Audio Engineering

Balanced and Unbalanced Transmission Systems (2)

The Music Telegraph | Text 2019/04/04 [15:13]
Audio Engineering

Balanced and Unbalanced Transmission Systems (2)

The Music Telegraph| 입력 : 2019/04/04 [15:13]

 

▲ Balanced and Unbalanced circuit

© Musician's friend



 

Balanced and Unbalanced Transmission Systems (2)

 

 

Balanced Transmission System

Since there are no practical shields which protect the unbalanced transmission lines completely from all types of interference, and in order to accomplish transmission of low level signals economically over long lines, a method of balancing transmission lines at both ends is used. This method of balancing has been in use for decades. The largest user of balanced lines is the telephone company. Almost all telephone installations and exchanges use balanced lines which allow use of unshielded wires with relatively low losses.

 

 

In order to balance the line, transformers (or operational amplifiers) must be used at each end of the line. Transformer windings connected to the balanced line are made with the center tap normally grounded, at least at one end (Figure 1). The idea behind this hookup is to keep both wires of the balanced line at the same potential with respect to the ground (hence the term "balanced"). External fields affecting these lines obviously affect both sides of the line equally at the same time. Signals arriving at the terminating transformer then will cancel each other out.

 

 

The effectiveness of a transformer balanced line depends on three factors. First, on the transformers and the symmetry of windings, second, on the effectiveness of the cable's outer shielding, and third, on the type of transmission line and the symmetry of the interference signals inducted in both sides of the line (Figure 2).

 

 

Pros and Cons of Each Transmission System

Balanced lines are be design less sensitive to the external interference, but require either transformers which are costly, bulky, and can pick up external fields themselves (or pampas which require a power source to operate). Unbalanced lines are more economical to use over shorter distances, but require much more care in connecting two pieces of equipment in order not to cause any harmful ground loops. Unbalanced lines are also more susceptible to external interference. Balanced lines are cheaper for longer transmission hauls because they do not require shielding. Their inter-wire capacitance as well as capacitance to ground can be kept small (if space is not at a premium) but they require good transformers which impose limitations of their own on frequency response, level, noise, distortion and phase.

 

 

Unbalanced lines are very costly on long hauls but very economical when used within a small system. They are very convenient when used with transformer less transistorized equipment and do not present limitations on electrical performance found in balanced circuits. A disadvantage (but many may think of it as an advantage) is the ability of a balanced line to have its phase inverted 180 degrees something that it is harder to do with unbalanced lines unless a transformer is used or a phase inverting amplifier.  

 

 

▲ Figure 1.

© The Music Telegraph


 

▲ Figure 2. A shielded balanced line using a doubleconductor within a shielded outer shell.

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