Wiring a Studio: THE GROUND RULES

7.  Balanced Wiring Systems

7.1  Balanced v Unbalanced Wiring Systems

Balanced wiring uses twin core shielded cable to interconnect equipment.  First of all, let's clarify that we're talking about the wiring method, not the equipment here.  Most studios which have a balanced wiring system still have a mixture of both balanced and unbalanced equipment.  In reality, a lot of equipment has both balanced and unbalanced inputs and outputs in random combinations.  Even with balanced wiring, the same ground loop issues apply whenever an unbalanced output is connected to an unbalanced input.

As we stated in Section 6.2  Eliminating Ground Loops in Unbalanced Wiring Systems, with unbalanced wiring systems, the shield of the audio cable performs three roles - electrostatic screening, acts as the second wire of the audio circuit connection and connects the audio common connections of the equipment together.  With balanced wiring, the only role of the shield is electrostatic screening.  The two inner cores of the audio cable carry the audio signal.  Equipment is grounded in star fashion to a central ground point, which is either mains ground or a technical ground.  Thus, the three roles of the shield in unbalanced wiring systems are now performed by three different mechanisms in balanced wiring systems.


7.2  Advantages of Balanced Wiring Systems


Balanced wiring
Balanced Wiring

In the above diagram, the audio signal path is shown in red and resembles the basic audio circuit described in Section 1.2  Audio Circuits.


7.3  Coping with Ground Noise in Balanced Wiring Systems

A fully balanced system, that is, an item of equipment with a balanced output connected to another item of equipment with a balanced input as shown in the above diagram, has the ability to cope with differences between two ground points.

The following diagram shows a balanced to balanced combination.  The shield wiring is not shown:


Balanced to balanced combination

In the above example, any difference between Ground A and Ground B and any external interference introduced into the wiring will cause equal currents to flow in the same direction in the HIGH and LOW connections, whilst an audio signal will cause currents to flow in an equal and opposite direction in the HIGH and LOW connections.

The ability of a balanced input stage to reject identical signals on its two inputs (interference) with respect to differential signals on its two inputs (the audio signal) is known as the Common Mode Rejection Ratio (CMRR) and is expressed as a ratio in dB.  For example, a device with a CMRR of 60dB will reduce ground noise and interference by 60dB with respect to the audio signal.

Combinations of unbalanced-to-balanced or balanced-to-unbalanced systems also have the ability to cope to a certain extent with differences between two ground points.

First of all, consider the following example where an unbalanced output is connected with balanced wiring to a balanced input.  The HIGH side of the balanced wiring is connected to the unbalanced output, whilst the LOW side of the balanced wiring is connected to the ground of the source equipment.  In the following diagram, the shield wiring is not shown:


Unbalanced to balanced combination

In the above example, when the line output stage is silent, its output is at the same potential as its ground - ground A.  The balanced line input stage receives the same difference between Ground A and Ground B signal on both of its inputs and tends to reject this.  When an audio signal leaves the output stage the line input stage receives it on its HIGH input only.  It sees this as a difference between its two inputs and therefore a signal.

In a similar way, fully floating balanced line output stages such as a transformer balanced output coupled with unbalanced input stages have the ability to reject ground noises:


Balanced to unbalanced combination

In the above example, one side of the output stage is connected to the line input of the unbalanced input stage, whilst the other side of the output stage is connected to the ground of the line input stage - Ground B.  When the balanced output stage is silent, both outputs are at the same potential.  Any difference between Ground A and Ground B will be added to the HIGH side of the balanced output.  Therefore, the unbalanced line input is held at the same potential as its own ground and it sees no input signal.  An audio signal is seen by the unbalanced input stage as the difference between its line input and its own ground - Ground B.  In other words, the unbalanced input stage sees only the difference between the two output connections of the balanced output stage as a signal.

Note that this only works for fully floating balanced output stages such as transformers or specially designed electronic output stages.  If the outputs of the line output stage are referenced to its own ground - Ground A, the above wiring would cause a ground loop.


7.4  Balanced Wiring Methods

A balanced wiring system can accomodate some combinations of balanced and unbalanced equipment as outlined above, as well as fully balanced equipment.

A balanced wiring system should be wired in a uniform way.  When an unbalanced item of equipment is encountered, the wiring is unbalanced at the connector - that is the LOW and the Shield are connected together to the case of the unbalanced connector.  That way, if the equipment is ever changed, the connector can be replaced with a balanced connector without any other change necessary to the remainder of the wiring.

A balanced wiring system still has limitations on specific combinations of input and output types.  For example, you cannot connect a grounded unbalanced output to a grounded unbalanced input without creating a ground loop.

Before going any further, we should take a closer look at the different types of audio input and output stages likely to be encountered in recording studios...


