In the operation of the telephone apparatus of the second and third groups of difficulty with carbon microphones is often observed the drop in sensitivity. This is due to sintering of particles of coal that causes disruption of clarity and distortion in the form of cod and the interference. There are currently a number of schemes to replace coal microphones. The sound quality is increased by an order of magnitude.
In Fig. 1 is a diagram using a dynamic microphone MDM-7. The amplifier in the three-stage scheme is executed, the transistors are preferably selected with high gain.
In Fig. 3 shows the amplifier circuit for a condenser microphone MKE-3. Scheme by repeating showed very stable operation. In Fig. 4 shows the amplifier circuit for a miniature microphone type FEM-B ("Pine"), and, finally, in Fig. 2 shows a diagram where as the amplifying element using a telephone key KA. With microphone MKE-3 circuit has a sensitivity of about 400…500 mV/PA provides a good hearing.
Fig. 1. Diagram of the carbon microphone replacement
Fig. 2. Scheme of replacement carbon microphone using a key KRKA
Fig. 3. Diagram of the carbon microphone replacement
Fig. 4. Diagram of the carbon microphone replacement
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