Abstract: A ground start loop current detector is used to monitor the loop current in a telephone subscriber loop when the subscriber requests service by grounding one line of the loop (ground start), when a busy subscriber discontinues use by opening the loop, and when a called subscriber answers in response to ringing by closing the loop. In order to operate in the presence of longitudinal (common mode) interference currents in the loop, especially in the ground start situation where only one line is monitored, the detector incorporates a differential amplifier which has its inputs connected to the loop through transformer windings that are phased so as to equalize the effects of longitudinal currents on the inputs of the differential amplifier.