In reality what we have is a microphone amplifier that works with 4 batteries and a magnetic earphone from 300 to 600 ohms. The microphone is made of crystal and the transistors can be BC558 for T1 and T3 and BC548 for T2. The circuit has low consumption. A 1M resistor between the collector and the Tl base can help to better polarize the transistor, regardless of the voltage generated by the microphone.
