Prima Luna Prologue One integrated amplifier
The theme continues inside, with point-to-point wiring that's carefully dressed and neatly soldered. All the hardware is bolted in place, not just stuck to the inside of the chassis with glue and hope, and well-finished metal shielding is installed wherever hum or RF interference might otherwise intrude. All the tube sockets are good ceramic ones, and the terminal strips are ceramic, too. Threaded parts are locked in place with a dab of red enamel. All the edges have been smoothed over. Inspectors' initials abound.
Here's where your music goes: Each channel's preamp tube is a 12AX7A dual triode, the two halves of which are tied together in parallel in the interest of current gain. From there it goes to the two halves of a 12AU7 driver, configured as a long-tailed pair. Then it's on to a push-pull pair of EL34s operating in "enhanced" class-A/B, with screen grids tied to the output transformer's primary so the tubes can deliver more power than if they were used in triode mode—yet also sound sweeter and exhibit a lower output impedance than if they were used in pure pentode mode. Tube-amp enthusiasts will recognize that as an Ultralinear output circuit, which the great David Hafler first described more than half a century ago.
Sounds pretty simple, doesn't it? It is—for the most part. Then again, because the Prologue One is aimed at the first-time owner of a tube amp, Prima Luna wanted to ensure reliable performance in almost any setting, with no need for adjustments. So chief designer Marcel Croese devised a new way to bias the output tubes.
In a normal fixed-bias amplifier, the output tube's cathodes are referenced directly to ground, and an independent negative voltage is applied to the signal grid: It's that bias that the AC music signal modulates, continuously varying tube current as it travels from the cathode to the anode, and allowing the high-voltage output to mimic the low-voltage input. But because temperatures can vary inside a tube amp, and because tubes, like people, begin to deteriorate from the moment they enter the workforce, the bias will require periodic correction. The bias may also need to be altered in the face of unusually loud or bass-heavy signals, to prevent the higher voltage from functionally adding to the bias—and thus limiting tube current and compressing the output. Croese's solution, called Adaptive AutoBias, is a circuit that continuously adjusts the bias voltage in response to changing temperatures and input signals. (Neither the J-FET op-amp at the heart of Croese's circuit, nor its supporting parts, is in the signal path. Adaptive AutoBias, which is the only major portion of the Prologue One that's laid out on a printed-circuit board, uses a reference signal derived from the amplifier's input.)
The result, according to Croese, is an increase in both performance and reliability—two things that don't always track one another in the minds of hard-core tube enthusiasts. And by taking the enthusiast out of the adjustment loop, the Adaptive AutoBias circuit ensures something else: moderation. "The adjustments made are very slight," Croese says, "within narrow margins, so that the tubes always operate in the parts of their range that are lowest in distortion."
For its part, the Prologue One's power supply is as traditional as they come: High-voltage AC from the power transformer is straightened out by a smallish rectifier bridge, and the bumps and valleys are smoothed by a pi filter centered around a sturdy-looking choke. Additional secondary taps lead to a separate filtering circuit for the tube heaters.
Again: All of this is contained in one of the nicest-looking enclosures I've seen in ages—one in which all the metal parts fit together, and the sheet metal doesn't flex under the weight of the trannies, and the feet hold the thing far enough off the shelf that the underside doesn't get too hot. It's also, coincidentally, one of the nicest-smelling amplifiers I've had in a long time—like wood smoke, which I find pleasant. Let's see if John Atkinson notices that, too, when he performs his measurements. (Don't worry: There were no signs of burning trannies—or burning anything else—inside the Prologue One.)
Although I enjoyed every day I spent with the Prologue One—my astonishment at the level of value it offers couldn't be more genuine—you may want to keep in mind that my reviewing environment is almost a worst-case scenario: Quad electrostatics on the one hand, high-sensitivity Lowther horns on the other. I heard just enough evidence of uncontrolled bass with the former, and shoutiness from an overpowered whizzer with the latter, that I couldn't help imagining that most real-world installations, centered around a dynamic loudspeaker of medium to medium-high efficiency, would sound even better than my own.
All right—let's dispense with this product's most notable shortcoming: Driving the Quad ESL-989s, the Prologue One had a very underdamped bottom two octaves. The bass drum in the Prelude of Ives' Symphony 4, performed by José Serebrier and the London Philharmonic (CD, BMG Classics 63316-2), was boomy, with an unrealistically long decay. Likewise the electric bass and the lowest-tuned floor tom on the Band's "Smoke Signal," from Cahoots (CD, Capitol 25391-2), which also sounded slow and rhythmically unstirring as a result. Roxy Music's "More Than This," from Avalon (CD, Virgin 5 83871 2), never really took off.
