Shields in internally balanced equipment

[dashbad]

I have searched and read numerous threads on grounding but not found an answer to this query.

I am planning a Control Room Monitor unit with input selection and volume control. I intend to keep the signal path balanced from input to output. This is to keep the signal as untouched as possible during normal operation.

I understand that it is recommended to connect the shield connections at the input and output to the chassis - thus making the chassis an extension of the shield of any incoming/outgoing cables. The Chassis will be aluminium.

Does this mean that I can run the balanced signal between PCBs inside the chassis as unshielded twisted pair without much issue?

What are/if any, the disadvantages with this approach? Should I be running shielded cables internally instead.

Signal wires will be kept well away from AC power and only likely to be close to low level DC.

Many thanks in advance

 

[epicyclic]

Connecting the XLR screen and using unscreened as your suggesting sounds OK to me but you will only really know by trying it .
For some very good info have a look at www.sound.westhost.com/articles/balanced-2.htm with very relevant info towards the very bottom ,figs 16 and 17.

 

[Simon B]

The ESP page that epicyclic links to is excellent and if you want to indulge yourself in more, much of it by Bill Whitlock, author of the material on balanced stuff on Rod Elliott's site, have a look at

JENSEN TRANSFORMERS, INC. - APPLICATION PAPERS AND SCHEMATICS

JENSEN TRANSFORMERS, INC. - APPLICATION SCHEMATICS

possibly particularly

Build the "Hummer" to Find "Pin 1 Problems" in Equipment (17KB PDF)

Proper Grounding Inside Equipment Avoids Ground Noise Coupling (aka "Pin 1 Problems") (15KB PDF)

Be warned, this site induced download frenzy in me when I went there, thought I'd grown out of that some time ago.....

 

[DF96]

If there is an interference source (e.g. PSU) within the box then you need to be careful. Of course, your balanced line receivers will reject common-mode noise but not differential noise.

 

[Simon B]

If there is an interference source (e.g. PSU) within the box then you need to be careful. Of course, your balanced line receivers will reject common-mode noise but not differential noise.

Wise words from DF96, as usual. Power supply rectification / mains noise ingress being possibly the most likely areas to watch.

 

[dashbad]

Thanks for the responses. Very useful. So it would seem that correctly grounding any equipment inside the case is key and mains/PSU noise will be of most concern inside the case.

How does this noise manifest itself in signals inside the case? Is it RFI that I can remove by shielding signal cables (with shield unconnected at either end)?

If all signal cables are twisted pair is it not likely that any injected noise will be common mode? In what situation might it be differential?

 

[DF96]

Mains and its harmonics are likely. Could be coupled via capacitance or magnetically. Good twisting will reduce magnetic coupling, which will be differential-mode. Good common-mode rejection will reduce the effect of capacitive coupling.