The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This circuit was found in a 2005 Linear Technology application manual. The circuit is for 48 V...
EZP-Q metallized polypropylene capacitors from Panasonic's can be purchased from Mouser...
We found this circuit in an American 80's documentation. The resistors must be of high precision...