Minelab GPX-4000 "Gain" Adjustment


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  • Admin

Hello All,

There has been some debate on the "Gain" Adjustment since the GPX-4000 has been out. Some are maxing out the "Gain," while others are running a lower "Gain," but running the Audio in Deep or Boost. I've done some brief testing on these adjustments, but curious what others are finding out?

Talk with you soon,

Rob Allison

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Rob, I'm the last one to listen too, but I'm trying the highest gain in whatever mode, yet with a smooth threshold. Where is the debate?

I'm still trying to figure out swing speed. Last weekend, doing a little cleanup, up on the hill, going very slow (speed about 6 secs per 6' swing); heard a slight target. In confirming, I ran across the target a little faster and got a much better signal. I'm hunting in "slow " motion, but beleive I'll try "very slow" tomorrow. Sound right?

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  • Admin

Hello Shep,

When I'm in the normal prospecting mode I've been in "Slow" mode, but when I know I'm in a patch I'm using the "Very Slow" mode and it seems to work better on deep targets.

I also like a higher "Gain" over a lower "Gain" with the "Boost or Deep" mode. Different ground conditions might have different results with these settings.

Talk with you soon,

Rob Allison

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  • 4 weeks later...

Rob,

This was a great question that didn't get much of a response. I have been mostly running a high gain with the less aggressive audio, but I wonder if this is always the best way. Should ground noise and EM be treated the same? Are there any conditions when boost and a lowered gain would be more productive?

I hope some of the more experienced folks will offer an opinion.

Bill

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Hi you guys,

I would not call myself deeply experienced, but I read

the heck out of all sorts of forums here and in Oz.

The concensus seems to be as you said Rob;

Run the highest gain you can first,

then make adjustments to it as the ground dictates.

I think audio would be the second most important function for depth

after gain.

Just my opinion from using a 4000 and reading alot.

Shep.

Swing speed is another very under-discussion-topic.

From what I have read (and what we tried up in Mariposa last time up), was a slightly

quicker swing speed with mono's... don't have specific's for DD coils.

This was gleaned from a post by JP a few weeks ago on a forum in Oz.

He said something to the effect that mono's need a slightly faster swing speed

to give the best signal.

I think that's right.

I see JP lurk here every once in awhile, maybe he'll jump in...

JP?

All the best,

and I agree, a very important discussion thread.

Flak

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  • Admin

Hello Guys,

Just have a few minutes to jump in here. If I remember correctly (need to take another look), the default settings for DEEP gold had a gain around 8-9. I know if you run your Audio in Deep, the GPX-4000 in the Deep setting and with a high Gain 13-15, the machine gets very noisy!

The settings I really like cause its extremely quiet is - Sensitive/Smooth, Quiet Audio and a High Gain (12-13). Found a lot of small deep nuggets and fair sized nuggets at depth.

I'm sure with all settings you're loosing something somewhere, but I like the settings above.

Talk with you later,

Rob Allison

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Hello Guys,

Just have a few minutes to jump in here. If I remember correctly (need to take another look), the default settings for DEEP gold had a gain around 8-9. I know if you run your Audio in Deep, the GPX-4000 in the Deep setting and with a high Gain 13-15, the machine gets very noisy!

The settings I really like cause its extremely quiet is - Sensitive/Smooth, Quiet Audio and a High Gain (12-13). Found a lot of small deep nuggets and fair sized nuggets at depth.

I'm sure with all settings you're loosing something somewhere, but I like the settings above.

Talk with you later,

Rob Allison

Hi Rob,

Many people fail to appreciate that depth and sensitivity is ultimately dictated not so much by signal strength but by the noise floor at the gain they are using. The optimal depth/sensitivity with all other things being equal occurs at the maximal S/N noise ratio. High gain always results in increased electronic and environmental EM noise. In adverse situations better depth/sensitivity is often obtained by reducing the gain which lowers the noise floor more than the signal. A good analogy is fog. To improve visibility we don’t make our headlights brighter (increase the gain) which generally makes visibility worse, we lower the intensity of the light (reduce gain) and perhaps change the wavelength of the transmitted light. If we use high gain in Pi’s there is always a price to pay even if we use sophisticated methods to reduce noise. For example to reduce noise levels at high gain in our Pi we might decide to use a wider sampling window and to integrate longer than at our usual settings. This will lower the noise floor by averaging it out but what else happens? After sampling and integration in a wide sampling window the average signal will be incredibly small or un distinguishable from noise for some nuggets because their signal decays very rapidly or is of very small initial amplitude.

