Click here to purchase the entire book in PDF format. Common Impedance Coupling Common impedance coupling occurs when there is a shared wire between the source and the receiver. Let's say, for example, that two units, a microphone preamplifier and a food processor, are both connected to the same power bar. This means that, from the power bar to the earth, the two devices are making use of the same wires for their references to ground. This means that, in the case of this shared ground wire, they are coupled by a common impedance to the earth (the impedance of the ground wire from the power bar to the earth). If one of the two devices (the food processor maybe...) is a source of noise in the form of alternating current, and therefore voltage (because the wire has some impedance and V=IR) at the ground connection in the powerbar, then the ``ground'' voltage at the microphone preamp will modulate. If this is supposed to be a DC 0 V forever relative to the rest of the studio, and it isn't, then the mic pre will output the audio signal plus the noise coupled from the common impedance.