  Isospin diffusion is probed as a function of the dissipated energy by
studying two systems $^{58}$Ni+$^{58}$Ni and $^{58}$Ni+$^{197}$Au, over the
incident energy range 52-74\AM. Experimental data are compared with the results
of a microscopic transport model with two different parameterizations of the
symmetry energy term. A better overall agreement between data and simulations
is obtained when using a symmetry term with a potential part linearly
increasing with nuclear density. The isospin equilibration time at 52 \AM{} is
estimated to 130$\pm$10 fm/$c$.
