May 2007

PrimaLuna ProLogue Seven Monoblock Power Amplifier
And ProLogue Three Preamplifier
Primo sound on a budget.

Review By Dick Olsher
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  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 — designed in the West and built in China — 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?

Sonic Impressions

If you have never heard a good KT88 based amplifier, then you are in for a treat. The midrange is definitely worth writing home about. Imagine diving head first into a tropical pond. The sensation of being enveloped in a warm, liquid, and inviting sonic cocoon was quite perceptible. Harmonic textures were consistently smooth but not to the point of fuzzing over ambient information and low-level detail. There was a time in the 1950s and 60s when you could confidently expect a tube amplifier to sound warm and inviting. The Seven, however, is not obsessive-compulsive about tubey euphonics. It strikes an admirable balance between detail and liquidity. In particular, the Seven re-kindled fond memories of the Harmon Kardon Citation II, though lacking the latter’s bass authority. It is fair to say that sonically this amplifier obeys the Goldie Locks principle, never excessive in its pursuit of tube sound.

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 Audio’s 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 KT88’s 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 presentation’s immediacy. The microdynamic ebb and flow of the music’s 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

The Three would have easily gotten thumbs up as a good value even with a price tag of $3K. But at $1,395 it represents an amazing value (an optional moving magnet module is available at $179). Note its weight. At 23.5 pounds this is one beefy line preamplifier. The weight is a result of an ambitious dual mono layout featuring independent power supplies. That’s right, there are two toroidal power transformers, choke filters, and two 5AR4 rectifiers, one per channel. Throw in premium caps, point-to-point wiring, an Alps Blue Velvet pot, DC tube heaters, and wideband no-feedback circuitry and you have a serious sonic contender at any price point.

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 Seven’s 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 singer’s 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 — ala the Dynaco Stereo 70. Aficionados of classic tube sound take note: you can now have your cake and eat it too!

Conclusion

Be sure to experiment with the various impedance taps, and I would not be in any rush to substitute alternatives for the stock Chinese "Genalex" KT88. Undeniably, the Three brings out the best in the Seven and you should give serious consideration to auditioning both components as a system. In my book, and in the context of budget tube amplification, the ProLogue Seven and Three add up to a perfect 10.

Addendum

Just the other day I happened to come across a small cache of GE four-star 5751 dual triodes, which have been collecting dust on a storage shelf. So what to do with them? A light bulb went off, as I recalled those Chinese 12AX7s in the Prologue Seven's front end. Yes, the 5751 is a lower gain version of the 12AX7 which would reduce the amplifier's sensitivity and drive voltage, but I thought it was worth a shot. A couple of minutes later, I settled down for a listen. In a nutshell, the results were very positive. Harmonic textures were smoother and the overall presentation more direct with enhanced focus and low-level detail. The moral of the story: rolling-in your favorite substitutes for the stock 12AX7s would be a good thing to try.

Specifications

ProLogue Seven

Type: Vacuum tube monoblock amplifier
Power Output: 70 wpc
Tube Compliment (per amp): two 12AX7, two 12AU7 and four  KT88
Frequency Response: 20Hz to 85kHz (+/- 0.5dB)
THD: Less than 1% at full power
S/N Ratio: 101dB
Input Sensitivity: 911 mV
Input Impedance: 100 kOhms
Power Consumption (per amp): 225 watts
Dimensions (per amp): 11 x 7.5 x 15.5 (WxHxD in inches)
Weight (per amp): 37.5 lbs.
Inputs: 1 pair RCA
Outputs: 2, 4, & 8 Ohm speaker taps 
Price: $2,695/pr.

ProLogue Three

Type: Vacuum tube preamplifier
Inputs: 4 pair RCA
Outputs: 2 pairs of pre-amp outputs + tape monitor out
Frequency Response: 4Hz to 85kHz (+/- 3dB)
Gain: 11.5dB 
THD: Less than 1% at full power
S/N Ratio: 109dB unweighted
Input Sensitivity: 200mV
Input Impedance: 100k Ohms 
Output Impedance: 3.5k Ohms 
Power Consumption: 48 watts
Dimensions: 11 x 7.5 x 15.5 (WxHxD in inches)
Weight: 23.5 lbs.
Price: $1,395

Company Information

Durob Audio BV
P.O. Box 109
5250 AC Vlijmen
The Netherlands

E-mail: durob@hifi-notes.com
Web: www.primaluna.nl

US Distributor
Upscale Audio
2504 Spring Terrace
Upland, CA. 91784

Voice: (909) 931-9686
Fax: (909) 985-6968

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