A small refinement of the phone with a carbon microphone according to the scheme in Fig. 1 will save owners from strong interference in the form of cod due to the low quality of the MIC capsule. When connecting this circuit to the apparatus to observe polarity. The gain of the transistor must be in the range of 10-15, but if it is, then you have to choose the resistance of the resistor R1.
Refinement collected by the circuit of Fig. 2, does not require polarity, but the charge should be placed in the tube under the microphone.
Author: K. K.-Yanchuk, Kyiv; Publication: N. Bolshakov, rf.atnn.ru