The difficulty level in doing what you propose is dependant on quite a few factors such as:
1. frequency of the AC current, or signal
2. the amplitude (or voltage level) and current of the signal
3. whether or not the cable carrying it is sheilded
(these are just some factors - not all)
The...
I've built similar circuits using the old LM567 PLL chip. It's cheap and VERY easy to use (8pin). It's been a while since I worked with it, so there may very well be a newer (better?) replacement, but the LM567 is still widely available and should do the job. In fact, I have one sitting here now...
