What is the difference between a balanced and unbalanced circuit?
A balanced circuit (also referred to as a “balanced line”) is a signal-carrying circuit with two active electrical conductors of equal impedance, each conductor carrying a different polarity. These conductors are generally twisted inside the insulator, and hence commonly referred to as a “twisted pair.” Usually, both conductors are enclosed within an overall metallic shield, which does not carry the signal. Balanced circuits are mostly used to reduce the pickup of noise/interference in audio system cabling. Typically, balanced circuits can be found on professional-level microphones, XLR inputs on a mixer, and balanced line connections between an amplifier and speakers.
An unbalanced circuit (or “unbalanced line”) is a signal-carrying circuit with one electrical conductor and an overall metallic shield. Unbalanced connections are typically found on professional-level instrument inputs and outputs, and such consumer-level products as turntable RCA connections, headphone outputs, and the 3.5 mm (⅛”) connections used for microphone outputs, PC microphone inputs, Mac line-level inputs, and some camera inputs.
Balanced circuits require additional materials and engineering in order to eliminate or reduce noise/interference in an audio signal. Unbalanced circuits, with their simpler design, are more susceptible to noise/interference, especially at longer cable runs. However, most products that utilize unbalanced circuits do not require the additional shielding and isolating material to perform properly at their respective cable lengths. Depending on the circuit and cable type, environmental use, and application, a long cable run can be anything from 6′ to 30′ and up. As an example, 6′ or more would be considered a long cable run for an unbalanced RCA connection. By comparison, ¼” unbalanced TS instrument cable would be considered a long run if it were greater than 30′.
Professional instrument connections and consumer-level connections have historically been unbalanced. They have stayed this way in order to maintain affordability and compatibility with their input devices. And at shorter cable lengths, unbalanced circuits/connections are perfectly acceptable to use. This makes balanced circuits more impractical for consumer-level equipment and some professional instrument equipment, and thus they have not been adopted for such purposes. But balanced circuits generally excel in professional applications, especially live sound and professional studio applications requiring longer cable runs. Additionally, since professional equipment features balanced circuits, balanced connections are required to maintain compatibility.
In order to properly convert an unbalanced signal to a balanced signal (and vice versa), an impedance matching transformer must be used.