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Positive Feedback ISSUE
as reviewed by Ed Morawski
A Plinius SA-102 power amplifier has been one of the foundations of my two-channel audio system for the past two years. My very first "audiophile" component, a Hafler DH-500, made me a firm believer in output power and headroom. Way back when, that old Hafler opened my eyes to the miracles that headroom could bring about with all genres of music. Even though 250 watts per channel seemed like overkill to many of my friends, I was stunned at the breathtaking heights to which the Hafler took my music playback from the first time I powered it up.
My taste runs to high resolution and low noise. Vinyl has the first of these characteristics, but I couldn't take the noise and all the preparation, so I went exclusively to CD and concentrated on finding the right player. After trying many, I settled on the Musical Fidelity NuVista. A few amps also came and went, but I was never really happy until I heard a few tube units and discovered that I liked their smoothness. The trouble was, tube amps lacked the sheer power I craved. Then I began thinking a tube preamp would be logical if it were combined with a good, high power, solid state amp. Although the Plinius SA-102 provides "only" 125 watts per side, it has such tremendous reserves and such silky smoothness that I truly felt it would remain in its place in my rack forever. Combined with my E.A. R. 864 tubed preamp, it imparted perfect tone to all of my music. While I changed CD players and speakers rather more than I should, I stuck with the SA-102.
Then, a short while ago, I acquired a nearly new Bel Canto eVo2 digital amplifer as payment for some photography work. Normally I would have turned this deal down, but I had been very impressed with a Panasonic XA-45 digital receiver in my home theater system, and was inclined to at least hear the eVo. I also figured I could turn it over on Audiogon without any trouble if things didn't work out. The day I picked up the eVo, my system consisted of the E.A.R. preamp and Musical Fidelity CD player, Empirical Audio cables, and DIY speakers that feature a serial crossover and ScanSpeak drivers. I was planning to replace the tweeters with a pair of the SEAS Excel Millenniums that I had heard in fellow PFO reviewer Victor Chavira's Marten Design Monk loudspeakers. I loved the delicate yet detailed highs of these tweeters, which I felt was the only deficiency in my system. Although I had received the SEAS drivers, I hadn't installed them.
I hooked up the relatively light (36 lbs.) and compact (17.5 x 14.5 x 4.5 in.) eVo2 in place of the SA-102. It took up a lot less space than the monster Plinius! Upon powering up the eVo, I prepared to listen carefully, but it turned out there was no need to do so, as the amp immediately overwhelmed me. Before acquiring the eVo2, I had done considerable research. I knew its price, I knew the specs, and I had read several reviews, including some glowing comments in Stereophile. I expected that it would not hold up in the bass, and I was also prepared to sacrifice some midrange. Nothing could have been further from the truth. The most stunning aspect of the eVo2 was in fact its midrange. It was like a veil had been lifted. Suddenly, unexpectedly, every vocal I played was clearer, more detailed, and richer than before. String instruments, most notably guitars, were also dramatically improved in detail and tone, but the surprises didn't end there. The bass was just as strong as before!
Why does the eVo sound so good? I went to Bel Canto's website and noted the key points. The amps are of a very simple design. Each one uses two Nchannel MOSFET output devices that are switched between the power supply rails. The switches turn on and off within 30 billions of a second, at a rate that averages over 600 thousand cycles per second (600kHz). The digital power processor adds small levels of high frequency dither to ensure that linearity is maintained from very low to very high output levels. Audio frequency information modulates the output stage by changing the time relationship between the positive and negative supply rails. When no audio signal is present, the ratio between the time at the positive supply and at the negative supply is balanced to provide no audio-frequency output.
The critical timing information is controlled by the digital power processor and the effective switching frequency is changed over a 200kHz to 1500kHz range. This spreads the digital energy created by the amplifier over a wide bandwidth, greatly reducing the energy at any one frequency. This permits using a simple 80Khz LC filter to remove the digital energy, and maintains low cost and excellent phase response. Feedback around the output switches is taken from the common node of the output switches before the LC filter and fed back to the digital power processor. This feedback is used to ensure that any variations in the switching speed of each output device are compensated for, optimizing the linearity of the output stage.
The eVo does not suffer from the distortion mechanisms of analog output stages; crossover distortion, thermal bias wander, and transconductance variations. The audio information is carried in the complex modulation of dithered switching edges. The typical compromises between output stage efficiency and complexity, and between linearity and cost, no longer apply. The eVo2 is over 90% efficient, and maintains extremely high linearity and low output impedance for good power delivery and control of the loudspeaker. The digital power processor insures that high frequency energy and audio band performance are excellent without the problems of older class-D type switching architectures. Important measures of amplifier performance such as THD, noise, and most importantly IMD, are extremely low across the audio band at all power levels.
The eVo2's very low and natural progression of harmonic and inter-modulation distortion products is achieved without heat, and without traditional analog feedback mechanisms. One of the least understood and difficult distortion mechanisms in solid state amplifiers is a result of changes in the power device's performance with temperature. This can cause bias point instability and changes in the amplifier's distortion characteristics with signal. The eVo2 is not subject to any of these distortions because the output stage has no bias current, and because it runs very cool. The result is stability in operation, heard as rock steady imaging and solid, dynamic bass performance.
