As promissed ...
** I'm only providing essential info for now, it's an untested build ...
http://www.lynx.net/~jc/vivaAnalogG100B.html
Depending on your chosen Zener you might have to lower R26 (3k9) to 2k7 or 2k2 when running off of 18v ... lemme know if you have any problems making this work ...
cheers, all the best ... jc