Stand Loudspeaker Reviews

Based in Bulgaria, European audio company Thrax has been active since 2009. Their ingenious and varied design approaches seen over several product lines have continued to intrigue me with their conceptual originality, innate musicality, and imaginative use of a broad spectrum of technologies. Their products range from valve (tube) amplification to digital audio and, more recently, loudspeakers. There are 16 products in the Thrax range.

Thrax founder and chief engineer Rumen Artarski packs a capacious scientific kitbag. His love of music and fine sound reproduction is self-evident. In the disputes that sometimes occur between measuring and listening, Rumen says, decisions made by the ear are final.

Writing for HiFiCritic in 2012, I supported Chris Bryant's evaluation of the all-tube Thrax Spartacus, a 40Wpc triode-based power amp. In 2014, Bryant examined the all-triode Orpheus phono preamp (footnote 1). You could say that we—Bryant and I—were both pretty much solid state diehards, even though we'd both enjoyed good experiences with tube designs over the years. In both the 2012 and 2014 reviews, we described how we were shaken up by these two thermionic designs, whose purist tube technology had been finessed using advanced electronic design principles to deliver a winning combination of tonal accuracy and transparency with the dynamics and musicality of low-feedback triode circuitry. We knew then that Artarski's Thrax was a force to be reckoned with. The range of distinctive high-end electronics has continued to expand to include a loudspeaker, the standmount Lyra, now joined by the smaller Siren ($13,600/pair), also a standmount and the subject of this Stereophile review.

Technology and engineering
Weighing a substantial 44lb (20kg), this standmount two-way marries an exotic, costly 6.5" (170mm) midwoofer to an unusual, semi-elliptical (biradial) horn of significant size. The horn "compression loads" a small, application-specific high-frequency driver, a 1" (25mm) ring-diaphragm design from pro audio specialist BMS.

Under the leather-covered central section, the construction is birch plywood reinforced by a cross brace. The walls of the 16 liter enclosure are not perfectly damped, but a knuckle rap is rapidly dissipated. Those substantial front and rear sections are milled from solid billet aluminum alloy. Fourteen stainless steel socket head bolts attach the rear panel. A filling of natural sheep's wool damps internal reflections. Artarski notes that the box/port system is tuned at 40Hz, with typical in-room extension to 33Hz at normal listening levels. The port's rear panel exit is mildly flared to reduce port noise at high air velocities.

Thrax prefers series topology for its crossovers, which is a little more awkward to implement than the more commonly used parallel form. In some designs, a series crossover may offer better acoustical blending of the driver outputs; simulation software has made the design task easier. As ever, crossover filter design is an interactive process involving theory and measurement, and must be partnered by meticulous listening.

Jantzen, a noted Danish manufacturer of audiophile-grade parts, was chosen to supply the costly Siren crossover components including three "zero saturation" wax-impregnated copper-foil, air-core inductors (no magnetic core to saturate!), plus several exotic film-and-foil capacitors (footnote 2) in each channel. The capacitors in the tweeter feed are the "Silver" type and enjoy pure silver lead-out wires. The whole crossover assembly is "sealed and potted" to damp vibrational disturbances from working drivers.

The horn is very efficient and imparts an unusually high intrinsic input impedance, which could otherwise add a trace of high-frequency excess with some tube amplifiers. So, an RC impedance-matching network placed at the input terminals helps maintain tonal balance when the Siren is used with tube amplifiers with high output impedance.

The Siren is single-wired with Aeco gold-plated copper alloy binding posts with 4mm plug, spade, and bare-wire connectivity. The DC input resistance measured precisely 4 ohms, a fair estimate of the "nominal" amplifier loading.

The deep-profiled custom flare of the BMS tweeter's horn was designed by Spherovox using electroacoustic modeling software. It blends seamlessly into the deeply milled solid alloy plate of the front baffle. The biradial transition is smoothly rendered and affords wider-than-usual horizontal directivity at higher frequencies. The driver element itself will never need to work hard, in view of its sensitivity: an exceptionally high 112dB/2.83V/m. That's some 25dB greater than the sensitivity of the completed Siren system (specified as 87dB/2.83V/m). Worked so lightly, it should remain free from compression overload at all sensible sound powers. It should also display low distortion and very good subjective dynamics and be free of dynamic sensitivity variations; some lower-sensitivity direct radiators, which draw more power, can compress due to thermal cycling during loud passages.