7.5  Input Stage Types


7.5.1  Unbalanced

This is the simplest type of input stage.  In balanced wiring systems, this is the biggest cause of problems, yet it is still commonly used.  An example is the insert point on consoles with a single stereo jack.  This is the one place where you may want to connect a wide variety of equipment and if the insert return is unbalanced, this places huge restrictions on what kind of output stages you can connect to it.  The line inputs on all Tascam consoles, older Yamaha consoles and some older low-end English consoles are unbalanced (i.e. they have a mono jack), making them useless for most modern installations.  You can save yourself a lot of grief when choosing a console by avoiding anything with unbalanced inputs!


7.5.2  Electronically balanced

In its simplest (and most common) form, an electronically balanced input stage costs only cents more than its unbalanced counterpart and there are no inherent sound quality issues.

Most newer low end consoles and some older English consoles such as the Soundcraft 200-1600 Series have balanced line inputs with stereo jacks.  You can also connect an unbalanced source to such inputs using a mono jack.  Unfortuately, many of these consoles still have an unbalanced insert point.  For simpler analogue installations, you might settle for using consoles like this and simply avoid using the insert point with output stages that cannot work with an unbalanced input.

Some consoles with jacks for their audio connections use separate rows of balanced insert sends and returns, normalled to each other.  An example is the Amek Langley Big console.  Higher end consoles such as Harrison, the Amek Mozart and SSLs have electronically balanced insert returns and a switch to bypass the insert point.

The down side of simple electronically balanced input stages is that their CMRR is usually not as good as transformers and in some cases this can be as low as 20dB.  The accurate matching of the resistors is critical for good CMRR.  Some designers add a trimpot to adjust the CMRR.  Unfortunately, most designers overlook the fact that with single stage electronically balanced inputs, the input impedance of the HIGH and LOW inputs is different and furthermore different again for common mode signals.  As a result, their effective CMRR is further reduced, making them more susceptible to the effects of external interference and any current flowing in the audio shield.  However, even simple electronically balanced input stages offer worthwhile benefits in reducing external interference.

It is possible to design fully symmetrical electronically balanced input stages using two or three op-amps to improve CMRR, but few manufacturers bother to do this.


7.5.3  Transformer balanced

Using transformers for input stages offers superior CMRR performance over most transformerless designs.  They also offer the best protection of sensitive input stages against high voltage spikes caused by nearby lightening strikes and electrostatic discharges, especially when long lines are involved.

However, they have inferior frequency, transient and phase response to transformerless designs.  An individual well designed transformer may not be too bad, but the cumulative effects of a large number of transformers in the audio path can be significant.  Input transformers are susceptible to magnetic hum fields and can be microphonic (i.e. pick up mechanical vibrations).

Many older consoles such as API and the class A Neves are have transformers on all inputs, including their insert points, making them almost immune to external interference.


7.6  Output Stage Types


7.6.1  Unbalanced

This is the simplest type of output stage.  In balanced wiring systems, unbalanced outputs will work with both transformer balanced and electronically balanced input stages, provided that the LOW and the Shield are connected together to the case of the unbalanced connector.  In balanced wiring systems, a combination of unbalanced outputs, balanced outputs and balanced inputs will all work fine together, provided that no unbalanced input stages are present.


7.6.2  Unbalanced, ground compensating

This type of output stage is designed specifically for feeding unbalanced inputs which are already grounded.  It works by using the LOW side of its output to sense any differences between the output stage ground and the ground of the input stage being fed.  This difference signal is added to the HIGH side of its output.  The resultant signal arriving at the input stage does not include the ground noise difference signal.  When feeding balanced inputs, it behaves as an unbalanced output stage.  It is therefore compatible with all types of input stages and there are no inherent sound quality issues.

An examples of this configuration can be found on DOLBY MH series noise reduction racks.  It was often used on outputs intended to feed power amplifiers back in the days when these often had unbalanced inputs only.  Harrison consoles and some speaker crossovers had these types of outputs.  In general, this type of output configuration is rare these days.


7.6.3  Electronically balanced, ground referenced

This type of output stage is essentially two unbalanced output stages operating 180° out of phase with each other.  It will not tolerate one side of the output being grounded and therefore must always feed balanced inputs.  This type of electronic output configuration therefore cannot work with an unbalanced input stage, so from a compatability point of view, is no better than an unbalanced output stage.

When working into a balanced input stage, the signals flowing in the HIGH and LOW lines are equal and opposite and this configuration offers potentially better interference rejection in long lines than an unbalanced - balanced combination.  There are no inherent sound quality issues with this type of output stage, provided that it is always used with balanced input stages.

This type of output stage has become very common and is used on most D/A converters, newer consoles, DAT machines and older English and Japanese equipment with balanced output stages.


