This ampli is thought to be a booster for a portable music player. It has a really small output power (0.12 W RMS on a 36 Ω load) and can be used as a tabletop ampli, just to avoid working with headphones plugged in the ears (and missing phone calls...). It consists in a common base input stage (polarization current 1.5 mA) with low impedance (about 15 Ω) for a best match with that of the device (the headset output). The amplification is about 150 and it drives the power section made up of a darlington assembly. The Power stage has a resistive load with a work current of about 200mA. The bias point is set via the trimmer R7 that needs to be regulated to place the emitter of Q1 (a 2N2905 in the present case) at half the supply voltage.
The circuit is mounted in a wooden cabinet that contains also the speakers, being the output power extremely limited.
The circuitry is installed in the central section of the cabinet: the picture below shows the ampli board on the left (the power transistors covered by the copper heat sink) whereas on the right we have placed the voltage supply. The transformer is contained in the wall plug visible in the previous picture.
