Description
Many Unison Research customers have expressed the wish for an all-tube pure class-A power amplifier with a power output significantly higher than any previous Unison product. In order to satisfy this request, the Unison Research design team has created a single-ended monoblock amplifier able to deliver 80 watts in pure Class-A. This product has become the first in a new line which embodies all our experience in valve amplifier design acquired over the years–the Reference Series. Having completed the power amplifier project, a preamplifier of similar quality and performance was added, and it is fully worthy of inclusion in the Reference Series. We have named it, of course, the Reference PRE. From the outset it was decided to create a circuit without compromise. The Reference PRE has a dual mono layout within a triple chassis to give full separation of the power supply stage from the line and phono stages. Input switching by two-stage relays has been added to eliminate any possible diaphony. The way we have labored over the details, which shows in every aspect of the design and appearance of the Reference PRE, is truly worthy of the finest Unison Research products. To briefly summarize the characteristics of the Reference PRE, the output stage employs one 300B tube and is able to supply an undistorted output voltage of 25V over a bandwidth which extends to above 100kHz. The volume control is a top quality ALPS model from their HQPro Audio Series, a truly excellent unit from a manufacturer that has been preferred by Unison Research from the very beginning. All power supplies are stabilized via isolated high voltage power MOSFET circuits with series stabilization for the output stage and double stabilization in series/parallel for the line and phono stages. Rectification is through Schottky fast diodes and series stabilization for the valve heater circuits (filaments). The whole amplifier uses no less than 18 signal triodes and 2 x 300B power triodes. From the beginning, this project proved to be an interesting challenge to the Unison Research design team, and only after a great number of tests with different configurations was the final circuit topology for the Reference PRE finally eventually chosen. The first parameter to be defined was the structure of the line level amplifier. The input stage uses one 6C45P triode, a valve noted for its low noise characteristics. This is followed by a driver stage using a twin triode ECC83. The two triodes are connected in parallel to provide the correct amount of gain for the stage and sufficiently low output impedance for good signal drive to the next stage. The final stage consists of a cathode follower, which allows particularly low output impedance and is realized, as mentioned above, with a 300B, a valve famous for its highly prized performance in audio amplification.
Specifications
– Type: Valve line preamplifier – phono, double chassis
– Class: Pure A
– Valve (line): 2 x 300B, 2 x ECC 82, 2 x ECC83, 2 x C45P
– Valve (phono): 4 x 6C45P, 2 x ECC83
– Inputs: 4 line RCA, 2 line XLR, 1 tape, 2 phono
– Outputs: 1 RCA, 1 XLR, 1 tape (active), 1 sub woofer (active), remote power for REFERENCE
– Output Impedance: 500 Ohm
– Input Impedance line: 47 kOhm
– Input Impedance phono: selectable
– Phono Gain: 45 – 55 dB
– RIAA: passive ± 0.2 dB
– Remote Control: radio for volume
– Frequency Response: 10Hz – > 100KHz @ – 0.3dB
– Feedback factor: 15 dB line, 0 db phono
– Distortion: 0.1%
– Power Consumption: 320 W max.
– Dimension (mm.): 451 x 560 x 210
– Net Weight: 66 lbs
