Verry interesting... Read this description of the little pod in the middle of the cable... It's more than just resistors! It's a filter that improves the sound...
FIG. 5 is a schematic diagram of the configuration of the junction unit 15 which connects the earphone cables 13 and 14 to the common cable 16 and which includes
special electrical filters for enhancing the performance of the earphones 11 and 12. One conductor 63 and one conductor 64 of the cables 13 and 14 are directly
connected to a ground conductor 65 of the common cable 16. A second conductor 67 of the cable 13, a second conductor 68 of the cable 14, and conductors 69
and 70 of cable 16 are respectively connected to pads 71, 72, 73 and 74 of a circuit board 76 which is formed with two circuits operative to increase the signal
applied to the receivers of the earphones 11 and 12 as a function of increasing frequency.
As shown, a capacitor 77 and a resistor 78 are connected in series between pads 71 and 73 while a resistor 80 is connected directly between pads 71 and 73, in
parallel with the series combination of capacitor 77 and resistor 78. Similarly, a capacitor 81 and a resistor 82 are connected in series between pads 72 and 74 while
a resistor 84 is connected directly between pads 71 and 73. It will be apparent that at very low frequencies, when the capacitive reactance is high, the series
impedances are determined primarily by the value of the resistors 80 and 84. As the frequency increases, the series impedances are reduced, increasing the
amplitudes of higher frequency components of the applied signals.
This electrical filtering operation is found to be highly desirable, permitting the use of an amount of acoustic damping sufficient to smooth out peaks in the acoustic
responses of the earphones 11 and 12 while obtaining optimum frequency response characteristics. By way of example, the value of each of the resistors 78, 80, 82
and 84 may be 100 ohms and the value of each of the capacitors 77 and 81 may be 0.22 microfarads. The circuit board 76 and the parts thereon preferably have
quite small dimensions. Each of the resistor and capacitor parts preferably has maximum dimensions of 0.150".times.0.300".times.0.100". These dimensions are
desirable to obtain a compact junction unit and are such that if desired, as when a single earphone is to be used, the filter for each filter might be located within the
earphone, e.g. between the receiver 18 and the end cap 29. In a two earphone assembly such as the illustrated assembly 10, however, it is generally preferable to
locate the filters in the junction unit 15.
