Thrax Audio Libra 300B Preamplifier Review

One of the more interesting announcements at the end of the 2019 was for me the all-new (50.000 EUR) Thrax Libra differential balanced 300b tube preamplifier. I do have a soft spot for anything 300B related and have owned over the years more than 30 different 300B based amplifiers, preamplifiers, etc. At one point I've just stopped with this particular enthusiasm as the financially it got out of the hands. 

Not mainly because of the prices of the hardware. The real cost was the 300B tubes themselves. At the time even the Western Electric tubes could be bought at "reasonable" prices compared to the current madness of hefty price tags. 

A lot was said about the 300B based amplifiers and the tubes. A little was written about the instability of the circuits and the costly consequences. This was the main reason for yours truly to stop with my passionate quest to reach the 300B heaven. 

When the Thrax Audio introduced first the all-new Spartacus 300B power amplifier my inner sensor has lit instantly. Imagine what the blurb did to me when I've read about six 300B tubes in two stages delivering deliver 50W of pure class-A triode sound! 

Then, Rumen Artarski revealed the Libra, a pure 300B tube preamplifier at point-blank.

A full 300B signal path directly heated triode preamplifier! This Thrax mind-boggling preamp is a take-no-prisoners approach to line-level preamplifiers (both tube and solid-state), based on the legendary Western Electric 300B tube.

Libra 300B implements two 300B tubes per channel in true dual-mono differential pure class-A triode operation. 

This forms a gain stage that is capable of revealing all of the Thrax Audio Spartacus 300 virtues with the continuation of the 300B fully designated path. 

Libra 300B!

Libra is not special and unique only because of the unique principle it comes also with many features, that are especially interesting for me as the reviewer and for anyone that hosts a multiple of high-end audio products in the system setup. 

Let us look at a few of the highlights...
  • Custom current production tubes for long life
  • Balanced differential topology for noise immunity and subtle dynamics
  • Auto bias for compensation of tube aging and correct operation of any 300B
  • Current sourced regulator for stable power allowing explosive dynamics and tonal purity
  • DC-coupled input and output for ultimate transparency
  • Microprocessor controlled operation for safety
  • Special filament heating circuit for harmonic richness and tube protection
  • Separate dual-mono power supply
Libra features two independent outputs that can be gain- offset from each other to allow for bi-amplification with different amplifiers for mid/hi and bass. 

A single-ended triode amplifier can be used for mids/highs and a powerful push-pull amplifier for lower frequencies. The offset feature lets you adjust the preamp for amplifiers with different gain and keeps their volume level in sync.

Thrax Libra also overcomes another limit with the tape loop feature. The fully balanced tape loop allows the use of professional tape recorders like Nagra, Studer, Ampex, and many to take full advantage of the quality of the master tapes.

Many owners of the upper-echelon high-end audio systems also have a dedicated home cinema setup. Libra comes with the bypass that allows the integration of the stereo system to the media room. The amplifiers can be powered on and off remotely for installations where they’re not easily accessible.

Like with all of the Thrax products, Libra was designed to convey the signal without degradation and saturation. Artarski has always designed his proud creations with a clear goal of non-constraining sonic transmission in absence of dynamics congestion. 

Libra seems to follow-up on the already established Thrax modus operandi and perhaps even more importantly it follows the Spartacus 300B logic.  

As explained by Artarski, Libra's inductively loaded heavy class A differential stage is canceling the 300b’s negligible 2nd harmonic leaving the spectrum clean.

DHT tubes demand a lot of associated electronics paraphernalia work properly. This is the very reason why Libra is split into two chassis. 

One box is reserved for a power supply and the other serves the duties of the active unit. This contains all mains AC and rectifier noise in a separate box while final regulation and filtering are still kept adjacent to each element in the amplification circuit. 

Everything is controlled with a complex microprocessor and an oversized color LCD was picked to show the unit’s status clearly from the seating position. 

I am sure that the Thrax Audio Libra, like mine, will capture the imagination of many of you. What makes it particularly interesting for me is that this particular 300B preamplifier is made by Thrax Audio... 

I have tried and tested a number of Thrax products over the last seven (plus) years, and I have used Dionysos and still use Orpheus MK2 (before MK1) as my reference phono preamplifier day in day out for years. I haven't had a single problem with the tubes or electronics, and that speaks for itself in terms of circuit stability and the design tested. 


