In the best case, you only need a cable like this, with the male jacks connected to the player and the ADC/sound card, and the headphones connected to the female jack. This should work as long as the output voltage of the device to be tested is safely below that of the maximum input voltage of the ADC (this is often 2 Vrms, like for the Xonar sound cards, but some devices may allow more, while onboard audio is often more limited). For most portable players, the 2 Vrms is enough, but not for portable amplifiers like the FiiO E11. To avoid an excessive signal level, use lower volume (start low, and keep it below the clipping level), or build a simple voltage divider that reduces the voltage to a level that works with your ADC. For a simple FR measurement, keeping the volume low enough should be sufficient.
If you only intend to test with RMAA, no sound file is needed, RMAA will generate its set of test signals.
I do recommend measuring the maximum output voltage and the output impedance, if possible, since these parameters are important for determining how well the device will drive a particular headphone.
Edited by stv014 - 2/15/12 at 5:47am
