Wiring a Studio: THE GROUND RULES

2.  Electrostatic Interference

2.1  How Electrostatic Interference Works

Consider an amplifier with a single exposed wire connected to its input.  This wire is in close proximity to another wire which is connected to an ac signal source.  The source and the destination both share the same common connection for their second connection.


Capacitive coupling

Even though the two wires do not touch, the voltage present on the source wire will have an effect on the destination wire.  The presence of a charge (voltage) on the source wire will either attract or repel electrons in the destination 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 destination 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:

Electrostatic interference is most dominant at high frequencies.  This includes frequencies above our range of hearing such as radio waves.  Although we cannot hear supersonic frequencies directly, if these get into audio equipment, they can cause problems with the audio signal.  In extreme cases, they can overload audio stages, causing audible side effects.  A common example of this is mobile phones placed close to audio equipment.


2.2  Screening out Electrostatic Interference

Using the example described above, let's place a metal plate in between the two wires and connect this plate to the common connection.


Electrostatic screen

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.  This type of shield is known as a Faraday shield.  In order for this shield to be 100% effective, it should fully enclose the destination wire.


2.2.1  Protecting Audio Signal Wiring against Electrostatic Interference

It is standard practice to enclose audio signal wiring in a shield to prevent electrostatic interference.  This shield is connected to the audio common connection.

Shielded cables 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 have double layers of braided or spiral wrapped shielding to improve performance.  However, these cables are thicker and less flexible.

Some cables use a thin conductive plastic shield inside a lighter braided shield.  This makes them more flexible, but they are not as tough as braided shield cables and are sometimes the cause of crackling problems when flexed.

Foil wrapped cables are 100% effective, since the wires inside are totally enclosed.  They contain a bare wire called a drain wire which is in electrical contact with the conductive foil and this is used as the shield connection.  However, foil wrapped cables deteriorate when the cable is flexed a lot, making them unsuitable for stage use.  Foil wrapped cables are ideal for fixed installations, and their stiffness makes them easier to loom.


2.2.2  Protecting Audio Equipment against Electrostatic Interference

Audio equipment is normally enclosed in a metal box which is connected to the audio common.  The metal enclosure acts as an electrostatic shield.  Non-conductive enclosures are often lined with some kind of conductive material connected to the audio common to act as a shield.

Sometimes metal shields are used internally to isolate individual audio stages.  Printed circuit boards often utilise large areas of vacant copper connected to the audio common to act as further shielding.  In addition, printed circuit board often utilise tracks connected to the audio common in between adjacent audio signal carrying tracks to prevent electrostatic coupling between them.

Sometimes, power transformers have an electrostatic shield between the primary and secondary windings to reduce electrostatic interference from the mains.


2.2.3  Protecting a Room from Electrostatic Interference

Professional studio equipment runs low impedance, high level (+4dBm) audio signals to reduce sensitivity to electrostatic interference.

Some audio signals such as the direct output from an electric guitar pickup are high impedance, low level signals which are very sensitive to electrostatic interference.  Furthermore, many guitars have internal grounding problems making them even more sensitive to interference.  We do not always have control over the 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 such as radio signals.  Note that this will have no effect on electrostatic interference being generated inside the enclosed area, such as from flourescent lights, computers and mobile phones.

A layer of chicken wire built into the walls, ceiling and floor, connected together and to the audio common can help a lot, but since it contains holes, it is not fully effective.  Foil-lined insulation in electrical contact with the chicken wire would help.  A double layer of copper gorse wire is often used in laboratories for this purpose, but the cost to do this for an entire studio would be rediculous!.  You would only go to the trouble of shielding an entire room if you knew you had trouble with external interference such as a nearby radio, TV or mobile phone transmitter.


2.3  Grounding to reduce Electrostatic Interference

Let's consider the hypothetical example of an amplifier - say a guitar amp - which has has been plugged into the the mains with the "earth" wire disconnected and has no other connections.  (I am not seriously suggesting that anyone does this, since this creates a potentially dangerous situation!)  There will always be a small amount of leakage between the amplifier mains connections and the amplifier common, due to imperfect insulation of the mains transformer, mains wiring and so on.  In addition, some equipment has built in mains filters which consist of small capacitors connected between the mains connections and the amplifier common (which is the metal chassis).  These capacitors are supposed to shunt high frequency mains-borne interference to ground and have very little (but still some) effect at mains frequencies.

Normally, this leakage is so small that if we were to touch the amplifier chassis, we would not feel anything.  Sometimes the leakage is sufficient to give us a slight tingling sensation.  With bad leakage or defective equipment we could get an electric shock.

In Australia, the mains neutral connection is connected to ground and the mains voltage is 240 volts AC at a frequency of 50 Hertz.  We can represent this situation with the circuit shown below, where "Active" = 240 volts AC, "Neutral" is ground and the resistors represent the leakage:


Mains leakage

For the purposes of this discussion, let's assume that the amount of leakage between the mains active connection and the amplifier common is the same as the amount of leakage between the mains neutral connection and the amplifier common.  We can redraw an equivalent circuit with the symbol representing a 120 volt (240 ÷ 2) 50Hz ac generator in series with a (hopefully) very high value resistor, as shown below.


Mains leakage equivalent circuit

In the above diagram, the symbol represents ground potential.  In other words, with an ungrounded amplifier connected to the mains, the amplifier chassis is floating at 120 volts AC!  Depending on the exact ratio of the leakage currents and the local mains voltage, the actual final voltage will vary from this figure.

So what happens if we were to connect a single exposed wire connected to the amplifier input and place this amplifier inside an empty room?  Let's assume that the room is built on the ground, and because the walls and ceiling are slightly conductive, the entire room is at a uniform ground potential.  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 Lone Amplifier

The above circuit can be translated into the equivalent circuit shown below:


Lone amp equivalent circuit

This circuit is similar to the one described in Section 2.1, How Electrostatic Interference Works.  Due to capacitive coupling, it is possible for any difference between the amplifier common and the room (ground) to find its way into the exposed wire.  In other words, we get hum!  As far as the amplifier is concerned, it is receiving electrostatic interference.

If we ground the amplifier, the leakage current now flows through the ground connection and the amplifier common is at the same potential as the room.  This eliminates this type of interference.

It is therefore important that the equipment common connection is connected to ground, so that it is held at the same potential as its surrounding environment to minimise electrostatic interference.

Obviously the other major reason for grounding mains operated equipment which can be connected to other equipment is for safety.  Some equipment (usually lower end stuff) does not have a mains ground connection and can usually be identified by a two core (often "figure 8") mains lead or a missing 3rd pin on its mains plug.  Some low end equipment is powered by an external plug pack and is not connected to mains ground.  In both of these cases, the mains is supposed to be "double insulated" for saftey.  Such equipment relies on being grounded via the shields of its audio connections.  Sometimes such equipment also has a ground terminal.

In this documentation, we use the symbol to denote a connection to ground, and the symbol to denote an equipment common connection.


2.4  Sources of Electrostatic Interference

Sources of electrostatic interference include:

Most equipment containing inductive loads (such as refrigerators and air conditioners) includes built in suppressors to reduce radiation.  Cars usually have suppressors on their ignition systems to prevent interference with their own radios.  Switched mode power supplies usually have suppressors on their mains inputs to prevent them from injecting interference back into the mains.  Computers usually have shielding to reduce radiation.  However, sometimes suppressors fail on older equipment and some newer equipment imported into Australia would fail to meet Australian safety and emission standards.


2.5  Electrostatic Interference Summary

Electrostatic interference is generated by the presence of alternating potential differences (voltage) between equipment.

Electrostatic interference can be minimised by: