You may find that when you hook up an external USB card, you hear a very annoying buzzing sound coming from your speakers. This buzzing is more commonly known as 60 HZ cycling, which has to do with the grounding points within your computer. A few troubleshooting tips may enable you to eliminate the hum that you may be experiencing. With the right instruction, you can enjoy audio without all the extra noise.
Disconnect all devices from your computer, except for the USB sound card that you are trying to troubleshoot. Unplug the mouse, external storage, network adaptors and printers if they are connected.
Listen to see if this has resolved the issue. If not, continue on to step three. If the problem is resolved, then you have identified the devices that are causing an issue. Contact the manufacturer of the device that is causing the 60Hz cycling, because they may request that you send the equipment to them directly for repair.
Unplug the computer's AC cable if you are using a laptop. Using battery power can isolate the problem by eliminating the flow of power that may be causing the cycling sound. If you are using a desktop computer, power off and unplug the monitor. If this resolves your problem, then your grounding issue lies with the monitor.
Put the pair of headphones on and listen for the cycling sound. Unplug the speakers from the audio interface, so that the sound is only coming through the headphones. If the noise has gone away, then the grounding problem lies with your speakers.
Take note of any changes to the level of undesirable hum that follow unplugging all of your devices. If any of these devices have been determined to be the culprit, you must lift the ground with the 3-to-2-prong adaptor. Lifting ground is a technique that eliminates ground hum by interrupting the ground loops between multiple electrical devices. Connect the 3-prong-to-2-prong adaptor to the device's AC cable, which has been identified as the source of the 60 Hz hum by process of elimination mentioned in the above steps. If you are still experiencing hum through your USB device, you will need to contact the manufacturer to have the equipment serviced. There may be a wiring defect in the circuitry, and home repair may void your warranty.
Test to hear if the humming has disappeared after unplugging each of the devices in step 1. This identifies the device in which you need to connect a 3-prong-to-2-prong adaptor.
Lifting the ground can be dangerous on certain pieces of equipment. Consult the manual's documentation or the manufacturer before doing this procedure.