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Chapter 17: Line Outputs > 17.4 Balanced Outputs

17.4  Balanced Outputs

Figure 17.3c shows a balanced output, where the cold terminal carries the same signal as the hot terminal but phase-inverted. This can be arranged simply by using an opamp stage to invert the normal in-phase output. The resistors R3, R4 around the inverter should be as low in value as possible to minimise Johnson noise, because this stage is working at a noise-gain of two, but bear in mind that R3 is effectively grounded at one end and its loading, as well as the external load, must be driven by the first opamp. A unity-gain follower is shown for the first amplifier, but this can be any other shunt or series feedback stage as convenient. The inverting output if not required can be ignored; it must not be grounded, because the inverting opamp will then spend most of its time clipping in current-limiting, almost certainly injecting unpleasantly crunching distortion into the crossover grounding system. Both hot and cold outputs must have the same output impedances (R2, R6) to keep the line impedances balanced and the interconnection CMRR maximised.


  

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