By Bruce Bartlett
Hum is caused mainly by
■ Cables picking up magnetic and electrostatic hum fields radiated by power wiring—especially if the cable shield connection is broken.
■ Ground loops. A ground loop is a conductive loop or circuit made of a cable shield and a power-ground wire. A ground loop occurs when two or more separated pieces of audio equipment are each connected to power ground through a 3-prong power cord, and are also connected to each other through a cable shield. The ground voltage may be slightly different at each piece of equipment, so a 50- or 60-Hz hum signal flows between the components along the cable shield.
These are the most important points to remember about hum prevention:
■ To prevent ground loops, plug all equipment into outlet strips powered by the same AC outlet or AC circuit.
■ Do not use an AC (electrical) 3-to-2 adapter to disconnect the power ground—it causes a safety hazard.
■ Some power amps create hum if they don’t get enough AC current. So connect the power amp (or powered speakers) AC plug to its own wall outlet socket—the same outlet that feeds the outlet strips for the recording equipment.
■ If possible, use balanced cables going into balanced equipment. Balanced cables have XLR or TRS connectors and two conductors surrounded by a shield. At both ends of the cable, connect the shield to a screw in the chassis, not to XLR pin 1. Or use audio gear whose XLR connectors are wired with pin 1 to chassis ground, not to signal ground.
■ Transformer-isolate unbalanced connections. If that is not an option, use the cable assemblies shown below in Figures A-3 and A-4.
■ Don’t use conventional SCR dimmers to change the studio lighting levels. Use Luxtrol® variable-transformer dimmers or multiway incandescent bulbs instead.
Even if your system is wired properly, a hum or buzz may appear when you make a connection. Follow these tips to stop the hum:
■ If the hum is coming from a direct box, flip its ground-lift switch.
■ Check cables and connectors for broken leads and shields.
■ Unplug all equipment from each other. Start by listening just to the powered monitor speakers. Connect a component to the system one at a time, and see when the hum starts.
■ Remove audio cables from your devices and monitor each device by itself. It may be defective.
■ Lower the volume on your power amp (or powered speakers), and feed them a higher-level signal.
■ Use a direct box instead of a guitar cord between instrument and mixer.
■ To stop a ground loop when connecting two devices, connect between them a 1:1 isolation transformer, direct box, or hum eliminator (such as the Jensen CI-2RR, Behringer HD400, Rolls HE18, or Ebtech He2PKG). See Figures A-3 and A-4 below.
■ Make sure that the snake box is not touching metal.
■ To prevent accidental ground loops, do not connect XLR pin 1 to the connector shell except for permanent connections to equipment inputs and outputs.
■ Try another mic.
■ If you hear a hum or buzz from an electric guitar, have the player move to a different location or aim in a different direction. You might also attach a wire between the player’s body and the guitar strings near the tailpiece to ground the player’s body.
■ Turn down the high-frequency EQ on a buzzing bass guitar track.
■ To reduce buzzing between notes on an electric-guitar track, apply a noise gate.
■ Route mic cables and patch cords away from power cords; separate them vertically where they cross. Also keep recording equipment and cables away from computer monitors, power amplifiers, and power transformers.
■ See Rane’s excellent article on sound system interconnections at http://www.rane.com/ .
By following all these tips, you should be able to connect audio equipment without introducing any hum. Good luck!
Bruce Bartlett is a microphone engineer, audio journalist, and recording engineer. A member of the Audio Engineering Society and Syn Aud Con, he holds a degree in physics and several patents on microphone design. He is also a musician and runs a 16-track digital studio specializing in live recording.He has written over 600 articles on audio topics for such magazines as Modern Recording, db, Recording, EQ, Mix, Recording Engineer/Producer, Radio World, Pro Audio Review, Audio, and the Journal of the Audio Engineering Society. He has also written a number of books including Stereo Microphone Techniques, Recording Music on Location, and the best-selling Practial Recording Techniques, all published by Focal Press. The new sixth edition of Practical Recording Techniques has just published! The above post is an excerpt from Chapter 4 – Equipping Your Studio.