Hi
This circuit looks pretty good for the typical diff input, CCS loaded single ended VAS, Darlington EF follower with global feedback type amp. IMO, I would stick with 3 pair of these outputs as they are common and cheap.....usually. I find a little over design in a diy audiophile circuit is a good thing.
According to the
datasheet for the TIP42C from On-Semi, the small signal Hfe is only 20. I'm thinking this may be a bit low with a VAS current of ~12mA. The input stage must supply ~0.6mA of base current for this transistor. It appears the input stage bias is only about 0.6mA per leg, which is too low for the MPSA42, and not really enough to drive the TIP42C. Try at least 1mA per leg, perhaps even 5mA per leg. If you do this, don't forget to change the value of R3 to maintain current balance
in the input diff.
Also, as Andy pointed out, the Cob is rather high for a VAS transistor with these circuit conditions. There are better suited transistors out there for this function. However, I don't see a problem with them used as drivers, other than they are kind of slow. There are probably better suited transistors out there for this purpose as well but as we all know, you have to use what you have available.
Just an after thought, a discrete Darlington VAS circuit might be better if you use such a large (relatively) transistor as the VAS.
