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The Laptop Buzz
Eliminating unwanted noise in your audio output
While many musicians are embracing the laptop computer as their new instrument of choice, a few have encountered an annoying problem. When a laptop is plugged into a mains outlet, it is not uncommon for a number of noises appear to at the audio output. Sometimes these noises manifest themselves as “zipper” noises that correspond with moving the mouse. Other times, there will be quiet clicking noises in the background. The most aggravating thing about these sounds is that you often cannot turn them down. Even if you have a mixer between the audio output of the laptop audio interface and the monitor speakers, turning down the volume on the mixer may still not solve the problem, as the sounds are not part of the audio signal. These noises have piggy-backed themselves on the ground signal that runs in the opposite direction of the audio signal.
In an attempt to minimize the size of laptops, some manufacturers have a common ground that connects all components in the laptop as well as any external devices that may be connected. The solution is to isolate the laptop from other grounded devices. The quickest way to do this is unplug the power cord from the laptop. The computer will now be running on internal batteries, instead of the mains connection that shares ground with other outlets in the room, so the sound will be gone. Still, how can this problem be solved without running on batteries all the time?
The laptop may be isolated from ground between the mains power and the laptop, or between the laptop and the other audio equipment. To isolate using the first method, you will need a 2- to 3-prong power adapter. Plug your computer’s power supply into the 3-prong end, and then plug the 2-prong end into the power outlet. In this method, the third ground plug of the power supply is not connected to the mains outlet. The ground for the system is then provided by another piece of studio gear, such as the mixer or speakers.
If you are wary about disconnecting the ground of your laptop’s power supply, you may employ the second method. You will need a direct box with ground lift for each channel of audio connected from the laptop to external gear. Plug an output channel from the laptop into the direct box. Plug the output of the direct box into your mixer, amplifier, or speakers. Repeat this for every channel of output from the laptop. Then engage the ground lift switch on the boxes and the noise will vanish. Actually, the sounds are still there in the computer, but by disconnecting the ground to your other gear, the signal cannot contaminate the rest of your signal chain. And since these noises are in the ground path
Stephane
