Wiring a Studio: THE GROUND RULES

4.  Common Path Interference

4.1  Common Path Interference in Ground Wiring

Consider an item of audio equipment which has been plugged into the mains, with no other connections to it.  As explained in Section 2.3, Grounding to reduce Electrostatic Interference, such equipment is normally grounded via the 3rd pin of the power plug, not only for safety reasons, but also to reduce its sensitivity to electrostatic interference.  The metal chassis of the equipment is connected to the the "earth" wire in the power lead which in turn is connected via the power point to the earth wire in the power circuit back to the main power switch board and from there to an earth stake driven into the ground.


Grounded equipment

Due to leakage between the mains wiring and the common connection of the equipment, a small AC current flows through this ground connection.  Without attempting to go into details here, there are also many situations where audio signals end up flowing between the common connection of an item of audio equipment and the mains ground when it is connected via audio leads to other equipment.  The ground connection is a common path for any mains leakage and any stray audio signals.

In reality, there is no such thing as a perfect conductor.  All wires have some resistance, even though this is usually very low in the case of ground wiring.  Let's redraw the above circuit, showing the resistance of the ground wiring:


Ground resistance

Due to the small AC current flowing in the ground wiring, there will be a very small AC voltage between the common connection of the equipment and ground.  This is because Ohm's law states that the voltage across a resistor is proportional to the current flowing through it.

In the case of a single item of equipment as in the example above, this voltage would normally be so small that it has negligible effect.  In a studio, you might have lots of equipment - a console, computer, amplifiers, outboard gear and so on - all ultimately connected to ground.  The cumulative current flowing through the ground wiring results in a more significant ac voltage difference between the equipment commons and ground.

Let's now look at two items of equipment, one plugged into a power point on one side of the room and the other plugged into a different power point on the other side of the room.  We'll assume that both power points are on the same electrical circuit.  We now have the situation shown below, allowing for resistance in the ground wiring:


Two items of grounded equipment

The second resistor represents the resistance of the ground wire between the two power points.  Due to the small AC current flowing in the ground wiring from the second item of equipment, there will be a very small AC voltage between the common connections of the two items of equipment.  Furthermore, because the AC current flowing from both items of equipment share the same common resistance to ground, they will affect each other.

Once again, if we are talking about only two items of equipment, all of these effects would normally be negligible.  But if we're talking about two groups of equipment, the AC voltage difference between the common grounds of one group and the common grounds of the other group can become significant.

You can also create a situation like this with extension leads.  For example, you might plug one rack into a power point and then run a lead from the first rack to a second rack and another to a third rack and so on.  This type of wiring is refered to as "daisy chaining" and it's bad news!


Daisy-chained grounds

The situation can be worse than this.  If there happens to be something like an air conditioner or refrigerator plugged into another power point further downstream on the same power circuit, suppressors in these units can pump current spikes down the same ground circuit, producing voltage spikes between the common connections of various equipment around your studio.

All of the voltages we are talking about here are very low, but any difference between the common connections of audio equipment can ultimately find their way into your audio signals, causing hum, clicks, pops and crosstalk.  We'll talk in more detail about how this happens later.


4.2  Grounding to reduce Common Path Interference

 
Ideal grounding

The ideal way of grounding equipment is to connect ground wires from each individual item of equipment to a central copper ground stake driven into the ground.  The ground wires should all be as short and direct as possible, therefore implying that the ground stake should be located centrally.  The ground wires form a "star" point at the ground stake.

As explained above, all wires have some resistance and due to the small AC currents flowing through them, there will still be very small AC voltages developed across them.  But since each item of equipment is grounded individually, the voltage differences between them are much smaller than if they were sharing ground connections as in the "daisy chain" model.

In practice, the above situation is difficult to achieve.  How many studio owners would like to have a dirty big copper stake sticking up in the middle of their control room?  Running lots of longer ground wires to a remote stake can become a problem, since the equipment is no longer bonded closely together.  A solution is to create a central ground point in the control room, and then run a single ground wire to the stake.  This central ground point becomes the star point, as shown below:


Technical ground

In Australia it is usually possible to rely on the incoming mains ground as the central ground point, provided that the studio equipment is run on its own separate power circuit, not shared with things like air conditioners.  This is because Australian Standards specify that a separate ground for each power circuit is run directly to the switch board, where it is "starred" with other power circuit grounds, the incoming Neutral connection and connected to a ground stake.

To use the incoming mains ground as the central ground point, all you have to do is bring all mains wiring back to a central power source.  Things like the console power supply should definitely be wired separately back to the central power source.  However, it would be impractical to wire every single item of equipment back to the central point.  For example, a rack full of outboard gear might contain a large number of different items.  A compromise can be made here.  Install a power distribution board in the rack and then run a single power cable back to the central ground point.  This makes sense anyway, since usually the chassis of each item of equipment is connected together by the metal strips that the equipment is bolted into.

 
Right: An example of power distribution for a large double rack.  A single incoming mains cable is connected to a 4-way board which in turn is connected to three 6-way power boards mounted on the sides of the rack.  The 6-way power boards act as a ground star point for a group of equipment.  The 4-way board acts as a ground star point for the 6-way boards.  The single power line to the rack is starred to the Studio Central Ground.  Finally, the Studio Central Ground is starred to the building central ground at the main switch board, where it is connected to earth by the ground stake.  The final result is a tree-like grounding structure.

Rack Power Distribution
 

If you have a recording area, any power outlets where guitar amps, other musical instruments or mic preamps might be plugged into should be considered part of the the audio equipment and should star to the same central ground point in your studio.  If you cannot rewire the mains outlets in the recording area, an option might be to run extension leads from your central power source.

