What is the safest and most noise free grounding method floating or non-floating

[larrygrobertson]

I am a hobbyist in the process of building an audio/video project in a 2U aluminum box. I have reached the point where I must choose which type of RCA & 3.3mm jacks to use before I drill the holes in the front and back panels.  I am using an Meanwell LRS-50-5 power supply https://www.mouser.com/ProductDetail/MEAN-WELL/LRS-50-5?qs=vDxCgdWo2h9hVrbBDwCVQA%3D%3D&gclid=Cj0KCQjwnMWkBhDLARIsAHBOftoTK-S-3O6f98SINotCcdsZPV8FJ6Vxe4LAA7dXasDB-o4X7zxwCosaAqk5EALw_wcB.

I have read countless documents and viewed schematics on this subject but am still confused when it comes to physically building a box and running all of the wires correctly and safely.

I drew a very simplified pictorial diagram (shown at bottom of post) to give a better context to my questions.

I have a 2U aluminum cabinet with basically 5 pieces front, back, left, right. and bottom which are all screwed together with M3 screws. Each piece provides two 6-32 ground studs so that AC shorts won't have to travel through the M3 screws. Inside the cabinet I have a 5VDC SMPS and a simple internal analog audio amplifier. The following questions relate to grounding only, the internal analog amplifier could be any amplifier and is not the focus of my questions.

I am getting a reading of 14.7 VAC from AC ground to signal ground when I float the signal ground. If I jump the two together with an alligator clip while the unit is still on and the headphone amp turned all all the way up full blast with no input signal, I can barely hear any noise or clicking when the alligator clip is connected and disconnected by tapping it in and out. I expected there to be a very noticeable noise but not at all. With music playing as an input signal, the sound is crystal clear even when I do the alligator test. This makes me think maybe it makes no difference, but is safety an issue?

QUESTIONS, please advise which scenario I should use with relation to external audio source devices shown in diagram:

Scenario 1 In the diagram, see "Scenario 1" isolates/floats the signal ground and uses insulated RCA1 and HEAD1 plugs to prevent the plugs from grounding to the aluminum case. All grounds will be run to an insulated ground star not connected to the case.

When connecting external input sources such as those shown in the diagram, will there be any noise or safety issues caused by floating the ground? Note I have shown an external source using a non-polarized AC cord and another external source using a 3-prong AC plug w/ground.

Scenario 2 In the diagram, see "Scenario 2" uninsulated RCA2 and HEAD2 plugs with metal cases grounding directly to the aluminum case. I will run a separate ground wire from the -5V output of the SMPS to the ground star G1 on this case. Is it necessary to run separate ground wires from each plug to the ground star G1, or will the aluminum case serve as ground without creating ground loops?

Will there be any noise or safety issues when connecting external sources while the AC ground and signal ground connected together?

[Roehrenonkel]

Hi there,
 
consider scenario 1.5:https://www.eevblog.com/forum/Smileys/default/wink.gif
Keep signal-ground and case-ground separate.
Connect these two by a resistor (1...10 MegaOhm) and parallel a cap (1nF 250V).

Should keep HF-noise away and the signal-ground at a safe level.

Good luck

[Conrad Hoffman]

A good general rule is to ask yourself, "does signal current flow in this connection?" If the answer is yes, it should have it's own path, not the chassis. I never chassis mount any input or output connectors to the chassis- all isolated. If you do have some hum or noise issue after that, it will be far easier to locate and correct. If everything is chassis mounted, not only are you likely to have a problem, it's darn near impossible to figure out what it is. In a perfect world, signal current would never flow in shields. Obviously we usually violate that with coax, though not always. Best books on the topic are from Ott. https://commons.princeton.edu/motorcycledesign/wp-content/uploads/sites/70/2019/08/otto_1988_noisereduction.pdf

[larrygrobertson]

Thank you so much, I will try the resistor and capacitor right away.

