After the acknowledgements we have received from the audiophiles and the press for our model PERFORMANCE, we have decided to design and produce a new, smaller valve integrated amplifier, the SINFONIA, close to the preceding model for technical characteristics and look.
Faithful to Unison.Research tradition, also this new amplifier is integrated. We feel that integration offers several advantages, both in terms of simplicity of use and attainable sound quality: an integrated amplifier requires less cabling than its separate counterparts, thus allowing a substantial reduction of the effect of cable impedance on timbre and dynamics, and a significant saving in production costs as well.
On the other hand, designing an integrated amplifier is a much more challenging task 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, preamplification 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 products which are today valuable classics, like the widely renowned ABSOLUTE 845.
The new SINFONIA, smaller than the PERFORMANCE, reaches an output power of 27-30W per channel. As usual for high quality integrated amplifiers, the new model is laid out in dual-mono configuration: within a single chassis there are two identical and fully independent anlplifiers so to avoid any possible electrical or electronic interference between the two channels.
Each amplification stage is entrusted exclusively to valves, and all operate in pure call A.
As the PERFORMANCE, the SINFONIA is characterized by the presence of two large heatsinks 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 bas been chosen to prevent the heat generated by the four KT88 from reaching the circuits located below, thus improving the stabilitJ, the sound quality and the overall reliability ofthe system.
Since its 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 two KT88 per channel in single-ended parallel ultra-linear configuration, in which the KT88 (beam power tetrodes) operate with good approximation like ideal triodes.
Preamplification and power stage drive are based on double triodes ECC83 and ECC82. These valves guarantee excellent linearity and nearly complete absence of odd order harmonic distortion components. Both the stages are constituted by triodes in parallel biased by automatic bias circuits. In this way the output impedance of the two circuits can be slightly reduced improving meanwhile the linearity of the stages. The signal to noise ratio and the total hamlonic distortion improve significantly too.
Special attention has been dedicated to the design of the output stage which is based, as stated above, on two KT88 tubes connected in parallel: this configuration gives very good results as far as concerning the sound perfonnances, but on the other hand it is extremely critical since it imposes the need for a rigorous and expensive selection and matching of the valves pairs. Moreover, the selection of the valves can only in part solve the problem, since the characteristics of the valves themselves change over time because of drift phenomena in a non unifoml way.
Our long observation and in depth understanding of valves behaviour has lead to the design of 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 perfonnances even after years, and contributes to slightly reduce the time it nonnally takes to the tubes to warm up after switching on and reach the optinlal temperature.
The direct current on the preamplifier valves filaments is supplied by an interesting circuit configuratiGn: it works directing the power stage valves cathode current, through a parallel regulator which operates like a zener diode, to the filaments temlinal.
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 have found an excellent compromise with a special anodic power supply circuit which features an output capacitance much higher than usual.
The SfNFONIA has a microprocessor-based circuit that manages the switching of the input selection rele and the volume level adjustment through the remote IR handset. The analogue volume regulation is operated by a high quality ALPS RK27 stereo motorized potentiometer.
Close to the “tape out” output connector, a low impedance output terminal has been added to allow the connection to an active sub woofer. This output line is connected through the volume potentiometer to the selected input, so it replicates the actual signal present at the preamplification stage. A buffer circuit based on operational amplifiers has been added to increase the impedance seen by the signal sources: it avoids the possibility that the connection of the sub woofer could in aoy way affect the performances SiNFONIA.
The output terminaJs are constituted by six gold plated universal colmectors, allowing the connection of 4 and 8 Ohm impedance loudspeakers. The input connectors are also high quality and fully gold plated.
In the SINFONIA nothing has been left to chance: from the choice of the components, the control knobs and the wood parts, to tlle design ofthe chassis and the circuits boards. Control knobs are machined from solid amagnetic stainless steel. Thanks to their weight, mass and balance, they provide smooth and precise control when operated manually.
The wood parts, besides the visual appearance function, also play another carefully calculated role: they contribute to damp 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 SINFONIA 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, the distribution and the layout of the PCBs have been accurately studied and optimized to limit electrical disturbances and prevent components overheating.
- Technical characteristics
- Type: Dual mono Stereo Integrated Valve Amplifier
- Output Stage: single ended parallel ultraliner A class
- Output Power: 25W
- Inputs: 4 line, 1 tape
- Input Impedance: 47 kOhm
- Outputs: 1 tape, 1 subwoofer, loudspeakers bi-wiring
- Output Impedance: 4 – 8 ohm
- Feedback factor: 14dB
- Frequency Response: 20 – 30000 Hz
- Valves: 4 x 6550/KT88, 2 x ECC82, 2 x ECC83
- Remote Control: IR inputs and volume
- Power Consumption: 500W max
- Dimension: 44 x 42 x 21 cm.
- Net Weight: 55 Ibs