An apparatus for generating AC electrical signals for application to a first pair of electrodes (25L/R) and a second pair of electrodes (25A/P), the apparatus comprising: an AC voltage generator having an output voltage; an electronic switch having an input that receives the output voltage of the AC voltage generator, a first power output and a second power output, wherein the electronic switch is configured to operate in a first mode that routes the output voltage of the AC voltage generator to the first power output and in a second mode that routes the output voltage of the AC voltage generator to the second power output, and the electronic switch is further configured to cycle through a repeating sequence that includes the first mode and the second mode; and a controller configured to synchronize the operation of the AC voltage generator and the electronic switch; characterized in that: the controller synchronizes the operation of the AC voltage generator and the electronic switch by either controlling timing of transitions of the electronic switch in switching to either the first mode or the second mode so that the transitions coincide with windows of time during which the instantaneous output voltage of the AC voltage generator has a magnitude that is below 5 V or controlling the AC voltage generator so that the output voltage of the AC voltage generator has a magnitude of less than 5 V whenever the electronic switch transitions in switching to either the first mode or the second mode; and the AC voltage generator is configured, within 20 ms after the electronic switch switches to either the first mode or the second mode, to provide that the output voltage of the AC voltage generator is at least 80% of the steady-state output voltage of the AC voltage generator. an AC voltage generator having an output voltage; an electronic switch having an input that receives the output voltage of the AC voltage generator, a first power output and a second power output, wherein the electronic switch is configured to operate in a first mode that routes the output voltage of the AC voltage generator to the first power output and in a second mode that routes the output voltage of the AC voltage generator to the second power output, and the electronic switch is further configured to cycle through a repeating sequence that includes the first mode and the second mode; and a controller configured to synchronize the operation of the AC voltage generator and the electronic switch; characterized in that: the controller synchronizes the operation of the AC voltage generator and the electronic switch by either controlling timing of transitions of the electronic switch in switching to either the first mode or the second mode so that the transitions coincide with windows of time during which the instantaneous output voltage of the AC voltage generator has a magnitude that is below 5 V or controlling the AC voltage generator so that the output voltage of the AC voltage generator has a magnitude of less than 5 V whenever the electronic switch transitions in switching to either the first mode or the second mode; and the AC voltage generator is configured, within 20 ms after the electronic switch switches to either the first mode or the second mode, to provide that the output voltage of the AC voltage generator is at least 80% of the steady-state output voltage of the AC voltage generator. the controller synchronizes the operation of the AC voltage generator and the electronic switch by either controlling timing of transitions of the electronic switch in switching to either the first mode or the second mode so that the transitions coincide with windows of time during which the instantaneous output voltage of the AC voltage generator has a magnitude that is below 5 V or controlling the AC voltage generator so that the output voltage of the AC voltage generator has a magnitude of less than 5 V whenever the electronic switch transitions in switching to either the first mode or the second mode; and the AC voltage generator is configured, within 20 ms after the electronic switch switches to either the first mode or the second mode, to provide that the output voltage of the AC voltage generator is at least 80% of the steady-state output voltage of the AC voltage generator.