Macs, hum, and unintended aerials

So you have just paid several hundred pounds/euros/dollars for an expensive audio system to connect to your Mac, and all it seems to do is hum. Do you dismount it and hook it up to your more traditional HiFi, or return it in disgust?

If you have a wideband and sensitive receiver and care to pass it around your Mac, you will realise that Macs, like any other computers, emit a mush of radio signals, ranging from some low down in the spectrum, up to plenty of different microwave frequencies. They are low power, so unless you put an aerial close to your Mac, they are not likely to cause any problems.

Sometimes, though, we do just that. Many even quite expensive analogue audio cables, particularly those connecting loudspeakers, unintentionally act as ad hoc aerials, as can any decent length of coaxial cable, etc. If you were to connect your wideband receiver to one of those cables, you would be amazed at their effectiveness.

Most radio-frequency emissions have no significant effect. Sometimes they happen to hit just the right frequency to cause you hassle; the most common problems arise from those which can result in audio interference, typically those from 40-60 Hz AC mains power, which is within most people’s hearing spectrum: hence the frequent issue of persistent mains hum at about that frequency.

Sometimes these parasitic signals shouldn’t cause a problem, but the equipment which picks them up demodulates them like a radio receiver, and you can end up hearing voices, music, or noise. This can happen if you are close to a poor quality, high-power radio transmitter, something which could include a taxi firm’s base station, for example.

This type of interference is confined to cables carrying analogue signals: it cannot (unless intense) affect digital data in USB, Thunderbolt, or FireWire cables.

There are two issues which you need to address when troubled by such interference: good electrical grounding of all electronic systems, and preventing inadvertent signal pick-up.

Internally, the circuit boards and components in your Mac have a common ground which needs to be connected to an external ground. This normally happens through your Mac’s power supply to the mains power cable, which in turn connects to the building’s ground system. In some countries, like the UK, the mains ground line is separate from the ‘return’ side of the mains supply, through a three-pin power plug and dedicated earth bonding in the electrical supply system. A professional electrician should be able to check the grounding of your mains supply, and ensure that it is properly bonded to earth.

Good electrical grounding is important for electrical safety, too.

Laptops and mobile devices are of course different, unless they are connected to mains power.

Other electrical components, such as peripherals, should operate with a common ground, such as the ground lines in USB and other buses. But not all devices are well grounded, and some peripheral cables do not have good grounding: these issues can be addressed, sometimes just by using a better cable.

A ferrite bead to reduce interference. By Karl-Martin Skontorp, via Wikimedia Commons.

Significant lengths of cable carrying power (AC mains or from a DC converter) should then have RF chokes fitted. These are inexpensive clip-on sheaths of ferrite, which filter out lower frequency interference, including 40-60 Hz ‘mains hum’. Sometimes, particularly with longer DC supply cables, it is better to wrap the cable around a ferrite torus a few times, which can choke out even quite powerful interference.

Ferrite chokes. Wikimedia Commons.

Similarly, all other lengths of cable which could end up acting as aerials should have RF chokes fitted, unless they are coaxial cables (such as those connecting intended aerials for TV or radio reception). The latter should have a braid breaker connected, which prevents the coaxial sheath from acting as an aerial.

For a small cost, and with little effort, you can silence that mains hum or other interference, and enjoy the audio quality which you paid for.

If you feel a little lost by all this, then contact a licensed radio amateur (ham), or approach your local ham radio club. All radio amateurs have to learn about interference and how to address it, and are usually only too happy to help others solve their problems.