I just purchased a Porta Corda amp from another Head-Fi'er, in "new in the box" condition. It has some sort of distortion, a warble-like effect, at normal and higher volumes. I've checked it with several different CDs and different headphones, including Ety ER-4S. This happens with crossfeed processor in or out.
I've compared it to a TAH I own, and the difference is significant. The TAH outputs crystal-clear sound.
I've searched the forums about this, and read that others have experienced this as well. I wonder if it's a defect in some units? I noticed that there were suggestions to increase the power to 18 volts, but (1) I can't figure out how to do this, and (2) I want to retain the small self-contained size of the amp.