Tube Power Amp Reviews

The Cosmos communicated music's inevitable flow with a relaxed precision that was seductive and absolutely addictive. The midrange had a creamy smoothness while never sounding thick, sluggish, or cloying. Nor did the richness of the midrange presentation obscure transient detail. Where my reference Nu-Vista 300 sounds a bit lean and lacking in bloom, the Kora blossomed in a most beguiling way. Live recordings, like Miles Davis' Live at the Blackhawk (Columbia C2S 820 "six-eye"), delivered their full atmospheric and musical potentials. Before the first note was played, I could sense the air in the room well in front of the speakers. When the music commenced, the reverberant field defined the space behind and to the sides, completing the full effect of three-dimensional musical images, air, and space, the totality of which created an amazingly three-dimensional "you are there" picture. The Nu-Vista 300 does this, as do other amplifiers I've auditioned, but the Kora Cosmoses did it more convincingly.

Another live recording that went from being very, very good to absolutely convincing was Luna Live (two LPs, Arena Rock ARE 017-1), a fabulous-sounding greatest-hits set recorded in concert. I highly recommend it to anyone who loves the sound of electric rhythm guitar, the Velvet Underground's more introspective moments, more electric rhythm guitar, and clarity of musical purpose delivered through a slightly drunken haze. The record sounds great through the MF Nu-Vista 300, but the Koras made it come to life as I'd never before heard it.

Could the amps really rock? I tried an original British Radar pressing of Elvis Costello's Armed Forces and was amazed to hear how the Koras were able to sort out and deliver all of the hard, ragged, Farfisa-drenched explosiveness of "Accidents Will Happen" without causing the temporary deafness or bleeding ears I sometimes get from listening to that track cranked really loud. All the detail and top-end extension seemed still to be there, but now cleanly rendered, free of edge, grain, and sibilant overlay. In fact, overall, I've never heard this track sound better—especially Pete Thomas' drum kit. The crack of the snare was magical. (And did you ever hear someone cough in the background of "Big Boys," during the repeated line "She'll build a wall"?)

Which brings me back to the "palpability" issue. The Cosmos's ability to capture texture and "feel" was unrivaled in my listening experience. Joni Mitchell's Court and Spark (DCC Compact Classics LPZ 2044) never sounded more "there" to my ears than it did through these amps. Her fingers on the keys and feet on the pedals, the hammers on the strings, were presented with a detailed, natural touch that brought the recording to life as I'd never heard it before. If the Cosmos had a higher noise floor than a good solid-state amplifier, I didn't hear it, nor was there a diminution of resolution of low-level detail. In fact, for whatever reason, I frequently heard new musical details revealed at the low end of the dynamic scale.

I haven't spent much time with low-powered, single-ended (SE) tube amplifiers, but recently I got a chance to hear a pair of Fi SE amps driving rear-horn-loaded speakers using Lowther full-range drive-units at Listener editor Art Dudley's house. I was impressed with his system's seamlessness of presentation and ability to produce palpable images. I think the Koras matched those SE amps in that regard while offering 100Wpc and far greater slam.

But no amp is perfect. If you demand the last word in bass dynamics to whip your music into shape, you might be disappointed with the Cosmos. I don't mean bass extension; that seemed to go down to the limits of my associated gear. And I don't mean bass control—the Cosmos offered exceptionally nimble bass definition and didn't sound mushy, nor was the midbass sloppy or bloated. There was just a noticeable lack of bass dynamics compared to a well-damped solid-state amplifier, or a hybrid one like the Nu-Vista 300.

Halfway through the review cycle, Peter Bizlewicz brought over double sets of Symposium Acoustics Rollerblocks and a pair of Ultra shelves. I didn't hear much of a difference with the shelves atop the acrylic ones that come with the Grand Prix Audio stands, but the double set of Rollerblocks (each amp sat on four Rollerblock sandwiches, each of which consisted of two blocks and a single ball) noticeably improved the bass performance. It wasn't so much more or deeper bass, but improved bass expression and definition—almost as if the Rollerballs allowed the Koras' bass to develop more effectively.

