Traditional Chinese Practical Hearing Aids, Memory Enhancers: Circuit Design, Parts List & Workshop Prototype Guide
In short: Traditional Chinese Practical Hearing Aids, Memory Enhancers. This entry documents a circuit & prototype taken from Chinese practical-technology literature, written for small-scale makers and home producers. The text below is the full English translation of the original Chinese workshop notes, covering materials and quantities, the step-by-step sequence, and the practical cautions recorded in the source. Roughly 2618 words of English technical text, translated and preserved without editing. It is intended for experimental and reference use, not for regulated or commercial certification.
Hearing aids, memory enhancersA small hearing aid can be assembled with some common components for use by the elderly or those with weak hearing. If you replace the hearing aid earplugs with headphones or double earplugs, it can also be used as a memory enhancer. Reading into a microphone, reciting texts or foreign words can assist in increasing memory and is suitable for general learning.The electrical schematic diagram of the hearing aid and memory enhancer is shown in Figure 150. It consists of a microphone and a multi-stage audio amplifier. The speech signal is picked up by a microphone, and after audio amplification, people with weak hearing can hear the other party's words clearly through earplugs. The microphone uses an electret condenser microphone commonly used in general radio recorders. It's small, but commercially available microphones have poor sensitivity consistency. In order to ensure that the whole machine has a large amplification amount, the amplifier circuit is composed of five transistors. In order to simplify and stabilize the entire circuit, the base bias of each tube is supplied through the collector load resistor, forming a voltage parallel negative feedback circuit. Therefore, the circuit distortion is small, the operation is stable, and the adjustment is simple.The signal from microphone BP is amplified by VT1 and directly coupled to VT2. The amplified signal is coupled to CT3 for amplification through C3. Potentiometer RP1 is used for volume control. The circuits of VT4 and VT5 are basically the same as those of VT1 and VT2. VT1 to VT4 are all NPN type 3DG6 silicon tubes, VT5 is a PNP type 3AX31 germanium tube, and the β value is >30. 3AX31 should be selected with small leakage current to reduce noise; other tubes can also be replaced by similar tubes with the same performance. The earplugs are commercially available 8Ω low-resistance type. The whole machine is powered by one AA battery.Figure 150The adjustment of this hearing aid is extremely simple. It mainly adjusts the three bias resistors R2, R6 and R7 so that the collector-to-ground voltages of VT1 to VT4 …
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Lei Cheng
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