VTL • TL-7.5 Series III Reference Preamplifier

by Paul Bolin | January 5, 2013

© www.theaudiobeat.com

It is not an exaggeration to say that the introduction of VTL's original TL-7.5 line stage in 2002 was a seismic event in the annals of high-end audio. The first tube line stage incorporating truly 21st-century thinking, the '7.5 went on to become one of the most acclaimed components of its era, and justifiably so.

The original '7.5 was even partially responsible for the beginnings of this very website. When a mutual friend put Marc Mickelson and me in touch with each other, the first thing we discussed were our reviews of the '7.5 -- his for Soundstage and mine for Stereophile. We heard exactly the same sonic character -- or lack thereof -- in the big VTL. Despite our very different writing styles, it was at that point we both recognized that we would be compatible in terms of sonic tastes and editorial goals, the TL-7.5 pounding in the golden spike, so to speak.

The first '7.5 was not just groundbreaking; it was a durable and utterly sound piece of audio engineering. The original design was not replaced by the Series II until 2006. At that time the gain-stage tube was changed from a 12AX7 to a 12AU7, which addressed some noise issues that arose when the '7.5 was paired with power amps that had a particularly high gain structure, though I never experienced any such problems in the years I used the original preamp as my principal reference, even with the notoriously high-gain amplifiers from Lamm Industries.

Several years on, Luke Manley, Bea Lam and their engineering team at VTL developed the Series III updates that have been applied to many VTL products, and the '7.5 received its makeover for introduction at the 2010 Rocky Mountain Audio Fest in the company of the Series III version of the venerable MB-450 Signature power amps, where they took home one of my two Best of Show commendations. Systems centered around the TL-7.5 III repeated this feat at the 2011 and 2012 Rocky Mountain shows.

Evolution is a subtle thing

Some things have not changed throughout the evolution of the '7.5. Luke Manley explained to me that the first priority in the rethink that resulted in the Series III was to maintain the basic architecture and time-tested integrity of the design while upgrading everything within. Given this, the Series III retains the two-box configuration of its forbears, with the control chassis housing all the power supplies and microprocessors (and their noise), while the audio chassis contains only the amplification circuitry, relays, and switching facilities. Connection to the clean audio box is via two 50-pin SCSI computer cables, one for each channel, with only a 5V DC signal traveling between the two chassis.

The power supply/control chassis holds a single knob, which can be switched to control volume level, input offset, and balance, simplifying the signal path and eliminating parts. Source selection and volume are 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 and tube sockets are shock-mounted to isolate them from their environment. Everything about the '7.5’s world-class interface and appearance will be familiar to anyone who knows VTL preamplifiers of the last decade.

What goes on inside the boxes is quite different indeed than in the earlier iterations, at least in the details where subtle and incremental, yet sometimes highly significant improvements dwell. Luke Manley provided me with a complete overview of the Series III's technical background, and it makes for fascinating reading, particularly in how VTL’s design team approached the process. As with its predecessors, the circuitry of the latest '7.5 is minimalist in the extreme, with a single tube per channel operating as the entire gain stage. This gain stage then drives a buffer circuit. VTL examined the design holistically and focused on improvements to the power supply, gain stage and buffer circuit.

The power supply came first, as it always should, for the quality of the power supply determines the ultimate ceiling for the quality of everything downstream. VTL investigated tube regulation and determined that though tube- and MOSFET-regulated supplies measured identically, the best sound came from the tube-regulated supply. VTL believes, however, that tubes are inappropriate for use as power-supply regulators because of their inherent current limiting. Tubes also wear and change as they age, which, in VTL's view, is not good for power-supply applications.

They discovered that the primary reason for the better sound of the tube-regulated supply was that the tubes had "far lower gain" than the MOSFETs and logically concluded that the high degree of amplification was negatively affecting the signal’s sound. VTL’s engineers subsequently experimented with various FET-based circuits and eventually developed a MOSFET-based shunt-regulated power supply and compared it to the MOSFET error-amplifier regulators used in the '7.5 Series I and II and the tube-based supply. Listening tests resulted in the shunt power supply being "far superior in dynamics and signal purity" to both.

Once the power supply had been addressed, the gain stage was "re-linearized" and the feedback loop was eliminated, resulting in a true zero-feedback circuit. The MOSFET buffer stage was examined, and a new, simpler design was used. VTL discovered that the power JFETs being developed for various green technologies such as solar panels and electric automobiles were just the thing they were looking for, and a JFET-like device served as the basis for the new circuit, which was then compared to the prior circuit and yet another tube-based design. Manley explained that "JFETs are more like tubes in their operation, insofar as they do not need to bias on, as MOSFETs do, but are normally on, like tubes are" and that this is why they sound better than MOSFETs. Furthermore, "no bias is required for [JFETs], and the circuit is simpler, with fewer components." The result, says Luke, is a buffer that is superior in "linearity and resolution, and able to drive loads . . . yet sounds very tubelike."

