Secondary circuit using transistor as silencer

Secondary circuit using transistor as silencer

The headphone circuit is one of the essential audio circuits for MP3/MP4 machines. Figure 9-31 shows a headphone circuit using transistors as muffler tubes
. The output audio signals (L, R) are sent to the headphone interface through coupling capacitors CT14 and CT16. There are muffler control
transistors . Q3, these two transistors are controlled by Q4. Q4 is a PNP transistor . When the base of Q4 has a low-level control signal, Q4 is turned
on, so the collector voltage of Q4 provides high voltage to the bases of Q2 and Q3 respectively through R88. level, Q2 and Q3 are turned on. The audio signal is shunted to the ground by Q2 and
Q3, with no output and signal, and is in a mute state. When no headphones are plugged in, the ④ pin of the headphone interface is connected to the right channel, and the right channel audio
signal is sent to the speaker amplification circuit. When headphones are plugged in, the signal to the speakers is cut off.
    In addition, the ground terminal of the headphone interface also serves as the antenna of the FM radio circuit . When the headphones are inserted , the shielding layer of the wire acts as
the external antenna of the FM radio to receive FM broadcast signals.