Tube Preamp Reviews

A quarter-century ago, when we were just getting into wine, my wife and I took a trip to Napa Valley. At one premium vineyard, we took a taste from the $20 bottle, then, for the hell of it, a taste from the $50 bottle. The first taste was nice; the second was alarming—an explosion of flavors, a gateway to sensory delights that we hadn't known could be had from a barrel of crushed grapes. We wobbled away, concerned that high-end wine might be a dangerous hobby.

I'm beginning to think the same thing about five-figure line-stage preamplifiers.

Early in 2015, I reviewed Simaudio's Moon Evolution 740P line preamp ($10,000), and grew so enamored of its pleasures that I bought the thing. Since late summer, I've been listening to Balanced Audio Technology's Rex II line preamplifier ($25,000), and it has stirred Proustian memories of that day in Napa, though on a multisensual and financially forbidding level. I can afford the Rex II even less than I could afford cases of $50 bottles of wine way back when—but it was such a pleasure to drink it in, to grasp a rung closer to the limits of what's possible.

Description and Design
In 1995, when Steve Bednarski, Victor Khomenko, and Geoff Poor started their high-end company, they called it Balanced Audio Technology because the circuits of their first products, the VK-5 line preamp and VK-60 power amp, were balanced throughout. So it has been ever since. The BAT Rex II has five inputs and three outputs, all balanced. (Balanced Audio Technology sent me some RCA-to-XLR adapters, which fit snugly and worked well with my unbalanced Nagra phono preamp, footnote 1.)

There's an array of consumer-friendly conveniences. The preamp's programming mode allows you to adjust the channel balance (to counter any left/right asymmetries in sound, whether caused by your system or your room) and set the volume to a fixed level (so that the Rex II can be bypassed in a home-theater system). With the push of a button on the front panel or the remote control, you can switch from stereo to mono, mute the volume, or invert absolute phase (the last a rare but useful feature, as many recordings unwittingly do just that).

But like the cliché about true beauty, the Rex II's distinctions shine from within. Victor Khomenko believes in power. He likes to say that eight cylinders drive a car better than four cylinders, a strong man lifts more objects (light or heavy) than a weakling, and 18 vacuum tubes push a preamp's circuits more effortlessly than some smaller number. Eighteen tubes require a lot of space, so the Rex II spreads its functions across two boxes: a 40-lb control module and a 36-lb power module, connected to each other by two detachable umbilical cords; each box has its own power cord (one per channel). Most designers of dual-chassis preamps put the controls and gain stage in one box, the power supply in the other. Khomenko doesn't like that approach, observing that it elevates impedance and restricts the power of the gain stage. The Rex II contains power supplies—two large toroidal transformers, for true dual-mono configuration—in each of its two boxes, one supply mirroring the polarity of the other. The control module contains the complete power supply for positive polarity; the power module has everything needed to juice negative polarity.

The control module, which contains a single gain stage with no global feedback, is powered by eight high-current, low-impedance 6H30 dual-triode tubes, which BAT's website calls SuperTubes, claiming that those eight 6H30s deliver the same current as 32 conventional 6922 tubes. The power module contains 10 tubes: two 5AR4 rectifiers, two 6C19 triodes per channel for the current source, and, in the AC shunt voltage regulator, one pair each of 6C45s and 6H30s. With a toggle switch, you can select between the latter pairs (more about that later), which means that the power module actually contains only 8 (not 10) working tubes, and the preamp overall 16 (not 18).

In any case, with all those tubes, these boxes run hot. The owner's manual recommends leaving at least 6" of space above each module; they are not to be stacked. Khomenko also suggests that, after the modules break in (it takes about two weeks), you should turn them off when not using them. (Another reason to do so: together, the two boxes consume about 350W.) I found that, even after being powered down for several days, they delivered fully warmed-up performance after half an hour of power—and sounded very close to that after just a few minutes. This was true whether I'd turned the preamp all the way off or switched it to Standby mode, which keeps the tubes' filaments on but withholds operating voltages from their anodes.