The Libra 300B was designed from the ground up to be a 21st century, expressive, balanced 300B tube preamp, and that's how it feels. 

The beautiful OLED screen features a nice big volume marker and active input. 

The Libra 300B is turned on and off by pressing and holding the left control knob for 10 seconds. A shorter press and hold of the left control knob opens the menu.

Diving into the menu can be done via the remote or directly from the controls on the front of the Libra. Everything is easy to use and adjust. 

There are many options to play around with, but perhaps most important is the adjustable gain for all inputs and the offset sensitivity of two XLR outputs. 

This is a great help in finding the right system gain for each component, and the XLR output offsets are ideal for bi-amping. 

Inputs are selected by turning the left knob and the right knob acts as a volume control/mute button. The volume is increased or decreased by 1 level (approx. 2 dB) per button press. If you keep the knob pressed, the volume is continuously adjusted and displayed from 0-100. 

  • 3x (pair) XLR - for standard balanced connections to sources, indicated 1-3 on the back panel
  • 1x (pair) XLR - for devices making use of the Tape Loop function, indicated as TAPE IN on the back panel
  • 2x (pair) RCA - for standard unbalanced connections to sources

  • 2x (pair) XLR - for standard balanced connection to power amplifiers, indicated 1 and 2 on the back panel
  • 1x (pair) XLR - for devices making use of the Tape Loop function, indicated as TAPE OUT on the back panel

When enabled, the Libra 300B HT mode sets a separate gain setting for a selected input. The HT mode for each input is stored in non-volatile memory so that this programming is retained in the preamplifier even after a power failure. 

Another interesting feature not commonly found in high-end preamps these days is tape loop, which connects a tape deck or digital processor to the tape inputs and outputs. It can be activated in the settings, where either TAPE for analog and PROCESSOR for digital can be set.

The menu also allows you to set the date and time, the brightness of the display, and to set the interval when the unit is inactive before the display is dimmed (0-60s). 

Another nice feature is the automatic power-off function. Libra can be set to turn off after 30 to 180 minutes. 

Last but not least, there is a tube timer that shows how long your tubes have been in use.

Thrax "burn-in" each Libra 300 preamplifier at the factory for 72 hours. The Libra stabilizes after about 150 hours of music playback and warms up after 15 minutes.

The Music

Thrax declares, "Libra should not be talked about but listened to."

Exactly. Yes, Libra packs impressive technology from the early twenty century (300B) packed into two boxes with current technology from the 21st century. But both old and modern technology and specs mean little without the real field test: the music. 

As expected and beyond, the Thrax Audio Libra 300B preamp showed its strengths and virtues straightforwardly and boldly from the first music intake. 

Two albums by Pat Metheny, which I listen to more or less regularly and which I know and appreciate to a greater extend, show very quickly and immediately what has changed with the use of the new component, and in what direction. 

The Way Up is perhaps one of the most underrated PMG albums. It is a modern symphony "orchestrated" by the two music mavens Myles and Metheny and there are so many layers hidden beneath The Way Up's three lengthy tracks.

With Thrax Libra 300B, I was stunned by the unique holographic presentation of The Way Up. Many aspects held my full attention and captivated my senses with new qualities upon prolonged listening. 

From "Part One" to "Part Three," Libra 300B managed to project a feathery lightness of detail that extended firmly into the room, always accompanied by the unrestrained power of sustain.

The other PMG treasure island album is the unmistakable eponymous white-covered first album of the Pat Metheny Group, released in 1978 by ECM.

Another gem I never tire of hearing, and a fascinating sonic challenge. With most preamplifiers, this album will require a preamp volume turned up all the way. And it still might not be sufficient to get a full blast. It was recorded and mastered differently than most albums, which allows for a very different dynamic extension but also places an unusual demand on the preamp. 

It was riveting to observe and record the different qualities of Libra in my listening notes with songs like "Jaco," "San Lorenzo," and perhaps the most memorable and PMG signature "Phase Dance."

A long story short; Libra 300 B's highly potent, never innocuous inner heart allowed the familiar track to be a fully soluble and nor sonic,  nor musical enigmas. 

Further on. The bass that was not puffed and disoriented, was certainly something I had come to expect to some degree based on my previous encounters with Thrax Audio. Not only did Libra preamp confine remarkable concentrating bass. Also, the spaciousness between the low notes has proven to be remarkably distinct. 

