  We study wave propagation in linear arrays composed of pairs of conjugate
waveguides with balanced gain and loss, i.e. arrays of the PT-symmetric
couplers, where the linear spectrum is known to feature high-frequency and
low-frequency branches. We introduce a domain wall by switching the gain and
loss in a half of the array, and analyze the scattering of linear waves on this
defect. The analysis reveals two major effects: amplification of both reflected
and transmitted waves, and excitation of the reflected and transmitted
low-frequency and high-frequency waves by the incident high-frequency and
low-frequency waves, respectively.
