This circuit is intended for underwater communications. Of course, the critical point is the shielding to avoid water. The circuit is from an Electronic Hobbysts Handbook from the 90s.
This filter with Butterworth response makes use of operational amplifiers 741 or equivalent and...
This Testa o Croce (name in Italian) was obtained from an Italian magazine in 1994. The circuit can...
This circuit was obtained in a 1979 publication, but it can be assembled with the BD135 instead of the...