| 摘要 |
PURPOSE:To prevent howling due to change in the connecting state of a telephone line by connectng a matching resistor to a 2-wire side of a 2-wire/4-wire converting circuit via a switching element switched by a DC signal from the telephone line. CONSTITUTION:A resistor R having an impedance equivalent to that of a telephone line L is connected to the 2-wire side of a hybrid coil HYB via a variable resistor element X and then the telephone line L is connected via a diode bridge DB. Then, the variable resistor element X shows a high resistance depending on a signal from the telephone line L and a low resistance when no signal exists. Thus, even if no telephone line is connected to a loudspeaker telephone set, the 2-wire side is terminated by the resistor R, the balancing is kept and no howling is produced. |