But with the same speakers, the Prologue One sounded consistently fine with smaller-scale music, and with music that didn't require so much in the way of quick, taut bass reproduction. The amp was engaging on good solo-piano recordings, such as the XRCD reissue of Thelonious Monk's Thelonious Himself (CD, Riverside/JVC VICJ-60170). The Prologue One didn't smooth over the deliberateness of Monk's style, but neither did it sound unduly mechanical—a common enough failing, in my experience, with other pentode amps. It tracked the album's continuously shifting moods, from the playful to the serious and everything in between: The music got bigger when it had to, without strain, and Monk's touch was never lost in the process. And while other amps may do certain things better—the purr and sheer scale of the low B-flat in the bluesy "Functional," for example—I've never heard another comparatively inexpensive amplifier get the humanness of Monk's piano sound this right.
The Prologue One's tonal balance seemed correct in a general sense—if anything, it was tilted somewhat toward the bottom end—and it reproduced instrumental and vocal timbres in a manner that was essentially neutral, but with a hint of pleasant warmth and thickness, especially throughout the midrange: It was, without question, a sweet-sounding amp, but not one I considered unpleasantly colored. When I took my attention away from the music and focused on the sound, I heard that Norman Blake's voice was slightly chestier than usual on "Greycoat Soldiers," from Fields of November (LP, Flying Fish 70004), and his vintage Martin D-18 was one color swatch darker. But the amp made up for it with a nearly SET-like presence and spatial believability on voice and guitar alike, and an equally believable sense of the space between them.
Conductors all focus on different things in the same music, and so, I think, do fine audio components. The Prologue One was acceptably good at conveying the scale and drama of orchestral music, and while it didn't pull subtle details from the mix as explicitly as I would have liked—as in the aforementioned Ives recording, which needs all the openness and clarity it can get—it was good enough that I could listen for hours at a stretch without frustration or boredom setting in. But this amp's strongest suit with good orchestral recordings was its tendency to find the color, substance, and textures of the instrumental sounds. Even on discs not known for their warmth, such as the 1962 Starker-Dorati recording of Dvoák's Cello Concerto (CD, Mercury 432 001-2), the Prologue One brought out the best in massed strings, preserving well their rich texture and timbral complexity. Simply put, this amp let that record sound beautiful.
The Prologue One brought those same strengths to well-recorded folk and pop music, too. It did a fine job with Tony Rice's guitar solo on the great Lester Flatt song "Why Don't You Tell Me So," from Cold on the Shoulder (CD, Rounder CD 0183), sounding especially twangy and right when Tony ventured down to the lower strings. And Ronnie McCoury and David Grisman's mandolins sounded real—and appropriately distinct from one another—on their fairly recent recording of Bill Monroe's "Roanoke," on Bluegrass Mandolin Extravaganza (CD, Acoustic Disc ACD-35). Another plus was the timbral and spatial realism of Del McCoury's backing guitar. On the minus side, the Prologue One made that recording and others like it a bit draggy, rhythmically: It missed a lot of the bounce and momentum that other amps seem able to find.
In fewer words: This chunky little tube amp sounded like a chunky little tube amp, for better and for worse. That it's so easy to buy and to use may steer you further from worse and closer to better.
Apart from its clever bias circuit, there's nothing new inside the Prologue One's handsome chassis, technologically speaking. Economically, however—or geopolitically, or however else you want to look at it—there's a much bigger story: An amp this good can't be made to sell for this little in America, Europe, Japan, or even Mexico. So the Prima Luna Prologue One, while designed in the Netherlands, is manufactured in the People's Republic of China. Its casework is fabricated and finished in China. Its components are wired together in China. And its original, Dutch-designed output trannies are wound in China—apparently quite well. (The Prologue One may not be at the very cutting edge of tube design, but it wouldn't sound this good if its output trannies were crap.)
Let's face it: China, when she's not busy buying up US currency, is busy making things, and making them well. And just like the clothes on your back and the flag in your yard, China makes them for a lot less than we apparently can. You are free to make of that what you wish.
I know how I feel about it. Because my first full-time job paid $96 a week after taxes, and my first integrated amplifier, a Sansui AU101, cost about $150, I've never shaken the notion that one's first very good amp should cost between one and two weeks' pay. And while I recognize that most things have changed since then—hi-fi doesn't mean the same thing to young people today that it did in the 1970s, when sequestering oneself with a record player was a way of turning one's back on all other media, most notably television—I can't help but feel that affordable or relatively affordable products are good for everyone: for dealers, for magazines, and even for people who make and sell music in the first place. Of course, I may whistle another tune if our economy tanks and I can't even afford the Christmas-tree ornaments at Wal-Mart on December 26.
But for now, boy oh boy, can I ever recommend this amp! It's not the liveliest-sounding thing, and if that's more important to you than such things as texture and color, you'll be better off considering something else. But if you're new to the world of tubes and you want to see what it's all about—and you're on a limited budget, and rolling your own is out of the question—then it's hard to see how you can go wrong with the Prima Luna Prologue One. Lustily, heartily, and enthusiastically (if conditionally) recommended.