VTL TL-5.5 Series II Signature Preamplifier and ST-150 Power Amplifier

Musical Soul

Equipment report
Tubed power amplifiers,
Tubed preamplifiers
VTL ST-150,
VTL TL-5.5 Series II Signature
VTL TL-5.5 Series II Signature Preamplifier and ST-150 Power Amplifier

While recently listening to András Schiff’s remarkable 2012 recording of Bach’s Das Wohltemperierte Clavier [ECM]—in which Schiff, rather unusually, and to enchanting effect, employs no sustain pedals—I was struck by how this great Hungarian artist seems to inhabit the very soul of Bach’s music, so beautifully and directly does he convey the essence of these intricate preludes and fugues.

Having now spent a great deal of time with VTL’s TL-5.5 Series II Signature preamp and ST-150 power amp, I’ve started to think of them as being rather András Schiff-like, as the crux of the pleasure they deliver lies in their ability to convey the essence of whatever music you play. Like a lip-smacking bottle of wine that leaves you anticipating the next glass, this VTL tandem will have you excitedly pulling out record after record.

VTL’s goal in designing the ST-150 was to create a relatively compact yet powerful stereo power amplifier employing 6550 tubes capable of driving a wide range of speaker loads, “as opposed,” VTL’s head Luke Manley explained, “to the EL34s that we use in other products in the Performance range.” Compared to EL34s, Luke says the 6550s offer the ability to operate in either tetrode or triode modes, and provide better bass wallop and linearity, and an extended well-defined treble. Manley described the $6000 ST-150 as a classic all-tube push/pull power amp delivering 150Wpc in tetrode mode, and, via the flick of a rear panel toggle, 70Wpc in triode operation, with a parallel 12AT7 input stage and 12BH7 long-tailed phase-splitter driving the push-pull output stage. “A fairly standard, simple circuit,” said Luke.

While I wouldn’t like to get in the habit of comparing the two tube operating modes every time I play a piece of music, I did find that, generally speaking, the more powerful tetrode state was the obvious choice for rock, large orchestral and opera, and hard-driving jazz, while the triode circuit, with its extra sense of bloom and the third dimension, was preferable for chamber music, folk, and generally lighter, more intimate forms of music.

The ST-150’s manual bias adjustment is fairly straightforward, even for a decidedly non-technical type such as myself. It does require removing the cage and a few tools such as a good voltage meter—that VTL was kind enough to provide—and I found that checking bias every three months or so is the best way to keep the ST-150 in top operating condition.

Finally I must say that, living in a small San Francisco house, I grew to appreciate the ST-150’s notably compact footprint (19" x 10" x 9"). This is a high-performance amp that can find a home in pretty much any size listening room.

As for the ST-150’s companion preamp, the TL-5.5 Series II Signature preamp was initially introduced in 1997 and has been seriously improved using technologies found in the flagship TL-7.5 Reference Series III ($20,000) and TL-6.5 Signature Series II ($13,500). These are, respectively, well over and nearly double the TL-5.5 Series II’s price. That said, the 5.5 II ’t’aint exactly chump change, selling for $8000 as a linestage only, and $10,500 with its optional internal phonostage, which is how I elected to review it.

Another difference between these three top VTL models is that the 6.5 and 7.5 are hybrid designs employing FET buffers, whereas the 5.5 shares their basic circuit, but uses tube buffers, which makes the 5.5 Signature II VTL’s top all-tube model.

The optional phonostage is derived from VTL’s standalone TP-6.5 phono preamp, and Luke Manley believes the performance of the 5.5 version comes close to that of the pricier separate unit. Indeed, during a visit from Manley and his wife, Bea, Luke popped the top on the TL-5.5 Series II in order to adjust (via jumpers) the load of the phonostage, revealing just how much space the phono section occupies within the chassis. It’s a surprising chunk of real estate, but as Manley said, “We felt it was important to have as much of the TP-6.5’s sound as possible.”

This topless view also gave me an appreciation for the overall build and layout quality of the design. Keeping with the fully balanced idea, the phono section delivers a balanced signal to the balanced linestage, which creates a balanced output from the single-ended input.

As Luke explained it, the volume control is largely responsible for delivering the linestage’s high-resolution performance. A chip-based control designed specifically for audio use, it has no crossing detection or internal op-amp buffers, which Luke tells me would degrade the sound. Instead VTL employs its own JFET buffer design, the same one used in its stand-alone phonostages. The volume chip operates at an unusually high 15 volts, which Luke explained allows for optimum signal headroom. Moreover, it’s a dual-chip design, operating in differential mode—one chip per channel—that controls both volume and balance, thus avoiding running the signal through the more commonly used, lossy, wiper-type volume control.

Via an e-mail exchange Luke added, “In designing the Series II version of the TL-5.5 we wanted to keep as much of the user interface and sonic capabilities of the more expensive TL-6.5 and 7.5, but offer a full-function preamp for people who require phono. We designed in the precision regulated power supplies in both the linestage and the phonostage that we use in the more expensive models, and I feel this gives the preamp its sonic precision and refinement.”