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Hi Ray,
Don't be afraid of increasing the gain quite a bit on both the diff integrator and the autotune amp if you are trying to build a detector to detect small gold. It will be unstable within the city limits, but out in the field, it should be much quieter.
Obvously, one can go too far, but changind the 100K resistors in either or both ckts to up to 470K is not unreasonable. I would increase the diff amp sensitivity to begin with and try it in the field. Then increase the autotune amp and try it also. I personally, would make the autotune gain adjustable just to see what might work best. It is much more difficult to make the diff integrator gain adjustable because both inputs have to be balanced, meaning if you change a 100K feedback resistor in the diff integrator ckt, you need to change the 100K in the + input leg also so they match. Both need to be tried in the field and in a very noisy environment.
Personally, I would also reduce the gain of the preamp to both widen the bandpass and reduce the recovery time. Make up the gain in the later stages.
Don't be afraid of reducing the size of the coil either. An 8" coil will be faster and you will not lose much depth. So, you will also see an increase in sensitivity to smaller gold, faster sampling times, and still have very decent depth.
Now, the best DD size I have found is the 11" round housing I got from Bill Hays. It is the least ground sensitive, is very fast, delay wise, and is very sensitive overall. It is one of my best coils
Finally, you do not need a perfect decay curve. If the curve is stable and flat at the time of sampling, then it should work fine, regardless of what it looks like at an earlier time. So, you could have an overshoot or a wiggle in the decay signal prior to sampling and everything will still work fine.
When I first started, I had an overshoot that would settle down before sampling and all worked ok. Later with refined coil windings, and using some of the techniques mentioned on this forum, I was able to make the decay curve appear more normal.
If you go back through some of the previous postings, you will find one where Eric Foster mentioned some tricks on how to take care of some minor overshoot problems using brass washers.
Now, I have used copper tape and lead foil tape to do the same thing. Some of my coils still have small pieces of it stuck on the outside.
Unfortunately, I can't find my HH folder so I don't know just how far I pushed the gains of the amps on my HH's. I do know, I did push the gains up a lot. The autotune gain was made variable also.
Now, as for your nugget, you might want to take a close look at it with a good magnifiying glass to see just what the structure looks like. If it is not extremely thin, wrapped around pieces of quartz, or crystaline in nature, then I would think that you should be able to detect it at a distance of at least 6 inches minimum.
If you have a VLF, see how far you can detect the small nugget with it. This will give you somewhat of an idea of what you should expect with a PI.
Reg
Don't be afraid of increasing the gain quite a bit on both the diff integrator and the autotune amp if you are trying to build a detector to detect small gold. It will be unstable within the city limits, but out in the field, it should be much quieter.
Obvously, one can go too far, but changind the 100K resistors in either or both ckts to up to 470K is not unreasonable. I would increase the diff amp sensitivity to begin with and try it in the field. Then increase the autotune amp and try it also. I personally, would make the autotune gain adjustable just to see what might work best. It is much more difficult to make the diff integrator gain adjustable because both inputs have to be balanced, meaning if you change a 100K feedback resistor in the diff integrator ckt, you need to change the 100K in the + input leg also so they match. Both need to be tried in the field and in a very noisy environment.
Personally, I would also reduce the gain of the preamp to both widen the bandpass and reduce the recovery time. Make up the gain in the later stages.
Don't be afraid of reducing the size of the coil either. An 8" coil will be faster and you will not lose much depth. So, you will also see an increase in sensitivity to smaller gold, faster sampling times, and still have very decent depth.
Now, the best DD size I have found is the 11" round housing I got from Bill Hays. It is the least ground sensitive, is very fast, delay wise, and is very sensitive overall. It is one of my best coils
Finally, you do not need a perfect decay curve. If the curve is stable and flat at the time of sampling, then it should work fine, regardless of what it looks like at an earlier time. So, you could have an overshoot or a wiggle in the decay signal prior to sampling and everything will still work fine.
When I first started, I had an overshoot that would settle down before sampling and all worked ok. Later with refined coil windings, and using some of the techniques mentioned on this forum, I was able to make the decay curve appear more normal.
If you go back through some of the previous postings, you will find one where Eric Foster mentioned some tricks on how to take care of some minor overshoot problems using brass washers.
Now, I have used copper tape and lead foil tape to do the same thing. Some of my coils still have small pieces of it stuck on the outside.
Unfortunately, I can't find my HH folder so I don't know just how far I pushed the gains of the amps on my HH's. I do know, I did push the gains up a lot. The autotune gain was made variable also.
Now, as for your nugget, you might want to take a close look at it with a good magnifiying glass to see just what the structure looks like. If it is not extremely thin, wrapped around pieces of quartz, or crystaline in nature, then I would think that you should be able to detect it at a distance of at least 6 inches minimum.
If you have a VLF, see how far you can detect the small nugget with it. This will give you somewhat of an idea of what you should expect with a PI.
Reg
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