Durob Audio, the Dutch design and marketing team that has already brought us the well-known and hugely successful PrimaLuna brand, has expanded its product offering in 2007 with the Mystère line. Comprising two integrated tube amplifiers, the IA11 and its bigger brother the IA21, the Mystères aim squarely at blending style, performance, and quality, while maintaining value for the end user. Finished in high-gloss piano black, the IA21 represents one of the most elegant examples of industrial design to have come my way. The gently arched faceplate beautifully complements the curvature of the tube cage. The latter is a massive affair with long parallel slits that to my mind resembles a medieval knight’s armored helmet. However, my recommendation is to remove the tube cage during use to improve heat flow; otherwise, ventilation is constrained and too much heat is retained within the cage.
Remove the tube cage and feast your eyes on octal power: two pairs of 6SN7GT dual triodes—my favorite driver tube. Nine-pin miniatures need not apply! Introduced in 1945, the 6SN7 is not only robust and linear, but capable of producing a big tone highly prized by musicians. There is no input voltage gain stage per se; the input signal is directly routed to the first 6SN7, which is connected as a long-tailed-pair phase-splitter. The second 6SN7 is used as a driver stage. The intent is to keep the signal path as simple as possible. The push-pull output stage uses a pair of KT88 power beam tubes per channel. All tubes are tightly-selected new production from the Chinese Shuguang factory. In a break from the currently popular ultralinear connection, the IA21 uses a true pentode connection with the screen grids held at a positive high voltage by a dedicated power supply. Historically, there has been much discussion over the relative merits of a triode versus pentode output stage as far as residual harmonic distortion spectrum and bass damping-factor. An ultralinear connection allows for a sonic character intermediate to that of pure triode or pentode and also facilitates switching to triode mode on the fly. Unfortunately, that is not an option for a pentode connection. Due to the lack of an input voltage stage, overall voltage gain was a concern and limited the amount of global feedback that could be applied in this circuit. This combination of a pentode connection and low overall feedback results in a rather large output source impedance. I was quoted figures of approximately 13 ohms at the 8-ohm taps and about 6.5 ohms at the 4-ohm taps—about an order of magnitude greater values than those achievable by either triode or ultralinear designs with adequate levels of global feedback. This has serious practical consequences in terms of bass damping-factor and load interaction, which will be discussed in more detail later on.
As with the PrimaLuna power amplifiers, an adaptive auto-bias circuit is used for each channel to continuously monitor the output tubes and keep them at their optimal operating point. If desired, EL34 pentodes maybe substituted for the KT88. The IA21 is clever enough to provide an external EL34/KT88 switch that recalibrates the auto-bias board for the selected tube. The output tubes are run quite conservatively, which, together with a soft-start circuit that powers up the amplifier slowly, guarantees extended tube life.
The front panel is adorned with a volume control and an input selector that controls four unbalanced (RCA) line-level inputs. Rather than use an inexpensive potentiometer for volume control, the Mystère line relies on a 24-step resistor-ladder attenuator. This is well worth the added cost. No matter how brilliant the subsequent circuitry, cheap pots can significantly degrade the input signal and compromise the output’s purity and immediacy.
My first impression was with the Venture Excellence III Signature speakers hooked up to the 8-ohm taps. The sound was unabashedly tubey: image outlines on steroids, a beefed-up midrange, closed-in treble, harmonic textures creamy and smooth to the point of being “chocolate”-flavored, and a midbass lacking control to the point of sounding flatulent. Switching over to the 4-ohm taps improved bass control to a point, while at the same time altering much of the overall sonic character. Image outlines lost some of their palpability but were better focused spatially, while the midrange sounded a tad less sweet. Subsequent listening sessions with the Venture speakers revealed a wealth of low-level detail and layers of orchestration, as if I were observing the soundstage through a microscope. All the while the IA21 maintained a smooth effortless delivery free of brightness or grain. I noted a slight loss in transient speed while bass lines remained overly ripe, no doubt a result of the poor bass damping-factor. Kick drum, for example, sounded far too woolly. The tonal balance emphasized the midrange at the expense of the treble, which served to glorify violin timbre and helped flesh out an orchestral foundation with commendable weight and authority. Personally, I find this sort of back-of-the-hall balance quite pleasant—the sonic equivalent of taking a warm bath. Yet, despite its perceived faults at the frequency extremes the Mystère was exceptionally adept at grabbing hold of the music’s rhythmic drive and propelling the melody forward.
I outfitted the amplifier with a quartet of factory tested EL34s to assess its sound versus that of the stock KT88 configuration. The sound of the KT88 in this context was, to use a wine metaphor, Merlot-like—smooth and refined. While the EL34 sounded a bit bolder in harmonic colors, it was not as immediate or as well focused. My preference was for the sound of the KT88 in this system context.
With the Esoteric MG-20, the midbass was poorly controlled on both the 8 and 4-ohm taps. Although the best bass definition was to be had of the 4-ohm taps, I actually preferred the 8-ohm taps because the mids evinced a more sensuous tone. Both acoustic bass and Fender bass lacked adequate pitch definition in the midbass, and kick drum was deficient in crispness. The midrange was emphasized and at times squawked a bit. On the plus side, when coupled with the PrimaLuna Eight CD player, image outlines were exceptionally palpable within a wide and spacious soundstage. The music’s energy and vitality were very much in evidence. And it should be noted emphatically that the Mystère served up a healthy dose of tube magic without sacrificing clarity or detail resolution. Microdynamics were retrieved fully intact, lending conviction to the music’s drama. However, there was a hint of compression during loud passages.
I was very curious to measure the frequency response of the MG-20 driven by the Mystère. It is well known that a power amplifier’s source impedance acts as a voltage divider in concert with a speaker’s impedance curve to modify the speaker’s frequency response. The effect is to reduce the speaker’s output the most where its impedance is lowest. And, vice versa, the least effect is at frequencies where the impedance is maximum. In most cases, the power amplifier’s source impedance is much lower relative to the speaker’s impedance making for minimal interaction. In the case of the Mystère, the source impedance is either about 6.5 or 13.5 ohms, quite significant relative to most speakers’ nominal impedance. Of course, with a hypothetical speaker with perfectly flat impedance curve, the speaker’s frequency response would not be affected. However, in the real world, a typical impedance curve is far from flat. In addition to peaks in the bass range (e.g., the twin peaks of a bass-reflex loading), there are often peaks in the midrange created by crossover networks. And it’s these peaks that appear to map over to the speaker’s frequency response. Measured at both the 4 and 8-ohm taps, the MG-20 developed a +3dB response peak centered around 1.2kHz, corresponding to an impedance curve peak at this frequency. The treble range was rolled off, most severely via the 8-ohm taps. There was also a 2-to-3dB reduction in upper-bass output followed by an emphasis at 80Hz, again corresponding to an impedance curve peak.
It should be obvious by now that the Mystère IA21 will likely sound different with every loudspeaker with which it is used. Its ultimate tonal balance and bass control are a bit of a mystery, but that’s no reason, however, to avoid giving it a listen, as the ensuing frequency-response differences may actually complement your speaker. The Mystère’s payoff is image palpability to die for, as well as verve and pace galore! At its best, outfitted with KT88s, the Mystère sings sweetly and satisfies the soul. I have no doubt that its unique blend of 6SN7 big tone and KT88 refinement is responsible for its smooth, fluid, and warm delivery.