Push-pull power amplifier on complementary darlingtons

Four diodes provide too strong bias. Together with positive temperature coefficient of output transistors and lack of thermal coupling, the result is sadly expected.

An option is to use three diodes, but the circuit is still thermally unstable until diodes are on the heatsink, and it has smaller but still very high crossover distortion.

^{simulate this circuit – Schematic created using CircuitLab}

Better solution is to use transistor. It must be mounted on the same heatsink, preferably on top of one darlington. Quiescent current should be adjusted to about 20mA (start with maximum resistence, measure mV on emitter resistor).