Consider an amplifier with a single exposed wire connected to its unbalanced input. This wire is in close proximity to another wire which is connected to an ac power source.
The source and the destination both have the same reference - that is, the second connection of both the source and destination is the same.
Even though the two wires do not touch, the signal voltage present on the source wire will have an effect on the exposed amplifier input wire (the destination). The presence of a charge (voltage) on the source wire will either attract or repel electrons in the amplifier input wire, depending on the polarity of the voltage in the source wire. With an alternating voltage in the source wire, there will be a consequential alternating voltage generated in the amplifier input wire.
The wires in fact form a crude capacitor and are referred to as being capacitively coupled. The amount of capacitive coupling between the two wires is governed by the following factors:
As far as audio equipment is concerned, electrostatic interference is most dominant at high audio frequencies. In addition, although you cannot hear supersonic and radio frequencies directly, if these get into audio equipment, they can cause problems. In extreme cases, they can overload audio stages, causing audible side effects. Sometimes, the overloaded audio stage acts as a crude radio detector, so you actually get to hear the radio interference!
Using the example described in Section 2.1, How Electrostatic Interference Works, let's place a metal plate in between the two wires and connect this plate to the common connection.
The source wire will cause current to flow between the metal plate and the common connection, due to capacitive coupling. However, as far as the amplifier input wire is concerned, it is now coupled to a stationary source due to the fact that the plate is connected to the common connection. The metal plate acts as a shield, preventing the source wire from having an effect on the amplifier input wire. In order for this shield to be fully effective, it should fully enclose the destination wire. This shield is referred to as a Faraday shield.
It is standard practice to fully enclose all audio signal wiring in a shield, to prevent electrostatic interference. This shield is connected to the audio common connection.
Shielded cables can vary in their effectiveness. Braided shields, which are usually used for microphone cables and other flexible audio cables are typically only 90% to 95% effective, since they can have small openings in them. Some audio cables have double layers of braided or spiral wrapped shielding. However, these cables are thicker and less flexible. Some cables use a thin conductive plastic shield inside a lighter braided shield. This makes them super flexible, but they are not as tough as braided shield cables and are often the cause of crackling problems when flexed on improperly prepared cables.
Foil wrapped cables actually can have 100% effectiveness, since the wires inside are totally enclosed. However, foil wrapped cables have inferior mechanical properties, making them unsuitable for microphone cables and cables which need to be highly flexible. Also, the foil can actually deteriorate when the cable is flexed a lot. Foil wrapped cables are ideal for fixed installations, and their stiffness makes them easier to loom. Foil wrapped cables also contain a bare wire called a drain wire, which is in electrical contact with the conductive foil so that the foil shield can be connected to the audio common connection.
Audio equipment is normally enclosed in a metal box which is connected to the audio common connection. The metal enclosure acts as an electrostatic shield. Sometimes plastic or non-conductive enclosures housing audio equipment are lined with some kind of conductive material connected to the audio common connection to act as a shield. Sometimes metal shields are used internally to isolate individual audio stages from each other. The printed circuit boards inside audio equipment often utilise areas of copper connected to the audio common connection to act as further shielding. This is referred to as a ground plane. In addition, a well designed audio printed circuit board often utilises tracks connected to the audio common connection in between adjacent audio signal carrying tracks to prevent capacitive coupling between them.
Sometimes, power transformers have an electrostatic shield between the primary and secondary windings to reduce interference from the mains.
Some audio equipment such as an electric guitar pickup is very sensitive to electrostatic interference. We do not always have control over this and other types of equipment that is brought into the studio.
Steps can be taken to reduce the general amount of electrostatic interference present within a recording environment.
By enclosing the entire room in an electrostatic shield, we can reduce external sources of electrostatic interference. Note that this will not have any effect on electrostatic interference being generated inside the enclosed area.
Often, fine chicken wire is built into the walls, ceiling and floor and connected to the audio common connection. This can help a lot, but since it contains holes, it is not fully effective. A double layer of copper gorse wire is often used in laboratories for this purpose. In theory, a layer of aluminium foil should work.
Consider an amplifier with a single exposed wire connected to its unbalanced input but with no connection to its common connection (in other words, not grounded). We place this amplifier inside an empty room. The room is built on the ground, and because the walls and ceiling are slightly conductive, the entire room is at a uniform potential (or charge) - the same potential as the ground. In the drawing below, the symbol represents the ground (room) potential and any difference between the amplifier common and ground is represented by the symbol.
The above circuit can be translated into the equivalent circuit shown below:
This circuit is similar to the one described in Section 2.1, How Electrostatic Interference Works. In other words, it is possible for any difference between the amplifier common and the room (ground) to find its way into the exposed wire connected to the amplifier input by means of electrostatic interference.
If the amplifier is a self contained battery powered guitar amplifier and there is no other source of electrostatic interference, there probably would be no difference between the amplifier common and the room (ground). Therefore, the amplifier would probably be silent.
If the amplifier is mains powered, the story is different. There is a small amount of leakage between the amplifier mains connections and the amplifier common. This leakage is due to imperfect insulation of the mains transformer and mains wiring. In addition, some equipment has built in mains filters which consist of small capacitors connected between the mains connections and the amplifier common.
In Australia, the mains neutral connection is connected to ground. For the purposes of this discussion, we'll assume that the amount of leakage between the mains active connection and the amplifier common is approximately the same as the amount of leakage between the mains neutral connection and the amplifier ground. In the drawing below, the leakage is represented by two resistors for simplicity.
In this example, assuming the two leakages are equal and that the mains voltage is 240 volts AC at a frequency of 50 Hertz, the symbol shown in our ungrounded amplifier model is equivalent to a 120 volt (240 ÷ 2) 50Hz ac generator in series with a resistor, as shown below.
For low leakage, this resistance is very high. Normally, the leakage is so small that if we touch the amplifier chassis, we do not even feel it. Sometimes the leakage is sufficient to give us a slight tingling sensation. With bad leakage (such as a large number of ungrounded devices with built in mains filters connected together or defective equipment) we can get an electric shock.
By the mechanism of electrostatic interference, a proportion of this 50Hz signal will appear at the amplifier input. In other words, we get hum!
To eliminate this source of interference, we can connect the amplifier common to ground. The leakage current now flows through the ground connection.
It is therefore important that all pieces of audio equipment are connected to ground, so that they are held at the same potential as their surrounding environment. In addition, all equipment shielding and equipment common connections should be connected together, to maintain them all at the same potential.
In this way, electrostatic interference due to ac differences between individual items of equipment and their surroundings are minimised.
In this documentation, we use the symbol to denote a connection to ground, and the symbol to denote an equipment common connection.
Sources of electrostatic interference include:
Electrostatic interference is generated by the presence of alternating potential differences (voltage) between equipment.
Electrostatic interference can be minimised by:
|© 2005 Colin Abrahams, Studio Connections, Sydney, Australia|