Here is a schematic and a parts list for a dual power supply I designed and built recently. It is 0-15 both + and - dual tracking with short circuit current limiting to about 400Ma. To change to 0-20V resistors R10, and R11 need to be changed to 300K 1% 1/4W and to increase the current limit to 1A change resistors R7 and R8 to 0.68Ω 2W.
The physical size of the two heat sinks needs to be increased.
The dual power supplys near the bottom were used as I used some LED DVM's for monitoring the voltage and the current...they required isolated supplies.