Causes of mains noise - electrical equipment
Your audio equipment probably shares its power network with hundreds of electrical-noise producing products. In recent years, there has been a huge proliferation in the use of switch-mode power supplies. These are now found in most items of consumer electronics, televisions, computers, mains-adapters and chargers, even high-efficiency lamps. Other sources of noise include motors and motor controllers in domestic appliances and power tools, dimmers and fluorescent lighting. Transient noise sources include thermostats, tube starters, any power switch and of course, electrical storms.
Causes of mains noise - radio frequency interference
In addition to noise coupled directly onto the mains, your house wiring is a good aerial, picking up the 'soup' of electro-magnetic energy from thousands of analogue and digital radio and TV stations, mobile phones and their networks, DECT phones, air-traffic, radar, microwaves, satellites. Anything you could receive with the appropriate equipment is being fed, in some measure, into the mains-lead of your hi-fi!
When is mains interference at its worst?
The amount of noise varies greatly, depending on location and time of day but nobody has 'clean' mains these days. If your hi-fi sounds better at 2AM than at 9PM, chances are it's not the wine, it's the mains! As you might expect, you get more noise during the day and early evening because that's when the most domestic or industrial electrical equipment is in use. The quality of the mains tends to be at its best very late at night and during those times at the weekend when electrical activity is low.
In many locations, audio units 'buzz' sometimes, not from the speakers but from the enclosure - that's also caused by problems with the mains!
Mains noise and the law
From 1st January 1996, European legislation introduced EU-wide harmonised limits to the maximum amount of noise coupled into the mains network by any single item of equipment. This is fine as far as it goes, the problem is that it doesn't go nearly far enough.
Limitations of the mains noise legislation
1) The legislation only covers frequencies between 150kHz and 30MHz. There are no limits covering the audible and ultra-sonic frequencies to which audio equipment is intrinsically sensitive. There are no limits to the VHF frequencies which often by-pass mains filters and power supplies, then get directly into sensitive audio circuits. Even humble transistors can amplify at hundreds of Megahertz, some small-geometry devices work at Gigahertz frequencies.
2) Some items of equipment pre date the requirements, some are exempt from the requirements, some don't comply under certain operating conditions, some just don't comply at all. Manufacturers can self-certify their equipment without even testing it! Prosecutions are very rare.
3) There are vast numbers of items of consumer equipment and the noise from each adds to the total.
