Check the offset of the last stages of your LM324. I will bet that it's 5 - 10MV negative. Replacing the LM324 with an LF347 will most likely fix it if that's the case. With a home built 10" coil I can squeeze 19"out of mine for a U.S. Quarter. Problem is, only about the first 12" is usable as it becomes unstable at high sensitivity setting, even with a stock coil.
Stock coil -> http://www.youtube.com/watch?v=BmwMIynbJSk
Homebuilt coils - > http://www.youtube.com/watch?v=qDyZXoC2lJk
http://www.youtube.com/watch?v=lIbAG...feature=relmfu
Don
Stock coil -> http://www.youtube.com/watch?v=BmwMIynbJSk
Homebuilt coils - > http://www.youtube.com/watch?v=qDyZXoC2lJk
http://www.youtube.com/watch?v=lIbAG...feature=relmfu
Don
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