Lastly, a thorough re-evaluation of the capacitors used throughout the '7.5 was undertaken, and polypropylene bypass caps were chosen for use over all caps in the signal path. While Teflon caps are used in power-supply bypass areas, Manley explained that using them in the signal path resulted in a sound too forward in the upper midrange -- though measurements did not show this, "resulting in an overblown soundstage, with high listener fatigue from an apparent over-presence in the upper midrange that was simply not accurate and is not found in any live listening that we have ever experienced." Not touted as a major improvement, but important for those who use high-gain power amps, the latest '7.5 allows the user to internally adjust the line stage’s overall gain structure.

As an aside, lest anyone think that such details are beyond human ability to hear repeatedly and critically, allow me to introduce you to VTL’s Bea Lam. I have known her for more than ten years, and her auditory acuity and perception are nearly beyond belief, though she is far too modest to admit it. Bea -- who has studied piano seriously for most of her life and owns a Hamburg Steinway -- has the last word on the voicing of every VTL product. During my 15-plus years in the world of audio journalism I have encountered no other listener who is her superior.

So what is the result of this massive rethink of a component that has already qualified as an enduring audio classic?

When even less is even more

On a stifling July day, near 100 degrees and with truly tropical humidity to boot, Luke Manley arrived at my Minneapolis home with a number of large, very heavy boxes in tow. They contained the TL-7.5 III and a pair of MB-450 Series III Signature monoblocks. Installation was quick, and Luke walked me through the basics of the highly flexible '7.5, some of which I had forgotten about since my time with its predecessor. I won’t recount them here, but it is safe to say that there is no system configuration that it cannot accommodate with a minimum of fuss and a maximum of ultimate user convenience.

One of the first recordings I popped on for serious listening after letting the '7.5 settle in was the Japanese LP of Jaco Pastorius Big Band’s Twins I [Warner Bros. P-11317]. The fruits of VTL’s labors were immediately apparent. The Series III had a singularly convincing way with spaces and the images located within them. Some years ago I posited that there are three sorts of space a component must successfully integrate into a seamless whole in order to create a convincing soundscape. These are the point source, the field source and the contextual source. The point source is defined by the outlines of the actual instruments or singers; the field source can be described as air energized by the instrument or vocalist in the close proximity of the vibrating sound source; and the contextual source is the sound of the venue or studio in which the recording was made. Few components get all of these sources in the proper balance, even with exceptional recordings. Right out of the box, so to speak, it was abundantly clear that the '7.5 Series III was one of that select group. Each brass instrument on Twins I was located with remarkable precision yet surrounded by a readily audible aura -- the field source -- and a superb sense of the venue’s sonic context. The sound was spatially complete in a way that is a very rare thing indeed. Get any one of these foci out of whack and images will be too sharp, as when the point source is exaggerated, where razor-cut cardboard ships will be heard to sail on the audio equivalent of a cardboard sea. Not enough inner focus and things become slightly amorphous.

Years of listening and several months of paying particularly close attention to the big VTL have led me to conclude that it is the field source that is the hardest to get just so. Even if it is not "just right," if a component is able to communicate the spatial placement and relationships of point and contextual sources, it can be quite spatially convincing. It is getting that energized aurora of the field source -- the way an instrument or a voice speaks into the space immediately around it -- that creates a vastly more believable illusion. Which was very much the case with the Pastorius LP; the '7.5 made every instrument pop into the room in a way that made the suspension of disbelief nearly effortless.

The quality of the spatial relationships among instruments was noteworthy in ways I have not previously experienced, and this was unsurprisingly revealed on the most demanding music -- the full orchestra. The Romantic Rachmaninoff box from Readers’ Digest, recorded by the legendary Kenneth Wilkinson and featuring conductor Jascha Horenstein, pianist Earl Wild and the Royal Philharmonic [Readers’ Digest/RCA RDA-29], is a spectacular example of Golden Age recording, with an all-star lineup from stem to stern. Wild’s pianism hearkened back to an earlier age of idiosyncratic, personality-filled playing, and the absurdly underrated Horenstein was one of the most meticulous and sensitive conductors of his time. Musicianship and interpretive skills of this level require nothing less than the finest sound to attain their maximum emotional and intellectual impact, and the '7.5 Series III pulled it off. The spatial relationships between the piano and the orchestra as a whole, and within the sections of the orchestra itself, in the Second Piano Concerto were exemplary, and quite possibly at the limits of what is available with this recording. The performance swept me away with an aliveness and presence that were immediately and utterly enthralling.

