A simple 15 watt power amp using darlington transistors
The circuit complexity for a final stage can be significantly reduced by the use of high-gain Darlington transistors. The 15Watt power amplifier shown here has even better performance in spite of lower effort, like the 15 Watt power amplifier presented elsewhere.
The LEDs cause a current limit in case of a short circuit in the loudspeaker line. Although this is not enough to secure the amplifier for longer-lasting short circuits, since then the allowable power dissipation of the transistors would be exceeded. But if the poles of the loudspeaker lines touch during operation only for a short time, the end transistors are so adequately protected. In practice, such occurs more frequently, e.g. if the wires are dissolved from the speaker terminals or if a jack plug on the box is disconnected during operation. For protection against longer lasting short circuits, a fuse should be inserted in the operating voltage line (1A ... 1,25A medium delay).
The transistor BD135 is used to stabilize the bias current at temperature changing. It has to be mounted on the heat sink for the power transistors.