Tube Power Amp Reviews

There was nothing soft or romantic about the Baldur 70's overall sonic presentation. While in terms of control, solidity, speed, and extension the bass naturally suffered compared to my solid-state reference, it was sufficiently well-defined, controlled, and extended to avoid mushiness—more so than I've heard from many tube amps. When I played rock reference recordings, such as an original UK pressing of The Clash's London Calling (CBS Clash3), the Baldur upheld its end of the bargain, reproducing both the prominent kick drum and bass on "Jimmy Jazz," for example, with enough rhythmic proficiency and tactile solidity to effectively propel the tune, although—no surprise—not with the same iron-fisted proficiency as the mighty Musical Fidelity kW amp.

There are a few drum strokes on Mickey Hart's Planet Drum (CD, Ryko RCD-10396) that can tell you just about everything you need to know about a speaker's ability to produce deep, tight, well-defined bass, as well as an amplifier's ability to adequately drive and control a speaker capable of delivering it. On this test—I don't care what amp you're talking about or how much power it can deliver—solid-state amplifiers have it all over tubes.

That said, and despite its relatively low power output, the Baldur 70 did an impressively credible job of capturing the impact, physicality, and weight of the drum strokes on Planet Drum. Unlike some tube amps of low to medium power, the Baldurs effectively controlled the Wilsons' woofers. (The Baldur's specs claim a damping factor of 20, which is relatively high for a tube amp, though I'm not sure such a spec has any real meaning.)

Was I conscious at all times of what was missing on the bottom compared to a megawatt solid-state amp? Yes, but it wasn't a deadly deficiency, and I could listen past what was curtailed to hear the Baldur's core strengths in the midrange and upper midrange, where it fleshed out and dimensionalized voices, strings, woodwinds—even electric guitars run through their own tube amps.

All this sounds good so far, and it was. Unfortunately, there was also an unremitting hardness in the upper midrange and top end, even at moderate SPLs, that was downright fatiguing to me and to visitors. Nor did it take experienced listeners long to pick up on it. It didn't sound like grain, and it certainly didn't sound like glare. Instead, it was an opaque wall against which high-frequency transients struck and stuck. It created that sensation of hardness while preventing the creation of delicacy and dimensionality—and, especially, natural decay in the upper octaves.

With the Baldurs' 70Wpc, the WATT/Puppy 7s' dynamics suffered, which caused me to want to listen at louder volumes—but the louder I cranked the system, the harder and more fatiguing it sounded. At lower volumes, the presentation was pleasant and more polite, but dynamics were so curtailed that the music's breath of life was extinguished—and it's that very sensation that's supposed to make tubes so enticing.

It could be argued that it was unfair to review the Baldur 70s with the Wilson WATT/Puppy 7s, which benefit from more power. Next up were Wilson's MAXX2s—but that was an even worse match. After doing some listening with the Baldurs and MAXX2s, I put this review on hold until after I'd finished with the MAXX2s and could start listening to the DeVore Silverback References that I reviewed in the March 2005 issue, which are both more efficient and have a remarkably flat and tube-friendly impedance (p.122).

The Silverback's somewhat more exuberant balance proved even more revealing of the Baldurs' somewhat hard, spotlit trebles. However, since the DeVore is easier to drive and I didn't have to turn the volume up as high to compensate for limited dynamics, and because the Silverback is somewhat reticent in the midbass in my room and the Baldur was a bit more robust in that regard, the Silverbacks proved better partners for the Baldurs in terms of frequency balance. On the other hand, the Silverbacks' superior imaging and soundstaging—especially in terms of depth—amplified the Baldurs' less than expansive depth production.

The Baldur's midband was delicate and reasonably "bloomy," but I've heard lusher, more rapturous midrange performance from tube amps—from the KT88-based Music Reference RM-200 stereo amp, for instance. In fact, switching to the RM-200 solidified my impression of the Baldur 70s: While tube-amp performance is somewhat dependent on the partnering loudspeakers' load, with three different speakers the Baldur proved that it does an unusually good job—again, for a tube amp—of woofer control, and thus of producing clean, notably tight, well-defined, and textured bass free of midbass bloat and mush.

