Thrax Audio Libra 300B differential balanced tube preamplifier

There was a lot of interest about the all new Thrax Audio (50k EUR) flagship Libra 300B preamplifier, so here is the official blurb about this statement differential balanced 300B tube preamplifier for the 21st century...

Libra uses two 300B tubes per channel in true dual mono differential pure class A triode operation to form a gain stage so unique that presents a tonal spectrum that is dense and clear revealing extra layers of information unexpected in the recordings. Libra paired with the Thrax Spartacus 300 allows users for the first time to experience a full 300b signal amplification path and be shocked by it. 

Libra features two independent outputs that can be offset to each other to allow for bi-amplification with different amplifiers for mid/hi and bass. For example an Single ended triode amplifier for mid/hi and a powerful push pull amplifier for the Low frequencies. The offset lets you adjust for the different gain of the amplifiers and keeps in sync the Volume on both. 

Another unique feature of Libra is the fully balanced Tape loop allowing the use of professional tape recorders like Studer, Ampex and many others where you can take full advantage of the quality of your master tapes. 

A home cinema bypass is also available for those wishing to integrate the stereo system in the media room. Remote power amplifier on/off is provided for installations where the power amplifiers are not easily accessible.

For us the task was to create a Preamplifier worthy of driving the Spartacus 300 power amplifiers known by their ability to preserve the tone and texture in the signal at all levels, not constraining the dynamic expression or imposing any coloration to it. Libra had to be much better than the 300b input stage of the Spartacus 300 in order not to affect it’s sound. But we already use one of the most liner tubes there, so the choice was an inductively loaded heavy class A differential stage canceling the Libra’s 300b negligible 2nd harmonic leaving the spectrum absolutely clean. 

DHT tubes for optimal operation require a lot of auxiliary electronics, so in this case we have split the unit in two parts. A power supply and the active unit. This keeps all mains AC and rectifier noise in a separate box while final regulation and filtering is still done next to each element in the amplification circuit. All is microprocessor controlled with a large color LCD sowing you the status of the unit and being visible from your seat. 

Libra should not be talked about but listened to..


  • 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


  • Post amplification volume control for lowest noise
  • Modular construction 
  • External dual mono power supp-ly with Choke input rectifier for lowest switching noise 
  • Programmable Balanced Tape loop
  • Stepped volume control for perfect repeatability of level setting
  • Interlocking solid aluminum chassis 
  • Inputs on RCA and XLR connectors with adjustable sensitivity 
  • Highest quality long life 300B tubes
  • Single gain stage
  • Dual balanced output for bi-amp with offset adjustment 
  • Home Theatre bypass feature for integration in Home Cinema systems
  • Microprocessor controlled opera-tion for safety
  • Special filament heating circuit for harmonic richness and tube protection
  • Current sourced regulator for stable power allowing explosive dynamics and tonal purity
  • DC coupled input and output for ultimate transparency
  • Balanced differential topology for noise immunity subtle dynamics
  • Auto bias for compensation of tube aging and correct operation of any 300B