This filter is from a 1977 publication. The circuit employs three 741 or other operational amplifiers. The supply must be symmetrical and the components with (*) are low tolerance.
The transistors of this amplifier for 8-200 headphones may be the BC548 or equivalent to replace...
This configuration appears elsewhere on this site with circuits obtained from other sources. This...
In this circuit of Texas Instruments an incandescent lamp is used as a light source, and lenses are...