But the Baldur's most distinctive sonic characteristic was its mid-treble and high-frequency performance: as fast as solid-state, yet somewhat reticent and lacking in sparkle and transparency, while also being spotlit and somewhat hard-sounding, especially as I pushed the amp near its limits.

While the Baldur 70 is claimed to output "70W," its spec sheet claims that its "maximum power" is 80W into an 8 ohm load at 1% distortion, or "0.3% THD (50W/100Hz–10kHz)." So if the Baldur produces 50W at 0.3% distortion, but only from 100Hz to 10kHz, how much power can it produce at 0.3% distortion from 20Hz to 20kHz? In other words, is the $3600 Baldur 70 monoblock really a 70W push-pull amplifier? It sounds as if it outputs less than 70W. Whatever the measurements show, those watts come fairly steep at $7200/pair, and the sound they produced was not that enticing.

Conclusions
In cooking, halving or doubling a given recipe by halving or doubling the amounts of the ingredients doesn't always yield a dish that tastes the same as the original recipe. The same may be true of audio amplifiers. The Baldur 70 derives from AudioValve's Baldur 200 Plus, a claimed 170W (conservatively rated) monoblock that uses eight 6AS7G output tubes and costs $21,000/pair—a formidable, highly regarded amplifier that has received uniformly outstanding reviews. (I haven't heard it.) The Baldur 70 uses the same circuit topology and build quality, which makes it, essentially, half of a Baldur 200 Plus.

When I first auditioned the Baldur 70 last winter, after a long break-in, with the Wilson WATT/Puppy 7s, the amp's bottom end was impressive for a tube design, but I couldn't work up much enthusiasm for what I heard overall. Dynamics were limited; the well-focused soundstage was notably compact laterally and vertically, and not particularly deep; and I found the top octaves bright and fatiguing at realistic SPLs.

Given the high regard in which this expensive amplifier is held in many circles (it was a Hi-Fi News "Editor's Choice" for 2004), I held the review back to listen to speakers better matched to the amp's power output. While the excellent-sounding, tube-friendly DeVore Silverback Reference proved a better match for the Baldur 70, I still found the amp's sound less than inviting overall, and just plain off-putting on top. Though the Baldur's all-tube design is notably quiet and detailed, and while its bottom end was first-rate for a tube amp and its midrange was harmonically rich, I continued to hear fatiguing, unpleasant upper-octave hardness at realistic volume levels, as well as less than satisfying dynamics.

With all three loudspeakers used in this review, when I inserted the $3450 Music Reference RM-200—a truly balanced hybrid stereo design featuring a bipolar transistor input and tube driver and output stages, and which I ran single-ended—the overall sound improved. There was more apparent power and greater dynamics, bass was equally well controlled, and the mids were supple and enticing. The Music Reference measures well in most ways and comes within 0.2dB of meeting its rated output of 100Wpc into 4 ohms. Up top, it sounded sweeter than the Baldur yet equally detailed, airy, and extended driving all three speakers I tried—including the Wilson MAXX2s. Listening to the RM-200 was more relaxing, inviting, and involving. While it requires carefully matched tubes and can't compare with the Baldur in appearance and build quality, it offers more power and costs less than half as much.

I imagine the Baldur 70 would sound best with a warm-sounding tube preamp and phono preamp, and/or driving speakers that are less revealing and detailed in the treble. I used more neutral-sounding (though hardly bright) hybrid electronics, but I did hear AudioValve's new phono preamp briefly at the Heathrow hi-fi show in fall 2005, and it sounded very much on the warm side.

I spent a lot of time listening to these amplifiers through three different speakers, one of which, the DeVore Silverback Reference, was surely designed for medium-output tube amplifiers like the Baldur 70. Yet the sound the Baldurs produced was dynamically limited and too hard and bright on top to be easily forgiven, especially given their steep cost. The Baldur's build quality is superb, and its unique automatic biasing circuit ensures peak amplifier performance as its tubes age. But what I heard makes it difficult to justify a price of $7200/pair—a high price that may soon be higher—for, at best, 70Wpc.