<<<IF the cable shield is separate from both signal conductors (that is, hot and return, as in a balanced mic), it's fine to separate return and shield. But if there's only the shield, it MUST go to the chassis. >>>
If one "ground" pin was dedicated to the chassis or cabinet ground, the other pin could be run through RF bypasses to the "cold" side of the audio input PC board. Then, in the case of a mic with an isolated ground return inside the shield, or even a balanced mic or unbalanced mic, the audio return could go to that pin. The shield could always go to the cabinet or chassis with the lowest possible impedance. No chokes, no bypasses. This is the same place the mic connector shell should go. If the mic had only the shield for an audio return, the shield and audio return pins could be bonded at the connector. This would be a simple solution that would cover all bases. For some reason this is an age old problem with Ham gear. Early tube gear used the case as a ground, but for some reason when the digital engineers got involved grounds started getting messed up. This is true for lightning grounds also. For example I did an antenna switching controller and made the chassis the common point for all grounds. The fellow writing the firmware did a whole new layout on his own where he isolated the digital and signal grounds. The result was a unit that failed CE testing for ESD between ports, and had significantly more RF out on leads from the clock. Strong RF would just kill the thing. I had a similar problem with a VOMAX speech processor. The RF power supply decoupling was on a single toroid and both negative and positive supply leads passed through it as a bifilar winding. While this provided reasonable common mode isolation, it offered virtually no differential isolation. The foil trace from the PTT lead (that had no RF isolation at the jacks) ran right along side the audio trace. Bypassing to the case as a common ground, and splitting the power supply leads into two isolated chokes, totally cured the VOMAX. 73 Tom ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