So far, nothing I've written in this section is new. Before the Rex II, there was the Rex, which Michael Fremer reviewed in the February 2008 issue, and which had all the features described above. And BAT first used—and thus introduced to the domestic audio industry at large—the Russian 6H30 SuperTube in their VK-50 SE preamp, way back in 1999.

Only one new feature distinguishes the Rex II from the Rex: the control module's output stage. In the Rex, the signal was capacitor-coupled to the preamp's output jacks, via BAT's Six-Pak of capacitors; in the Rex II, the output signal is transformer-coupled. The gain stage in most preamps operates at low current; the Rex II's can operate at very high current because of this transformer, which is large, and heavily shielded from stray magnetic interference from the nearby power transformers. The Rex II's output transformer, called the T-Rex, was custom-built by a Swedish company and features an amorphous-core design. Khomenko claims that replacing capacitors with a transformer—with this transformer, anyway—has two effects: It enhances the preamp's ability to drive low-impedance loads; and, for any load, it improves dynamics, transparency, top-to-bottom coherence, and, as BAT's website puts it, "the organic portrayal of music."

Not having an original Rex for comparisons, I couldn't verify this claim. To paraphrase Donald Rumsfeld, a reviewer goes to the listening chair with the preamp that he or she has.

Setup
I played LPs on a VPI Classic turntable with an Ortofon Cadenza Blue cartridge mounted in VPI's JMW Memorial tonearm, connected to my Nagra BPS battery-powered phono preamp. I spun silver discs in a Krell Cipher SACD/CD player. All signals went through a Simaudio Moon Evolution 860A power amp into a pair of Revel Ultima Studio2 speakers.

At first, I strung all the components together with Nirvana interconnects and speaker cables, as usual. Then Geoffrey Poor, BAT's marketing director, suggested that the Rex II's transparency might be better illuminated by Nordost cables. Nordost kindly lent me a set of Frey 2 speaker cables and interconnects, which changed the sound considerably. To check whether this change was due to the cables, the preamp, or some interaction between the two, I borrowed from Stereophile's editor, John Atkinson, some AudioQuest Wild Blue and Kubala-Sosna Elation! interconnects. As recounted below, each swap-out of wire made a difference, in some cases a significant one. After I'd found the best mix of cables, I switched back to my Simaudio Moon Evolution 740P line preamp, then to the Rex II, to parse which overall improvements were due to the cables and which to the BAT.

Finally, toward the end of the listening period, I opened up the Rex II's power module and, with the flick of a switch, changed the shunt voltage-regulator tubes from the default pair of 6C45s to the alternative 6H30s. (Not to spoil surprises, but after listening to a couple of songs, I swiftly switched back.)

Sound
It took me a while to figure out just what the Rex II sounded like—what it brought to the show on its own, as distinct from how it affected, or was affected by, the associated components, especially the cables. (This is always a puzzle, to some degree.) I began with my Nirvana interconnects and speaker cables, and, after playing music for a couple weeks, began listening closely to the BAT. It sounded wonderful (details to follow), except for two things: a darkness in the lower midrange that, curiously, didn't much affect the translucence of higher frequencies, and a slight tubbiness in the bass. Did the Rex II need to break in a while longer? Was something else in my system at fault?

This was when, unprompted by me (unless something is dreadfully amiss, during the reviewing process I keep mum about what I'm hearing), BAT's Geoff Poor suggested the switch to Nordost wire. One connection at a time, I swapped out my Nirvanas for Nordost Frey 2s, listened, then swapped out more. With each day of playing music, the darkness had lightened and the bass had tightened (a sign that more break-in time had indeed been needed). But with each replacement of Nirvana wire with Nordost, the upper midrange and highs, though sounding steadily clear and transparent (Poor had been right about that), also sounded more bright, edgy, and thin.

---

Footnote 1: A Balanced Audio Technology representative informed me that the company no longer makes the adapters that were included with Fred Kaplan's review sample.—Art Dudley