Vacuum Tube Logic TL-7.5 Reference line preamplifier
But VTL also has a long history of building fine, if somewhat under-appreciated, line stages and preamplifiers, the excellent TL-5.5, reviewed for Stereophile by Chip Stern last November, being only the most recent example. For some reason, the light has never shone quite as brightly on VTL's front-end electronics, perhaps in part because it's been so long since the company attempted a headline-grabbing, all-out assault on the state of that art.
More than five years ago, Luke Manley, president of VTL, decided to rectify that situation, and began work on what was intended to be a world-beating line stage. The result of VTL's efforts, the TL-7.5, was introduced to the world at Home Entertainment 2002. One look at the TL-7.5 and there was no doubt that here was a line stage prepared to compete with, and quite possibly best, anything the competition had to offer. When Manley agreed to make a TL-7.5 available for review, I responded much as I would to learning that Lucy Liu was at my front door: immediately, and with no small enthusiasm.
Two chassis, no waiting
In early 1998, Luke Manley set his engineering team a formidable challenge: Develop a line stage that would not only outperform any existing preamplifier design, but would also have the test-bench measurements to back it up. The primary goals were to "deal with the sonic bottlenecks that occur in the multiple stages and functions of known preamplifier designs," and to completely isolate the circuitry from all external environmental influences, including AC condition, load, source components, noise, and physical vibrations. Manley wanted the TL-7.5 to be able to drive any length of cable, even a punishing 600 ohm load, and to provide the full gamut of switching and control facilities and an easy-to-use interface.
The VTL team took a unique approach to the TL-7.5's circuit topology. The gain is provided by two 12AX7 tubes run in a completely differential circuit with very small amounts of global feedback. Nothing too unusual about that, but the output buffer stage uses high-voltage proprietary MOSFETs in a push-pull circuit. As the output stage is a buffer and provides no voltage amplification, the very low output impedance and high-current capabilities of the solid-state devices allow the preamp to drive virtually any type and length of cable with no sacrifice in performance.
According to VTL's white paper, the output stage is capable of "maximum signal swing on both phases of the signal" so that, when the circuit clips at saturation, that clipping is perfectly symmetrical. The massive power supply and large coupling capacitors ensure that low output impedance is maintained at low frequencies; the preamp is thus "virtually unaffected by load impedance" and can swing 30V into 600 ohms before saturating.
The most obviously different feature about the TL-7.5 is its two-box package. As with the Mark Levinson No.32, the control chassis houses all the power supplies and microprocessors (and their noise), while the audio chassis contains only the amplification circuitry, relays, and switching facilities. Multiple, fully regulated power supplies are used for each stage of amplification, and extensive mains filtering is provided to block RF and other grunge that can sneak in to contaminate the signal path. The weight of the control box indicates that the power supply has plenty of muscle. Messages are relayed to the clean audio box via two 50-pin SCSI computer cables, one for each channel, but no digital signals are used to control the audio box—only a 5V DC signal travels between the boxes.
The control section contains but one knob, which can be switched to control volume level, input offset, and balance, simplifying the signal path and eliminating parts. Source selection and volume control are all controlled with specially selected low-signal, instrumentation-quality reed relays. The volume control offers 95 steps of 0.7dB each, and the signal passes through only one relay-selected series resistor at any setting. The dual-mono circuit boards are shock-mounted to isolate them from their environment.