Microphones are a special case, as they're hanging out there alone, not connected to earth via a U-ground utility plug. Between devices that are, care is needed to avoid the inevitable voltage drop causing a 60Hz current to flow in a shield that induces hum electro-magnetically in signal conductors. For these interconnects, shields are connected to female XLR pin 1 at the source end, but kept open at the destination end male. An exception is in high RF environments (radio transmitters) where at the destination the shield is connected through a capacitor that shorts the RF, but blocks LF hum. Finally, it’s up to the device designer to collect all pin 1’s to the common ground, usually one point in the power supply.
OTOH:
A Practical Interference Free Audio System (Part 2)
https://www.emcstandards.co.uk/file...oise_for_emcj_may_02_pdf_version_4_may_02.pdf
10 Conclusion
For many audio engineers that have been continuously working in the industry for as long as I have (nearly 40 years), most of the information in this article will seem like sacrilege. I too, thought that it was all pretty far-fetched, when I first learned of the techniques necessary for designing electronic components and systems to be able to pass the EMC Directive. You will know all the arguments against such concepts, and I have personally rehearsed them too. My interest in even trying out some of the techniques described here, was brought about by my sheer frustration in spending hours and hours chasing hum, buzz and RF interference noises during component or system design, testing and commissioning, year-in and year-out, rather than keeping within the law.
It was with much trepidation that the first basic system was put together with all I/O cable shields bonded at each end, and mesh grounding techniques used between the console, local and remote racks. Many people attended the first installation, just to get a look at my face when the hum got so loud that loudspeaker cones would leave their cabinets horizontally. We were all in for a big surprise. Instead of loud and uncontrollable noise - there was silence. After some moments, a distant voice echoed my own thoughts: “Is the system turned on yet?” Three different voices answered “yes”. The owner of the dissenting voice who muttered: “It can’t be.”, pushed a fader to “+10” and pressed the appropriate play button. Everyone ducked as high performance audio filled the room, very loudly indeed.
None of the information in this article is just “theory”. All of the techniques described here are based on practical experience with practical components and systems. I have put in some additional physics to help explain why the techniques work, and probably got a bit “carried away” with the extra information in sections 8 and 9. But the more I learn about how to use these techniques, the more detail I find necessary to include in the explanation of
why it works.
Note that I started with the safety aspects of electronic system design. We all need to be able work safely. Star ground power systems can provide good safety, but only at power frequencies. Today’s audio systems must be able control interference phenomenon many tens of MHz above the required audio pass band, due to the necessity of incorporating processor controlled devices that have become part of the system specifications. Unfortunately, even well designed and maintained star ground systems cannot help us with high frequency interference control. But few star ground systems remain safe over the longer term. They tend to degenerate into multi-ground-loop nightmares. I come across star ground systems that have deteriorated so badly over the years, that engineers have to disconnect the protective ground conductors on the AC power input cables of some equipment, just to be able to obtain a reasonable noise floor, disregarding the safety risks this introduces. This problem is certainly exacerbated by large numbers of audio equipment still being designed with incorrect shield termination methods (and other EMC design deficiencies). However, the old methods of eliminating ground loops, but leaving the driving potentials that cause the current to flow in place, must now be considered to be a retreat from good design practice.
Many audio experts still insist that bonding audio cable shields to ground at both ends produce noise artefacts called SCIN (Shield Current Induced Noise), due to the imbalances in balanced audio cables. In practice, the input and output filter techniques discussed in section 7, have no difficulty in controlling such problems, as well as providing excellent EMC. Now, even if it could be proved that lifting the shield at one end gives measurably better results for audio band noise tests - this is no use to those of us that
must use processor controlled boxes within an audio system (analogue or fully digital).
Only fully shielded, filtered, and mesh bonded cable structures stand any chance of being able to control the overall interference problems associated with this type of audio/digital control system, give good sound quality,
and produce a safer working environment. Such systems are also easier to design and less costly to test and commission [3 and 6]. Better still, you don’t have to “fiddle about” trying to figure out which end of the cable shield to bond to ground, or which to “lift”. Much less frustrating, and a more efficient way of working.
Customers who specify complex computer controlled audio systems, get rightly upset when interference couples with the cable system and causes the installation to malfunction. At the same time, they insist on the highest quality audio performance, and a safe working environment. Since the techniques described in this article achieve all three criteria extremely well, they should now be considered to be the best engineering practices for pro-audio systems.
References
[1]IEC 61000-5-2:1997 “
Electromagnetic Compatibility (EMC) – Part 5: Installation and mitigation guidelines – Section 2: Earthing and cabling”
[2]“
EMC for Systems and Installations”, Tim Williams and Keith Armstrong, Newnes, 2000, ISBN 0 7506 4167 3 (
www.newnespress.com)
[3]“
Bonding cable shields at both ends to reduce noise”, Keith Armstrong and Tony Waldron, EMC + Compliance Journal, May 02 (
www.compliance-club.com).
[4]Journal of the AES: Volume 43, Number 6, 1995
[5]
“The Keith Armstrong Portfolio”, EMC + Compliance Journal,
www.compliance-club.com/KeithArmstrongPortfolio
[6]
“Designing for Interference-free Audio System Components”, Tony Waldron, EMC + Compliance Journal, July 02 (
EMC Information Centre - The EMC Journal (Free in the UK))
Tony Waldron is Technical Manager for CADAC Electronics PLC. Sound Designer at The Royal Danish Theatre in Copenhagen (1979-81) and Head of Sound at The National Theatre, London (1983-89).[email protected] Home - Cadac
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