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Audio feedback howl when connecting a notebook to a projector or amplifier
Published Date : 26 Feb 2008   Content Ref: TEC1117644  





Symptoms

If you connect a notebook to external speakers or an audio amplifier, under some conditions you may experience a high-frequency howl or a loud low-frequency hum.


Cause

This issue is common when unearthed devices (or devices which have a weak connection to earth) are connected together. Many notebooks and projectors have a double-insulated power supply and when connected together (eg projectors or audio amplifiers or external speakers) a ground-loop may be set up which can result in unwanted electrical noise which can be heard at audio frequencies.

You may find that a certain model of notebook, or notebook AC adaptor or amplifier exhibits this issue, but not other similar types. This will be due to the different way the earth connection has been made within these devices and may also depend on the internal power circuitry within each device.



Requirements

  • Ground loop isolator


Procedure

Check that you have a ground loop issue

Set up your equipment so that you can hear the unwanted noise (squeal or hum). Then, if possible, disconnect an appliance from the mains supply or disconnect other leads or cables. For instance, disconnect the notebook AC adaptor from the notebook and run it on the battery, or unplug the projector VGA cable or projector mains cable.

If you can establish that when a certain item of equipment or cable is unplugged and the unwanted audio noise disappears, then the issue is almost certain to be due to ground loop affects. If it is not possible to unplug any equipment or cables without breaking the audio playback circuit, you will have to assume that the issue is caused by a ground loop.


Fit a ground loop isolator

A simple fix for a ground loop is to fit a ground loop isolator. These are small unpowered devices which usually have four phono connections - typically left and right phono plugs and left and right phono sockets.

Connect the isolator into the audio 'chain', typically between the notebook audio output and the amplifier and speakers. You may need to experiment to find the best place to insert the device if you have more than two devices connected.

Notebook audio output - Ground loop isolator - Projector - Amplifier - Speakers

Ground loop isolators are inexpensive (approximately £10) devices which can be obtained from most online electronic retailers or your local car accessories shop.

You can either install one isolator in each room, or keep the isolator with each notebook. You may also need to purchase a converter cable to convert a 3.5mm audio jack to a phono connector.



More Information

A ground loop isolator contains an isolation transformer for each channel. These transformers usually have a 1:1 ratio which neither boosts nor cuts the audio level. The audio is magnetically coupled through the transformer's core. Since direct current cannot flow across the transformer, the earth path is cut and the feedback loop is broken.

Each channel (left and right) has two connections. An amplifier channel should amplify the voltage difference between these two connections. However, many devices also connect one of these connections to earth (or 'ground'). If two devices are connected together, the 'ground' of one device is thus connected to the 'ground' of the other device. If there is any small DC electrical noise or variation of voltage in the 'ground' of one of these devices, this may be detected by the amplifier as a 'difference' and amplified. The amplified signal causes an even greater disturbance of the 'ground' level and because the 'ground' of the amplifier is effectively connected to the 'ground' input of the audio source (eg notebook), the larger noise is then transmitted again to to the amplifier and speaker. Again, this slightly greater noise is amplified which causes and even greater noise on the amplifier 'ground' line and so on and so on. Because this happens at high speed (eg 10,000 times a second) we hear a squeal on the speakers.

If both devices have one of the signals directly connected to 'ground', there is not normally an issue. However, if one or both devices do not have a direct low-resistance 'ground' connection, this feedback loop can cause issues. In some situations you may only hear this issue when you connect a third device (eg a projector VGA cable).

An audio isolator breaks this loop by not directly connecting the two devices together - it only transmits the audio frequencies.



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Document Keywords: howling, squeal, screech, whistle, loud high pitch, mains hum, whiteboard, projector, amplifier, external speakers, ICT, EL81, nbook, notebook, laptop


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