Terry Bateman, the now legendary designer of Rega electronics, has spent the last ten years thinking about a CD player with a tube output stage. Tubes have been used extensively in the amplification of musical instruments and recording equipment since the 1950s, so it made sense to develop a tube CD player.
This approach has resulted in very linear circuitry, this is because musical instrument amplifiers are not hampered by a certain folklore that seems to prevail in some aspects of tube audio design. The result is the combination of the technical and sonic attributes of tubes in a differential instrument amplifier circuit driven by a differential DAC output stage with a passive LC filter.
The tube Isis shares the digital and USB sections of the state version but the analog stage is based on tubes with passive filtering. This stage uses two 5814A (ECC82/12AU7) triple mica triodes driven by the revolutionary Wolfson WM8741 DAC. The output buffer and transformer stage use two ECC88 triodes (6DJ8/6922). This, as with the solid-state version, is an unorthodox design.
The aim was to take a different approach than "let's use tubes that give a warmer sound" and to use industrial tube circuits to build a well made tube CD. Each component of the digital and analog signal path has been carefully chosen to ensure signal integrity.
Careful PCB design ensures isolation not only of the digital and analog sections but also of the motor, display and user interface processor.
All critical capacitors were bypassed with polypropylene or polyester capacitors. Furthermore, even electrolytic capacitors with important values have been by-passed with audio grade electrolytic capacitors.
The supply impedances in the DAC are kept very low by using solid polymer capacitors. All power supplies use custom Rega K-Power capacitors, coupled with fast diodes used throughout power rectification. Isis uses improved and optimized control code for processor and display control. This speeds up initialization due to a more rigid control interface between the micro and the CD processor.
• The DAC's differential output drives a tube differential amplifier (ops), with modest feedback to stabilize operating conditions. The 5814A differential amplifier, tailed by a solid-state current source, takes advantage of the differential output from the DAC. This pre-amplification stage is followed by a second-order passive low-pass Butterworth filter followed by the output stage made with two ECC88s which drive both outputs, unbalanced and balanced, through two very high quality transformers.
• It was understood during design that unnecessary high THD levels would outweigh the qualities of the Isis digital stage. The THD is usually 0.06% and the second harmonic is predominant. This gives the sonic dynamics of the tubes without dominating the sonic qualities of the digital section.
• After many hours of tube testing we found that the 5814A differential amplifier position was better for microphonic, sonic and reliability qualities. Both HT & LT power supplies are fully regulated and use the same power supply parts used in the solid state Isis. As with the solid state analog stage, the Valve Isis has its own dedicated 50VA current transformer, which ensures galvanic isolation between the digital and analog sections of the player.
• The Isis has an isolated balanced output and in keeping with the tube circuit topology we have chosen to use a transformer in this position. This transformer has the same standards as the one used in Ios MC.
We hope you enjoy this product as much as we enjoyed building it
All these features amalgamate to create a supreme level of performance and make it a perfect match for the Rega Osiris amplifier.