This circuit was found in American documentation from the 90's. The circuit applies to any operational. The power supply can be between 5 and 15 V depending on the operational.
The figure shows three circuits for using headphones in amplifiers with speakers that do not have...
In this charger, the charging current is basically limited by the LEDs. The transformer is a low...
This configuration is from an old integrated circuit manual. The circuit can be applied to modern...