In some occasions ( such as conference call meetings) needs to be enlarged voice phone for people to listen to, not only this, sometimes the content needed to record the phone . The circuit can meet both requirements, making changes when the phone without the use of time as long as you can on the phone at . When the voice coil within the current in the telephone ( earphone coil or induction coil machine ) in the passage , in the vicinity and the voice on the existence of consistent current variation of the magnetic field , if time plus two ends of a coil L will be sensitive in its Out with the same magnetic field variation of the signal voltage , then amplifies the signal processing , you can push for people to listen to the speaker .
Induction telephone amplifier circuit shown, the figure will phone in the voice coil L leakage magnetic field into electrical signals , passed into the control potentiometer RP1 formed by the transistor VT1 pre- amplification stage , and then Through the capacitor C4 and the potentiometer RP2 into the power amplification stage by IC1 ( audio integrated amplifier LM2896) for power amplifier , the output from IC1 pins sound signal to promote the speaker BL1 . When the speaker impedance of 412 hours , IC1 ‘s output power up to 5W more than 100m2 in a room or meeting room , available for dozens of people to listen . To be able to phone , record , IC1 ‘s output signals are part of the coupling through the capacitor C10 , resistors R8 and R9 partial pressure limit , by the plug XP recorder line input to the input of real-time recording.