My attempt at designing a discrete mic pre

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It can work (sometimes a rarity around here).

BC212AP seems to be (info is slim) a TO92 part rated for 0.3 Watts? Here you have 60mA at 24V which is like 1.5 Watts?

What is the gain/phase relationship? Looks like the middle is rolled-off to unity near 35MHz. That's not huge margin on the BC212's fT of 200MHz. OK, that may work for minimum gain of 10.

I like the Garbage Principle. Each stage of a chain should build on the one before. Little 13g trash cans go in big 40g trash can. Big can goes to 5ton street truck. Street trucks dump to 20ton highway trucks to haul the stuff out of town. At first glance, you have 1mA in first stage, 6mA in middle, 60mA in last stage. BUT you have these puny 0.3mA Darlington Inputs working at low fT. That's like dumping the street-truck into a breadbag, then back into the truck.

And Q10 does not seem to drive Q11's base current, certainly not with authority.

Why the Sziklai at the output?? These things love to sing radio waves. It isn't like PNP and NPN are different prices. (And used to be Si NPN was the cheaper.) A simple Darlington is tamer, makes Q9 and Q11 the same part, saves an order-line, an inventory, and a place for the worker to get mixed-up.

Any unprotected output, I like to ask "what happens if...?". Actually there is some limit of damage, between current-limit Q9 and the cap on the output. Direct-coupled allows more trouble. When things go bad, Q11 melts then Q10 melts. At this point Q7 tries to pull the whole load current. Without CCS and C3 (normal power amp), current is limited only by R15 47r, so could be a half an amp, which Q7 may not like any more than Q11 and Q10 did. Yes, the gig is over in any case. But I like to see "some" limit on how many burnt parts I will have to pull and replace before I can get paid again.

You bias all three stages with one diode-string. The impedance here is low but not zero. There can be sneak interaction across the 3 stages. (Maybe not a lot because none are class B.) I see 2mA in the diodes and near 0.7mA in the three bases, so the diode current is not "much larger" than the loads it is trying to control. I'd pencil 3K at R4 and see if the boss objects to the extra power and heat.
 

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His book Audio Power Amplifier Design might be of interest.  If you want more equations The Art of Electronics has many including a section about op amps.
 

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