An improved arrangement for removably coupling a replaceable cartridge to a housing of a dispenser with the cartridge carrying a piston pump with a movable piston element to be coupled on insertion with an actuator plate of the housing. Occurrence of an overflow or an underflow in the buffer as a result of DA conversion of the audio data synchronously transmitted is prevented. With this arrangement, the current from the guard electrode defocuses the measure current from the measure electrode as it enters the formation, and at greater distances, the current from the measure electrode is refocused by the effect of the current from the pad. Numerous features of the pump are disclosed.