The Cosmos lacked the last word in bass oomph and drive. But even though I listen to a lot of rock and jazz, I never missed what was lacking until I put the Nu-Vista 300 back in the system. Then the ballsy foundation returned, but everything that had made the Cosmos so seductive, revealing, surprising, and comforting was either gone or diminished. The Nu-Vista 300 is a great amp, but it's not a pure tube amplifier, and it's clear to me that there's something special about a properly designed pure tube amp. I heard that something special when I reviewed the VTL 300s and the Conrad-Johnson Premier Twelves (February 1999), and I heard it again here—but the Kora Electronic Concept Cosmos seemed lighter on its feet, faster and more detailed.

Before regretfully shipping the pair of Kora Cosmos amps off to JA for measurements, I listened very carefully to an original British Track pressing of Tommy that I've owned since it came out in 1968. It's been played hundreds of times, so it's got some noise and wear—but I've taken very good care of it, and it still sounds much more dynamic and extended than the wimpy digitally mastered remix issued last year on CD.

I played all four sides at very high SPLs and was absolutely thrilled by the Cosmos' rendering of this rock classic (not the first "rock opera," which was S.F. Sorrow, by The Pretty Things). Keith Moon and John Entwistle pack a potent rhythmic punch on this very-well-recorded set, and these tube amps did not disappoint in any way. Despite the bass-dynamics problem, there was sufficient low-end slam, extension, and definition to drive home the musical point. In almost every other performance parameter I could think of, these relatively inexpensive medium-powered tube amps either met or surpassed my reference. They were fast, detailed, dynamic, and just plain fun to listen to.

Returning the Musical Fidelity Nu-Vista 300 to my system and playing Tommy again, I noted far more bass power and punch, but not greater extension or control—and the 300 excels at those. The Nu-Vista 300 sounded great, but it wasn't up to the Cosmos' creamy, relaxed midrange and spectacular, ultra-detailed (but not etched or mechanical) high-frequency performance. Nor could the Nu-Vista deliver quite as expansive a soundstage or produce as much midband bloom. Record wear that was somewhat audible through the tube amps was more obvious through the Nu-Vista 300, but what was gained in terms of high-frequency musical performance? Nothing that I could hear. Maybe the measurements will show a rolloff of a few dB at the frequency extremes. But unless your speakers are really soft and rolled-off on top, you won't miss a thing.

Conclusions
While the Kora Electronic Concept Cosmos' seamless sonic presentation was a high point, the pinnacle was the amp's very top end. Here was airy, incredibly detailed high-frequency extension and fast transient response, delivered free of grain, grit, edge, or mechanicalness. Forget about this tube amp sounding rolled-off on top—its high-frequency performance reminded me of the Hovland HP-100 preamp, which detractors say sounds bright. They might say that about the Cosmos as well, but to my ears it sounded just right on top.

Rhythmically, the Cosmos could rock, but it could also pass along all of the delicacy and nuanced touch you'd want and expect from a tube amp. Dynamically, it would be hard to fault the Cosmos' performance at both ends of the scale, save for a deficiency in the very bottom end (compared to solid-state amplifiers). This is one tube amp that I can heartily recommend no matter what kind of music you listen to, but I suspect some listeners will be more bothered by the weight loss on bottom than others. Be sure to audition before buying.

As for the issues of reliability and your comfort in buying an amplifier from an offshore company with a limited US presence, I can't give you an answer. Nor can I tell you whether 100Wpc will be enough to drive your speakers. That was more than enough power to drive the Sonus Faber Amati Homages and the Audio Physic Avanti IIIs—and that was driving them single-ended. Even better results might be obtained with a balanced preamplifier.

I encountered no reliability problems during the review period, and my understanding is that the power tubes are long-lived, reliable, and relatively inexpensive to replace in matched pairs. Still, I'd make sure I bought my Koras from a reliable dealer, whether he or she is down the block or across the country.

If Kora's Electronic Concept Cosmoses cost $7500/pair, I'd still consider them a good deal. At $4795/pair, they look like a steal. Even at $5500/pair (after January 2002), it seems to me that you get tremendous value for your money. How do they measure? I'll be reading along with you!