Information density -- the resolution of more music from both digital and analog sources --- was an ongoing source of surprise, even with music I have been listening to for far longer than I care to remember. In "Meeting of the Spirits" from the Mahavishnu Orchestra’s The Inner Mounting Flame LP [Columbia KC31067], the main section that follows the explosive introduction opens with John McLaughlin’s electric twelve-string playing an ostinato that sounds like a ball of writhing snakes. McLaughlin’s complex, sinuous, vaguely sinister riff loops back on itself and is quickly joined by Jerry Goodman’s dancing electric violin, Jan Hammer’s fleet keyboards and the ferociously complex drums of the great Billy Cobham. The combination of the '7.5 and its sibling, the TP-6.5 phono stage (the example sent along by VTL was equipped with a transformer to step up the moving-magnet input, because Luke Manley believed my reference Dynavector XV-1s cartridge prefers to see a transformer -- and he was correct) unraveled this furiously fast, supremely virtuosic music more effectively than any electronics I have auditioned. It should be pointed out that Tri Mai’s mid-December installation of his Tri-Planar Mk VII UII tonearm on the Kronos turntable ratcheted up the resolution by another few vital ticks. The sheer amount of information that poured through the Series III made profound emotional connection with music second nature. From the other end of the complexity spectrum, Natalie Merchant has one of those voices that can do bittersweet yearning so as to carve itself on your soul. She does just that on "Cherry Tree" and "Don’t Talk" from 10,000 Maniacs’ In My Tribe (LP, [Elektra 9-637081]). With the '7.5, these lovely songs were absolutely heartbreaking, rather than just pretty and affecting.

Timbrally, the Series III was as revelatory as it was with space. That the two are closely interrelated was something else that was clarified in a hurry. The revisions described to me by Manley correlated exactly with the sound of the '7.5. There was an unmistakable sense of a profoundly natural and very extended treble, a midrange that was consistently as lifelike as the source material allowed, and firmly controlled but expressive and bloomy bass. Janos Starker’s traversal of the Bach suites for solo cello (CD, [Mercury 432 757 2]) is a real acid test for tonal naturalness and completeness. Mercury’s recording team placed Starker and his cello in an appropriately sized venue. They also miked him at a proper distance, instead of in a manner that presented a room-sized cello. It is a wonderfully natural-sounding recording with a rich, unaffected ambience. The result is a recording with every last detail and nuance of his sophisticated, superbly controlled sound and matchless technique that came through in the fullest of measures. Even more impressive was how the most ordinary of CDs and LPs revealed subtleties I’d heard hinted at but never openly revealed, even with the cutting-edge Kronos turntable, through lesser electronics.

But when the recording was superb, as is the case with Neville Marriner/Academy of St. Martin in the Fields' performance of Tchaikovsky’s Serenade for Strings [Argo ZRG 584], the Series III revealed a constant fundamental neutrality, from double-bass fundamentals to the highest of violin overtones, that leaves me searching for appropriate descriptors. It simply sounded like music. Though there must be some sins of omission somewhere, they were not audible in my room with my system. On a clean, well-made rock recording like Ian Dury’s New Boots and Panties [Stiff Records SEEZ 4], Dury’s alternately warm, growling, crooning and screaming Cockney voice was totally true to form. Norman Watt-Roy’s melodically funky bass-guitar work and Charley Charles’ taut and ever-thoughtful drumming had a natural, unprocessed, live-in-the-studio sound. My listening notes kept returning, time after time, to "what I am hearing is exactly what is on this record, no more and no less," and I’ve been listening to New Boots since it came out in 1978.

Dynamics set a new standard. Whether it be the hugely dynamic, live Pastorius LP, the thundering science-fiction oratorio of Magma’s Mekanik Destruktiw Kommandoh LP [A&M SP-4397] (composer/drummer Christian Vander’s sensibilities at the time of this 1974 recording combined the musical style of Carl Orff, the ambitions of Richard Wagner, the melodic sensibility of Frank Zappa’s avant-garde works and Sun Ra-style arrangements; furthermore, he invented his own language for his works, and it sounds a lot like Klingon), or my much-loved Otto Klemperer rendition of Wagner’s Meistersinger Prelude (Klemperer Conducts Wagner LP, [Angel 3610 B], gold "Stereo" stamp), the Series III always seemed to be loafing along, with multiple higher gears constantly available for yet bigger moments. The '7.5’s ability to place every instrument in the most precise and accurate place with respect to its fellows, discussed above, was astonishing with the Wagner, particularly after the Tri-Planar arrived on the scene.