We found this circuit in a 1973 publication. The circuit lets frequencies below 480 kHz pass through with an output impedance of 1k. The circuit can be implemented with current operational.
This alarm with sensors in series and parallel was found in the 1995 Encyclopedia of Electronic...
This integrated amplifier was used in televisions from the 70s, with good sensitivity...
This source, found in a publication from the 1980s, uses an integrated circuit that is uncommon today....