I played many CDs during the following week, and the eVo2 never failed to impress. One of my favorite artists is Alison Krauss, who possesses one of the purest voices in music today. Even more rare, she has a band, Union Station, whose talents neatly match hers. On the eVo2, Krauss was just heavenly. The amp accurately recreated every lilting detail of both her voice and the accompanying instruments. After a few more days, I switched back to the Plinius amp. Moving the nearly 80-pound monster back into place was a chore, and its prodigious heat soon began filling the room. (The eVo2 had not produced any heat.) With the Plinius, the midrange detail fell off, and again was masked, as if foam had been placed in front of the speakers. Nevertheless, I noted two important improvements with the Plinius in place. The bass was a bit better, and the soundstage was wider and more distinct. Nevertheless, after replaying many of the CDs I had just finished playing through the eVo2, my conclusion remained that the SA-102's midrange lacked detail and richness.
Other matters prevented my listening for a few days, so when my next opportunity to return to the Plinius arrived, I was able to listen with fresh ears. It sounded great, but I could still hear the veil, and I missed the richness of Alison Krauss' voice. Back went the eVo2, and I hadn't been imagining things—the vocals came alive, and I knew I was hooked. The final question: Was I willing to give up the wonderful, wide soundstage and deep bass I had worked so hard to achieve in my difficult room? Studying the eVo2 manual, my curiosity was aroused by the ability to bridge the unit. My speakers are nominally 6 ohms, so according to Bel Canto, a single eVo2 was producing around 180 watts per side. Bridging it would produce nearly 550 watts per channel into 6 ohms! That surely was overkill in my room. I must admit that I am of the school that thinks, "If a little is good, more most surely be better," so I began looking for another eVo2 on the used market. I again figured that since the eVo2 was so popular, I could turn it over quickly if things didn't work out. Within two weeks I had another eVo2. Bridging one simply entails moving the positive speaker lead and pushing in a tiny switch on the rear of the amp.
After turning the preamp volume down, I powered up the two eVo2s and let them settle in for a few minutes. Now came the moment of truth. How did this powerhouse sound? My instincts were correct! The soundstage widened considerably, and the separation was of another magnitude. The bass also took on an authority I had never experienced before. The only drawback was that I would have to keep the second eVo2! In retrospect, though, the cost of the two Bel Cantos was nearly the same as that of the Plinius. Although the current list price of the eVo2 is $3290, I had spent around $3900 for both of mine. A few more weeks zoomed by, and I was really enjoying the eVo2s. I listened very critically to many different genres of music, always on the lookout for etching, harshness, brightness, and other forms of "digital" distortion. It seemed possible that my admiration for the eVo2s was a result of using the E.A.R. tube preamp, which imparts smoothness to any solid state amplifier. I have had no opportunity to listen to the eVos with a solid state preamplifier, so I don't know if my opinion would change.
By now I was anxious to put those SEAS tweeters to use, so I restarted my speaker modification project. Using Krauss' New Favorite, I began the task of integrating the new tweeters into my speakers. My first experiment had been with the Excel Tweeter and SEAS Millennium mid/woofer in a new enclosure, but that combination proved a tad light in the bass, so I decided to try my ScanSpeak SS8535 mid/woofers with the SEAS Tweeters. This was much better, and after a slight change in the value of a crossover component, the speakers came alive! I now had exceptional bass definition and weight, combined with clear highs and a marvelous, lush midrange.
While New Favorite is one of Krauss' best-known recordings, I feel So Long, So Wrong is her best to date,and I couldn't wait to try it with the dual Bel Cantos and revamped speakers! The title track is an exhilarating arrangement of acoustic guitar, mandolin, and fiddle, with a driving beat and a very up-tempo Alison Krauss. I started at a low volume, then cranked it up, letting the music wash over me. The considerable power of the two eVo2s brought me a "live" performance. Every member of Union Station was in his or her proper place on the stage, with Krauss slightly off center as her glorious, dulcet voice filled my room. On the third track, "Deeper than Crying," she perfectly matches her vocal power with a pounding electric bass. This emotion-filled song came through clearly, while the beat tracked my own heartthrobs.
The best track on So Long, So Wrong, and perhaps my favorite of all her songs, is "It Doesn't Matter." Though the verses are deceptively simple when you see them written, this song is a masterpiece of simultaneous simplicity and complication when played by Union Station. It is a fairly slow tune, with Alison and an acoustic guitar starting off. A little over halfway through, the tempo abruptly increases while a mandolin kicks in and the guitar shifts downward an octave or two. At high levels, this can really test your speakers. Most of the ones I've tried boom horribly in my small room. This time, though, the hair stood up on the back of my neck. This is what our hobby is about, and why I keep seeking the holy grail of perfect music reproduction! At the end of the song, Alison's voice again slows, tapering off to complete a song drenched in the emotion of lost love, but I had found my new love in the Bel Canto eVo2s.
During this time, I picked up a pair of Usher X-719 speakers for my home theater system. My better half had been complaining about the plain-Jane boxes I had been using, so I wanted something nicer looking, and the Ushers look great with their piano black finish and real wood sides. They sound great, too! I had them in my listening room for two weeks before I was forced to put them to the use for which they were intended. I was not only impressed by the Ushers' high quality and sound, but they proved that the eVos sounded fantastic with commercial speakers.
For those of you who like to cut to the chase and skip to the conclusions of a review, I'll accommodate you. (I do it too.) The Bel Canto eVo2 produces power that certainly belies its small size and weight. Although its lack of heat may seems like a minor issue, it's a big plus for those of us with smaller listening rooms. On the negative side, the eVos require care in choosing upstream and downstream components. The wrong combination could easily result in some brightness and "digitalitis." I feel that the eVo2s combine with my E.A.R. 864 tube preamp and Musical Fidelity NuVista CD player tube to sidetrack these complications and make for a wonderful system. If the eVo2's only advantage was its lack of heat, it would be worth consideration for that reason alone. The fact that it sounds better than amps costing twice as much makes it a keeper in my book. Ed Morawski