The Boulder Linux User Group mailing list had a short but interesting discussions today. Karl Horlen asked how he could reduce a constant buzz he was receiving from his power amp/speaker connected to his laptop. The buzz only occurs when the power supply is plugged in. I found this problem interesting since I was getting similar noises in recordings I made to my laptop when I played them back. Audio from music and video files did not have this noise.
Steve Queen made the observation that this is likely a ground loop problem. In layman's terms this means there are two competing ground points: one on the laptop plug and one through the power amp. To get rid of the buzz you have to eliminate one of those points. Steve also noted that a more distant power outlet for the laptop would probably aggravate the situation.
One possible solution is a three-prong to two-prong converter for the laptop which you can buy at local hardware stores. This is usually a fairly inexpensive solution. An alternative would be to use more expensive isolation transformers.
Another possible cause of a buzz from your audio is EMI – electromagnetic interference. This can come from the electronics within your laptop. Switching to battery powered speakers on your laptop can help deteremine if this is a ground loop problem or EMI. If the buzz goes away with the battery powered speakers it's likely a ground loop problem. If not, you have some EMI problems. Solving the EMI problems may require an external usb connected sound devices that do A/D conversion outside of the laptop (instead of using the internal sound system).
It was also noted by Mat James that, in his experience, laptops don't always have a ground or the ground from the power brick doesn't propogate to the laptop. That would mean the noise is not being caused by multiple grounds. Fortunately, plugging and unplugging a few fairly inexpensive devices (the power cable on the laptop and battery powered speakers) will help make that determination.
While the ground loop problem may address Karl's problem, I'm pretty sure it doesn't address mine. I haven't isolated the buzz yet though I suspect it's just a cheap microphone. It's also possible that the noise comes from the ground loop problem because the external stereo the laptop speaker port is connected to is active when I record. So I'll just have to dig around a while till I figure that one out.