Partnering that custom horn is a 170mm, 6.5" midwoofer sourced from European "new-tech" company Purifi, affiliated with Bruno Putzeys of class-D amplifier and high-oversampling DAC fame. Purifi was founded in Denmark in 2019 by Putzeys and Lars Risbo with long-established audio entrepreneur Peter Lyngdorf. They spent several years on primary research. This Purifi driver has an unusual and highly distinctive cone design, with a circumferential brace and central reinforcement, while the assembly is supported on a distinctive, aggressive-looking, sectionally corrugated surround (see Sidebar 1).

This long-throw design incorporates an ultralinear moving coil motor system with deep excursion. The cone is fitted with a huge, sophisticated motor system built on a free-breathing die-cast alloy chassis. Purifi claims exceptionally low distortion, outstanding power handling, and a very low incidence of the usual menu of coloration-inducing mechanical misbehaviors. The driver's linearity and distortion are claimed to measure more like a good amplifier than a deep-excursion, spring force–limited, oscillating mechanical piston. It is easy to imagine the Purifi team planning their attack on the primary moving coil driver issues, then relentlessly advancing on less well-known problems, the process continuing to a practical vanishing point. Such complexity does not come cheap, as the Purifi driver costs perhaps four times as much as typically favored contenders.

The Purifi engineers have developed comprehensive mathematical models to simulate the key components of a drive-unit: the motor, inner suspension, cone, and surround suspension. Close examination of the motor section led to new insights about coil and magnet geometry, the selection of ferromagnetic materials, and their detailed configuration. Such complex mechanico-acoustic models allowed the engineers to analyze and control issues that previously were inexorably intertwined, facilitating the optimization of each source of error.

Purifi addressed distortion of all kinds arising from normal cone movement, in combination with excursion limiting so that the driver cannot easily be overloaded in the bass. The surround/suspension geometry is configured to radiate as little as possible, minimizing its unwanted (distorted) contribution. More subtle errors such as hysteresis distortion have been suppressed. There is no magic bullet but rather a systematic grinding down of well-known problems to a lowered baseline.

Typical moving coil drive units suffer 0.25% to 0.5% distortion at average sound levels, with better examples aiming for 0.15%; Purifi is aiming for a massive, 10–20dB reduction to just 0.05%, including intermodulation products. Over the important midband, the potentially whiney-sounding third harmonic reads just –76dB—0.015%, which is a remarkable achievement.

But how will this all fit together? Overall sound quality should be improved right away. The most obvious, immediate benefit would be an order of magnitude gain in clean bass power for drivers of this size.

Siren's song
After a few days' warm-up following a deep chill during their cross-Europe transit, and following trials with stands and placement, the Sirens were put to the test. At first hearing they proved to be well-balanced frequency-wise and not too critical of position. As expected, the best stereo imaging was with the speakers placed in a free-space location. Positioned about 6' apart, 4' to each sidewall, 4' 6" from the front wall, and 4' from my seat, they were toed in just enough that I could see down the inner sides of the enclosures. While optimal, this set up was not exacting: The fine imaging and overall quality proved relatively uncritical of their precise location.

At first, nothing in particular stood out; no special character drew my attention.

This was, in a sense, very promising, suggesting that the primary sound characteristics were in balance. Gradually I became aware of something special about them, firstly in the high frequency range. What I heard here did not sound like a tweeter of the usual kind; indeed, it was not aurally identifiable as a working mechanism but rather an open window into recordings. It's hard to explain. It sounded as if the tweeter was not present as a distinguishable entity but rather a blended extension of the fine midrange.

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Footnote 1: HiFiCritic: Spartacus: Oct–December 2014; Orpheus Jan–Mar 2015 Vol.6 No.4.

Footnote 2: Not the usual metalized plastic film.