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Yet another ground debate

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  • #16
    I'm with Enzo on this. If it works, it's a good grounding scheme. What makes grounding confusing is that for any given situation, there are many, possibly an infinite number of grounding schemes that give very low hum/noise; likewise, a possibly infinite number that don't work well.

    In designing or modifying a grounding scheme, you have a few different ways to go. You can go with tradition - that's the way we've always done it, don't rock the boat. You can go with easter-egging (I wonder what happens if I move this wire over there...). Or you can try to think about what the voltages and currents and electromagnetic fields do - "lessee here - I want to write down Maxwell's equations with complex differential boundary conditions and then solve the resulting partial differential equations with complex-number matrix inversion... ".

    Or you can go with a few guiding principles and then tweak it in. I like the few-principles approach. For the question of how/whether to ground pot cases, I go with the principle of the Faraday cage. A Faraday cage is a complete conductive shell surrounding some volume. The million-equations approach shows that no electromagnetic field outside the cage (* with the exception of a DC magnetic field!) can cause interference inside the cage. A cage with holes only lets in EM radiation with a quarter wavelength shorter than the biggest dimension of the holes. So for 5/60/100/120/200/240... Hz, a metal enclosure is pretty darned good. If you run circuits inside there that run to the outside world, you want the circuits to be using a ground reference tied to the shield.
    But to keep surface currents from inducing voltages in your circuit, you want to attach the enclosing shell to ONE AND ONLY ONE POINT on the shell. This is the ideal case.
    Where in the circuit or on the shell the single connection is made does not matter much. You can pick your one point to be the input jack. Works. You can pick it to be your output jack. Works if you don't connect the input jack too, and don't run return/ground currents through the shell. This just goes on and on.
    So picking the input jack as the shield enclosure shell ground is OK.

    Meanwhile, back at the original question - what to do with pot shells: Pot shells can be made to form part of the Faraday cage both to keep outside signals out of the circuits, and to keep AC hum fields from the inside of the chassis from getting into the signal path. They ought to be grounded to the enclosure shell. One traditional way is to use star washers to the chassis. Works well if the shell crimping makes firm electrical contact. To bolster this, some chassis use a copper/brass strip inside the chassis that the bushings go through. To preserve the shielding effect while not mixing in Faraday cage currents, it's important to NOT connect any pot lead that happens to be carrying signal ground to the pot shell. Don't bend the pot terminal over and solder it to the pot shell. Instead, run that signal ground wire back to the part of the circuit it came from. If you do this, any enclosure voltages/currents can't inject signal onto your signal ground. This makes pot shell grounding fairly uncritical. Generally the shell crimping is fine for a long time. Or you can make sure by soldering a wire to the pot shell.
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.