This circuit was found in a 1980 publication. It uses only 3 easily obtainable transistors. It is a two-channel intercom with the two output transistors connected in the Darlington configuration with which a very good output volume and excellent sensitivity are obtained. The power supply must supply a voltage of 6 V and 4 batteries can be used, which will have considerable durability since, according to the author, the consumption of the device is only 20 mA. In view of the system operating at low impedance, the interconnection line does not need to be shielded, but it must not be more than 50 meters in length. The transformer used in this device has a primary winding of 2000 ohms and a secondary winding according to the speaker.
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