790 Chapter 27
A high degree of linearity is achieved in modern line-level audio stages by utilizing high
open-loop gain op-amps and very large feedback factors. The only issue remaining,
once the choice of op-amp has been made, is available headroom. This is almost always
determined by choice of power supply rails. Taking a typical audio op-amp (TL072, for
example), maximum output swing is usually limited to within a few volts of either rail.
So, furnished with 12-V positive and negative supplies, an op-amp could be expected to
swing 18 Vpk-pk. This is equivalent to 6.3 V rms, or 16 dBV, easily adequate then for
any circuit intended to operate at 0 VU 10 dBV. In a professional line-output circuit,
like that shown in Figure 27.21 , this voltage swing is effectively doubled because the
signal appears “ across ” the two opposite phases. The total swing is therefore 12.6 V rms,
which is equivalent to 24 dBu. For equipment intended to operate at 0 VU 4 dBu,
such a circuit offers 20 dB headroom, which is adequate. However, 12-V supplies really
represent the lowest choice of rail volts for professional equipment and some designers
prefer to use 15-V supplies for this very reason.
Looking again at the output stage circuit illustrated in Figure 27.21 , note the inclusion
of the small value resistors in the output circuit of the op-amps. These effectively add
some real part to the impedance “ seen ” by the op-amp when it is required to drive a long
run of audio cables. At audio frequencies, the equipment interconnection cable looks to
the output stage as a straightforward—but relatively large—capacitance. Also, a large
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