I wanted to illustrate the above points with an actual example of how the E7 can help improve the sound from a player's headphone output:
Take the Cowon D2 player's output - suppose you have a 16ohm headphone, here's the performance you can expect:
http://rmaa.elektrokrishna.com/Single_Players/Cowon%20D2%20-%2016%20Ohm%20Load.htm
I am unsure of the E7's input impedance, but suffice to say it is higher than 600ohms. Presenting the D2 with a 600ohm load gives you the following performance:
http://rmaa.elektrokrishna.com/Single_Players/Cowon%20D2%20-%20600%20Ohm%20Load.htm
You get a significant boost in signal quality across the measurements in terms of frequency response, noise, distortion, crosstalk, and IMD. Just to be thorough, here's the D2's performance into "no load:
http://rmaa.elektrokrishna.com/Comparisons_%28No_Loads%29/Cowon%20D2-O2%20-%20No%20Loads.htm
The above graph actually shows two poings: 1) there is again more raw performance from the D2's headphone output at no load versus 600ohm load, the E7 's input impedance is closer to no load rather than 600Ohm, if its input is standard 10k Ohms. 2) the graphs show the expected increase in performance across the board as the D2's volume is increased, most significantly an increase of 12dB in signal to noise ratio! This is what I was referring to when I wrote in my previous post that "the source volume can be set sufficiently high to ensure a good signal quality".
I couldn't find a RMAA plot for a production E7 quickly, so here's one based on a pre-production sample:
http://rmaa.elektrokrishna.com/Various/FiiO%20E7%20%28Aux%20vs.%20USB,%20Vol.%2060,%20No%20Bass%20Levels,%20Var.%20Loads%29.htm
As can be seen, the E7 is not nearly as sensitive to load changes as the D2, thus giving you far superior signal to a 16ohm headphone versus directly from the D2's headphone jack. Using the E7 following the D2's headphone output gives substantial improvements in sound quality, even though the ultimate volume may only be up to 4dB louder. I believe this is the true fundamental benefit of a separate headphone amp: superior quality amplification, not just more amplification.
Jack
Edited by Jack-Micca - 7/14/10 at 7:16pm