Since I have a 24V/210VA transformer available this is my starting point.
U1-3 will stay TL081, I have them already.
The 24VAC will be about 25.5VAC without a load. Then the peak voltage will be 36V and the unregulated voltage will be 34.7VDC. Most of the opamps use the positive unregulated voltage plus the negative 5.6V supply for a total of 40.3VDC. The absolute max supply voltage for a TL081 opamp is only 36V. There are a few inexpensive opamps rated at 44V available such as the MC34071AP and the TLE2141CP which would be more reliable for your project.
The losses in the circuit will produce a max regulated voltage output of only about 25VDC at full load because the transformer's voltage is too low for 30VDC. The 25VDC might have high ripple. A 30VAC transformer and high voltage opamps are needed for 30VDC at full load.
The power rating of your transformer is fine for a load of 5A.
3x 2N3055, each with emitter resistors of 0,1R-0,27R/min. 2W (all will get the same value). Q1/Q2 will be TIP31A or BD241 depending what I can find in my boxes or what is available at the store.
Yes, but use 0.33 ohm/2W emitter resistors. People reported that the 2N3055 transistors have a wide range of current gain and the higher resistance allows them share the current better.
If I got it right the absolute max. current is despite the power dropped over the output transistors depending on the voltage over R7 which is 1,41V (I hope I remember this value right?!) for full current limit with current potentiometer set to max. current?
People found the current too high since the pot has a very wide tolerance. A 0.27 ohm resistor for R7 will produce a max current of up to 7A which will cause the 3 2N3055 transistors, Q2 and your transformer to melt if the output voltage is low. People add a preset trimpot in series with the adjustment pot for calibration to exactly 5.0A for the max current They also add a trimpot in series with the voltage-adjust pot to calibrate the max voltage which is also too high..
One thing i cant figure out about it: C10 10uF/50V. I cant find it anywhere...what did I miss? Is it a good addition to the original layout, whereever it will go ?
There isn't a C10. It was a mistake.
Audioguru, guru now! (as everyone can see, theory is important!)
Some people think so. I hope that I have helped you.