But the controlled bass is not just a love affair reserved for the deep electronic notes and the thundering tones of timpani, etc. It is a fundamental quality that evokes a certain clarity that is especially evident in the double bass. 

Alone Together Jim Hall - Ron Carter Duo, a telephonic union of two talents that takes Jim Hall (for many the godfather of modern jazz guitar) together with Ron Carter into a somewhat different and unexpected journey. 

Hall's legato lines and tone were a new discovery, but as for the bass, Carter's bass notes and his playing offered a newfound sense of the primary formations of the lower notes with a mesmerizingly involvement. 

"Autumn Leaves," which begins with a two-note motif and develops into a complete song, was rendered without blurs or smudges and free of spatial contractions that can destroy not only the sense of being on the spot but also the atmosphere, which is subordinate above all to the density of the acoustic nodes.

Transparency and clarity can all too easily be compromised by sonic patina, and classical music is always a perfect medium to delve deeper into this matter. 

Even with top-notch preamps, the music can sound like it's coming through the ebb. Nevertheless, Leopold Stokowski - Rhapsodies Smetana: Vltava; Liszt: Hungarian Rhapsody No. 2; Romanian Rhapsody No. 1 with Thrax Libra 300B in play enabled this timeless recording expertly recorded by Bob Simpson on a 3-track Ampex tape recorder to fully expand.

Anything that holds up the music detracts from the sheer drama and emotional engagement that takes place here.

I am always taken by the sublime rendition of "Tristan und Isolde, Act III: Prelude." This romantic masterpiece with an enticing Tristan chord slowly builds the tempo in non-tertian harmony.

I was shocked at how incessantly the Thrax Audio Libra 300B preamp brought "Tristan and Isolde, Act III: Prelude" to fruition. 

Almost poignant, unfinished cadenzas, deceptive resolutions with "fallible" chords that often have nothing to do with the main key are Wagner's real leitmotif, and all of this requires a preamp that can overcome many obstacles in the sonic quest. 

Thrax Libra 300B not only fused elements of the orchestra with ease, but portrayed soft, palpable, vibrant, and hypnotic strings that extend deep into the sonic space, with a hologram-like projection and an underlying, almost eerie narrative. 

The sheer, slowly unfolding drama was constructed on a much larger scale than I am used to. Libra 300B framed the epicenter of the orchestral energy with a unique, wondrous quality, creating a compelling connection that can recall almost poignant melancholic emotions of longing and unfulfillment. 

The Wagner intrigue is far deeper than is readily apparent even on repeated listenings, including deceptive resolutions. This is perhaps more true of the mystique that's also encrypted and starts with the first two chords of the better known, one of the most popular standards, "Stella by Starlight."

Another song covered by Joe Pass that Libra 300B portrayed so well. Pass's phrasing was grandly performed, with no artificial gap between notes. 

Although a guitar solo, "Stella by Starlight" can sound too quickly like a sentence full of expletives. 

Stacking notes, layering notes, and properly spreading dynamics may sound like a basic task. Like the human voice and the piano, guitar reproduction is far from it. 

Here, the Thrax Libra 300B preamp has once again shown how far from flustered duplication it is and that it can present music with a rare and unique, kind of cosmic clarity, and never even come close to sounding like a sonic quack nostrum.

There is a certain magic in the way the designers use the even harmonics and how they mask the odd higher-order harmonics. That can be the clincher, and that leads me to Julie Steinberg & David Abel from the album Beethoven: Violin Sonata, Op. 96 - Enescu: Violin Sonata, Op. 25 (Wilson Audiophile Recordings). 

A soulfully playful Violin Sonata No. 3 in A minor, Op. 25, "Dans le caractere populaire roumain": I. Moderato malinconico" unmistakably Balkan melodramatic melos cuts deep into the soul, through the sensual sentinels. 

This is the record and the song that is about emotional bonds that either take place or not. 

The witty interplay between piano and violin creates rapid mood changes and continuous tempos that require aptitude in the preamp that goes beyond a quick bounce. 

This is where the analogy with analog recording comes in, mirroring the sound engineer from the past who follows the musical score with the movements of the fader with the score in front of him. It's also one of the mysteries of why these iconic classical masterpieces sound so real and compelling. 

