Telephone amplification and recording additional circuit

Telephone amplification and recording additional device In some special occasions, the telephone sound needs to be amplified for multiple people to listen, and sometimes the telephone content needs to be recorded. The amplification and recording additional device introduced in this article can meet these two purposes. There is no need to modify the telephone when making it, and it only needs to be placed next to the telephone when using it. working principle   　　　　　  　　

The circuit working principle of this device is shown in Figure 1. When the voice current passes through the internal coil of the telephone (the handset coil), there is a magnetic field near it that is completely consistent with the changing pattern of the voice. If an external coil L is added at this time, a changing pattern of the magnetic field will be induced at both ends. The same signal voltage and then amplifying that signal can drive the speaker for multiple listenings. In the figure, after the coil L converts the leakage magnetic field of the telephone voice into an electrical signal, it enters the preamplification stage composed of the transistor VT through the control of the adjustable resistor RP1, and then enters the power amplification stage through C4 and RP2, and the power is performed by the IC. Amplified, the signal output from IC ② pin drives the speaker BL to emit a loud sound. Among them, R5 constitutes the deep negative feedback of the power amplifier stage, which can significantly improve the sound quality. The network composed of R6 and C7 is mainly used to protect amplifiers and speakers. The output power of the LM2896 integrated circuit can reach 5W on a 4Ω load, which can be listened to by dozens of people in a 100-square-meter room. In order to record the phone content, part of the output signal of LM2896 is coupled through capacitor C10, current limiting by R8 and voltage divider by R9, and then input into the line input of the recorder for real-time recording. The component selection and production component list is shown in the table below. No. Name Model Quantity No. Name Model Quantity R1 Resistor 1M 1 C5 Electrolytic capacitor 10u/16V 1 R2 Resistor 8.2K 1 C6 Polyester capacitor 50P 1 R3 Resistor 470Ω 1 C7 Polyester capacitor 0.1u 1 R4 Resistor 510Ω 1 C8 Electrolytic capacitor 150u/16V 1 R5 resistor 51K 1 C9 electrolytic capacitor 1000u/16V 1 R6 resistor 1Ω 1 C10 electrolytic capacitor 4.7u/16V 1 R7 resistor 51Ω 1 C11 electrolytic capacitor 220u/16V 1 R8 resistor 10K 1 VT transistor 9014 1 R9 trimmer resistor 47K 1 IC power amplifier Integrated circuit LM2896 1 RP1, RP2 adjustable resistor 47K 2 L homemade telecoil 1 C1 polyester capacitor 0.01 1 S toggle switch 1 C2 electrolytic capacitor 0.47u/16V 1 BL speaker 4Ω 10W 1 C3, C4 electrolytic capacitor 1u/16V 2 The IC uses the low-voltage high-power audio power amplifier integrated circuit LM2896, and only one channel is used in this circuit. It is characterized by medium noise, low distortion, wide operating voltage range, and its power supply voltage can be selected in the range of 3-9V. The coil L is densely wound with 1000-1500 turns of high-strength enameled wire with Φ=0.06mm on the flat medium-wave magnetic rod. You can also use the output transformer in the tube radio to modify it, remove the "I" shaped silicon steel sheet, leave the "E" shaped silicon steel sheet, insert it in one direction, and wind it with 2500-3000 turns of high-strength enameled wire with Φ=0.07mm. After the two methods are prepared, it is best to dip them in paint. After the circuit is soldered, you can connect the battery to adjust RP2, and touch the input terminal ⑤ pin of the IC with your hand. There should be a loud AC "buzzing" sound in the speaker BL, indicating that the power amplifier is working properly. Then touch the center pin of RP1 with your hand, and adjust RP2 at the same time to make the sound clear and the noise minimal. When making a call, you can place the pickup coil L close to the earphone in the microphone, and the speaker BL should have a loud and pleasant voice. If you think the sound is distorted or the noise is too loud, you can adjust RP1 or RP2 appropriately. The sensitivity of this induction coil is extremely high, and strong medium wave signals may be "received". When using it, the angle and direction of L should be adjusted so that the sound emitted by the speaker is the largest and the interference sound is the smallest. When recording, insert plug CT into the line input jack of the recorder. Except for L and CT, the entire circuit and speaker BL can be made in the form of a speaker. When amplifying, just put the pickup coil L close to the telephone headset and close the power switch S.