The direct-coupled design of USB and Firewire and Thunderbolt audio interfaces can be susceptible to grounding anomalies that are difficult to discern. This can manifest in a variety of audible ways including hum, artifacts, and noises which can modulate with activity from the mouse, hard drive, wireless network, printer activity, or other functions of the computer. One way to begin troubleshooting is to monitor the outputs of the audio interface with headphones. If the noise isn't audible then it's likely that a grounding issue exists. If you look at the components of a computer based recording system, grounding noise can emanate from a multitude of sources. A few examples of these sources include:
- Input (Instrument, line-level device, etc.)
- Audio cables
- Audio interface power supply (if applicable)
- USB/Firewire cable
- Computer's AC
- Attached peripherals/devices
- Powered monitors or amp
A grounding problem could also be caused by the power, AC circuits, and power plugs of the room, or even the building you are in. To troubleshoot the issue you'll want to systematically remove, add or replace components in the system until the device (or devices) introducing the grounding issue has been discovered.
- Because variables can exist within the same space from one circuit or AC outlet to another. Experiment moving devices around to different AC outlets. The preferred setup in a audio/computer system is for the power to be provided from the same AC circuit/outlet.
- If a MacBook is being utilized, typically the AC power adapter (Magsafe) includes a 3-prong power cable and 2-prong adapter. Try utilizing the 2-prong adapter (North America). Does running the MacBook on batteries (=floating) solve the problem?
- Utilize balanced cables (1/4" TRS or XLR) when possible particularly when the audio interface is equipped with balanced inputs or outputs. Balanced circuitry is less prone to ground loops as the ground does not carry signal.
- When unbalanced cables (RCA, 1/8" or 1/4" TS) are utilized keep the length of the cable to a minimum, under 10 feet (3 m). Longer lengths can amplify and exacerbate grounding noise.
- Separate AC power wiring from audio cables.
- Ground lifting the AC of a device may resolve the problem. This should be done with extreme CAUTION, as grounding exists for safety. You'll want to consult with the manufacturer of the device about the implications of configuring the power in this manner. In North America this can be accomplished cheaply with a 3-prong to 2-prong ground lift adapter attached to the AC plug. In other countries a power strip without a ground may suffice.
- Depending upon the severity of the issue another product may have to be implemented to resolve it. This could be a DI with a ground lift option (for input sources like a keyboard, guitar, etc.), a transformer based solution like those offered by Jensen and EBTECH, power conditioning or regulating device.
If you have questions, are unable to resolve this or any issue contact Apogee Support for assistance.