IMPROVED CIRCUIT FOR TELEPHONE SET 



617 



>!5 



UJZ 

 OR 



0.02 



0.04 



0.16 



0.18 



0.06 0.08 0.10 0.12 0.1 



LOOP CURRENT IN AMPERES 



Fig. 5 — Level of 500D telephone set at various loop currents with and with- 

 out equalization. 



amount for long loops to a maximum for short loops. It is important to 

 note that the greatest portion of this loss is the ac shunting loss intro- 

 duced by the varistors. 



SIDETONE BALANCE 



Before considering the matter of sidetone balance, it will be helpful 

 to examine some of the features of the telephone set circuit which 

 are required for good receiving and transmitting performance. For sim- 

 plicity, the induction coil is shown in a hybrid arrangement in Fig. 3. 

 The speech currents received from the loop pass through the windings 

 A and B of the induction coil which are so connected that they produce 

 additive voltages in winding C which is connected to the receiver. 

 These additive voltages would cause a resultant current to flow in the 

 network resistance (68 ohms), were they not opposed by an approxi- 

 mately equal voltage 180° out of phase which results from the receiving 

 current in winding B. Therefore, there is little or no voltage drop across 

 the network resistance. Maximum receiving levels are thus obtained 

 without appreciable power loss in the network resistance. 



The transmitter of the set is made low in resistance because it is in 

 series with the loop and influences the permissible loop range of the 

 set from the standpoint of supervision. The impedance Zi looking toward 



