Libra

DIFFERENTIAL BALANCED 300B TUBE PREAMPLIFIER

Statement differential balanced 300B tube preamplifier of the 21st century.

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 settings
Interlocking solid aluminum chassis

RCA and XLR inputs with adjustable sensitivity controls
Highest quality long life 300B tubes
Single gain stage
Dual balanced output for bi-amping 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

The Libra employs two 300B tubes per channel in a true balanced differential pure Class A triode configuration, creating a gain stage so unique that it delivers a tonal spectrum both dense and crystalline, revealing layers of detail in recordings you never knew were there. When paired with the Thrax Spartacus 300, the Libra completes a full 300B signal amplification path, offering an experience so stunning it redefines what’s possible in high-fidelity audio.

Libra features a dual-mono design with two independent outputs per channel, each adjustable for level offset. This allows for bi-amplification setups, where you can pair different amplifiers for mid/high and low frequencies—for example, a single-ended triode amplifier for the mid/high range and a powerful push-pull amplifier for the bass. The level offset ensures seamless volume synchronization between amplifiers, regardless of their gain differences.

Another standout feature is the fully balanced, isolated, and buffered Tape Loop, designed to integrate professional tape recorders like Studer and Ampex. This lets you fully harness the quality of your master tapes, bringing their richness and depth to life. For home theater enthusiasts, Libra includes a home cinema bypass, making it easy to integrate your stereo system into a media room. Additionally, a trigger output allows you to automatically power on or off hard-to-reach amplifiers when using the preamp.

Our goal was to create a preamplifier worthy of driving the Spartacus 300 power amplifiers, renowned for their ability to preserve a signal’s tone and texture at all levels without constraining dynamics or adding coloration. To achieve this, Libra had to match or exceed the performance of the 300B input stage in the Spartacus 300. We accomplished this with an inductively loaded, heavy Class A differential stage, which cancels out the negligible 2nd harmonic distortion of the 300B tubes, leaving the audio spectrum pristine and uncolored.

Directly Heated Triode (DHT) tubes require extensive auxiliary electronics for optimal operation. To address this, we split the Libra into two units: a dedicated power supply and the active unit. This design isolates all mains AC and rectifier noise in the power supply, while final regulation and filtering occur near each element in the amplification circuit. The entire system is microprocessor-controlled, with status displayed on a large color LCD visible from your listening position. A comprehensive menu systemprovides access to advanced features, keeping the front panel clean and aligned with Thrax’s signature minimalist design.

The Libra isn’t meant to be talked about—it’s meant to be listened to.