This circuit was obtained in a Popular Electronics of the 90s. The circuit uses integrated from the time, uncommon today. Note that the speaker has a higher impedance than ordinary speakers.
This amplifier for hearing aid was found in a 1968 documentation using an integrated of the time....
We obtained this circuit from a 1993 documentation. The circuit has 9 current ranges that are...
In the figure we have the transmitter of a data transmission system with optical fiber suggested by...