Balanced circuit

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A balanced circuit is a common method of transmitting many types of communication signals between two points on two wires. In a balanced circuit the individual signals on each wire are inverses of each other as referenced from an arbitrary midpoint between the two signals.

As the signals travel the wires, noise is induced into both by the surrounding environment. Due to the proximity of the wires, this induced noise is very much the same in each wire. At the end nodes, the two out-of-phase signals along with the common noise that each wire had induced into it over the transmission path, are captured. One of the two signals, "signal" or "signal inverted", is fed into an inverter. The resulting wave is combined with the unaltered signal. The noise on each signal is the inverse of the other, and the signal wave is imposed on itself. The result is twice the signal, and essentially no noise.

This explains the lack of significant noise in properly balanced audio circuits. Two common examples of this type of connection include RJ-11 and XLR connectors. Not all XLR connectors are balanced, but there are good odds to it. Also 1/4" connectors may also be used to create a balanced circuit. When done so they are known as TRS connectors or tip ring sleeve.

A common and simple method of designing a balanced circuit is to drive both ends of the loop circuit with matched transformer output (and input) coils. As stated above, the two wires forming the basis of the circuit traverse the same environment and, if twisted, can reduce interference and stray radiation to a minimum. Coaxial lines by design are unbalanced in that the center conductor and the shield are not referenced to each other but to other conductive external systems. A triaxial cable or a twin-axial with two center conductors can be made into a balanced circuit by proper driving devices (usually transformer windings, which should be reasonably matched.)

For ordinary purposes it is not necessary to attempt to 'balance' the two sides of the circuit to the nth degree. Most practical applications can be successfully designed using similar transformers at each end. Stray unbalances can usually be filtered out for a very small price.

Many persons whom I have advised to use balanced circuits resisted the idea because they expected that they had to spend a great deal of time and money to make the two sides of the circuit EXACTLY balanced. In reality, the term "balanced circuit" actually refers to a circuit that is not "single-ended". The single-ended circuit is one which relies upon external means to complete the circuit. An example from old telephone use: when there was trouble on rural lines, often only one side of the metallic loop circuit was affected. The remedy: string one-half the circuit along the barbed-wire fence which keeps the cattle in the meadow. The combination of fence wire and earth ground would usually return the circuit, albeit unbalanced, and the phones could be used until true repairs were in place. Old telephone history also gives us an example of controlling lightning damage to outside plant. In Milwaukee a telephone maintenance man was assigned to care for the large cable which runs (or ran) from Milwaukee eastward under Lake Michigan to a telephone station on the west coast of Michigan. Well, every time Wisconsin had a severe thunderstorm the lightning would find its way into the telephone cable and run along until it was several feet out into Lake Michigan and then it would promptly burst through the insulation and into the water of the lake, leaving a large hole for water to enter and destroy the communications carried in the cable. After several repeated efforts at repairing the submerged cable, the repairman became clever. In each instance he would have to cut out the damaged portion of the cable and splice in a piece which had to be connected to each end where he had cut out the lightning damaged portion. He reasoned that if he were to supply an extra length of cable on the shore end, when he was ready to splice, he would need to pull additional cable from only one side (the shore end) and then he would need to make only ONE splice. So what? you say. Well, when he stacked up and extra length of cable on the shore, he provided enough cable so that he had to coil it up and stack it where he could get convenient access to it and where it would be enough for several splicing jobs. Now the punch line; are you ready for this??? He NEVER had to make another splice! It seems that Mother Nature helped out Ma Bell (old name for the telephone company) by designing lightning so that it did not like to travel through coils. So the coiled supply of cable acted as an inductive choke and prevented lightning energy from using the telephone cable as a means of traveling beneath the waters of Lake Michigan.