Well, it's not quite an AC signal... Without delving into the gruesome details of the experiment I'm doing, I guess I should have been a little more clear: the signal is of varying _low_ frequencies, and is actually DC at times, so I can't filter out DC components entirely.
And the offset is big: about 7 volts. I'm actually detecting variations in the output from a phototransistor operating between +15 V and ground. The device operates most linearly around 7 volts, so that's where I want my output to be nulled at.
But thank you for your suggestions. I was hoping to be able to work with what only things I have on hand, but I'll give the IC in-amps a thought now.