S9015 is a PNP  type of Bipolar Transistor. This transistor is primarily used for the amplification of low-gain signals. When a low gain signal is amplified to a high gain signal, the noise associated with the signal is enhanced as well. However, this transistor has a low noise feature that filters the noise from the amplified signal, resulting in a low noise and high gain signal at the output. If a small few mill watt input signal is provided, the transistor’s maximum collector dissipation is 400mW, which is enough to drive a small 2 to 3 inch 4Ohms or 8Ohms speaker directly.