The ProLogue Six is part of an expanding range of valve amplifiers based on a common circuit architecture and external design. It's available in two versions, with either KT88 or EL34 output valves. The KT88-equipped model (not tested here) commands a $1500 premium.

Although the brand was only recently established (in 2003), the three individuals behind PrimaLuna have been involved in high-end audio for a long time in retail, service and design. The company is of Dutch origin but Chinese manufacture helps keep prices in check, and value for money is written all over the PrimaLuna range in big letters.

If any valve amplifier was designed for the first-time valve user, this is it. The design is fundamentally simple (although there are some novel tweaks), with a phase splitter and driver stage. Rated at 70 watts, each monoblock includes a complement of two 12AX7 dual triodes and two 12AU7s in addition to the four EL34s. The output valves work in an enhanced Ultralinear Class AB mode, which is presented as offering performance and power output somewhere between conventional pentode mode and pure Class A. There is no black magic here – just choose the appropriate output tap (two, four or eight ohms), connect your input to the single phono socket and you're ready to rock.

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In common with other PrimaLuna designs, the ProLogue Six goes all out for practicality, particularly with Adaptive AutoBias, which constantly adjusts bias levels in the output stage according to circuit conditions. This is the only major part of the circuit that is produced on a board, rather than hard-wired. Adaptive AutoBias eliminates the need for manual biasing of the output valves, and is claimed to reduce distortion, increase valve life, and eliminate the need for matched valves in the output stage. It is also said to eliminate the need for conventional negative feedback, but if this means that the amplifier has a higher than normal output impedance, the amplifier will be load sensitive. This is because its output impedance will interact with the reactive component of the loudspeaker impedance, with the possibility of unpredictable sonic balances when switching between speakers. Some of the listening tests were designed to assess whether such effects would be audible.   

It would be remiss not to acknowledge build quality, which at the price is exceptional, both in terms of finish, general solidity and even such thoughtful details as the cleverly decoupled valve cage cover. Internal highlights include a heavy power supply and hardwired construction. The only user control is the power switch, which is awkwardly positioned on the left hand side of the unit. The socketry all looks like WBT-sourced components, though they may be pattern parts.

SOUND QUALITY

Different speaker designs expose changes in system voicing, due to the high output impedance. During our test, for example, Mordaunt-Short's Performance Six sounded light in balance through the midband and upper bass, while Monitor Audio's GS10 was significantly sharper and more substantial.

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But there are common factors, too. The amp has a smooth, easy-on-the-ear style, with a warmish and slightly loose balance – a quintessentially ‘valve' type of sound. In every system combination tested, there was a lack of articulation and separation. There is no real shortage of detail, but the overall effect was as though listening to the music through a very faint fog. Nor is there a global lack of dynamics, despite some smudging of the subtle differences that separate out members of an instrumental group. In its favour, the amp sounds fluid and easy going, and is electrically quiet, with no obvious indications of microphony.

But ultimately there is not enough excitement, either. It's a clean and presentable sounding amplifier, and an engaging one in its own way. Power output is impressive, though the amplifier can become shrill in the midrange and treble when played at too high a volume (not a traditional valve characteristic). What it doesn't do musically, however, is to explore the depths and complexities of really fine recordings, represented for much of this test with large-scale orchestral and small-scale female vocal material. Ultimately it doesn't really make music sing.   

This is a highly interesting design, which has many features to its credit, especially when it comes to price and build. It also has certain qualities that will not endear it to some.

PrimaLuna ProLogue Six ¦ $4900 per pair ¦¦ www.referenceav.com.a u
For: Fine build quality.
Against: Sound lacks separation and articulation of the best in class.
Conclusion: Great material value for a valve monoblock.
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