Having some kind of switch board for your studio can be convenient.  You can break up circuits into amplifiers, console, computers, outboard gear and so on.  In this way, you can power up things in sequence.  For example, you would normally power up your amplifiers last and turn them off first.  Sometimes you might just want to do work on the computer, without powering up the rest of the studio.

For small studios, a power distribution board with switches on each outlet can act as a sub board and the central ground point for the studio.  It is important that all audio equipment is plugged into this single power source, with no exceptions.  In most cases, the total power consumption of a small studio is well within the maximum rated power of a single power point.  In Australia, most power outlets are rated at 10A = 2400W.  If in doubt, add up the power ratings of the all individual items of equipment to estimate your maximum power consumption.

It is also important that this power distribution board is used exclusively for your studio equipment and not shared with anything else.  This applies especially to high power consumption items like electric heaters, which ideally should be connected to a different circuit.

Power Distribution Board

4.3  Grounding the Console

Since the console is usually the heart of a studio, correct grounding is of utmost importance.

Most larger consoles have one or more external power supplies.  The power supplies themselves are grounded in the normal way via the 3rd pin of their mains plug.  In most cases, the main console ground is either isolated or grounded via an optional link in one of the power supplies.  It is usually best to ground the console directly and disconnect any link in the power supply if necessary.  Indeed, with some consoles, the ground link in the console power supply can actually cause problems, since it is in close proximity to a large power transformer radiating electromagnetic interference!

To check whether the console ground is isolated, you can measure for continuity between the console frame and the 3rd pin of the power supply power plug when it is not bolted in a rack.

To ground the console, you need to find the main ground connection.  Sometimes there is a terminal provided for this purpose.  Sometimes you might have to look for a power distribution strip and find the ground connection.  To ground the console to the studio central ground, you might hard wire a ground wire into the switch board.  For studios with a power distribution board, you can wire the ground wire to the 3rd pin of a power plug and simply plug it into the board.

Console Ground Connection
A typical console ground connection
Connecting ground to a standard power outlet
Connecting ground to a standard power outlet

4.4  Is the Incoming Mains Ground Good Enough?

In the past, some local wiring standards such as used in the USA relied on grounding via metal conduits rather than using an actual ground wire.  Some old wiring standards relied on metal water pipes for a ground connection rather than using a dedicated ground stake.  For this reason, it was sometimes necessary to run a separate grounding system with its own ground stake in order to get a satisfactory grounding situation.  In countries outside Australia, you should check your local mains wiring standards.

Unless it is absolutely necessary, I strongly advise against creating a separate technical ground for your studio.  With two different grounding systems in the same area, there can be severe grounding problems if an item of audio equipment is plugged into the wrong circuit.  In the event of a fault occurring in either of the two ground systems, a potentially dangerous situation could be created if something (or someone!) gets connected between them.  It could also be illegal in some countries.

It is permissible to have more than one star point in a building, provided that the equipment in the studio uses only one star point.  For example, in large buildings, there might be one main switch board feeding several sub boards on different floors.  The grounds of the power circuits leaving the sub board are starred together at the sub board, and the sub mains circuit grounds from each sub board are then starred together at the main switch board.  It is desirable, however, to ensure that a sub board feeding a studio feeds only studio equipment, and not other electrically hostile items.

If you are leasing a space, you do not always have control over how the power is connected, but usually a space has its own separate circuit(s) and possibly its own sub board.

In private situations, you might be setting up a studio in the shed out the back or in a separate area in your house.  Chances are that there is already a separate power circuit for that area.  Things like air conditioners, stoves, hot water sytems and lights normally have their own separate circuits.  In situations where a power circuit is shared with other things in the house, it is advisable to get an electrician to install a new power circuit, wired directly back to the main switch board, with its own sub board for the studio equipment.


4.5  Should the Entire Studio be Wired on the Same Phase?

This is a myth which is often raised when planing studios!  For a small studio, since all audio equipment would be powered from a single power circuit, it will obviously be on the same phase.  For larger studios and studio complexes where there might be a 3 phase switchboard, there is no problem if some gear runs off different phases, provided that all the audio equipment circuit grounds are connected together at the same central ground point.

Indeed, it could be argued that by putting all audio equipment on one phase and perhaps all lighting on another in a large studio complex can actually create problems!  This is because most audio gear contains power transformers which are predominantly inductive loads on the mains.  On the other hand, flourescent lights are predominantly capacitive loads.  By putting one predominant type of load on the same phase causes distortion of the mains waveform, which is more likely to interfere with our precious audio gear!  Furthermore, any imbalance between the loads on the 3 phases causes heavy currents to flow in the normally benign incoming neutral line and since this is joined to the main ground at the switch board, this will also cause heavy currents to flow in the wire to to the ground stake.  The wire to the ground stake becomes a common path with heavy currents flowing through it, resulting in a voltage difference between the entire building ground and "real" ground.  Any electrostatic interference an item of equipment picks up will include noise from this "dirty" ground.  It is for this reason that electricity authorities like to randomly mix up loads on phases to keep the overall load balanced between the phases.


4.6  Common Path Interference in Unbalanced Wiring


4.7  Common Path Interference Summary

Common path interference is caused by different AC currents sharing the same wire.  Because all wires have some resistance, this results in a corresponding small AC voltage across the wire.  In ground wiring, this can result in hum, clicks and pops being present between different items of equipment.  In unbalanced wiring, this can cause crosstalk between audio signals.

Common path interference can be minimised by: