Autobias Vbe with output current sensing
The important thing to understand is that "dynamic bias" gimmicks are mostly useless. Let's do a simulation:



"V4" is a voltage source which drives this output stage. I've replaced the CFP lower transistor with a darlington, besides that it should work like the original.

The diff pair Q15,Q16 compares the voltage on emitter resistors R16,R17 with reference V3. I labeled both emitters e1,e2 on the schematic so the voltage between them is v(e1,e2). I set reference V3 to 66mV, R16+R17 in series make 0.66 ohms, so we have idle current of 100mA.

The output of this diff pair drives Q13. When v(e1,e2) is too high relative to desired bias voltage V3, Q13 conducts more and shunts some current, discharging C4, which reduces v(e1,e2).

When v(e1,e2) is too low, Q13 conducts less, thus voltage on C4 increases, also increasing v(e1,e2).

This works nicely until a load is connected to the amplifier and it draws enough output current to pull the amp out of class A. 

As long as output current is low enough to stay in class A, that is as long as both both output transistors conduct, then this scheme will keep v(e1,e2) constant as desired. This would make it pretty good for a pure class A amplifier.

However, when output current exceeds what class A can provide then one of the transistors will turn off, and voltage on the other emitter resistor will increase as current increases. Say Q9 turns off, then v(e2)=v(out) since there is no current in R17, and all the output current flows through R16. If output voltage increases, and output current increases, then v(e1,e2) will be greater than the reference voltage... and Q13 will turn on hard, discharging C4. Since C4 can only charge through current source I2, until it is charged again to the desired bias voltage, the amp will be underbiased and run in class B, producing huge crossover distortion as shown on the simulation below:



Curves from top to bottom:

Input and output voltage (showing crossover distortion)

Emitter current of both output transistors. At the beginning, they are biased at 100mA each without signal.

v(e1,e2) is the voltage on both emitter resistors

v(c4) is the bias voltage

ie(Q15) and ie(Q16) are currents through the diff pair... it does clip...

ie(Q13) is the current through the shunt transistor Q13, which becomes quite chaotic as soon as output current exceeds Class-A current...

In other words, this would work for class A, but definitely not for class AB, so unless you want a space heater this is only for headphone amps... I'm sure Self mentions this, right?