I've made a few prototypes with the first schematic and they seemed to work OK, the only problem is that i don't have a nice big 5K pot, only a tiny pcb mount one, so there's no way of putting a knob on it. I only have 20K and 50K pots so i've thought of using an OpAmp.
The opamp i chose is NJM4556A which would use a LM7805 as a voltage reference. I'm sure this Opamp can make the two transistors work without much hesitation. From my calculations i need the max of 1.3 gain on the opamp.
D2 will most likely be a W574 Zener diode ( can be something else ).
Now i'm a bit stuck with the two potentiometers ( R2 and R3 ), i also think that R2 should be put on the other side of R3.
I would like to know if i'm going to the right direction and if i need to change, add or remove anything from the schematic.
Edit:
After some more playing around on the bread board i've decided do this without the opamp. Been thinking about stuff, making calculations, and finaly after a couple of hours of tracing came up with this masterpiece.
i chose to add fixed 12V and 5V rails just for the sake of it. From what i tested the output on this supply configuration, output voltage changes at about 0.01 V for every 1.5V on the AC input. The KT872A transistor seems to have a pretty low gain ( about 1.2 ) so my calculations on the opamp part were a complete nonsense. And with this configuration i should get about 0-35 V output.
I think it should work. Connections A, B and C are going to the pot on the front panel, GND is common for every output, although there is a separate ground pad and +12V pad for the internal fan ( small 1.3W one ) which can move just enough air and is pretty quiet ( no thermal speed controller BS required ).
I'll take a look at the layout one more time tomorrow and make the PCB and assemble it, i hope it doesn't go boom.