I've become interested, lately, in looking for gold nuggets along stream beds, with a "vlf" metal detector. They have limited sensitivity. One reason is that they have a tone (about 500 hz I believe) that varies in amplitude when a target is found. That's all fine for coins and large targets, but your ear can only detect changes of 6 db's or so. However, one's ear can detect changes of just a few hz in frequency. I thought I might be able to rectify the tone coming from the "vlf" detector and apply it the Ad537 that was "talked about here on this site. Then maybe run that to an op amp and apply that my headphones. I'm a fair tech, but my engineering skills are limited. Again any ideas would be appreciated. Thanks,Ron.