How do metal cases work with connectors & isolation?

Question

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

And pot resistor with metal on it's case

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.

Answer 1

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.

Answer 2

The simple answer is to use a plastic enclosure. The next simplest answer is to buy parts that don't make electrical connections to the case. You may have to look at data sheets to figure this out. With some connectors, like BNC connectors, there may be a description of "Isolated" in the part name somewhere.

Other approaches would really require you to look at the data sheet for each part as you buy it, know what electrical connections get made to the case, and be ready to handle them. For some parts, insulating hardware for case mounts is available. For others, no such luck, and you have to jury rig your own. If every connection to the case is Ground, for example, you should be OK (but there may be electrical safety concerns).

For best safety, Plastic enclosures and no metal parts that can be touched by a user is the way to go (double insulation).

Answer 3

You're upsetting the load seen by the amplifier's output stage, probably by adding shunt capacitance, so you're causing an oscillation, at the resonant frequency of the speaker inductance and the total shunt capacitance (or some other frequency derived more complexly).

Solution: isolate the case completely from all connections and components. That potentiometer looks like it may carry connections through to the metalwork.

Another possible solution may be to add a Zobel network across the output inside the amplifier, typically 10R in series with 1nF. Tube amplifiers generally don't have these: solid state ones almost invariably do, and some also have a small series inductor in the output path.

Answer 4

This is perhaps a comment, but I hate plastic enclosures.

Why can't you ground the metal box? If the whole thing is floating with a lot of gain, then it should sing when you touch it... (maybe it would help with the florescent lights turned off?)

Ground is both your friend and enemy. The good news is that if you make a nice metal box there is both an inside ground and outside ground. (above a certain frequency... and only electrically, not magnetically, unless it is a thick steel thing.)

Getting it all right is a learning experience.... I could tell a story about a steel lock washer on the wrong side.

Edit: I should add that I have seen EMI leaking down metal shafts when you touch them.
(our solution is a trade secret, and would cost at least a beer. :^)

Answer 5

The sleeve of a phone plug and the outer conductor of a COAX connector should be connected to the ground/case of the equipment. If you have a poorly-made 1/4" phone plug cable that interchanges the shield/outer connection and signal/center connection you would likely have the problem you describe.

The case of your accessory should be connected to the case/ground of the equipment it is used with.