Here's a thorough explanation of the audio connector bonding problem from K9YC:
http://www.audiosystemsgroup.com/Pin_1_Revisited.pdf
The problem is ripe when circuit board sub-assemblies are used as a direct
interface to the outside world. It's especially problematic when using an
extruded aluminum chassis. Typically, a circuit board with audio connectors
slides into the retaining fins of the chassis. But, there's no cost-effective
nor practical means to bond the connector to the chassis end-plates with a
bonding jumper after assembly. Many manufacturers ignore the potential for
hum/buzz, and settle for convenience and cost-effective assembly over noise
performance as discussed in K9YC's article.
Paul, W9AC
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