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.