Collection Schematic

1. The connection of the opamp’s output to ground is slightly unusual, but enables the base current for output transistors T1 —Tz to be drawn from the supply rails, rather than from the opamp.
2. The current ‘· through the voice coil and R12 v develops a voltage across the 1 resistor. A negative feedback · loop is created by feeding this reference voltage to the inverting input of IC.
3. The quiescent current of the amplifier is of the order of 50 to 100mA for class A operation, and is determined by Ra-R4 and R5-Rs.
4. The complementary power transistors should be closely matched types to avoid fairly large offset currents (and voltages) arising.
5. Capacitor Cs functions to set the roll-off frequency at about 90 kHz.
6. Some re-dimensioning of either Ra or R4 may be required to achieve the correct balance for the power output stage.
7. This amplifier is based on current feedback to ensure that the current sent through the voice coil remains in accordance with the input signal.
8. In the present case the amplification is 16 times (Z1./R1=8/0.5=16).
9. The emitter current of T1 and T2 is about 500 mA when the amplifier is fully driven. The harmonic distortion of this ampflier is less than 0.01% at P=6.25 W and U= $18
10. The overall amplification of the circuit depends on the ratio of the ·loudspeaker’s impedance, Z1., to the value of Rv.
11. In multi-way speaker systems, this difficulty is overcome by appropriate dimensioning of " the crossover filter, but a different approach is called for when there is but one loudspeaker.
12. It l is very much evident, therefore, that the power delivered by such amplifiers is inversely. proportional to the loudspeaker impedance, since the cone displacement of a loudspeaker is mainly a function of the current through the voice coil, whose impedance may vary considerably over the relevant frequency range.
13. The majority of modern AP power amplifiers drive the speaker(s) with a voltage that is simply a fixed factor greater than the input voltage. 

Source: Téxas Instruments Linear Applications.  

Current Corrected AF Amplifier Circuit is shown below: