This filter for frequencies from 300 Hz to 3.4 kHz was found in the February 1995 Radio Electronics magazine. Other operational amplifiers can be experimented, and the source must be dual.
This is the project found in a 1981 publication, but it is still viable. It is a simple AM...
This commercial earphone receiver was manufactured in the 1960s in Japan. It was powered by a 9 V...
This circuit was found in a European publication from the 1980s. The transistors can be the BC548...