The differential output of the digital to analogue converter drives a valve differential (operational) amplifier, with a very modest amount of feedback to stabilise the operating conditions. The 5814A based differential amplifier, with a solid-state current generator in its tail, takes full advantage of the differential output from the digital to analogue converter. This is followed by a passive Butterworth second order LC filter. Finally, this signal is buffered using a low anode resistance ECC88 triode to drive the output and balancing transformer.
In design it was felt that unnecessarily high levels of THD would unduly overwhelm the qualities of the digital stage of the Isis. The overall THD is typically 0.06% being prominently even order, where the 2nd harmonic predominates. This will give the sonic dynamics of the valves without dominating the sonic qualities of the digital section.
After many hours of tube rolling we found the 5814A differential amplifier position was best on the grounds of micro-phonics, reliable operation and sonic qualities. Both the HT & LT power supplies are fully regulated and use the same power supply parts as used in the solid state Isis. Like the solid-state analogue stage, the Valve Isis has its own dedicated 50VA mains transformer, ensuring galvanic isolation between the digital and analogue sections of the player.
The Isis has an isolated balanced output, and in keeping with the spirit of the valve circuit topology we have opted to use a transformer in this position. This balancing transformer is made to the same exacting standards of that found in the Ios MC amplifier.
We hope that you enjoy this product as much as we have enjoyed creating it.
All these features amalgamate to create the supreme level of performance found in the Isis and provide a perfect partner for the Rega Osiris amplifier.