Dear mat,

Thanks for the reply. I tried this too. It is basically a variable power supply with a low budget. Since the input is more than 40 volts I have to level shift the supply for the op-amp (a resistor connected to supply Ground and clamped by a zener, that is the negative supply pin of the opamp is connected to the negative unregulated supply). So using a difference amplifier did not work , level shift, high common mode voltage. May be you can help with some schematics.