Hearing aid. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electronics in medicine
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The design of the hearing aid described in the article is intended for people with hearing loss. On fig. 1 shows a schematic diagram of the design of the hearing aid.
It has the following technical data
gain 5000;
maximum output voltage with a load resistance of 60 Ohm 0,5 V;
operating frequency band 300-7000 Hz;
supply voltage 9 V;
current consumed in silent mode, 7 mA;
maximum current consumption 20 mA.
The device's amplifier is three-stage. The first two stages are covered by negative DC feedback in order to stabilize the gain. The frequency response in the high-frequency region has a blockage, which is carried out by turning on the capacitor C3 between the collector and the base of the transistor T2. In order to reduce the inherent noise of the amplifier, the first stage is made on a low-noise transistor. From the collector of transistor T2, the signal is fed to the potentiometer R7, which acts as a gain regulator. The final stage is assembled according to the floating operating point amplifier circuit, which makes it possible to drastically reduce the current consumed by the stage in silent mode. The DZ LED serves as an on indicator.
The hearing aid is assembled in a plastic housing with an internal dimension of 82 x 56 x 22 mm. The amplifier is mounted on a printed circuit board made of foil-coated getinax or fiberglass. A gain control and a microphone are also attached to the board. The microphone is wrapped with foam rubber and hung on a thin elastic band in a window cut out in the board. The walls of the microphone must not touch the board and the body of the hearing aid.
All resistors used are MLT-0,125. An SP3-3a type resistor was used as a gain control (for example, a transistor receiver volume control). Electrolytic capacitors-K50-6. Capacitor C3-KLS or KM-4a. Capacitors C1 C7, C8 - KM-6a, they can be replaced with electrolytic capacitors of the same rating K50-6 or others, while, however, you will have to change the pattern of printed conductors. Diodes - series D9 or D2 with any letter index. The hearing aid uses an electromagnetic microphone from a commercially available apparatus BK-2 (601). Phone-TM-3 or TM-4. The hearing aid is powered by a 9 V battery.
The adjustment of the hearing aid amplifier begins with setting the operating modes of transistors T1 and T2 for direct current with resistors R4 and R6, then with resistor R8, with the microphone turned off, the quiescent current of the final stage is set to 2-2,5 mA. A signal with a frequency of 3 Hz and an amplitude corresponding to the maximum amplitude of the signal at the collector of transistor T1000 is supplied to the base of the transistor T3 from the generator. Resistor R9 achieve undistorted signal amplification. In this case, the collector current of the transistor should have a value of 15-17 mA.
Establishing the first two cascades for alternating current comes down to selecting the capacitance of the capacitor C3 according to the most pleasant sound, the absence of sharp "metallic" sounds.
Author: V. Muravin; Publication: cxem.net
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