The invention relates to compound of the formula ; or a salt thereof, wherein the substituents are as defined in the specification; to its preparation, to its use as medicament and to medicaments comprising it. The microphone device includes a buffer stage with an output FET. The system comprises an implantable array of electrodes and a pulse generator, wherein at least one electrode is a cochlear effecting electrode adapted for disposition in the associated Eustachian tube in the proximity of the associated fenestra rotunda.