UNISON RESEARCH TRIODE 25 USB

Tube integrated amplifier with USB DAC incorporated.

There are three amplifiers that can be considered the symbol of Unison Research’s success: the Simply Two, the Triode 20, and the famous Absolute. On the occasion of the 25th anniversary of Unison Research’s founding, it was only natural to reintroduce these three iconic devices: the Simply Two and the Absolute in a limited and numbered "Anniversary" edition, and the Triode 20 in its new and significantly enhanced version, the "Triode 25".

Like its predecessor, the Triode 20, the Triode 25 is an amplifier with a push-pull output stage using EL34 valves. The push-pull topology features a final stage with two power tubes, each dedicated to amplifying the positive and negative halves of the audio signal. The accurate reconstruction of the signal is handled by the output transformer, which, thanks to its symmetrical structure, allows for an extremely linear summed output.

The benefits of a push-pull structure are numerous. The main advantages include:

• Higher output power
• Better efficiency and reduced heat generation, with positive effects on valve lifespan and significant energy savings
• No static biasing of the output transformer, resulting in improved performance of this stage

To ensure high sound quality in a push-pull amplifier, each amplification stage—especially the two output half-amplifiers—must be carefully designed to prevent the generation of odd-order harmonic distortion. This is equally true, if not more so, for the output transformer.

The Triode 25 is the result of extensive design and experimentation, despite the experience already gained with the Triode 20. It can operate in two modes: triode and pentode. In pentode mode, it delivers over 45 watts per channel into a 6-ohm load, with extremely low harmonic distortion across the entire audio bandwidth.

The amplifier's construction includes gold-plated RCA connectors, a gold-contact dustproof rotary selector, and a motorized ALPS RK27 blue series volume potentiometer, controlled by a microprocessor via infrared remote control. The signal passes through an initial stage using a dual-triode ECC83, followed by a stage with an ECC82 for phase inversion and low-impedance, high-linearity driving of the power stage, which features a pair of EL34s and a specially designed output transformer, ensuring a bandwidth exceeding 30 kHz at full power.

In the Triode 25, negative feedback can be selected between two preset levels. Even at the maximum setting, it does not exceed 12 dB, allowing for natural and fatigue-free listening even during extended sessions.

During development, particular attention was given to the drive current required by the push-pull output stage. The driver stage was carefully dimensioned to present a low output impedance. A phase-splitting topology was adopted to generate two robust and highly symmetrical out-of-phase signals, with sufficient amplitude for driving the output stage under all operating conditions.

The output stage uses a pair of EL34 tubes. Through extensive testing, the ideal bias level was defined to achieve the best balance between dissipation, maximum output power, and minimal distortion. Bias adjustment is approximately 20% self-regulating, with the remaining portion adjusted manually using an easy-to-read analog meter.

The power supply stage is also critically important. The power transformer is a low-loss toroidal type with magnetic shielding to minimize stray magnetic fields. After rectification (via an 8-amp bridge), a first LC π-filter supplies the 500V anode voltage. This is followed by independent RC filtering cells for the left and right channels. All filament voltages (except those for the power tubes) and service circuit voltages (including remote control) are stabilized.

Technical Specifications:

• Output stage: Push-pull, ultralinear, Class AB
• Output power: Approx. 45 watts per channel (pentode configuration), approx. 25 watts per channel (triode configuration)
• Output impedance: 6 Ohms
• Input impedance: 47 kΩ / 50 pF
• Negative feedback: 5 dB or 1.8 dB
• Tube complement: 2 x ECC83 (12AX7), 1 x ECC82 (12AU7), 4 x EL34 (6CA7)
• Power consumption: 300 watts (max)
• Fuses: T6.3A for 100V to 130V, T3.15A for 220V to 250V
• Dimensions: 30 cm x 45 cm x 20 cm (height)
• Net weight: 20 kg