he ran the company at the time, had recently emigrated from the UK to California. I asked Luke how he liked the West Coast. "Great," he replied. "Much better parts availability." This was about 30 years ago, when I was just immersing myself in high-end audio at a high-toned level. Our exchange gave me a taste of the obsessions ahead, though Luke Manley's single-mindedness through the decades since has been more dogged than many—and, at the same time, less dogmatic.

Once a tube purist (the initials VTL originally stood for Vacuum Tube Logic), Manley realized early on that solid-state had advantages in amplifying current, so he began designing output stages and power supplies based on metal-oxide-semiconductor field-effect transistors (MOSFETs) instead of tubes. When the results struck him as sounding precise but a bit heartless, Manley replaced the MOSFETs with a type of FET that behaves more like a tube. Manley won't discuss just what this FET is, nor can you take a look: it's inside an output module, the casing of which you can't remove without affecting the service contract.

Description & Design
The shift from MOSFETs to FETs is one feature that distinguishes Manley's newest line-stage preamplifier, the TL6.5 Series II Signature ($15,000), from the original TL6.5, which hit the market in 2007. Then and now, the TL6.5 is a one-chassis version of the two-chassis TL7.5, and, throughout their evolution, the singles have trailed the doubles in a good way: Improvements wrought in the TL7.5 Series II were applied to the first TL6.5, and now improvements built into the TL7.5 III are showing up in the TL6.5 II. In other words, the cheaper, single-box preamps have stayed, in one sense, a generation ahead of the costlier, two-box models.

Like the TL6.5, the TL6.5 Series II is powered by tubes: one 12AU7 per channel, only in the gain stage, where voltage trumps current and where, therefore—according to Manley—tubes produce better, more linear results than solid-state devices. The preamp's circuitry is of fully balanced, differential design, and is thus claimed to be less susceptible to noise. The volume control is constructed from relay-switched resistors, which Manley says provides more headroom and higher resolution than a potentiometer. The chassis is made of heavy steel, and the sides are stiffened with an aluminum faceplate, to shield the circuits from external noise and vibrations. There is also internal shielding to isolate the power transformers from the audio circuitry. To that same end, both tubes in the gain stage are shock-mounted on rubber O-rings.

The Series II's new features—upgrades from the original TL6.5—are considerable. First is the shift, noted above, from MOSFETs to FETs in the output stages. There are now two toroidal transformers (the TL6.5 had just one). But the Series II has the additional benefit of zero global feedback (the original's feedback amounted to 2dB). The elimination of feedback required the gain stage to be, in Manley's words, "completely redesigned," meaning that he readjusted all the voltage and current operating points for the tubes and revoiced the circuits for the capacitors, in some cases switching to different caps or, in the power supply, bypassing the electrolytic caps with Teflon—a move that, he says, extended the high frequencies. Finally, the volume attenuator has been moved back, farther away from the transformers, thus providing still more shielding.

Manley says his goal has been to design a circuit that's as simple as possible while keeping impedance low enough to drive amplifiers of all loads and to handle vast signal swings across all interconnects, regardless of length.

The owner's manual outlines scads of convenience features, including bypasses for home theater and ways to standardize gain levels across all inputs. The remote-control handset is elegant and extremely legible (why can't more high-end companies make such clear, clean remotes?), with buttons for muting and for inverting absolute phase, as well as the other usual functions.

The tubes are expected to last 1500 hours or more, their life expectancy prolonged by a soft-ramp filament circuit that avoids the "flash" that can otherwise occur when a long-cold unit is turned on.

The TL6.5 Series II preamp is heavy (48 lbs), owing to its steel chassis and two transformers, and its casing is a bit clunky, but it sports a cool look, with a nifty three-tone faceplate and asymmetric edges.

Setup
Throughout my time with the VTL TL6.5 Series II Signature, I played LPs on a VPI Classic turntable with an Ortofon Cadenza Blue cartridge, hooked up to a Nagra BPS battery-powered phono preamp. I played CDs and SACDs on a recently purchased dCS Puccini, the best single-box CD player I've heard. The power amp was a Simaudio Moon Evolution 860A, and speakers were a pair of Revel Ultima Studio 2s. For comparisons, I frequently swapped out the TL6.5 II for my Simaudio Moon Evolution 740P line-stage preamp. All electronics were plugged into hospital-grade power outlets wired to a dedicated 20A circuit.

I let the TL6.5 II warm up for a couple of weeks by leaving it powered on all the time and playing it, even at low volumes, as much as possible; after that, I usually turned it off when it wasn't in use. It would sound quite close to peak after about 10 minutes of warmup, and fully up to snuff within an hour. I settled on Nirvana S-L interconnects from the turntable and phono preamp, AudioQuest Wild Blues from CD player to VTL, Kubala-Sosna Elation!s from VTL to power amp, and Nirvana S-X speaker cables to the Revels. I briefly ran Nordost Frey 2 interconnects from the turntable and phono preamp, but this was a poor match: highs sounded harsh and lows sounded tepid, maybe because, in my limited experience, I find Nordosts tend to tilt the frequency balance upward, which the VTL didn't need. I placed three of Black Diamond Racing's Mk.4 Cones under the TL6.5—two in front, one at back-center, all points down—which noticeably tightened the bass. The comments below do not reflect when the Nordosts were in use; they do reflect use of the BDR cones.

Sound
I never heard the original TL6.5—reviewed by Larry Greenhill and Wes Phillips in the June 2007 and July 2008 issues, respectively—so I can't judge whether, or to what extent, the Series II marks an improvement. But by any measure, this is a very fine preamp.

Luke Manley stressed in several e-mails that his No.1 goal has long been to achieve linearity throughout the audioband, and even before I read those observations, that was the first thing I noticed while listening to the TL6.5 Series II. It was the smoothest-sounding preamp I've heard in my system—and I don't mean smooth in a pejorative sense. What I heard wasn't at all your stereotypical lush in the midrange, flabby in the bass, clipped in the highs "tube sound": instead, from bottom to top, the TL6.5 II's sound was very nearly seamless and neutral. (I'll elaborate on the qualifier "very nearly" soon enough.)

This was especially true on top: High frequencies stretched higher than I'd thought my system (or some of my records) capable of—and not in any boosted way, the overtones zapped with some homogenous fizz. No, these highs were pure, natural, clearly of a piece with the instruments and ambience that produced them.