OptiValve® analogue output stage

Typically, so-called valve equipped compact disk players only use thermionic electron valves as an add-on in the final buffer stage. For most, the preceding analogue stages use generic solid-state operational amplifiers (Op-Amps) drawn from textbook circuit designs. The valve sections often appear tacked on at the end seemingly as an after thought as in "shouldn't there be some valves in this?"

The CD-77 has stayed true to AMR's uncompromising approach as the complete analogue output section is a full "ground up" design comprised of thermionic electron valves in all functions. Not only this, but AMR's use of New Old Stock Valves is rare in this day and age. New Old Stock Valves were produced throughout the original golden age of valves or later by specialist military suppliers to much higher standards than most current production valves. They were usually stocked for military or telecommunication purposes and despite being of limited supply, significant stocks still remain available to this day.

Several factories in China, Russia, the former Yugoslavia and the Czech Republic currently produce new thermionic electron valves (and seem set to continue to do so for years to come), which are fairly inexpensive and are often of adequate quality. Some have even been labeled with prestigious brand names from the golden age. However these valves rank as reasonable seconds when compared to the best from the likes of Siemens, Telefunken, Philips, Mullard, Brimar, General Electric, Western Electric and others.

Of course, being limited in supply and not renewable in most cases New Old Stock valves usually carry a substantial premium over current production valves, which is why they are rarely seen in the production of high performance audio equipment. AMR has sought out the best quality valves of which significant stocks remain (to ensure the supply of spares and for consistent sound quality in production) and purchased them, in the knowledge that the premium paid is money well spent.

From the very beginning of our analogue circuit (the initial power supply rectification stage), instead of solid-state diodes, New Old Stock thermionic electron valves are used. Thereafter the CD-77 employs a unique and revolutionary output stage, unlike most thermionic electron valve circuits, it uses zero negative feedback, boasts extremely low distortion, low output impedance, high gain and unrivalled linearity, all in a simple direct-coupled circuit. The design parameters were specifically tailored to optimise synergy and operation with the Philips TDA1541A DAC chip.


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