The net result is a loss of depth and sensitivity.

Doug

http://groups.yahoo.com/group/ProspectinginOz

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Thanks for the replies

I too have been using quiet, but not smooth. Some crude testing showed smooth to suffer a great loss of depth/sensitivity. Perhaps I should re-evaluate.

Doug,

If lower gain reduces the noise floor more than the signal, would it then be advantageous to run in deep or boost, or would the added boost lead to a decrease in signal to noise ratio similar to increased gain? I assume that gain is increased near the front end of the receiver and boost is amplification later in the recieve ???

Bill

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Thanks for the replies

I too have been using quiet, but not smooth. Some crude testing showed smooth to suffer a great loss of depth/sensitivity. Perhaps I should re-evaluate.

Doug,

If lower gain reduces the noise floor more than the signal, would it then be advantageous to run in deep or boost, or would the added boost lead to a decrease in signal to noise ratio similar to increased gain? I assume that gain is increased near the front end of the receiver and boost is amplification later in the recieve ???

Bill

Hi Bill.

Some that are using the 4000 ( I only have a 3000 by the way) on my forum have found that deep at high gain can be noisy . As to your last question I can't say as noboby including me knows the ML circuit topology. Your explanation however seems feasible, the deep setting may just be some filtering of the audio output to make the reponse to deep targets more obvious? Quite a few people here(oz) are now runing 4000's with DD coils which improve s/n as they are more immune to em noise than mono's and have must less ground feedback because their dual windings tend to cancel out ground signals(and EM noise). They are running in normal soil timings and vaying the gain ( and other menu settings as well) according to the ground and em noise levels. I think DD coils are very underrated and under some conditions will outperform mono's simply becasue of their better s/n. The depth advantage of mono coils over DD's tends to dissappear when you have to run them at much lower gains than DD coils to achieve an acceptable S/N. Some mono addicts here are running in sensitive/ extra but often with very low gain ( as low as 2-3). Some I think will quickly wear out the function select and setting knobs very quickly I suspect!

Doug

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Doug,

With such different conditions in Oz from the areas I hunt it's a lot like trying to compare apples to oranges. I regularly use sensitive/extra with a gain of 10 to12 with what you would probably consider a moderate size mono. I once was able to turn the gain to 15 with a 8" and maintain a rock steady threshold. Thanks for the info. It has given me plenty to think about.

Bill

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Hi all,

You guys are causing me to think again, and sometimes that be dangerous.. :(

I was thinking about the port on my GP3000 that is used by the Minelab repair folks to check the detectors correct function.

My question is, is this an informational access only, for the Minelab techs to use, with their fancy instruments, or can they make adjustments to the machine via this port without opening up the control box?

If adjustments can be made using this port, why couldn't some sharp guy come up with a means to attach a small aftermarket digital screen and allow the owner/user to perform some fine tuning such as gain and sensitivity for the 3000, or the 3500?

Hell, I'm no electronics whiz, but it seems to me that something like this might be possible!

I might even pay a dollar three eighty for the setup, too!

Jest thinkin' outside the sand box. ;)

It's getting to be a LONG winter.. B)

~LARGO~

Patent application applied for...

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Hi all,

You guys are causing me to think again, and sometimes that be dangerous.. :(

I was thinking about the port on my GP3000 that is used by the Minelab repair folks to check the detectors correct function.

My question is, is this an informational access only, for the Minelab techs to use, with their fancy instruments, or can they make adjustments to the machine via this port without opening up the control box?

If adjustments can be made using this port, why couldn't some sharp guy come up with a means to attach a small digital screen and perform some fine tuning such as gain and sensitivity for the 3000, or the 3500?

Hell, I'm no electronics whiz, but it seems to me that something like this might be possible!

I might even pay a dollar three eighty for the setup, too!

Jest thinkin' outside the sand box. ;)

It's getting to be a LONG winter.. B)

~LARGO~

Patent application applied for...

Now this is a question that I can't answer! Only ML I suspect know the answer. Why not contact them or ask JP?

Doug

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