Tracking: 24 Ways To Bust HUM

You patch in a piece of audio equipment, and there it is—hum! This annoying, low-pitched sound is typically a tone of around 60Hz (50Hz in Europe and other parts of the world), as well as multiples of that frequency. Hum is caused mainly by ground loops. A ground loop is a closed loop made of two separated pieces of audio equipment each connected to power ground through a three-prong power cord, and also connected to each other through a cable shield. The ground voltage may be slightly different at each piece of equipment, so a signal can flow between components along the cable shield. Other causes of hum can be guitar pickups, cables, and mics that pick up hum fields radiated by power wiring and transformers—especially if the cable shield connection is broken.

Here are the most important points to remember about hum prevention:

1. To prevent ground loops, plug all equipment into outlet strips powered by the same AC outlet. First, make sure that the sum of the current draws of the equipment does not exceed the breaker’s or outlet’s amperage rating (typically 15 or 20 amps).

2. Never use an AC 3-to-2 adapter to disconnect the power ground, as this is a safety hazard.

3. Use short cables.

4. Some power amps create hum if they don’t get enough AC current. Therefore, connect the power amp’s (or powered speaker’s) AC plug to its own wall outlet socket—the same outlet that feeds the outlet strips for the recording equipment.

5. If possible, use balanced cables going into balanced equipment. Balanced cables have XLR or TRS connectors, and two conductors surrounded by a shield. The cable should use twisted-pair wires to reject magnetic hum fields, and it should have heavy braided copper shields to reject electrostatic hum fields. Optional: At both ends of the balanced cable, connect the shield to a screw in the chassis—not to XLR pin 1. Or, use modern audio gear whose XLR connectors are wired with pin 1 to chassis ground, rather than to signal ground. There should be a continuous connection from one chassis to another through the cable shields.

6. Transformer isolate unbalanced connections. To stop a ground loop when connecting two devices, connect between them a 1:1 isolation transformer, direct box, or hum eliminator— such as a Jensen Iso-Max CI- 2RR, Behringer HD400, Rolls HE18, or Ebtech He2PKG.

7. Don’t use fluoresecent lights. Don’t use conventional SCR dimmers to change the studio lighting levels. These clip the AC waveform and generate lots of harmonics. Use Luxtrol variable-transformer dimmers, or multi-way incandescent bulbs instead.

8. If you are working with a recording that already has hum on it, apply narrow notch filters at 60Hz, 120Hz, and 180Hz (or 50Hz, 100Hz, 150Hz in countries with 50Hz power). Raise and lower those frequencies slightly to find the best hum-rejection points.

9. If hum is coming from a direct box, flip its ground-lift switch. Usually, but not always, you may need to lift ground if the musical instrument has a power cord, or is connected to an amplifier.

10. Check cables and connectors for broken leads and shields.

11. 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.

12. Remove audio cables from your devices, and monitor each device by itself. It may be defective.

13. Lower the volume on your power amp (or powered monitors), and feed them a higher-level signal from your mixer or audio interface.

14. Use a direct box instead of a guitar cord between instrument and mic preamp.

15. If you use a snake box to route mic signals in your studio, make sure it is not touching metal, which can cause a ground loop.

16. To prevent accidental ground loops, do not connect XLR pin 1 to the connector shell.

17. Try another mic. Dynamic mics have a coil of wire which can pick up hum radiated from power wiring. Some dynamics have a hum-bucking construction.

18. Turn down the high-frequency EQ on a buzzing bass-guitar track.

19. To reduce buzzing between phrases on an electric-guitar track, apply a noise gate with a fast attack and moderate decay.

20. Route mic cables and patch cords away from power cords. Separate them vertically where they cross. Also, keep recording equipment, instruments, and cables away from computer monitors, power amplifiers, and wall warts.

21. If you hear a hum or buzz from an electric guitar, have the player move to a different location, or aim the guitar in a different direction. The guitar pickup is sensitive to magnetic hum fields, which are directional, so moving about can sometimes do the trick.

22. If you’re playing a guitar with single-coil pickups, see if another instrument with humbuckers can do the job.

23. If the electric-guitar hum stops when the player touches the strings, that indicates the player is picking up hum radiated from the AC wiring, and is re-radiating it into the pickup. When the player touches the grounded strings, their body becomes grounded, and no longer radiate hum. In this case, ask the guitarist to keep at least one hand in contact with the strings while playing.

24. When you’re recording a live concert, power all instrument amps and audio gear from the same AC distribution outlets. This is also a safety issue, as electric-guitar players can receive a shock—or cause hum—if they touch their guitar and a mic simultaneously. This occurs when the guitar amp is plugged into an electrical outlet on stage, and the mixing console (to which the mics are grounded) is plugged into a separate outlet across the room. If you’re not using a power distro, these two power points may be at widely different ground voltages, and current can flow between the grounded mic housing and the grounded guitar strings. Electric guitar shock is especially dangerous when the guitar amp and the console are on different phases of the AC mains. If you lack a power distro, run a heavy extension cord from a stage outlet back to the mixing console (or vice versa). Plug all the power-cord ground pins into grounded outlets. That way, you prevent shocks and hum at the same time.