The FA9 marks the first time FiiO has employed Knowles EJ balanced armature drivers, commonly used among high-end IEMs. FiiO and Knowles collaborated to come up with the custom EJ-33877 driver for use in the FA9 for excellent mid-frequency performance with hauntingly realistic vocals.
In order to achieve the purest sound performance and to ensure the multiple drivers do not interfere with each other, the FA9 employs a specially-designed, ultra-long sound tube 80.6mm in length. The sound tube acts as a low-pass filter that effectively filters out unneeded extra mids and highs from the drivers to ensure a seamless frequency response. The sound tube also ensures the mid-bass is smooth and clean while enhances bass texture in general.