Factless

I quickly realized that the LM723 was the closest thing I could easily find to fit my needs, with better regulation than the 78xx series and since I would need a pass transistor anyhow, the ~100mA limitation of the chip itself is not a concern. I will actually use the LM723C which main difference is its rated operating temperature range.

I also added two suggestions to hopefully improve the voltage regulation, first I added a zener diode based pre-regulator, which drops the input voltage from the bridge rectifier and filter capacitor to a minimal amount needed for the voltage drop of the pass transistor and LM723 regulator, which is about 3V. So I selected a 28V zener - a 1N4751 with its own pass transistor to handle the necessary current for this pre-regulator zener diode which gives roughly 1V "extra".

The other thing I did was try to reduce the noise on the output of the LM723, by adding a circuit based upon an interesting article from Wenzel Associates entitled Finesse Voltage Regulator Noise! I don't remember how I found this article (maybe from the time-nuts mailing list), but I thought it was a gem. Maxim/Dallas Semi have a similar Application Note, AN3656.

The one thing I didn't add to the design was using an external shunt voltage reference, like the LM399 or cheaper LM329. It would of only required the 3/4-legged IC plus a FET as a constant current source (set with a resistor) - total cost of about $1-10 depending on which reference IC used. This would of reduces the output drift of the LM723 due to ambient temperature change, by an order of magnitude or two. I didn't add it because I could not figure out how to add the reference without messing up the non-inverting input when used with a PNP series pass transistor.

Although I didn't need it, I found the explanations for the fold-back current limiting confusing, as I could only find one equation to specify the value of two resistors...(R3, R4 in figure 6 of the National Semi LM723 datasheet)