There are only two electrical output terminals for an unbalanced output—signal and ground. However, the unbalanced output stage in Figure 21.1a is fitted with a three-pin XLR connector to emphasise that it is always possible to connect the cold wire in a balanced cable to the ground at the output end and still get all the benefits of common-mode rejection, if you have a balanced input. If a two-terminal connector is fitted, the link between the cold wire and ground has to be made inside the connector, as shown in Figure 20.2 in the chapter on line inputs.
The output amplifier in Figure 21.1a is configured as a unity-gain buffer, though in some cases it will be connected as a series-feedback amplifier ...
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