  We report a comprehensive study of the low-level substitution of Mn$^{3+}$ by
Fe$^{3+}$ effect on the static and dynamic magnetoelectric coupling in
TbMn$_{1-x}$Fe$_x$O$_3$ ($x=0$, 0.02 and 0.04). The cationic substitution has a
large impact on the balance between competitive magnetic interactions and, as a
result, on the stabilization of the magnetic structures and ferroelectric phase
at low temperatures. Low-lying electromagnon excitation is activated in the
cycloidal modulated antiferromagnetic and ferroelectric phase in TbMnO$_3$,
while it is observed up to TN in the Fe-substituted compounds, pointing for
different mechanisms for static and dynamic magnetoelectric coupling. A second
electrically active excitation near 40 cm$^{-1}$ is explained by means of Tb3+
crystal-field effects. This excitation is observed up to room temperature, and
exhibits a remarkable 15 cm$^{-1}$ downshift on cooling in Fe-substituted
compounds. Both electromagnon and crystal-field excitations are found to be
coupled to the polar phonons with frequencies up to 250 cm$^{-1}$. Raman
spectroscopy reveals a spin-phonon coupling below TN in pure TbMnO$_3$, but the
temperature where the coupling start to be relevant increases with Fe
concentration and reaches 100 K in TbMn$_{0.96}$Fe$_{0.04}$O$_3$. The anomalies
in the T-dependence of magnetic susceptibility above TN are well accounted by
spin-phonon coupling and crystal-field excitation, coupled to oxygen motions.
