To complete the series of all-tube integrated amplifier, including successful models like the Performance and the Sinfonia, Unison Research introduces the Preludio.
This amplifier, though maintaining the distinctive technical elements, sound characteristics and winning look of the preceding models, features original solutions which are the result of Unison Research great design and production experience gained over the years. Introducing the Preludio. As the other models of the same series, and faithful to a consolidated Unison tradition, the Preludio is an integrated amplifier. Integration offers several advantages, both in terms of simplicity of use and achievable sound quality: an integrated amplifier requires less cabling than its separate counterparts, thus allowing a substantial reduction of the effect of cable impedance on sound timbre and dynamics, and a significant saving in production costs as well. On the other hand, designing an integrated amplifier is much more critical and challenging than designing a separate pre-power combination: let us mention, for example, how carefully all the possible interferences between different components and different stages (power supply, control, preamplifier and output stage) have to be considered and limited.
Our experience in the design and production of integrated amplifiers has grown over the years, allowing us to create successful models which are still today valuable classics, like the widely renowned Absolute 845. The Preludio, though small in size, reaches an output power of 14-15W per channel.
As the Performance and the Sinfonia, the Preludio is characterized by the presence of two large heatsincs mounted on the sides, which are required by the high voltage stabilized power supply circuits, and by a wide stainless steel cover. The cover material is amagnetic 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 b ased on a single toroidal transformer, especially 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: high costs, short life-expectancy, large size and weight, etc.
We considered this problem in depth, in the attempt 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 much higher than usual.
The dc current for the preamplifier stage tubes heaters is supplied , as in the Sinfonia, 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 to use the energy that otherwise would be dissipated in the power stage valves cathode resistances.
In this way, in addition to a significant improvement of the system efficiency, it is 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 imply the simplification of the power supply transformer through the elimination of one secondary winding.
Conversely, the power stage tubes heaters receive an ac current previously regulated by balancing nets.
As far as concerning the amplification circuit, each channel of the Preludio is constituted by a preamplifier stage, a power stage and a dedicated output transformer.
Preamplifier and power stage drive are based on a double triode ECC82. This valve guarantees excellent linearity and 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, based on two KT88 tubes: we conceived and tested in our laboratory 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 performances even after years, and contributes to slightly reduce 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 valve amplifiers performances are closely connected to the quality of these complex components. Thanks to a particular elaboration of the output transformers it has been possible to design for the Preludio a power stage constituted by a single KT88 per channel in single-ended ultra linear configuration, in which the KT88 (beam power tetrode) operates with good approximation like an ideal triode.
The Preludio has a micro