also there is one thing: if i put the inductor in series before the caps (right after the transformer) what happen when power switches off? the magnetic field should collapse and create a big spike damaging the caps and the rest of the circuit?
also can you show me how to calculate the inductor?
No it won't hurt anything. The best way is to have a PI filter. After the rectifier theirs a Capacitor, Inductor (choke), Capacitor. The first capacitor is the "Bulk" The 10mH is the Critical Value (Minimum value).
You can use this formula
Load resistance/1200 for 60Hz
Load resistance/1000 for 50hZ
Load resistance is equal to peak output voltage divided by peak current R=E/I
So for example 30VDC and 3A we have 10 ohms, divide by 1200 = 8.3 milliHenries critical inductance
or 30V/3A=10ohms 10/1000=0.01H or 10 milliHenries critical inductance
I'm using a 2mH 30A choke with 2 50000uF capacitors on a 40V 20A project and get No ripple at 40V 20A load. Without the choke theirs over 2V of ripple. Chokes work.
The need for a massive choke is a throw back from the vacuum tube days with very high voltages and low current. With low voltages and higher current the choke value needed, drops significantly.
Now the reason you get sagging in your voltage has nothing to do with the choke. Its all about the transformer. The RMS voltage of the transformer Must be equal or greater than the peak output voltage of the power supply. So for example, If your required peak output voltage is 30V you should use a a transformer rated at 30V or greater. If your using a 24V transformer, the Peak voltage after rectification will be around 33V. But most of the 9V above the 24V RMS is basically unusable under high load.
This is why I suggested a different op amp that works at a higher voltage. Unfortunately the LT1637 is fairly expensive.
There are other less expensive ones that will work at 44V, but require an adapter PCB to plug into the provided sockets.
I suggested the LM741, but in hind sight I realized that the maximum input voltage is only ±15 V which won't work. You need an Op amp with an input voltage that is much closer to the supply voltage.
The power supply design in your first post doesn't match the original design from before the 1980's . The original design had per-regulation for the opamps. Pre regulation insures that the supply voltage never exceeds the maximum rating of the opamps. The original design works very well. In fact that basic architecture is used in many reliable linear supplies on the market. Unfortunately these cheap kits have given the original design a bad reputation.
You can still use the kit, but it will work better using different opamps, regulating the opamp supply, using an adequate transformer with the addition of a PI filter.
You can still use the TL081. but you will need to regulate the supply voltage to the opamps to 30V maximum. I also suggest reducing the negative supply Zener Diode to 3.3V to 4V. This will keep the op amps within their operating voltage.