Know if Hi-Fi Separates are Earthed or Ungrounded to Avoid Hum
Is it worth being aware that mains-powered audio equipment comes in two main variants when it comes to tracing hum problems in domestic Hi-Fi systems? There is no performance difference – it is perfectly possible to make good sounding audio equipment which either has a mains ground connection or to make it without a ground connection.
However, using more than one item grounded to mains earth in a domestic audio system can lead to ground loops (also known as earth loops) which can lead to hum in the system. It is, therefore, necessary to identify whether Hi-Fi equipment is grounded or ungrounded.
How to Identify Audio Equipment that Does Not Have a Mains Ground (Earth) Connection
The easiest and most sure-fire way to identify a piece of ungrounded equipment is to look for a two-pin mains plug. A two-pin mains plug by definition has no earth connection, this piece of equipment will, therefore, be ungrounded.
In some countries such as the UK, nearly all mains plugs are three-pin, making this harder to see. For equipment with detachable mains cables, there is another plug in the system – the chassis-mounted plug on the equipment itself that the mains lead plugs into. If this is two-pin then the piece of equipment is ungrounded.
Other things to look for are a mains cable that has just two cores – these are often of a flat or oval cross-section rather than round cross-section. In the UK if the equipment is described as Class II or double-insulated it has no ground connection. Audio equipment that is powered from a wall-wart or plug-in power supply almost always does not have a mains ground, though there are some very rare exceptions.
How to Identify Audio Equipment that Has a Mains Ground (Earth) Connection
Unlike the infallible test for ungrounded equipment, it is not possible to conclusively state that a piece of equipment is grounded just because it has a three-pin power plug. However, if it is described as Class 1 insulated in the UK it is definitely grounded. Likewise, equipment that has a round cross-section non-detachable mains cable is likely to be grounded. The only way to know for sure is to perform a multimeter test.
How Electrical Safety Works in Grounded and Ungrounded Audio Equipment
There are two accepted ways of keeping people safe from mains power. One is to connect all exposed metal to the mains safety earth, which is carried on the third prong of the mains plug. The alternative, depending on the particular country electrical code, is to ensure that all mains power connections, live and neutral, are isolated by at least two insulating layers, one of which may be air. Such equipment may have just a two prong power cable.
How to Avoid Ground Loops in Hi Fi Equipment
Make sure that only one piece of equipment in the Hi Fi system is grounded – preferably the amplifier or control unit/pre-amplifier. If more than one piece of equipment is grounded, then plug the grounded components into the same distribution board or wall socket pair to minimize the ground loops and susceptibility to hum.