Just one more question tomb....
All I need to charge a couple of Ni-MH/Ni-Cd 9V batteries is just that simple circuit above and a wallwart of 24V and 500mA???? Nothing else to take care of???
I mean, I don't mind reducing battery life, I'm planning to build a small portable amp (something similar to a47) and I was planning just to take out the batteries and recharge them when necessesary (I want the amp as small as possible), but it might be worth a little more space on the board if I could just recharge them inside.
About the walwart - it's gotta be DC, and the batteries can't be more than the 8.4V variety - a 24V walwart is somewhat close at that rating (with 2X batteries, of course). If you used the 9.6V batteries that are available places, it won't cut it. Of course, the paragraph above assumes 250mah batteries. Those are pretty easy to get by a number of e-bay sellers. (I think the Thomas and MAHA's are way too expensive.)
Jameco has a very nice 24VDC at 500ma linear, regulated walwart for about $13. It gives pretty good sound when connected to a PIMETA - the bass is a tad "squishy" compared to a full-blown TREAD or batteries alone. If you're using Grado's, you may not even notice that deficiency (no offense).
I can't predict for your amp, though - the 500ma is a typical size walwart for headphone amps. The charger itself, as noted above, only uses 12ma. Adding double buffers and biasing probably brings a PIMETA up to 150-200ma, but maybe not that much. Your A47 will be less than that.
Please let me recommend a PIMETA. Tangent's design is genius in its simplicity. If you are contemplating something more than a CMoy, but less than a PPA/M3, the PIMETA is it. It is far superior to an A47 and can be built very cheaply.