The DC bias voltage at the output of the unity gain, inverting preamplifier B, precisely matches that of the input of preamp A. When the telecoil is placed between these two pins, no DC current will flow through the coil as the potential at both ends is equal. With stage B configured as unity gain, its output appears as a virtual ground to AC signals. The lower -3 dB corner frequency of the amplifier is set by the ratio of the telecoil resistance to the inductance, given by the equation: RCOIL