Deaf hearing aid five

Circuit Operation Principle The deaf hearing aid circuit consists of a large output amplifier circuit, a volume control circuit and an audio power amplifier circuit, as shown in Figure 9-81.

The input amplifying circuit is composed of a microphone BM, resistors R1-R4, a capacitor C1, and a transistor V.
The volume control circuit is composed of a field effect transistor VF, a volume potentiometer RP, resistors R5-R7, capacitors C2, C3, C5 and a diode VD.
The audio power amplifying circuit is composed of an audio power amplifier integrated circuit IC, capacitors C4, C6 and a headphone BE.
After the power switch S is turned on, the whole machine is energized. The BM receives various sound signals and converts them into electrical signals. The electrical signals are amplified by V and VF, and then added to the 7th pin of the IC through C6. After further power amplification by the IC, the headphone BE is sounded.
A part of the IC output signal is rectified and filtered by C5, RP, VD, and C3, and a negative voltage is applied to the gate of the VF, and the input and output signal voltages of the IC are adjusted by controlling the impedance between the drain and the source of the VF. Achieve automatic control of the volume.
Adjusting the resistance of the RP can change the level of the negative voltage of the VF gate, thereby changing the volume of the headphone.
Component selection
Rl-R6 uses 1/8W metal film resistor or carbon film resistor.
The RP uses a small band switch (power switch S) potentiometer.
Both Cl and C4 use monolithic capacitors; C2, C3, C5 and C6 are all ultra-small aluminum electrolytic capacitors with a withstand voltage of 6.3V.
VD selects 2AP9 type é”— ordinary diode or 2AK series é”— switching diode.
V selects S9014 or 3DG6 type silicon NPN transistor.
VF uses a 3DJ6 junction field effect transistor.
The IC uses TDA2822 or D2822 type audio power amplifier integrated circuit.
BE uses 8Ω high-fidelity headphones.
The BM uses a high sensitivity electret microphone.