Back then, no limiters or compressors were used. And unlike modern classical productions, which are often cut and pasted from different takes, these historic gems were usually recorded in one take to capture the essential beauty of the music and the skill of the musicians. And that's exactly how the Thrax Libra 300B made Julie Steinberg & David Abel sound. Free of dynamic constraints and without false emotional attachment mockingly ignited at the last minute. 

It seems that cubic and quadratic nonlinearities are well in hand with the Thrax Audio Libra 300B preamp. When the 2nd and 3rd harmonics are properly processed, they positively disguise the higher orders, escaping analytical or bog standard acoustic presentation. 

Since the Thrax Libra 300B is a differential balanced preamplifier, some of the even orders are certainly canceled. Perhaps this (among many other aspects) is the key to the Libra 300B's non-tube sound, giving it some of the solid-state qualities not normally present in tube preamps. 

Although solid-state preamps have their substantial differentiae, their topology offers a different kind of sonic journey by default. Even with all the highly praised traits of solid-state amplifiers, and they can be remarkable, highly pronounced higher order distortions that do not hide the lower order distortions can cause that familiar and unappreciated fatigue.

Most properly tuned tube preamps just sound right and true to the sound even without spending a fortune. 

That said. In ultra-high-end audio and at the price tags that allow entry into the exclusive club, it's simply not enough to cover the bases. Nor is it enough to simply go beyond the expected. 

The missing key in many modern, high-end luxuriously priced preamps is actual performance.

Our hearing and the subsequent decoding center in the brain are very sensitive to phase and timing, both of which are critical to a credible reproduction of the sonic illusion. 

Many exotic, or even highly regarded of-the-shelf volume controls can create a delay effect due to their non-linearity, which does not just project the sound. This creates a delay phenomenon that was (and still is) well used for studio effects boards. In high-end audio, the usual aspiration is for perfect reproduction, however, the music was recorded. And over the years of evaluation, such inconsistencies in gain handling, no matter the price tag, were not a rare occurrence. 
We do not react immediately and promptly only to the golden trinity of timbre, tone, and color. We react subtly or disconcertingly to almost everything alien to us. 

The absence of these unwanted anomalies leads to, among other things, no fatigue, and the Thrax Audio Libra 300B proved once again that it is one of the very top conqueror of aural mastery with The Poll Winners. 

The eclectic playing of Barney Kessel (guitar), Ray Brown (bass), and Shelly Manne (drums) simply must be experienced to be truly appreciated.

"Satin Doll" showed that the Libra 300B has all the audiophile attributes. But also that it does not stop there. The Thrax Audio Libra preamplifier is simply more than just a remarkable audiophile device that transcends both solid-state and tube designations. It punctures directly and deeply into the music, presenting it with unsurpassed authority.

What follows is too often-enriched glucosing: "I am rediscovering music in a whole new way." I am the last person to want to use this cliché, as it has been so obtusely overused over the years. With that exception, I am willing to pull the joker card this time and claim it for the Thrax Libra 300B preamp. The Poll Winners were truly a different kind of sonic affair and showed that the Thrax Libra 300B is not just a preamp that commands attention. 

There is a big difference between a natural sounding component and a pleasant or melodious sound, and the Libra 300B preamp has an uncommon, defining focus that allows the music to be presented with tremendous energy and vibrancy. 

The Thrax Libra 300B preamp has never heralded music with a faux articulation. Rather, it presents it with all the inherited laurels and spell-binding crux that contains more than enough enchantment brew to build an uninhibited connection. 

This cannot be overstated, as it is a rare thing and something that every high-end preamp and manufacturer strives for (or should strive for). 

The Libra 300B has also proven in many well-known albums and songs that it is not bound to a particular concept (which happens too often) of genre, recording, or mastering, but defies and surpasses audiophile nomenclature from track to track, album to album.


Some high-end and ultra-high-end components excite the listener from the first encounter. Not many hold the same effect as a forte quality after the initial excitement is gone. 

This is the familiar phenomenon of bitter aftertaste after a brief listen at high-end audio shows or in showrooms, and then waking up to reality with undiminished returns after the purchased product is not acting as expected. As BB King would put it; The thrill is gone. 

As a reviewer, it is important to observe this phenomenon and respond to it objectively. It's something I always pay attention to, and that's why I always tend to spend a longer time with the component being evaluated.

