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 l...