The Better Audio Coupler
Improved version over the simple one. Added is a click suppressor using 4 each 1N4004 diodes, a capacitor, 0.022 MFD, to prevent RF (Radio Frequency) interference, and a 470 Ohm matching resistor. The transformer is the high cost one that will withstand a high voltage.
In this design you still need to keep the telephone off the hook. The 0.15 MFD capacitor can be increased to 1 MFD but the audio level in the telephone may drop.
Three coils are shown above. (1) The large Western Electric Type 120 Repeat Coil, (2) The small 1:1 Transformer (similar to the TU-016), and (3) A round Loading Coil, which is a 150 Ohm/150 Ohm Repeat Coil, but not designed as one. A House Key is shown for size comparison.
More versions of this will be added when I get a round2it.