Wiring a Studio: THE GROUND RULES

6.  Unbalanced Wiring Systems

6.1  Ground Loops and Unbalanced Wiring Systems

Normally, single core shielded cable is used to interconnect an unbalanced output to an unbalanced input.  When a ground loop is present, there is always a difference between equipment common grounds.  Unbalanced wiring systems have no ability to cope with these differences.  In the following diagram, we'll ignore the shield connection:


Unbalanced to unbalanced combination

When the line output stage in the first item of equipment is silent, its output is at the same potential as its ground - ground "A".  The line input stage in the second item of equipment sees any variation in its input with respect to its own ground - ground "B" as a signal input.  Therefore, when the output stage is silent, the input stage sees any variation of ground "A" with respect to ground "B" as a signal.  When a real signal leaves the output stage, it is combined with the noise between ground "A" and ground "B".

Normally in unbalanced wiring, the shield joins ground "A" and ground "B" together, but when a ground loop is present, there is still noise between ground "A" and ground "B".


6.2  Eliminating Ground Loops in Unbalanced Wiring Systems

Getting back to the ground loop scenario described in Section 5.2  How Ground Loops Work, we saw how when a ground loop which involves the shield of an audio cable is present, it "brute forces" noise between the two ends of the shield.  With unbalanced wiring, the shield forms part of the audio signal path and therefore this noise is added to the audio signal.  However, there is a solution.  If we were to disconnect the mains ground from second item of equipment, we break the ground loop:


Ground lifting

Now, the second item of equipment is grounded via the shield of the audio cable to the mains ground of the first item of equipment.  Ground "A" is now closely bonded to ground "B".  Even if there is noise on ground "A" with respect to "real" ground there is still negligible difference between ground "A" and ground "B".  As far as the input to the second item of equipment is concerned, it is now unaffected by any ground noise.

Let's review some of the things we have talked about in previous chapters in the context of the unbalanced wiring model above:

In unbalanced wiring systems, the shield of the audio cable performs three vital roles:


In unbalanced wiring systems, disconnecting one end of the shield is never an option for resolving ground loop problems.
 
In unbalanced wiring systems, only one item of equipment can be connected to the central ground.  All other items of equipment must rely on the shield of the audio cable to ground them.
Unbalanced grounding
 

6.3  Ground Lifting

Most equipment with unbalanced audio connections is usually double insulated with no mains ground connection to prevent ground loops occurring in the first place.  On unbalanced equipment which does have a mains ground connection, it is usually best to choose this as the ground point for the whole system.  If you have more than one item of equipment like this, it becomes necessary to disconnect the mains grounds on the others.


WARNING

On equipment with the mains ground lifted, always ensure that the mains plug is disconnected first, before connecting or disconnecting the audio connections.

Never leave the mains connected to a ground lifted item of equipment which has had all of its audio connections removed.  Even if it is switched off, it could be live!

If you do disconnect the mains ground from an item of equipment, you also need to ensure that it is not getting grounded by other means.  For example, if it is bolted into a metal rack which is grounded by the cases of other equipment, you would need to insulate the equipment from the rack.

When disconnecting mains grounds from a number of items of equipment, each item must be done individually for this to work properly.  Never be disconnect the mains earth from an entire rack of equipment!  The cumulative mains leakage from multiple items of equipment becomes significant and I have heard many reports of people receiving electric shocks after doing this or seeing sparks fly when they go to patch something in!

In general, I do not recommend disconnecting the mains ground from equipment unless it is absolutely necessary.  Anything you do plan on lifting should be carefully checked first for any potential safety issues.  I would never propose disconnecting the mains ground from anything which contains mains filters, since these usually shunt any mains-borne interference into the ground wiring, which is a bad thing for unbalanced wiring systems.  If in doubt, don't even think about it!

With some unbalanced equipment, there is an alternative to lifting the mains ground.  Such equipment contains either an internal or external link which connects the audio common connection to the chassis ground .  By removing this link, you can have the best of both worlds!  The chassis is still safely grounded, but the audio common connection is isolated and you can then treat the equipment as if it was not grounded.

It should also be mentioned here that there is a special type of unbalanced output stage known as a ground compensating unbalanced output, which permits connection between two items of mains grounded equipment.  In other words, it allows you to break the grounding rules we have talked about for unbalanced wiring.  Such output configurations are rare these days, but include things like the old Dolby "A" multitrack noise reduction system and the original DBX 160 compressors.  This is described later.


6.4  Limitations of Unbalanced Configurations

In simple audio setups such as the example above which involves just two items of unbalanced equipment, unbalanced wiring can be fine.  A classic example is the home hi-fi system where the amplififier is usually grounded and all other items of equipment such as the CD player, turntable and so on are ground lifted.  This makes sense since the amplifier is the central unit, with all other items "starred" to it via their audio leads.

With a home theatre system, we have to be a bit more careful.  The TV will almost certainly have a switched mode power supply and must be grounded via the 3rd pin of its power plug.  This means that all other items of equipment, including the amplifier system must be ground lifted to avoid ground loops.

In a small home studio situation you might have a laptop computer, a sound card with unbalanced line inputs and outputs and some self-powered speakers.  None of these may be connected to mains ground and you can freely plug between them.  Larger self powered monitors probably have mains grounds connected, so you might settle for these as the ground reference for the whole system and just make sure that all other items of equipment are ground lifted.

If you want a patch bay, the only way to do this is to join all the common connections of the audio connectors together at the patch bay.  This must be the Central Ground Point for the system, meaning that all other equipment - including the console - are ground lifted.  (You cannot rely on the patch cords themselves to connect the grounds, since unpatching an item of equipment effectively ground isolates the equipment.  In some cases of main powered equipment, this could be a safety issue.  Patching things in could produce loud thumps, give you electric shocks and even produce sparks!)  All audio wiring between the patch bay and a console is sharing common paths.  Depending on exactly how the console is wired, this can lead to crosstalk between different audio signals.

What happens if you add a mains powered computer with a non-opto isolated link to an unbalanced sound card?  It's not an option to ground lift it, since it has a mains filter and switched mode power supply.

As your setup grows, it becomes harder, if not impossible, to satisfy the rules for unbalanced wiring.  Even if you do manage to ground lift every item of equipment, you start to run into issues of cumulative mains leakage into the shields of the audio wiring and get ground differences between the grounds of different groups of equipment, manifesting as hum, "tones on the mains", clicks and pops.  Due to inevitable common paths, crosstalk becomes a problem.  You may have supersonic frequencies running around your audio system, degrading audio quality.  You may hear computer noises.

Unbalanced wiring systems are suitable only for the simplest of setups.