The temperature- and frequency-dependent dielectric susceptibility parallel to the ferroelectric axis of DMAASD crystals was studied in the ferroelectric phase. The dispersion corresponding to a critical slowing down of the thermal dipole relaxation was observed in the vicinity of $T_c$ in the frequency range 1 - 10 MHz. Besides, a dispersion which is related to the dipole reversal within the domain walls was found at lower frequencies. The latter is much more pronounced in the deuterated sample and is thought to be connected with the dynamics of hydrogen bonds.