Balanced outs

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If you're going to use balanced audio cables you need a balanced output from the source driving those cables. That source can be generated from an output transformer (as we do in DirectStream DAC), a phono cartridge (sometimes), or a piece of audio kit properly configured. Of all the tasks and challenges for the design engineer, as associated with balanced audio, getting the output balanced is by far the easiest (though to be honest none are hard). As discussed previously the requirement is for two outputs, each of the opposing phase. How this is accomplished with active circuitry, as opposed to passive—like a transformer, ranges from a glued on device to the right way. Both deliver the same signal but they don't sound the same. Let's take it slow and easy so we truly grasp what turns out to be a rather interesting concept. The simplest way to take an existing single ended amplifier and add balanced outputs is to tack on another stage to invert the original output. OpAmp Phase Splitter See in the drawing where it says "input"? This feeds the top triangle and represents the original single ended amplifier with its +out. (These simple triangles are how we engineers call out an amplifier block. This block can range from the very simple to the complex, tube, solid state, op amp, SET, whatever. In this drawing it is a simple op amp) The "+out" is our standard RCA output for single ended signals. If we want balanced, we need only add a second amplifier block. This block (the lower amplifier triangle) is called an inverting amplifier because it flips upside down what is fed to it. The "-out" is the second of our two balanced outputs needed. Tacking an additional amplifier stage onto an amplifier works. It is not the preferable way to do this when sonics matter. There is a better way, one we'll discuss tomorrow. Your takeaway for today. What you see pictured is how 90% of all high end preamplifiers, DACs, sources, of all kinds get their second out of phase output to make them "balanced".
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Paul McGowan

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