This circuit is used in the analog part of the telephone lines. The same circuit can directly coupled in series with the line. T1 is formed by winding 50 turns of 28 AWG wire on a ferrite rod, resulting in terminals 1 and 2, 200-1000 fine wire coil (32) for winding 3 and 4. The circuit can be powered by two or four batteries and the knob adjusts the volume to better undistorted sound quality. It can also be used a small power transformer and the low voltage winding connected to the points 1 and 2. The circuit does not work on digital lines.
