The input differential, i call it a CFP, although not done with two BJT but FET/BJT. Feedback to the emitter / source ("current feedback" as the OpAmp maufacturers are calling this feedback scheme), gain=1. Am I wrong?
Well, in principle a compound feedback pair is just a combination of two devices. I have heard it calls "Inverted darlington", however I think this designation is not entirely correct.
Here the First device is a low Transconductance J-Fet (2SK246), the second a BJT.
As it is designed the J-Fet operates at around zero Tempco and has around 0.8mA/V Transconductance at that current. Compared to (say) 2SK170 it has however much lower input capacitance (and let's not forget that Zero Tempco thing - which means thermal distortion is defeated).
The BJT being a 2SA970 running at 0.8mA has around 30 Ohm Emitter impedance (or 33mA transconductance) and around 9K input impedance, so the effective load on the J-Fet is around 2.4k and the gain of the Fet around 2, so the combined transconductance becomes around 66mA/V.
In other words, we now have a J-Fet with 66mA/V transconductance, a 5pF reverse and 9pf input capacitance and a bandwidth that is more limited than either BJT or J-Fet alone, however, as the Ft for the 2SA970 is 100MHz the "slower" J-Fet is likely to still dominate.
How we connect this new compound device is up to us. It be a follower or even a common gate amplifier.
In the Amplifier Circuit is connected as differential pair with degeneration. As the effective source impedance is around 15Ohm, the 560 Ohm source resistors apply around 32dB degeneration, which helps to linearise the circuit, as does the cascode. Incidentally, the cascode incidentally is bootstrapped to the common mode signal (top of the tail), so common mode distortion is minimised
In addition, the low current in the J-Fet (> 2V) means we have (compared say to a BJT differential) an enormous dynamic range available to the differential circuit.
I might have applied a FET/BJT Compound to the output stage as well (then we can dump overall feedback), but the existing circuits are incredibly ill suited to that sort of modification and I do not fancy making new ones.
I found the PSU primary quite interesting, especially that you want to get rid of DC component.
I can imagine that especially with all that switching mode psu's around, loading the grid asymmetrically, there can be a DC component.
Did you experience less hum when applying this cap?
First, this circuit is quite old. You will find it (confirmed) in Bryson and Lamm Amplifiers (also almost all of my own designs). It helps with transformer hum if it is caused by DC (not always the case), but mostly it makes the transformer much more efficient and reduces temperatures.
You mentioned TINA-TI as a good program. I like the optical appearance of the schematic.
But the simulation tool (although i do not think i will use it often) and especially how external models can be "loaded" isn't clear at all to me.
You cannot "load" external models. You need to put the default component in (say P-Channel J-Fet) onto the schematic and then select the correct Model (Tina allows amny options) and then enter the parameters. You can then copy the component to all sorts of schematics. Each component has basically it's own local model in Tina.