Ins and outs

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I think one of the problems people face understanding balanced audio has to do with how it is applied. The terms that get thrown around, like true balanced, are so generalized they lose meaning. They get inaccurately applied across the board. In tomorrow's post I will show you how a balanced signal works and why. But for today, I wanted to focus on the broader scope of application. Let's assume there are three main points in a circuit where balanced audio can be applied. The input, the middle, the output. These three generalized areas where the term balanced can apply, are all very different and need to be separated if we're to achieve any sort of clarity of understanding. Balanced input This is the most most common application of balanced audio. Any properly designed amplifier featuring an XLR balanced input is often said to be a balanced amplifier. This is misleading and does not mean what you might think. Just because an amplifier sports an XLR connector doesn't mean it is a balanced amplifier. All it means is that it has, as its name implies, a balanced input. The distinction is an important one. A balanced input has the advantage of lowering incoming noise from the connecting cable. Noises picked up by the connected cable are reduced in the balanced input, through a process called Common Mode Rejection, a subject we shall delve into tomorrow. For the sake of brevity let me suggest simply that a balanced input's advantage is confined to reducing noise from the connecting cable. It is not a balanced amplifier, not does it enjoy the advantages of a balanced amplifier. It is, simply put, exactly what its name implies. Balanced amplifier Strictly speaking, a balanced amplifier does not necessarily have a balanced input or output. It usually does have both, but as in point number one, the term's accuracy does not demand it, nor does every design benefit from it. There are numerous schemes to build a balanced amplifier. The most common is a series of differential pairs, a design we use a lot, and one I will detail in the coming days. An amplifier qualifying as balanced is said to have two identical signal paths, each handling a signal out of phase with the other. I would argue this is not what we mean when we think of balanced audio. Instead, a better description would include one additional requirement. Common Mode Rejection—the benefits of which are huge—including lower distortion and noise. To be a balanced amplifier in my book means to be a differential amplifier—an amplifier that internally amplifies only differences, ignoring anything in common. More to explain later. Balanced output This is very common that amplifiers have balanced outputs. Any design, whether single ended or balanced, can have balanced outputs. An amplifier sporting an XLR connector at its output, presenting two signals, each out of phase with the other, can be said to have a balanced output. This is the simplest form of balanced for design engineers to achieve and provides zero benefit to the device that has it. The only benefit of a balanced output is to be found later, when connected to a balanced input.
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Paul McGowan

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