This amplifier for hearing aid was found in a 1968 documentation using an integrated of the time. The power is from a battery, the consumption of 1.65 mA and the power of about to 1 mW.
This alarm does not interrupt its operation when the power is cut. A battery is kept at a constant...
The transistor of this circuit can be a BD135 or TIP31. The load current is given by resistor R1....
We found this circuit in a 1973 publication. The circuit lets frequencies below 480 kHz pass...