Understanding Role Of Capacitor In Telephone Interface Circuit

Thread Starter


Joined Feb 9, 2015
I've seen this same circuit with many different capacitor values. The values range from 1.0uF to 47000 pF. Which would be the most appropriate for good quality sound? I understand that the capacitor is used to block DC in the circuit and keep the line from going off-hook. I don't understand the range of the capacitors used nor the use of the resistor. Any insight would be appreciated. :)phone_circuit_schematic.jpg


Joined Aug 21, 2008
The resistor is there to limit the current to a reasonable value. In the Bell System in the United States in the 1960's this was 50 milliamps or a little less. The resistor, capacitor and the inductance on the primary determine the low frequency cutoff. Generally, I used the largest non-polarized capacitors (in case something gets wired backwards) I had that were rated at 125% or more of the DC voltage, allowing that the capacitor will drop some additional voltage during the ring signal. From memory the value was 2.2 uf .


Joined Dec 13, 2013
the capacitor blocks dc while passing the audio and ring signals. the dc circuit on the origional phones was to indicate oh hook and off hook, and power the carbon button mirophones.


Joined Aug 1, 2013
Depending on the impedance that the transformer presents to the phone line at audio frequencies, the resistor might be needed to prevent excessive AC signal loading. The tradeoff is that it also forms a voltage divider with the transformer primary, reducing the signal amplitude at the earphone jack. If the transformer has a high impedance winding, the resistor is not needed in your application. 2.2uF is indeed a "standard" value. Western Electric used 2.0uF 200V caps in many applications. One way to approach the capacitor value is to consider that the phone system is derived from 600 ohm characteristic impedance design rules, and has a nominal bandwidth of 300 Hz to 3 KHz or 3.3 KHz or 4 KHz, depending on who's talking. 600 ohms and 2.2 uF give a corner frequency of 120 Hz, a little over one octave below the 300 Hz lower frequency response corner for good design margin.