I have read of some people plugging their headphones directly in the Line Out jack of the ODAC.
The line out is designed to input 2V into a 5K ohms input which is perfect for an amplifier.
However, the designer of the ODAC recommends NOT to use the Line Out with headphones.
Most likely, the ODAC output impedance is too high for most headphones which leads to distortions in the frequency response.
I have searched the ODAC designer's website, Head-Fi and a few other forums to find out what the ODAC output impedance is.
The funny thing is that this specific spec doesn't seem to be published anywhere.
It is even more surprising when you know that the ODAC designer's trademark is based on extensive objective measurements.
Does anyone have the output impedance figure for the ODAC or is able to perform the measure?