[larrygrobertson]

Roehrenonkel,
I ordered the capacitor & resistor from Digikey it should be here in a couple days.
Capacitor: https://www.digikey.com/en/products/detail/kemet/C711U102MYVDAAWL35/17838288
Resistor: https://www.digikey.com/en/products/detail/stackpole-electronics-inc/CF12JT1M00/1741064

It is possible the capacitor is built into the LRS50-5 SMPS, but the resistor is defiantly not built in because the resistance is infinity.  Would it hurt to add both just external to the SMPS in case?

Here is the internal block diagram of the SMPS:

[larrygrobertson]

Thank you Conrad,
That book contains valuable information on the subject. The book looks pretty old and that's what I like, you just can't get good books like that any more.  I will definitely read every word.  All of my books I bought in the mid 70s-80s they include chip bibles from TI Motorola and National Semiconductor. 

[Roehrenonkel]

Hi larrygrobertson,

Roehrenonkel,
I ordered the capacitor & resistor from Digikey it should be here in a couple days.
Capacitor: https://www.digikey.com/en/products/detail/kemet/C711U102MYVDAAWL35/17838288
Resistor: https://www.digikey.com/en/products/detail/stackpole-electronics-inc/CF12JT1M00/1741064

It is possible the capacitor is built into the LRS50-5 SMPS, but the resistor is defiantly not built in because the resistance is infinity.  Would it hurt to add both just external to the SMPS in case?

Good choise for the cap, the resistor could be bigger in size to withstand voltage-spikes (imho).
One could even make the grounding selectable (hard/floating/soft) by switch or banana-connectors.
You never know what will be connected to your box..... ;-))

The cap in the smpsu is to get rid of all the switching-noise to pass the emc-test,
maybe there is even one form the positive rail.

Good luck

[larrygrobertson]

Roehrenonkel,
I'll bump up the wattage of the resistor from 1/2W to 1W.

What is the difference between "hard" and "soft" grounding?

Is "hard" grounding a direct connection between the AC ground and signal ground?

Is "soft" grounding  using the resistor/capacitor?

Is "float" just leaving them totally isolated with no resistor/capacitor?

Would this be a good switch? https://www.digikey.com/en/products/detail/cw-industries/G-1128S-0055/164374?utm_adgroup=Slide%20Switches&utm_source=google&utm_medium=cpc&utm_campaign=Shopping_Product_Switches_NEW&utm_term=&utm_content=Slide%20Switches&gclid=Cj0KCQjwnMWkBhDLARIsAHBOftppf4AIcYpinrR8S_9cybLHknFx8zE1ivWsaLOSgjmBuhyjZRXNzvEaAn8sEALw_wcB


What would the current rating need to be for a switch or banana jumper?

Thanks

[Roehrenonkel]

Hi larrygrobertson,
 
yes, you got the point.

Your switch of choice is not the worst (70-ies Japan Radio-style).
Ideally no current should be flowing through the switch, but murphy is everywhere. ;-)
Reliable contact is the primary factor.
 
Experiment, listen, modify, hear...... :-))

[larrygrobertson]

Thank you so much for all of the information Roehrenonkel  |8)

[larrygrobertson]

Hi Conrad,
Thank you!
I was so excited about the book that I ordered a hard copy of it and the second edition as well.  It will arrive on 6/23/2023.  This is just what the doctor ordered for my tree of knowledge to grow one more branch.

Noise Reduction Techniques in Electronic Systems [Good condition (MD)] $12.29
https://www.thriftbooks.com/w/noise-reduction-techniques-in-electronic-systems_henry-w-ott/304295/#edition=3335827&idiq=4765916

Noise Reduction Techniques in Electronic Systems 2nd Edition [Good condition (GA)] $42.09
https://www.thriftbooks.com/w/noise-reduction-techniques-in-electronic-systems-2nd-edition_henry-ott/273447/#edition=3705910&idiq=4850182

[jonpaul]

Ott is classic and excellent.

Specific for audio is the fine SynAudCom og Jim Brown and Bill Whitlock.

https://www.prosoundtraining.com/2010/08/25/30-years-of-synergy/

See also  similar thread   cable shielding -- connect to ground on either or both ends

https://www.eevblog.com/forum/beginners/cable-shielding-connect-to-ground-on-either-or-both-ends/ here;

Bon Chance


Jon