This design gives an output power of about 4.1 watts rms into an 8 ohm load, but requires a supply of 22 to 24 volts at 1 amp. This gives an efficiency of about 19% at best, less than a third of the efficiency of many Class B designs.

Circuit in lines (Click to enlarge)

Q1 is used in the common emit­ter input stage, and is direct coupled to the output stage via emitter follower buffer transistor, Q2.

The latter is needed because of the fairly high drive current required by the output stage. Q5 is the out­put transistor, employed in the common emitter mode. It has a constant current source as its col­lector load, formed by Q3, Q4, and R7.

The latter sets the output cur­rent of the circuit at just under 1 amp. The constant current genera­tor load gives better efficiency than a load resistor and also gives good linearity.

R3 gives virtually 100% negative feedback over the amplifier at DC, giving unity voltage gain. By bias­ing the input to half the supply voltage using R1 and R2, the out­put is also biased to the required level of half the supply voltage.

R5 and C3 decouple some of the feedback at audio frequencies, giving the circuit a sensitivity of about 380 mV rms for maximum output. C2 and C4 aid stability, while C1 and C5 provide DC blocking at the input and output respectively.Q4 and Q5 must be mounted on a substantial heatsink.