This filter for receiving audio in telegraphy was found in a magazine 73 for radio amateurs, in an edition of the 80s. The circuit is tuned to 700 Hz and has a 200 Hz bandpass.
This circuit was obtained in a 1975 magazine and can be implemented with BC548 transistors. The...
Found in Popular Electronics, this '90s circuit generates harmonic-rich signals for testing audio...
This configuration can be applied to other operational amplifiers to increase the current...