If you are unfamiliar with the PrimaLuna line, then most likely you have been living under a rock for the past couple of years. PrimaLuna is hot; its ascent to an eminent position in the budget tube amplification arena having been paved with a chorus of critical acclaim. There have been other "East meets West" lines ó designed in the West and built in China ó but none as compelling as PrimaLuna. A major factor in its success and product reliability is no doubt attributable to direct oversight of the manufacturing process.
The ProLogue Seven is billed as a classic Ultralinear circuit based on a quartet of KT88 beam power tubes (70 wpc) used in push-pull topology. Chinese new production copies of the venerated Genalex KT88 are standard. As with other PrimaLuna amplifiers, a soft-start circuit extends tube life and an Adaptive AutoBias circuit monitors and automatically adjusts tube bias current for improved performance and reduced distortion. Hence, there are no messy adjustment pots and burnt fingers to contend with. And best of all, forget about matching output tubes. PrimaLuna even gives you a green light to experiment with a variety of power tubes. The following types may be substituted for the stock KT88s: 6550, EL-34, 6CA7, 6L6GC, KT66, or 7581.
Some feedback is used both inter-stage and globally to reduce both harmonic distortion and output impedance. The power supply incorporates a toroidal transformer and rectifiers are fast-recovery solid-state diode types for reduced noise. The output transformers are said to be custom designed for wide bandwidth. Impedance taps are provided for 2-, 4-, and 8-ohm loads. Wiring is high-quality point-to-point, and in general, parts quality is excellent featuring Nichicon and Solen caps. The chassis is made of heavy gauge steel and finished using a five-step automotive finish. A removable tube cage is included at no extra charge. In terms of parts and build quality, what more could you ask for at a $2,695/pr price point?
The Seven, throughout an extensive audition period, possessed a high quotient of fatigue-free listening; mellow, and never bright and obtrusive though the presence region, but a tad too laid back (i.e., ever so slightly bleached harmonic colors) for my taste through the upper midrange. I should hasten to add that results will likely differ in a manner dependent on the power tube complement. To forestall pesky reader questions about power tube substitutions, and partly to satisfy my own curiosity, I broached the subject with the US distributor, Upscale Audioís Kevin Deal. He was kind enough to provide me with a set of Russian (SED brand) 6L6GC tubes to experiment with. The Russian 6L6GC turned out to be crisper sounding relative to the Chinese KT88. By that I mean that third-order distortion products were more pronounced, which only served to highlight the KT88ís more vintage character. I am quite partial toward the Chinese Genalex KT88 copy, at least in this application. In terms of reliability, I only experienced one mishap. One of the KT88s arced and had to be replaced. But other than that, the Seven performed flawlessly even when driven hard into a difficult speaker load such as the MartinLogan Vista.
Image outlines were tightly focused with good dimensionality. But the soundstage was not fully illuminated so that its inner recesses were not clearly resolved, which when combined with a slight loss of clarity somewhat blunted the presentationís immediacy. The microdynamic ebb and flow of the musicís harmonic tapestry were nicely reproduced. And there was no problem revving up into higher gear, though there was some compression evident while scaling the range from loud to very loud relative to much more expensive amplifiers.
The usual tube amp suspects were noted at the frequency extremes. The treble was slightly closed in, lacking a bit of finesse, and the bass range lacked the ultimate in control and visceral impact. Note: it really pays to experiment with various impedance taps to optimize the overall sound for a particular speaker load. Overall, the Seven appeared to perform no worse at the frequency extremes than other comparably priced tube amplifiers.
Enter the ProLogue Three
There is no balance control. Given the dual mono layout, I would have preferred to see dual pots, one per channel for independent tweaking of the volume level. On purist grounds, no cathode followers are used at the output stage, which for this circuit translates into a high output impedance of about 3.5 kOhm. I would recommend a matching power amplifier input impedance at least a factor of ten higher to minimize treble rolloff. Since the Sevenís input impedance is 100 kOhm, this criterion is easily satisfied.
This is one clean, liquid, and sweet sounding line preamplifier. Tube rectification bestows a vintage character to the sound that is hard to match in a world run amuck with solid-state rectifiers. If you think your system needs an injection of a high-carb midrange additive, then the Three would be the ideal vehicle for it.
There was undeniable synergy when the Three was introduced into the system, which is the main reason for combining these two components into one review. The focus now was clearly on the midrange, which took on a smoother and sweeter luster. Harmonic textures bloomed to a degree that is rare for a push-pull tube amplifier. And it was not just about the sugar. Low-level modulations in pitch, volume, and rhythm that convey a singerís emotions were more clearly captured. As a consequence, reproduction of female voice now bordered on the spectacular, being more emotionally charged. Soundstage spatial outlines benefited as well in terms of enhanced depth and width perspectives. Transients were a tad softer sounding, which is not necessarily a bad thing in the context of a bright digital front end or a loudspeaker with a relentless treble range. Again, it is important to emphasize that despite its classic tube virtues, this combo did not fuzz over low-level detail with an overly liquid or gooey sound ó ala the Dynaco Stereo 70. Aficionados of classic tube sound take note: you can now have your cake and eat it too!
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