Precision Half Wave Rectifier:
This circuit will rectify AC signals right down to a couple of millivolts. That is, it gets over the 0.7V drop across a silicon Diode. I use this circuit to drive Mechanical VU meter displays (-20dB to 3dB) in amplifiers or Mixing desks, etc. It can also be used with an LM3914/15/16 bar graph display driver. Use an LM324N for stereo mechanical meters*. Also, for greater control in calibrating the meter, use a 10k trimmer for the top 10k resistor (feeding the 2nd op amp from the input).
* The balistics of mechanical meters are such, that a faster Op Amp like the LF347 or TL074 will not make any difference to the response of the meters.