some info for Diogo:
R8 and R9 are there to limit the current through the diodes, they can also be replaced by a current source
the diodes are needed to avoid crossover diistortion, also other devices are common.
Q5 is the voltage amplifier stage (C3 reduces higher frequency oscillations), the gain is equal to R12/R13
Q3 and Q4 is a differential amplifier (R3 and R4 reduces little differences between Q3 and Q4). A differential amplifier only 'allow' different signals to go through. (i.e. if the same signal is applied to the base of Q4 and to the base of Q3 there won't go a signal to the base of Q5). When no signal applied, the diff. amp. is 'in balance' => the current through Q3 (I3) is equal to the current through Q4 (I4). I ~ (12V - 0.6V)/R5 = I3 + I4
I3 = I4 so I3 = I/2. Calculation of R7: R7 = 0.6V(Ube of Q5)/I3.
HB.