This LM380-based circuit was found in documentation from the 1980s. The components are current, and the transistor may be a general-purpose equivalent. Note the use of a medium impedance speaker.
Being able to operate with frequencies up to 100 kHz, this circuit was found in a 90’s...
The integrated circuit used in this filter is not very common today, but the reader can use...
This circuit doubles the frequency of signals from 10 to 50 MHz. We found it in a 1969 Electronic...