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it could be nice to make a circuit that detect what volatge you want a the output to set the lower input voltage for that ouput. I think LM317 require 3V to run.
if you want 10V as output and your input is set to 35V we will have a lot of power lost ( heat disipating )
but if the input was just 15V that's should be great...
my idea is take a center tap transformer with a relay but how to detect the posisition of your voltage command (maybe a potentiometer )
You can make a preregulator with two SCR-s or SMPS. The control voltage coming from dissipative supply output, and hold always 4V difference between analog supply in-out.With this method can build the supply without great heatsink, because the max. dissipation is always 20W, and - thank for analog section - the noise is very low.
I recommend L4970 from ST micoelectronics. It`s a 10A switching regulator (SMPS) See the datasheet for complete application.
For wovk: SCR=thyristor. If You apply two SCR-s as rectifier with center-tap transformer, with variable-delayed trigger pulses, You can control the voltage on buffer-capacitor.