A couple of ideas come to mind, neither are very energy efficient but far better than nothing.
Use a typical series regulator circuit with a zener for reference, an op-amp for control and a nice chunky transistor as the pass element to control the voltage.
I suggest either a rail to rail op-amp, or use a +/- supply for the op amp so that it can control down to zero volts.
Use a big low voltage battery (portable ?) say 3v for the supply to the pass transistor, and a separate higher voltage supply (9v battery) for the op-amp.
Or, you could use a shunt regulator circuit with maybe a power MOSFET as the shunt controlling element. Again using an op-amp etc for the controls.
As I have been writing, I vaguely remember a variation of the LM317 circuit which can control down to near zero volts. I am not sure without going searching whether it could be made to handle 1.5 amp.
JimB