A hearing aid comprising: a first part configured to be positioned at or behind the ear of a user and an assembly (10, 300) comprising a second part (12, 340) configured to be positioned at or at least partly in the ear canal of the user and a third part (16, 328) configured to mechanically connect the first part and the second part (12, 340), the second part (12, 340) comprising an output transducer (34,326) configured to provide an acoustic signal to be provide to the user's ear canal, the third part (12, 328) comprising a transmission path configured to provide electrical connection between the first part and the second part (12, 340), the transmission path being at least partially established via or on a flexible substrate (102, 108, 114. 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338) having a plurality of electrically conductive paths, wherein the third part further comprises a protective member (24, 90) mounted along the length of the flexible substrate (102, 108, 114, 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338), wherein the hearing aid further comprises a wireless interface and an antenna, and wherein at least part of the antenna is formed along the flexible substrate (102, 108, 114, 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338), wherein the antenna extends at least in part of the length of the flexible substrate (102, 108, 114, 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338). a first part configured to be positioned at or behind the ear of a user and an assembly (10, 300) comprising a second part (12, 340) configured to be positioned at or at least partly in the ear canal of the user and a third part (16, 328) configured to mechanically connect the first part and the second part (12, 340), the second part (12, 340) comprising an output transducer (34,326) configured to provide an acoustic signal to be provide to the user's ear canal, the third part (12, 328) comprising a transmission path configured to provide electrical connection between the first part and the second part (12, 340), the transmission path being at least partially established via or on a flexible substrate (102, 108, 114. 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338) having a plurality of electrically conductive paths, wherein the third part further comprises a protective member (24, 90) mounted along the length of the flexible substrate (102, 108, 114, 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338), wherein the hearing aid further comprises a wireless interface and an antenna, and wherein at least part of the antenna is formed along the flexible substrate (102, 108, 114, 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338), wherein the antenna extends at least in part of the length of the flexible substrate (102, 108, 114, 120, 126, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 251, 252, 253, 256, 280, 290, 338).