In some occasions, we need to amplify the sound during a call for a speaker , and sometimes the contents of the calls need to be recorded. This circuit describes this public address and recording add-on devices with a same telephone. The working principle of the circuit shown in Figure 1 .When the current passing through the telephone coil is in the same variation as like the voice, then we can even add a multi-speaker to listen.
In the figure, the voice signal in through the telephone coil L will be converted to electrical signals. After the magnetic field leakage through the adjustable resistor RP1, the control is accessed from the transistor-level preamplifier which composes of VT, and then the signal enter through the C4 and RP2 power amplifier stage, which is carried out by the IC power amplification. From the output signal pin IC , the promotion of BL emit loud speaker’s voice. R5 constitute a class in which the depth of the negative feedback amplifier can significantly improve the sound quality. R6, C7 a network mainly to protect amplifier and speakers. LM2896 is a 40 pin IC load, the output power up to 5W, in a room of 100 square meters available to listen to dozens of people. In order to be able to call the contents of recording, LM2896’s output signal through the capacitor C10 are part of the coupling, R8 and R9 limit partial pressure, enter the recorder’s line input in real-time recording.
The IC uses a low-voltage high-power audio amplifier IC LM2896, and this circuit uses only one channel. It is characterized by noise, low distortion , the use of a wide voltage range, its supply voltage can be within the 3-9V choice.
The circuit can be connected to the battery to adjust RP2, hand-to-IC input , speakers should have a greater exchange of BL in the “buzzing” sound,which indicates that the power amplifier is working correctly. RP1 and RP2 are used to adjust the sound clarity to the smallest noise. C
If sound distortion or noise is too large, its enough to re-adjust the RP1, also adjustable RP2. For recording, it will be inserted into the CT plug the recorder’s line input jack. In addition to L, and CT, the entire circuit and speaker BL can be made into the form of a speaker, public address, just the pickup coil L close to the telephone headset, power switch S can be closed.