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Preludio
Integrated amplifiers





Technical Cgaracterists
Type: Stereo Integrated Valve Amplifier
Output Stage: single ended ultraliner A class
Output Power: 14W / channel
Inputs: 4 line, 1 tape
Outputs: 1 tape, 1 sub woofer, loudspeakers
Input Impedance: 47 kOhm
Output Impedance: 6 ohm
Feedback factor: 10dB
Frequency Response: 15 – 50000 Hz
Valves: 2 x 6550/KT88, 2 x ECC82
Remote Control: IR for volume
Power Consumption: 200W max
Dimension: 37.5 x 40 x 18cm
Net Weight: 40 lbs
Tests
Preludio WhatHi-Fi02/2007

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To complete our series of all-tube integrated amplifiers, including successful models such as the Performance and the Sinfonia, Unison Research now introduces the Preludio.
This amplifier maintains the distinctive technical elements, sound characteristics and winning look of the preceding models, and features original solutions which are the result of Unison Research’s innovative design and production experience that has been gained over the years. Along with the other models of the same series, the Preludio is an integrated amplifier which is faithful to the traditions here at Unison Research.
Integration offers several advantages, both in terms of simplicity of use and the sound quality that can be achieved at the price. Since an integrated amplifier requires less cabling than its separate counterparts–therefore allowing a substantial reduction of the effect of cable impedance on sound timbre and dynamics—there is a significant savings in production costs as well. On the other hand, the design of an integrated amplifier is much more critical and challenging than designing a separate pre-power combination. For example, we must carefully consider all the possible interferences between different components and different stages such as the power supply, control, preamplifier and output stage.
Our experience with the design and production of integrated amplifiers has grown over the years, allowing us to create successful models, such as the widely renowned Absolute 845, which are still considered classics today. The Preludio follows this tradition. Though small in size, the Preludio reaches an output power of 14-15W per channel and still mates well with a wide variety of loudspeakers.
As with the Performance and the Sinfonia, the Preludio is characterized by the presence of two large heat sinks mounted on the sides; these are required by the high voltage stabilized power supply circuits which are protected by a wide stainless steel cover. The cover material is non-magnetic and has low thermal conductivity: it has been chosen to prevent the heat generated by the two KT88 from reaching the circuits located below, thus improving the stability, the sound quality and the overall reliability of the system.
The power supply of the Preludio is based on a single toroidal transformer which is specifically designed to minimize the magnetic flux dispersion. In an audio amplifier, the anodic voltage should ideally be supplied by a battery, but this is obviously not possible for a number of reasons such as high costs, short life-expectancy, large size and weight and more.
We considered this problem in depth when we attempted to obtain a power supply stage whose electrical characteristics were the closest possible to those of a battery bank. We found an excellent compromise with a special anodic power supply circuit which features an output capacitance which is much higher than usual.
The DC current for the preamplifier stage tubes heaters is supplied, as with the Sinfonia, by retrieving a part of the anodic voltage of the power stage tubes. The correct level of the applied voltage is ensured by a dedicated regulator circuit. This interesting solution allows the amplifier to use the energy that otherwise would be dissipated in the power stage valve cathode resistances.
In addition to a significant improvement of the system efficiency with this design, it is now possible to reduce the dimension of the components, to optimize the circuit layout and to limit the heat produced inside the amplifier. Moreover, this solution simplifies the operation of the power supply transformer through the elimination of one secondary winding. Conversely, the power stage tubes heaters receive AC current that was previously regulated by balancing nets.
In terms of the amplification circuit, each channel of the Preludio is comprised of a preamplifier stage, a power stage and a dedicated output transformer. The preamplifier and power stage drives are based on a double triode ECC82 valve. This valve guarantees excellent linearity and results in a nearly complete absence of odd order harmonic distortion components.
Each amplification stage is entrusted exclusively to valves, and all operate in pure A class.
Special attention has been dedicated to the design of the output stage, which is based on two KT88 tubes. In our laboratory, we conceived and tested an original circuit which, through the control of the KT88 bias, ensures a real time compensation of the parameters affected by drift.
This solution guarantees the best sound performance even after many years, and also contributes to a slight reduction in the time it takes to all the tubes to warm up after switching on and reach the optimal temperature.
Since the beginning, Unison Research has been directly involved in designing, developing and building the output transformers for its devices. We feel that a valve amplifier’s performance is closely connected to the quality of these complex components. Thanks to particular innovation of the output transformers, it has been possible to design a power stage for the Preludio that is comprised of a single KT88 per channel in single-ended ultra linear configuration, in which the KT88 (beam power tetrode) operates with good approximation to an ideal triode.
The Preludio has a microprocessor-based circuit which allows the user to control the volume by using the remote IR handset. The analogue volume regulation is operated by a high quality ALPS RK27 stereo motorized potentiometer.
The output terminals are constructed with six gold plated universal connectors, allowing the connection of 4 and 8 Ohm impedance loudspeakers. The input connectors are also high quality and fully gold-plated.
In the Preludio nothing has been left to chance, from the choice of components, the control knobs and the wood parts, to the design of the chassis and the circuit boards. Control knobs are machined from solid non-magnetic stainless steel. Thanks to their weight, mass and balance, they provide smooth and precise control when operated manually.
The wooden parts, aside from their obvious visual beauty, also play another carefully calculated role: they dampen out induced resonance in the metal chassis. Thanks to this structural damping, the valves themselves, which like all vacuum tube devices are susceptible to microphony, are much less liable to generate unwanted resonance-related effects.
For the best reliability and long term musical quality, all the components used in the Preludio have been chosen with care from the non-inductive resistors and the filter capacitors, to the pure ceramic valve bases and the vetronite printed boards. The dimension, distribution and lay out of the PCBs have been accurately studied and optimized to limit electrical disturbances and prevent components from overheating.


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UNISON RESEARCH
A.R.I.A. Advanced Research in Audio
via Barone, 4
31030 Dosson di Casier (Treviso)
ITALY
Tel. +39 0422 633547
Fax +39 0422 633550

ABOUT
Unison Research for more than 20 years has been admire and appreciate around the world. From very beginning our purpose has always been to provide enjoyment throught listening to music. To that deceptively simple goal we bring a lifetime of experience in both valve and hybrid electronics design - and a genuine passion for making equipment that will give lasting pleasure.