Design the hearing aid described in the article, is designed to people with reduced hearing. On Fig. 1 shows the schematic schematic structure of the auditory apparatus.
It has the following technical data
the coefficient gain 5000;
the maximum voltage at the output of the resistance load of 60 Ohms 0.5;
bandwidth 300-7000 Hz;
a supply voltage of 9 V;
the current consumed in the mode silence, 7 mA;
maximum current consumption 20 mA.
Amplifier the three-stage apparatus. The first two cascades covered negative feedback DC for the purpose stabilization coefficient gain. Frequency feature in the area upper frequencies has a blockage that is the inclusion of the capacitor C3 between the collector and base the transistor T2. The purpose reduce noise of the first amplifier the cascade is made on a low noise transistor. With the collector of transistor T2 the signal is sent to potentiometer R7, role the gain control. The final stage is assembled by the amplifier circuit floating the working point, that can dramatically to reduce the current consumed in cascade the mode of silence. Led The DMZ serves as an indicator inclusion.
Hearing the device is assembled in plastic housing internal size 82 x 56 x the Amplifier 22 mm. mounted on a printed Board of foil Micarta or fiberglass. On the Board also, the regulator is attached amplification and microphone. The microphone wrap foam and thin the gum was suspended in cut at the pay window. The walls of the microphone should not touch Board and the housing a hearing aid.
All applied resistors-MLT-0,125. In quality controller gain used resistor type CP3-3A (for example, the regulator the volume of the transistor receiver). Electrolytic capacitors-C50-6. Capacitor C3-KLS or Km-4A. The capacitors C1, C7, C8 - km-6A, they can to be replaced by electrolytic capacitors of the same the C50 value-6 or other, in this case, however, will have change pattern of the printed conductors. Diodes series D9 or D2 with any letter index. In a hearing the device is installed electromagnetic microphone from mass-produced apparatus Bq-2 (601). Phone-TM-3 or TM-4. Powered hearing aid battery "Krona" a voltage of 9 V.
Establishing amplifier hearing apparatus begin with installation modes transistors T1 and T2 by DC resistors R4 and R6, then resistor R8 at mute set the quiescent current end of the cascade is equal to 2-2. 5 mA. The base transistor T3 with generator, signal a frequency of 1000 Hz and amplitude, appropriate the maximum amplitude of the the signal at the collector transistor T3. Resistor R9 achieve undistorted amplification signal. The current the collector of the transistor should have a value of 15-17 mA.
Establishing the first two stages on alternating current is reduced the selection of capacity capacitor C3 at the pleasant sounding, the absence of sharp "metallic" sounds.
Author: B. Muravin; Publication: www.cxem.net