| 摘要 |
In a telephone instrument, especially when using modern transducers such as electret devices or moving coil devices, it is desirable to have a volume control which is manually adjustable. This is done by a resistive network (9, 10, 11, 12) between the microphone wires and one of the id receiver wires which is grounded. This network is preferably at the handset. At the instrument body the inputs to the transmitter preamplifier (3) are connected via similar resistor to a common mode amplifier (8) the output from which goes to a multiplier (7). The multiplier (7) varies the gain of the audio signal according to the output from the common mode amplifier (8), which in turn is controlled by the variable resistor (9) in the resistive network. The output of the multiplier (7) goes to the input of the final receiver amplifier (6). <IMAGE> |
