A Faraday cage is simply a shield that wraps around the component / circuitry. In extreme measures, it is basically a "tank-like" enclosure with very minimal entrances and exits and in other situations, it can be something as simple as loosely spaced screening.
Proper design of an audio component would have the chassis acting as a Faraday cage. That is, the chassis would be electrically isolated from the hot and neutral AC line yet tied to Earth ground. This in turn would act as a shield for the active circuitry, minimizing the potential for external RFI and EMI to corrupt the signal path.
Many designs integrate the chassis into the active electrical circuitry via using it as a common connection point for the AC neutral. This is done for sake of convenience and lower production costs. This is a poor design in my opinion. Anything that touches the chassis therefore becomes part of the active circuit in such designs. Hopefully, you can see how / why this would be less than optimum, let alone have the potential for greater health / safety risks. Sean
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