Tube Power Amp Reviews

It seemed logical to begin by comparing the ProLogue Three preamp with the CAT SL-1 Ultimate, keeping the Audiopax amps in the system. In evaluating preamps and power amps, I always do some matched-level comparisons, which in this case was complicated by the fact that the CAT preamp has a switched-resistor volume control with fairly large steps. What I did was to set the CAT's volume at a moderate level, measure the voltage at the amplifier speaker terminals when playing the 1000Hz tone on Stereophile's original Test CD (Stereophile STPH-002-2), and set the ProLogue Three's continuously variable volume control to match the CAT's as closely as possible, which was just slightly greater than ±0.1dB.

Changing over from the CAT to the ProLogue Three, the first thing I noticed was that the sound was slightly softer, with the mid- to high treble less prominent—not muffled or rolled off in any obvious way, just somewhat on the laid-back side. The subtle percussive sounds on track 3 of the Chesky Records Jazz Sampler & Audiophile Test Compact Disc, Vol.1 (Chesky JD37) had a crispness as well as delicacy, but they had just a bit more in-the-room presence when reproduced through the CAT. In other respects, the sound of the ProLogue Three was what I'd call "typically tubelike," which to me means smooth, open, transparent, with an easy-on-the-ears quality that does not exaggerate, and may even attenuate, harshness in the source material. Compared to the CAT, there was some enhancement of midbass warmth that would be welcome in a system that tends toward leanness.

Did I say "typically tubelike"? Auditory memory is notoriously unreliable, but the ProLogue Three reminded me of the first tube preamp I owned, a Conrad-Johnson PV-2ar, which sounded so much more like music than the squeaky-clean solid-state unit with DC-to-light frequency response that it replaced in my system. I've heard some excellent-sounding solid-state preamps since then (the latest being the preamp section of the PS Audio GCC-100, which I reviewed in the January 2006 issue), but every time I listen to a tube preamp I'm drawn into that warm, engaging presentation that somehow puts the emphasis on the music rather than on the technology used to reproduce it. The ProLogue Three had this quality in spades.

Was the ProLogue Three as good as the CAT? No. The CAT combines this musical quality with a higher level of resolution, greater three-dimensionality of soundstage, and a sense of greater extension at the top and bottom of the frequency range. It also costs more than four times the price of the ProLogue Three. For $1395, the ProLogue Three turned in an outstandingly good performance.

If the ProLogue Three represents an excellent buy—and it does—the same can be said of the ProLogue Seven—doubled. In fact, somehow, the limitations of the ProLogue Three's performance in comparison with the CAT SL-1 Ultimate were much less in evidence when it was paired with the ProLogue Sevens rather than the Audiopax Eighty Eights. There really is something to this synergy business...

That is, once I'd managed to deal with a nasty ground-loop buzz/hum that appeared as soon as I connected the ProLogue Three to the ProLogue Seven. This had not shown up when I was comparing preamps using the Audiopax amps, but that didn't necessarily mean there was anything wrong with the PrimaLuna amps. In my experience, ground loops often involve idiosyncratic interactions between components, and floating the grounds of the ProLogue Sevens (the usual solution to a ground-loop problem) didn't get rid of the noise, which would suggest that the amps were not at fault. What did work was floating the grounds of all the components in the system, which completely eliminated the problem. The ProLogue Three-Seven combination was quite sensitive to interconnects, with the lowest noise level and overall best sound produced by the modestly priced PS Audio xStream Statement, a triple-shielded design. With the ground-loop noise fixed, and using the PS Audio interconnects, the noise level was very low—nearly as low as I've heard in my system.

Perhaps the most enduring debate in the audiophile world is the one between those who want reproduced sound to be "accurate" and those who want it to be "musical," with fans of solid-state equipment typically aligned with the former position and tubeophiles with the latter. And then, of course, there are people like me, who want the sound to be accurate and musical. (I like to think that we're in the majority.) Of course, if the sound produced by an audio system were, in fact, a 100% accurate reproduction of the original sound, the debate would have to be over, but I don't think we're anywhere close to that, even with the best systems. It's this failure to achieve 100% accuracy that results in audio designers and audiophiles opting for various subtly different approaches to that presently unobtainable ideal.

How did the sound of the ProLogue Three-Seven combo fit into this picture? Well, those aspects of the sound of the ProLogue Three that I described as being "typically tubelike" became less so when the Three was paired with the Seven; now the sound had a more optimal combination of accuracy and musicality. Assuredly, the ProLogue Seven didn't sound like a beefy solid-state amp, but it had less of the "typically tubelike" quality than, say, the Audiopax Model Eighty Eight. Combining the ProLogue Three with the ProLogue Sevens maintained the open, transparent quality that I had admired when the ProLogue Three was paired with the Audiopax, but the upper midrange and treble acquired a bit more presence. The result was that music became more exciting to listen to, with a greater sense of drama. Those percussion instruments on track 3 of Chesky's Jazz Sampler were now more in the room, the highs still staying well this side of shrill or exaggerated. Dynamics—a strong suit of the Avantgardes—were simply stupendous, with a "suddenness" and sense of unstrained power that made recordings of large-scale orchestral works most exciting. Bass was firm and extended, and there was a good sense of rhythmic drive on appropriate recordings.

Two amps in one
The ProLogue Seven comes supplied with KT88 power tubes, but, as mentioned earlier, it will also take EL34s, in which case it becomes, more or less, the equivalent of PrimaLuna's ProLogue Six monoblock ($2295/pair). A set of eight EL34s can be had for as little as $16 each from Upscale Audio; the ones Kevin Deal sent me, bearing the PrimaLuna logo, sell for $20 each, for a total price of $160. Not bad for getting what is, in effect, a different amplifier!

Once I felt I had a handle on the sound of the standard ProLogue Seven-Three combo, I removed the KT88s, substituted the EL34s, and let them burn in for a couple of days before doing any serious listening. Matching levels with the listening sessions two days apart probably isn't too important, but I was interested in whether there was any change in gain with the EL34s, so I checked the amplifier's output voltage at my usual listening level. Although I hadn't touched the volume control, with the EL34s installed instead of the KT88s the output level was slightly higher (320mV instead of 300mV), which I then compensated for by turning down the volume slightly. Correspondingly, when I switched back to the KT88s, I had to turn up the volume control a bit to get the same output level.

With EL34s instead of KT88s, the ProLogue Sevens didn't sound all that different. The noise level remained low—maybe even lower than with the KT88s—and the sound had the open, dynamic quality that characterized the Sevens with the KT88s. Overall, I preferred the Sevens with KT88s: the sound had a greater sense of depth, the highs seemed more extended, and the dynamics seemed superior. But the differences were small, and it's possible that the EL34 tubes may not have had enough time on them to be at their best. And, of course, not all brands of EL34s are alike, and there may be some that sound distinctly superior to the stock KT88s. For folks who are inclined to tune the sounds of their amplifiers by trying various tubes, the ProLogue Seven's Adaptive Autobias circuit lets you do this without the hassle of having to manually rebias them.

The sweet spot
As you ascend the price ladder of any product category—cameras, cars, refrigerators, golf clubs, what have you—you reach what some call the "sweet spot." This is where performance is at a high level but short of the very best available, and where any additional increments in performance will require spending disproportionately larger sums of money. For those in the market for a tube preamplifier-amplifier combination, the PrimaLuna ProLogue Three and ProLogue Seven hit this sweet spot head-on. With sound quality that gives up little to much more expensive products, the ProLogue Three and Seven are beautifully built, and, like the ProLogue One, reviewed by Art Dudley in the February 2006 issue, they represent outstanding value.