John Schmidt, P.E., is principal of John Schmidt Audio Video Systems Design in Hempstead, N.Y. For 36 years until retirement, he worked for ABC Television in New York as a senior audio video systems engineer, designing and supervising the installation of audio systems.
After reading a recent Workbench column, John offered a few suggestions to help the uninitiated create more problems than they solve when working with microphone cable shields.
Individuals involved in connecting analog audio equipment should familiarize themselves with two Audio Engineering Society standards: AES 48 and AES 54. These describe best practices in dealing with cable shields.
John’s first suggestion based on the guidelines is that the microphone cable shields need to be connected — typically to Pin 1 of an XLR connector, as seen in Fig. 1 — in a continuous, preferably isolated path from the mic to the input of the mic preamp.
Keep in mind that mics that utilize phantom power will not work without a shield (Pin 1) connection, as the cable shield provides a return path for the phantom power that operates the electronics inside the microphone.
Even if the mic does not use phantom power, failure to connect the shield may leave the mic circuit subject to hum or buzz due to capacitive coupling to one or the other of the active conductors, or interference due to RF pickup.
Now the issue moves to the line-level interconnects. Here there is a conflict between the practicality of the interconnecting equipment — which may not have been designed with best practice for grounding the shield connection (Pin 1 on the XLR) at the equipment interface — and requirements for RF immunity.
If all equipment was designed with proper termination of the shield connections for both its inputs and outputs, in accordance with AES 48, one would be advised to connect the shields of all input and output cables where they interface with the equipment at both ends. But as many of us have learned, this sometimes creates hum in the form of a ground loop.
At the expense of losing some RF immunity, connect the shield at only one end.
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If breaking the shield at one end solves your hum/buzz issue but leaves the equipment RF susceptible, try connecting the unterminated end of the shield to the metal case of the equipment through a small capacitor.
Transformer boxes can be useful, but John recommends specifying transformers with internal shields between the windings. Otherwise the capacitive coupling between the windings can pass interference.
Also keep in mind that some transformers have very poor frequency response when fed from a low-impedance source. If your circuitry is unbalanced, with the shield grounded at both the output and the input, and is carrying the return side of the audio signal, all bets are off, and you may really have a ground loop.
Finally, regarding analog video: Yes, the coax is fed with an unbalanced signal; however, most professional video equipment uses differential receive amplifiers, where the shield of the input is not grounded. The same is true for the unbalanced “composite” input of many FM exciters.
As readers can see, this is not a trivial issue.
John Schmidt’s website is www.john.schmidt.audio/main/.
A better way to model
On Oct. 29, Radio World reported on a proposal by Dielectric and other antenna manufacturers to allow computer modeling of FM directional antennas.
The FCC has opened a notice of proposed rulemaking to take industry comments about this. It says more than 2,000 full-service FM stations, more than 20 percent of them, use directional antennas. The change would allow any of them that replace existing antennas to avoid the expense of field measurements. It would apply not only to applicants for new FM facilities but to FM licensees applying for facility modifications.
John L. Schadler, a friend and former co-worker of mine when I was at Dielectric, outlined the studies that support this proposal in a presentation for the Broadcasters Clinic in October.
John’s presentation is available online and is fascinating. Of particular interest are the computer simulations John shows of FM coverage. Adjustment of parasitics or spacing yields nearly instantaneous display of the FM coverage pattern. These measurements, done physically on a range or in an anechoic chamber, currently take hours or days. In just a few minutes, patterns can be reliably modeled, modified or adjusted to meet coverage restrictions.
Final thought
Kuala Lumpur engineer Paul Sagi has followed our discussion of converting to LED lighting. He writes that heat is not the only enemy of LEDs; the wrong type of power supply can also shorten their life. LEDs work best on a constant current supply, rather than a constant voltage supply, which can also reduce their service time.
Got a great ideas? Share it! Tips for Workbench qualify for SBE recertification credit. Email [email protected].
John Bisset, CPBE, is in his 31st year of sharing reader tips in Workbench. He handles western U.S. radio sales for the Telos Alliance and is a past recipient of the SBE’s Educator of the Year Award.
