This circuit was obtained from a 1968 documentation and can be adapted to use current equivalent components. The maximum frequency depends on the components used.
August 2016 - Mouser Electronics already has in stock the new MCP1501 voltage references from...
This voice scrambling scrambler was published in the August 1982 issue of Radio Electronics...
Found in a 90s Electronic Design, this suggestion to have a higher voltage in a fixed regulator...