Noise reduction hearing aid

You don’t need to buy that connecting terminal, it’s for positioning. If you don’t want to use this switch, you can fly the wires (see the schematic diagram, there are only two wires)

By the way, the two microphones should not be too close to the board, otherwise they will not be welded together. Use a pipe to cover them as mentioned below (I plan to use the pipe to heat the melted glue)

In fact, the potentiometer is 47k, but I couldn't find it, so I used 50k, there is no difference.

The right side is the main circuit of the HA-5 hearing aid assembled with TDA2822 M or D2822 integrated circuits , which is currently relatively cheap and highly popular . It is essentially a BTL power amplifier composed of TDA2822 M integrated circuit. The left side of the dotted line is the added "noise suppression preamplifier circuit". Only this part will be highlighted below.

Noise-suppression hearing aids have a strong ability to suppress environmental noise and can be used in situations where the noise is as high as 90dB. The voice is clear and not noisy, with only weak background noise in the headphones.

This part consists of a low-noise transistor VT as the core to form a preamplifier. RP is used to adjust the volume, and the noise suppression function is completed by two electret microphones connected in series. The electret microphone (BP) is composed of a condenser microphone capsule with polarization voltage provided by electret material and a special field effect tube. Its internal structure and external wiring diagram are as shown in the figure. The capacitor film is connected between the gate G of the field effect transistor and the ground, and the signal is output by the field effect transistor. This kind of microphone has two types: source output mode and drain output mode. When the drain output mode is used, the phase of the output signal is opposite to that of the input signal; when the source output mode is used, the phase of the output signal is the same as the input signal. The signals are the same. The design of this circuit cleverly takes advantage of this.

  In the circuit shown in the figure, BPA is the source output mode. At this time, BPB can be regarded as the source resistance of BPA. BPB is a drain output mode. In the same way, BPA can also be regarded as the drain resistance of BPB. If the same external signal acts on two electret microphones A and B at the same time, they will output two signals with the same waveform and opposite phase at the positive terminal of C1; thus "cancelled" and cannot be sent to VT through C1 base, so the signal cannot emit sound from the headphones.

  Component selection and production:

BP is best to purchase matching microphones used in two-channel recorders, fix them on both sides of the printed board, and then put a plastic sleeve on the two microphones, with one end close to the hearing aid and the other end close to the printed board. . It is best not to make holes in the printed circuit board to prevent noise from entering the hearing aid box, causing resonance or causing an "acoustic short circuit" and weakening the noise suppression effect. BP has polarity, so it must be wired as shown in the figure to work properly.

Generally, hearing aids only have one microphone, while noise-cancelling hearing aids have two microphones. One microphone (BPA) is installed on the front of the hearing aid box, and the other microphone (BPB) is installed on the back of the box. It can be seen from practice that ambient noise can be considered to fill the entire room, so the intensity in front and behind the hearing aid box is almost equal, which makes its amplitude the same and opposite in phase and cancel it out. If the user points the BPA toward the speaker, then microphone A receives speech (stronger than BPB) plus noise, while microphone B still receives speech (but weaker than BPA) plus noise. The signal sent from the positive terminal of C1 is only Voice strength and weakness signals of BPA and BPB. As can be seen from the above, no matter how large the absolute value of the noise signals sent to microphones A and B is, their sum is always equal to zero.