| 摘要 |
PURPOSE:To adjust steplessly a speaker volume and a telephone receiver volume with one variable resistor and to release further automatically a receiving volume variable condition with on-hooking by connecting and changing over automatically the variable resistor for controlling the volume at a speaker side to a telephone receiver side and abolishing the volume changing over switch of a conventional example. CONSTITUTION:In a receiving buffer amplifier 10, the fudback resistance of an arithmetic amplifier 32 is formed by three routes including a switch 25, switches 30 and 31 and a switch 26 respectively. A variable resistor 20 is for controlling volume. When a receiving signal is outputted from a speaker 23, the switch 25 is turned on by the inverting signal of a control signal 41 from a CPU, when the receiving signal is outputted from a telephone receiver 17 with a variable volume, the switch 26 is turned on with a control signal 42 and the switch 30 is in such a same way turned off with the inverting device signal of the control signal 42. The switch 31, at the time of the telephone receiver call, is always turned on by the control signal 41. As mentioned above, respective routes are selected in accordance with the condition of a button telephone set. At the time of completing the call by a receiving volume variable mode, the routes are changed over automatically to a receiving volume fixing mode, etc.
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