The transistor can be the BD135 and this circuit, found in a 1978 publication, can be powered by voltages from 1.5 to 6 V. The resistor must be changed to obtain the best gain without distortion. The circuit can receive radio signals and MP3, but if the signal is taken from the headphone output, a 47-ohm to 100-ohm resistor must be connected in parallel. The speaker is 8 ohms.
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