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load resistor and 8k2 emitter resistor.
The voltage acrossthe 6k8 will never be more than 0.7v due to it being between the collector and emitter terminals of a BC 557 transistor.
This means the collector of the BC 547 will sit at 3v - 0.7v = 2.3v.
Thebase voltage will be 2.1v and the emitter will be 1.4v
The sounds picked up by the electret microphone cause a charged mylar diaphragm to vibrate and re-distribute some of the charges. Thisre-distribution is detected by a FET transistor (Field Effect Transistor) inside the case of the microphone and its resistance changes.
This causes a varying current to flow and this causes a varying voltage toappear across the 10k resistor.
This waveform is picked off the 10k resistor and becomes the volume control for the hearing aid.
The whole circuit for this project is directly-coupled and thismakes it very difficult to diagnose and test, if something goes wrong. That's why you have to understand the circuit fully before you start working on it with test gear.
We have explained how the...
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