Grounding and shielding question

[tempus]

Hey all;

I'm designing an effects switcher. It will be housed in a metal box, and all my pedals will have their ins/outs mounted to the box. For proper shielding, does the box have to be grounded to the signal ground, or could it be grounded to the power (earth) ground)? I'm asking because I had hoped to keep the signal ground floating and use a 3 prong plug to for AC power so i dont have to use a wall wart. I'm also hoping to avoid ground loop issues, both with that 3rd prong and the multiple connections to the metal box.

Thanks

[R.G.]

1. If you bring AC power line into the box, then the box MUST be connected to the safety ground. No exceptions. There are several interesting ways to die if you mess this up. Mother Nature administers punishment for violations of Her laws without mercy. Wall warts do have the saving grace that they completely encapsulate all the safety regulations and considerations into one easy to handle blob. Unless you're already skilled at AC wiring, it's better to use a wall wart for safety reasons.
2. You may keep the signal ground floating. However, this increases the chance for grounding hum (not ground loop hum) if you can't guarantee that the signal ground is connected to AC ground somewhere. It's probably best to 'ground' it the way you 'ground' your guitar strings for safety - through a big resistor (maybe 220k) and a 0.01uF/1kV ceramic cap. This at least gets rid of a lot of fluorescent buzz.
3. Hoping to avoid ground loop issues is a good thing. Doesn't always come true, even with the best of efforts, though. You might want to include a switch which hard-grounds the signal ground to chassis, or lets it float on the RF network. However, lifting the ground like this means you're open to electrocution from faults elsewhere in the signal chain.
4. There is a trick involving using a high-current diode bridge and a power resistor to let the AC power ground float by about +/-1.4V which gets rid of hum in many circumstances; however, no safety agency in the world would pass it as "safe" so it can't be recommended for safety reasons.

[tempus]

Thanks RG. I just need to clarify a couple things:

2. You may keep the signal ground floating. However, this increases the chance for grounding hum (not ground loop hum) if you can't guarantee that the signal ground is connected to AC ground somewhere.

By AC ground do you mean the safety (3rd prong), neutral, or ground on the secondary side of the transformer?

It's probably best to 'ground' it the way you 'ground' your guitar strings for safety - through a big resistor (maybe 220k) and a 0.01uF/1kV ceramic cap. This at least gets rid of a lot of fluorescent buzz.

Do you mean connect the signal ground to the chassis through a resistor and parallel the cap across it?

[R.G.]

By AC ground do you mean the safety (3rd prong), neutral, or ground on the secondary side of the transformer?

Yes, third wire safety ground. Neutral is not ground. Secondary on the transformer is not ground. Safety ground is what conducts any AC power shorts back to ground, holding the metal chassis at ground and keeping you from letting the power flow through your body.

Do you mean connect the signal ground to the chassis through a resistor and parallel the cap across it?

I mention that as a possibility I've seen. It does not meet recognized safety regulations so it can't be recommended. But if you did it, yes, you could connect the resistor and capacitor in parallel as you say.

[tempus]

Thanks again RG.

OK, so...

How does keeping the signal ground floating affect shielding effectiveness? What I was thinking of doing was either grounding the metal box to the safety ground (3rd prong), and isolating the jacks from the chassis, or using a wall wart and grounding the chassis to the signal ground at one place only. In the first case, the signal ground would not be connected to the chassis at all. In the second, the chassis would not be connected to the safety ground because I would be using the wall wart. Does either of these have a shielding advantage over the other?