Editing Repeating coil (section)

{{short description|Voice-frequency transformer}}

In [[telecommunications]], a '''repeating coil''' is a [[voice-frequency]] [[transformer]] characterized by a closed [[magnetic core]], a pair of identical [[balanced line|balanced]] primary ([[line (electrical engineering)|line]]) [[winding]]s, a pair of identical but not necessarily balanced secondary (drop) windings, and low [[Attenuation|transmission loss]] at [[Voice frequency|voice frequencies]].   It permits transfer of voice [[current (electricity)|current]]s from one winding to another by [[magnetic]] [[Electromagnetic induction|induction]], matches line and [[Drop (telecommunication)|drop]] [[Electrical impedance|impedance]]s, and prevents direct [[electrical conduction|conduction]] between the line and the drop.

It is a special application of an [[isolation transformer]], and is often used to prevent [[ground loop (electricity)|ground loop]]s or earth loops, which cause humming or buzzing in [[sound reproduction|audio]] [[Electronic circuit|circuits]].  It also prevents low [[direct current]] [[voltage]]s from passing.