7.6.4  Electronically balanced, floating

This is a refinement of electronically balanced, ground referenced output stages.  It emulated a transformer balanced output and will feed balanced or unbalanced input stages with no ground loop or level problems.  When feeding balanced input stages, it acts like an electronically balanced output stage.  When feeding unbalanced input stages, the LOW output stage is effectively shorted to ground and a feedback loop adds the "missing" signal to the HIGH side, increasing the HIGH output level by 6dB.  This maintains the same level from the output, regardless of whether it is feeding a balanced or unbalanced output.

This configuration can, however, be prone to instability when feeding long lines and usually has a slightly higher output impedance.  The simplest design tends to have slightly unbalanced output levels when feeding a balanced input and some designers add a trimpot to adjust this.  In addition, electronically balanced inputs with unequal impedances on their HIGH and LOW inputs can cause unbalanced levels.  However, this unbalance is only significant in terms of interference rejection in long lines.  When feeding unbalanced inputs, it has 6dB less headroom than when feeding balanced inputs.

I first saw this type of output stage on MCI JH-24, JH-110B, JH-110C tape machines and JH-600 console line outputs.  The circuit turned up in an encapsulated form in Sony broadcast equipment after they bought out MCI.  The SSM2152 balanced line driver IC has a similar design and is used in the Amek Langley Big console and some other consoles such as the Otari Status manufactured in the 1990's.  Some manufacturers tried to refine the idea.  The Otari MTR-10 has an elaborate output stage that does not work well into unbalanced inputs, although it includes an internal switch to unbalance the output.  Also, some later Valley People modules and the Model 610 has a design which appears to have distortion problems when working into an unblanced input.  The Tascam DA-98 has a design which had stability problems in some installations.


7.6.6  Transformer balanced, floating

This configuration is fool-proof in terms of compatability and can feed both balanced and unbalanced input stages with no headroom or instability problems.

Speaking of headroom, the old rule of thumb with professional equipment was that it should have a minimum of 20dB headroom.  That meant that a +4dBm line driver should be capable of delivering at least +24dBm.  An unbalanced line driver running off ±15V rails is only capable of delivering around +22Bm and for this reason, a lot of older high end equipment ran off higher voltages such as ±24V or more.  Electronically balanced line drivers can theoretically deliver +28dBm when running off ±15V rails and feeding a balanced input.  Transformer balanced output stages are basically an unbalanced line driver with a transformer on the output.  With a 1:1 transformer, the same headroom restrictions apply, so many older transformer balanced devices with 1:1 transformers ran off higher voltages.  For example, the MCI JH-400 console had ±24V rails, Quad-Eight consoles ran of ±28V, while the MCI JH-500 consoles ran off ±36V, yielding a massive headroom of 26dB!

Another approach with transformer balanced output stages is to use the output transformer to "step up" the output level.  For example, API gear runs off ±16V rails and uses 1:2 output transformers to double the output voltage to get the headroom.  The down side of this is that the unbalanced output stage must be able to work into a load of 150 ohms for the transformer to be able to drive a 600 ohm load.

Output transformers introduce frequency and phase response and ringing effects on transients.  The transformer needs to relatively big to handle higher output levels cleanly.  Transformer output stages usually need to operate into the correct load for best performance.  For example, output stages designed to work into 600 ohm loads should be terminated with a 600 ohm resistor when connected to typical input stages used in modern equipment which have an input impedance of 10K or higher.  To minimise ringing effects, some manfacturers such as Neve wire a resistor/capacitor network - known as a snubber network - across the output.

Some manufacturers use electronic feedback around the output transformer to improve performance.  An example of this is the line output stages on Harrison consoles.  However, as with all devices which have output stages with transformers such as valve power amplifiers, there is a limit to the amount of electronic feedback that can be used, due to phase shift problems.  This prevents them from working as well as transformerless designs.


7.7  Input/Output Configuration Compatibility Chart

The following table summarises the various combinations of input stages and output stages which will work in a balanced wiring system.  When a piece of equipment falls outside the requirements outlined, the problem must be addressed by either modifying the equipment concerned or adding balancing amplifiers or transformers.


Transformer balanced input Electronically balanced input Unbalanced input
Transformer balanced floating output
Yes
 
Phase reversible
 
Optimum for long lines
Yes
 
Phase reversible
 
Optimum for long lines
Yes
 
Phase reversible
Electronicaly balanced floating output
Yes
 
Phase reversible
 
Optimum for long lines
Yes
 
Phase reversible
 
Optimum for long lines
Yes
 
Phase reversible
Electronically balanced ground referenced output
Yes
 
Phase reversible
 
Optimum for long lines
Yes
 
Phase reversible
 
Optimum for long lines
No
Unbalanced ground compensating output
Yes
 
Phase reversible
Yes
 
Phase reversible
Yes
Unbalanced output
Yes
 
Phase reversible
Yes
 
Phase reversible
No
 
Key to drawing
Not permissible
Permissible
Phase reversible by interchanging the HIGH and LOW connections
Optimum noise rejection for long lines