Transistor amplifiers are fun to build. Here is a 2.5 W stereo audio amplifier built with common transistors and medium power BD139/BD140 output stage. Schematic, PCB and simulation file provided.

Single channel schematic

Amplifier simulation in LTspice

Component outline

Built stereo amplifier

This amplifier is easy to build using common discrete parts. A channel uses only four general purpose transistors (3 NPN like BC547, 2N2222, 2N3904 and 1 PNP like BC557, 2N2907, 2N3906) and two medium power complementary transistors like BD135/137/139 with BD 136/138/140 or MJE200 with MJE210. Almost any pair of complementary transistors will do as long as hFE is greater than 60, maximum collector current at least 1 A and dissipation at least 10 W.The power transistors must be fitted on a heatsink! They have their collectors electrically connected so you don't have to isolate each other. The heatsink should have an area of at least 40 square centimeters. Note that the heatsink voltage is neither ground or supply voltage so do not connect it with anything else besides transistors. Be careful not to connect it with chassis ground. If you build the other channel for stereo use, do not electrically connect the heatsinks of both left and right channels.After building the amplifier you must adjust the DC operating point. In order to do this, connect IN to GND with shortest possible wire and connect a 4 ohms speaker to the output. Using a voltmeter, measure voltage on the collector of the power transistors (or heatsink) and adjust R5 trimmer until you read 6 V (half of supply voltage).The maximum output power is delivered without distortion with a 5 mVp-p input signal. A LTspice simulation proves ~ 2.5 W medium output power.Below is the component outline on PCB. You will have to build two of those PCBs for a stereo amplifier.Here are the two PCBs with heatsinks for a stereo amplifier.'s article from Romanian electronics magazine, 1985 Almanac, pages 68-69.