Radio with amplifier (CB20486E)

We found this circuit in a publication of 1991. The audio amplifier is an operational that admits equivalent.

Variable low pass filter 741 (CCB5600E)
Using the 741 operational amplifier this circuit was found in the August 1993 Electronics Now...
Solid State Autotransformer (CB6573E)
We found this circuit in a 1967 documentation. The circuit can be designed based on current...
Transistor Schmitt trigger (CB7801E)
Found in Popular Electronics, this configuration can be powered by voltages from 6 to 12 V....