Hearing aid system for the in-situ fitting of hearing aids, said system comprising a separate control device, and a least one hearing aid, adapted for communication with each other, said hearing aid comprising at least one microphone, a signal processor for generating an output signal to a receiver, and means for receiving control signals form the control device. During the in-situ fitting the control device is in communication with said hearing aid for the activation of generation of test signals, which test signals are delivered to said receiver and emitted therefrom as acoustic test signals. Further, the hearing aid comprises a switch means which when said hearing aid is in communication with the control device may optionally be switched between at least a first and a second position, said switch attenuating in the first position the output signal to the receiver using a voltage dividing resistor network, and said switch bypassing in the second position said voltage dividing resistor network so as not to influence the output signal to the receiver.