How do metal cases work with connectors & isolation


I have troubles understanding how do metal cases behave in some cases.

Case 1:
I made passive attenuation box for amplifier(basically L-pad resistor), simple configuration:
Amplifier => attenuation box => speaker
It has 1/4 jack sockets on input and output like this one
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And pot resistor with metal on it's case
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Everything works fine, but as soon as I touch case or any metal part of pot with my hand it starts screaming from a speaker high-pitch sound. Why does it do that? Other commercial equipment uses the same jacks with metal cases and it has no problems with touching the case. And I can't ground it obviously because it is passive and input is floating from transformer.

Case 2:
On other powered devices without ground connection(example – guitar FX pedals) you also have metal case with metal jacks and it doesn't have any problems with touching either metal case or pots.

In both of these cases I don't see what's the trick here, at first I though I wire something in a wrong way, but there is no way to wire socket in such a way so that sleeve would not touch case – since the outer plane of socket touches case, obviously sleeve of connector will touch it no matter how you wire stuff internally.

So basically the questions are – how to properly wire metal parts that touch the case in order to make case out of the circuit.
I really tried to google that by myself, but it seems that most of people have no troubles at all with it, maybe I miss very basic thing.

Thank you for reading such a long question.

Best Answer

It's a common problem any time you get a high power output signal anywhere near the instrument level input signal. It's an induction/radiation problem, and it basically causes feedback. Always keep your instrument and its cabling away from your speaker wiring.