The Thrax Audio Libra 300B preamplifier belongs to a rare breed of high-end audio tube preamplifiers that escapes the fading effect of instant thrill. It also does not fit into any particular audiophile pigeonhole. 

It sounds neither like a typical tube preamp nor like a solid-state preamp. It combines the qualities of both sides and adds a whole set of its own virtues. 

I hate to repeat myself when it comes to this topic. However. The importance of proper system gain is still too often neglected and is one of the main reasons why high-end audio setups fail to perform as expected.

And the role of the preamp is not just to switch inputs and act as a fancy, expensive volume/balance control. Its first and most important job is to act as a pass-through medium between different front ends and amplifiers. 

There are many analogies used for the role of the preamplifier, but perhaps the most practical is the one that explains the conversion of the energy pulse through the gear shift in the car. The more refined, the less taken away, the more effective, useful, and enjoyable the experience. 

This type of gain conversion or transfer is usually reserved for transformer volume control (TVC) passive preamplifiers. But!!!

It is also the key advantage and strength of the Thrax Libra preamp. It has the speed, foundation, and transparency of a TVC-based preamplifier, but without the gain-loss or mechanical hysteresis often associated with transformer winding. 

Such transparency, coupled with fully controlled low tones that extend to the submarine depths, an enchanting tone, timbre, and color, a refreshingly unexcited three-dimensional extension with unrestrained horizontal and vertical expansion, followed by unsurpassed dynamics and speed - all this is in this two-box preamplifier and clearly defies any ordinary high-end audio labeling. 

The Thrax Libra 300B preamplifier exceeds all expectations and sets a benchmark and remarkable foundation for a true ultra-high-end preamplifier of the 21st century. Century. 

Thrax Libra is anything but a high-end product, slowly evolving over several versions. As is so often the case with software, the consumer acts as a beta tester, polishing the turd until it becomes something of a gem or not... 

On the contrary. The Thrax Audio Libra preamp comes with a mature preamp that starts out as a giant, not one that rests on the shoulders of giants or performs as David against Goliath. It is a genuine giant to start with.

The foundation of Thrax Libra starts with its high flexibility, input gain adjustment, etc. But even more important is the pristine sound in the complete absence of any kind of saturation or sound coloration. The Thrax Audio Libra preamp delivers music with a rare reverence and without artificial glare. 

Yes, all the familiar 300B qualities that any 300B aficionado will recognize and the same attributes that have always excited me are unmistakably present, but that's about it. Thrax Audio Libra is a state-of-the-art, fully balanced 300B preamplifier for the 21st century that simply demystifies the 300B tube implementation at its core with its unique music unfolding/deconvolution and gain potency. 

From the very beginning, the Thrax Audio has already escaped the familiar trap of allowing a single model to define the brand, with a constant output of genre-defining products, but the Libra 300B preamp certainly pushes the bar on the brand's efforts and uniqueness and constitute this one-of-the-kind tube preamplifier as Artarski most stand out products so far.

It is impossible to return to the familiar modus operandi with Libra's impact and lasting, higher-order impressions. I am not talking about the effect that lingers and is tied to a specific attribute that usually condemns itself as a kind of fatigue in the long run. 

I am talking about the unpretentious sound quality that defies a theoretical sonic journeying. The Thrax Audio Libra 300B preamp simply sculpts the music, without false enhancement or chiseling. It is thoroughly true to the nature of the music. 

For what it represents in terms of music, value, and performance, the Thrax Audio Libra 300B is the very first 2022 winner of the seldom Mono and Stereo Editors' Choice Award. 

With its overall flexibility, remarkable sound, and sensational performance, Libra has proven to be a game-changing preamp in a variety of system configurations. Regardless of the front end, Libra helps decipher what's hiding in the black grooves or data encoded with zeros and ones. 

Matej Isak


  • 50.000 eur 

Technical specifications

  • Weight: Main 26kg, power 34kg  
  • Dimensions 430x450x150mm 
  • Power   80W 115/230V 50/60Hz 
  • Max. input level 5V Rms 
  • Max Output level 10V Rms 
  • Gain +12db 
  • Input impedance 90k Ohm  
  • Output impedance 80 Ohm 
  • Signal to Noise 90db 
  • Frequency responce 20-20,000Hz +/- 1db 


Thrax Audio
251 Okolovrasten Pat
1766 Sofia

Tel: +359 2 988 9555