[goodsound]

Dont know if this has been done before or not but here's some measurements I did on the nano and the stock earbuds. First, the frequency response of the i-pod. I put the load (stock earbud) only on the LEFT channel to get an idea of no-load v/s load.

FR:-

Headphone FR:-

Headphone Z:-

As you can see the low freq rolloff of the device isn't really that bad. Under load (34ohm) -3db is still 20hz. -1db is 35hz. So the output caps are pretty well sized. I think its the headphone that looses steam in that area. The huge hump in the 200hz region explains the annoying resonance and boomy nature of the mid-bass. Listening to it with Grado SR-80 and KOSS KSC-75 eliminated this problem and the bass was plenty! I found it lacking in absolutely no way in the opening bass drums in Hotel California (Hell Freezes Over version).


Also, the response gets shelved by barely -1db under load which gives the impression that the output opamp is able to deliver the current required by a 34ohm load reasonably well.

Next up are the THD and IMD results:-
I did THD and IMD measurements for four different scenarios-
1. No Load
2. Load (stock earbud) at 75% volume.
3. Load (stock earbud) at 60% volume.
4. and with the KOSS KSC-75 headphone.

Now I have combined four different scenarios into one screen shot so please bear with the large sizes. In the following two pics this is the order of the charts -
1, 2
3, 4

Nano THD:-

Nano IMD:-

As you can see the thd and imd run wild in #2 and #3. but tames a lot in #4.
So the conclusion I draw from this is that a high-impedance (light load), high-sensitivity (less power) headphone might work best. I wonder why the low frequency IMD jumped up in the KOSS. Could it be the cable picking up hum ?

Here's a close up of the high frequency IMD, for your enhanced viewing pleasure.

 

[goodsound]

I should have mentioned how I did these tests.
For the headphone impedance and FR I used Speaker Workshop and did a "nearfield" response(mic virtually stuck to the earbud).

And for the i-pod's fr, thd & imd I used RMAA. I copied a WAV file produced by RMAA to the i-pod and played it from there and let RMAA record it. (now as I type this I wonder if using an AIFF instead of WAV would have produced any different results or otherwise.hmm..)