This circuit was found in documentation from the 1980s. The integrated circuit is from the time and the voltage depends on the desired power, and the component's datasheet should be consulted.
This circuit was found in a 1992 publication, using an operational amplifier not very common...
This is a amplifier for XTAL high impedance earphones or piezoelectric types. Current drain is...
This circuit is from the LM111 manual of the 1980s. The component no longer exists, but...