Thrax Libra 300B - The ultimate preamplifier!?

One of the more interesting announcements at the end of the last year 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 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. 

So, yes. Count me highly interested and enthusiastic about this out of the box tube preamplifier. 

As it turns out, there is one option Thrax Libra that will be reviewed in the not too distant future. 

Stay tuned!

Matej Isak