Many customers, already familiar with our the current products, have expressed the desire to see a new model in the catalog: an all-tube, pure Class-A power amplifier, with a power output much higher than anything so far seen from Unison Research.
With the aim of satisfying this requirement, the Unison Research team began to evaluate the possibility of producing an amplifier which would incorporate the best of what we have learned from many years of experience with valve amplifiers. The idea was certainly an ambitious one: to design a single-ended amplifier with twice the output power of the most powerful product in the catalogue, and make it truly worthy of inclusion amongst models that have made the Unison famous throughout the world.
Since this demanding project was to be developed entirely by the Unison team, it was necessary to make several important decisions concerning the overall design. Our first realization was that the Reference power amplifier needed to consist of a separate monoblock pair of amplifiers.
As mentioned above, the Reference project began with a definition of the outline scheme. Being a power amplifier, the first question was to decide on the required output power. We decided on 80 watts from a quartet of 845 triodes in parallel, a level roughly double that of the most powerful amplifiers in the Unison Research catalogue. This requirement also dictated the choice of a two chassis solution, as a monoblock amplifier pair. The obvious advantage of this solution lies in the maximum separation between channels, but equally important was the need to keep the size and weight of each to a manageable level.
The next step was to define the circuit. In view of the limited amplification required from one power amplifier, maximum power is obtained with 1 volt (rms) at the input. Considering also the small amount of feedback typical of a Unison Research amplifier, the choice fell to a first stage consisting of two twin-triode ECC82s. The two triodes in a single valve operate in parallel giving reduced anode impedance which helps to give wide bandwidth, good driving ability, and distortion and noise levels which are generally reduced. The first pair of triodes in parallel is used for voltage amplification of the input signal, which is passed on to the second stage. This too consists of a pair of ECC82 triodes in parallel but this time the triode bias has greater current to enable these valves to act as drivers for the power triodes. A power supply of around 600V allows this stage to have an adequate voltage swing. The output from the driver stage feeds the four grids of the 845 triodes, whose anode current in turn feeds two output transformers working in parallel.
The 845 valve was chosen in this project first and foremost for its renowned sound quality, but also because it is a recognizable symbol used in many classic products from Unison Research including the famous ABSOLUTE 845.
Next we outlined the structure of each single amplifier. We decided on an input stage based upon the twin triode ECC82 used as a voltage amplifier, followed by a driver stage again using a twin triode ECC82 and biased in such a way as to be able to adequately drive four directly-heated 845 triode valves. Finally, it had to have two very large output transformers working in parallel, and the whole amplifier should have a minimum amount of feedback.