Originally Posted by Chu Gai
Yuriv, can you elaborate more on how the connections were made when using the headphones? Also, in your estimation, would it be fair to say that the performance of the players becomes signficantly better when the impedance of the headphones is greater? That's not to say you'll necessarily hear more bass seeing as the fit to the ear plays a signficant role.
I split the headphone output with a Y cable. One end goes to the headphones, and the other goes to the recorder. Normally a splitter cable is used to connect two pairs of headphones in parallel. But here the headphones are in parallel with the 1820m's line in, which has a relatively high input impedance.
There's a joke in Futurama
about changing the outcome by measuring it. The line input's high Z ensures that we don't change the iPod's behavior too much. 60 ohms in parallel with 10+ kohms is still around 60 ohms. I have another recorder that has a 1 Megohm input impedance just in case.
I think it's safe to say that the player's performance improves as the load impedance increases. The bass roll off can be explained by a voltage divider and the iPod's increasing source impedance at lower frequencies. The higher the source impedance, the more the voltage drops across that impedance, resulting in a lower voltage across the load headphones.
The iPod's output capacitors are responsible for the rising source impedance at decreasing frequencies. The caps eventually dominate this figure, and at 0 Hz, the impedance is very, very high because caps almost completely block DC. How fast the impedance increases depends on the size of the capacitors. The 1st generation iPod Mini has smaller blocking caps, and therefore a more severe bass roll off with low-impedance earphones.
Does THD decrease with increasing headphone impedance? You be the judge:
0.034% JVC HA-FX33
0.030% Sony MDR-EX90LP
0.111% Shure E4
0.025% Sennheiser PX-100
0.025% Grado SR125
0.047% Etymotics ER-6
0.021% Koss KSC-75
0.018% Sony MDR-7506
BTW, Yesterday we got to test a friend's 16-ohm Etymotics ER-6i earphones and a HeadRoom AirHead headphone amplifier. The iPod bass roll off is worse on the ER-6i than on the Shure E4. Yet the ER-6i has much more bass than the ER-6, which has a much flatter response with the iPod. Etymotics Research and others probably designed their lower-impedance earphones around a source that they knew would have bass roll off. More on that later.