Audio Engineering

792 Chapter 27

The line-level input stage on professional equipment therefore has to perform a
complementary function to the output stage to derive a single-ended signal from the balanced
signal presented to the equipment. Conceptually, the simplest circuit is a transformer, like that
shown in Figure 27.22. In many ways this is an excellent solution for the following reasons:
it provides electrical isolation, it has low noise and distortion, and it provides good headroom,
provided the core doesn’t saturate. But, most important of all, it possesses excellent common-
mode rejection (CMR). That means that any signal that is common (i.e., in phase) on both
signal phases is rejected and does not get passed on to following equipment. By contriving
the two signal conductors within the signal cable to occupy—as nearly as possible—the same
place, by twisting them together, any possible interference signal is induced equally in both
phases. Such a signal thereafter cancels in the transformer stage because a common signal
cannot cause a current to fl ow in the primary circuit and cannot, therefore, cause one to fl ow in
the secondary circuit. This is illustrated in Figure 27.22 as well.

Another advantage of a balanced signal interface is that the signal circuit does not include
ground. It thereby confers immunity to ground-sourced noise signals. On a practical level
it also means that different equipment chassis can be earthed, for safety reasons, without
incurring the penalty of multiple signal return paths and the inevitable “ hum loops ” this
creates. However, transformers are not suitable in many applications for a number of
reasons. First, they are very expensive. Second, they are heavy, bulky, and tend to be
microphonic (i.e., they have a propensity to transduce mechanical vibration into electrical

Signal input

Signal

source

Phantom

power

Connecting

balanced-line

Interfering noise-source

Noise ‘cancels-out’

48V

Figure 27.22 : Balanced input circuit, and CM rejection.