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Not As Simple As They Look?

Identifying & Solving Microphone Problems

By Bruce Bartlett January 14, 2016

Besides sound quality, there really isn’t much to think about when it comes to microphones, right? Well, guess again!

Like all elements of a sound system, mics present their own unique set of special problems.

Fortunately, a lot of these problems are relatively simple to solve. It’s just a matter of identification and appropriate action.

For example, most mic handles include a set-screw near the connector, with many models using this screw to ground the mic handle. If the handle seems to be picking up hum when touched, check that the set-screw is fully secured down (turn clockwise until tight).

Inside the XLR connector on a mic cable is a ground lug, offering option of tying it to pin 1 or leaving it floating. If this ground lug is connected to pin 1, the connector shell is grounded. Then, if the shell touches a grounded metal surface, a ground loop can occur, causing hum. So, a better approach is to float the shell.

Lighting cables and AC power cables radiate strong hum fields, which mic cables can pick up, so keep them well separated from lighting and power cables. If the cables must cross, do so at right angles to reduce the coupling between them. In addition, vertically separate the cables.

Keep a set of “tweakers” handy to tighten down mic screws. (Mic image courtesy of Audio-Technica)

If your situation produces severe hum pickup when using dynamic mic models, try ones that include humbucking coils. In addition, twisted-pair mic cable can reduce pickup of magnetically induced hum. The more shield coverage, the less pick up of electrostatically induced hum. Braided shield generally offers the best coverage; double-spiral wrapped is next best, and spiral-wrapped is worst.

Also routinely check mic cables to make sure the shielding is connected at both ends. For outdoor work, tape over cracks between connectors to keep out dust and rain.

Shocking But True
At times, electric-guitar players can receive an electric shock when they simultaneously touch their guitar and a mic. This is caused when the guitar amp is plugged into an electrical outlet on stage, and the mixing console (to which the mic is grounded) is plugged into a separate outlet across the room.

These two power points may be at widely different ground voltages, so a current can flow between the grounded mic housing and the grounded guitar strings. This occurrence is especially dangerous when the guitar amp and the console are on different phases of the AC mains.

Twisted-pair cable reduces pickup of magnetically induced hum

It helps to power all instrument amps and audio gear from the same AC distribution outlets. That is, run a heavy extension cord from a stage outlet back to the mixing console (or vice versa). Plug all of the power-cord ground pins into grounded outlets. That way, you prevent shocks and hum at the same time.

Also, put a foam windscreen on each vocal mic to insulate the guitarist from shocks. As a bonus, a foam windscreen suppresses breath pops better than a metal grille screen.

If you’re picking up the electric guitar direct, use a transformer-isolated direct box and set the ground-lift switch to the position with the least hum. Using a neon tester or voltmeter, measure the voltage between the electric-guitar strings and the metal grille of the microphones. If there is a voltage, flip the polarity switch on the amp or reverse its AC plug in the outlet.


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About Bruce

Bruce Bartlett
Bruce Bartlett

Recording Engineer
   
AES and SynAudCon member Bruce Bartlett is a recording engineer, audio journalist, and microphone engineer. His latest books are “Practical Recording Techniques 5th Ed.” and “Recording Music On Location.”
http://www.bartlettaudio.com

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