> For what its worth, I'd suggest deriving the bias voltage from the 
> regulated 5V supply instead of the battery

For that matter you could generate V+ for the op-amp with the
PWM module

Principles -

http://ece.wpi.edu/courses/ee2201/2201%20LAB%202.doc

Figure 2-8


555 as voltage doubler

http://www.reconnsworld.com/power_voltdoubler.html

555 as +12V to -12V converter

http://sound.westhost.com/project95.htm

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