I took the +/- 11.8 V figure from the "O2 Details" (listed under "Popular Posts") page of the designer. The O2 uses 12 V regulators, but the voltage is reduced slightly by the diodes that isolate the two sources of power supply (these are special low voltage diodes that drop less than the typical ~0.6 V), and the MOSFETs in the under-voltage protection circuit. The "Op Amp Measurements" page from August 2011 includes graphs that shows the THD+N vs. voltage performance of the gain stage with a few op amps (the stock one is NJM2068), both on battery and AC power, and at 2.5x and 7x gain. These can be used to find out what the exact clipping level is. Of course, the supply voltage will have a small amount of random variation.
Edited by stv014 - 11/9/12 at 10:03am