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Offline towro
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« Reply #10 on: June 25, 2009, 06:55:56 am »
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Thanks for the input. I had no idea what this knob on my machine did either

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« Reply #11 on: March 22, 2016, 02:52:57 am »
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All excellent, clear, and easily understood...

Thanks for all us 'Newbs' or is it 'Noobs'  Cheesy

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« Reply #12 on: March 22, 2016, 12:04:14 pm »
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Actually The Sensitivity control only Adjust the Listening side of the machine which is why when you turn up the Gain / (Sensitivity) too much it blinds or over loads the machine and some machines will either tell you on the screen or produce a sound that alerts the user that it is over loading,

The fact is 99% of machines have a fixed Gain / Sensitivity (POWER OUTPUT) and when you Adjust the Control called SENSITIVITY or GAIN all you are adjusting is the Volume of what the machines hears from the ground or from what is in the Ground and by adjusting the received signal correctly (sensitivity) (gain) you allow the signals to stand out from all the other noise that the machine picks up..

hope this helps

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« Last Edit: March 22, 2016, 12:32:20 pm by Ridge Runner »
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« Reply #13 on: March 22, 2016, 10:54:24 pm »
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Ridge Runner,  if what you say is true, why does my White's GMT not only have a gain adjustment but also an audio boost "for those faint signals"?  It will boost the received signal by 4X (from manual).  The battery pack has a regulated output to the system to keep the detector operating at optimum performance, does this mean the coil output is always the same?  It would seem plausible the gain would add current to the coil increasing the strength of the flux lines and depth, and the sensitivity would decode the reflected signal coming back. Do I have the their functions backwards? 

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Online GoldDigger1950
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« Reply #14 on: March 23, 2016, 12:10:00 am »
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This can be a confusing issue. Since the sound in almost every metal detector is based partly on the signal return from the coil then turning the knob does affect both the power and the sound. If we didn't rely on the coil for our headphone sound we wouldn't ever be able to hear the difference between a piece of delicate chain and a quarter or 20 pence. The transmit coil in a dual coil machine is also very close to the receiver coil meaning to the clear thinker that the receiver coil will interfere with the transmit coil. They are normally even overlapping. So how to compensate?

We have a sensitivity knob and circuit for that. If we lower the receiver sensitivity, we lower the preamplification of the return signal from the buried object (there are several levels of amplification in a detector). When we transmit a pulse or signal into the earth, the metal becomes part of the transmitter magnetic field for a moment while our detector receiver is blanked out. It's turned off duing the output pulse or signal. If it wasn't, it would blast our head off from the volume of sound. The transmit signal then momentarily magnetizes the metal object(s) in the ground. In the case of ferrous or iron objects, the magnetization runs through the target internally aligned with the longest or most massive shape. In the case of non ferrous objects, the magnetization still happens but it only happens on the surface in what we call eddy currents. On the surface of these objects there are tiny, circular patterns of magnetism. This happens even on aluminum foil. Remember, this only happens for a tiny bit of a time slice. Microseconds. Then the transmit coil is turned off but the magnetism in the target stays for a tiny bit longer.

This is where the receiver is turned on and we sense the tiny currents in the target(s). The magnetization from the transit pulse is even strong enough to mildly magnetize the soil itself but that goes away almost instantly. Almost. If our sensitivity or our amplification is too high, we get a return from that so we crank it down a little to keep that from happening. This may sound to some like ground balancing but it is not at all the same. Ground balancing is a compensation technique while this is a signal enhancing technique.

My explanation here is frozen in time at this point and I want to continue with it that way in your mind. So, our metal object has a bit of magnetization either inside of it or on the surface. At this moment the receiver has been turned on and our coil, being a simple electro-mechanical device has no choice but to see the signals from our targets. The preamplification is an extremely high gain circuit. The signal from our receiver coil can be amplified from 500 times to 600,000 times depending on your circuitry. You can probably begin to see why we need to lower the sensitivity or the gain of the amplifier to allow for better, more refined detection.

OK, let's move forward in time. Our metal is momentarily magnetized and we are sweeping over the target while the receiver comes on again ready to see what the transmitter "lit up" for us to find. In ferrous metals, the magnetic field lasts longer so they always seem larger no matter what size they are. The gold, silver, bronze, brass, copper and other non ferrous targets have those little circular eddy currents on the surface for the briefest time you can imagine. The residual magnetism in ferrous metals can last a long time relatively, theoretically for centuries. That is how tape recorders work. But the signal in the earthbound metal does deteriorate eventually and is more like seconds in duration rather than centuries. Sweeping back and forth can build it up and that is why it seems larger when we find one. Frustrating sometimes when you have a pile of dirt and only find a paper clip.

Back to our target now. The receiver coil is excited by the magnetic fields in the ground objects and in simple detectors all targets look alike. When the signal starts to deteriorate, it changes in intensity and the receiver coil notices that dutifully sending the diminishing signal to the amplifier. This signal is sent to at least two places. One is a voltage to frequency circuit that creates our sound that we hear. It is also sent to signal enhancing circuitry. That, along with ground balance, is another post. I could write a book on those two topics so we stop here.

This has been a long post and for those who made it to this point, I hoist a cup of coffee in toast to you for that! It's 2:00am here in Connecticut as I type this. To sum this up, the orginal question has been answered and the volume control issue is much simpler. In most modern detectors the audio circuit is set for user comfort. Automatic gain control is often used to set the output sound to an average level. This is why many of us use amplified headphones. In others, the gain or loudness is set by a volume control and needs no amplified headphones. So that feature is a choice of the designer of your detector.

To sum it up, your sensitivity affects your receiver gain and in turn this effects sound which is where the confusion arises.

For those who are wondering about those poor eddy currents, before you hear the sound, they are mostly gone. In ferrous materials they can and do last longer but eventually they go on vacation too. For the engineer and designers out there, I apologize for the oversimplification but you know why we have to do that.

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« Last Edit: March 23, 2016, 12:16:25 am by GoldDigger1950 »
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« Reply #15 on: March 23, 2016, 05:10:26 am »
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Quote:Posted by Don in SoCal
Ridge Runner,  if what you say is true, why does my White's GMT not only have a gain adjustment but also an audio boost "for those faint signals"?  It will boost the received signal by 4X (from manual).  The battery pack has a regulated output to the system to keep the detector operating at optimum performance, does this mean the coil output is always the same?  It would seem plausible the gain would add current to the coil increasing the strength of the flux lines and depth, and the sensitivity would decode the reflected signal coming back. Do I have the their functions backwards?  


With the GMT on the Gain control it gives you up to 16 X Boost on the Original signal and when you flick that boost switch IF you were running you GMT at number 10 = (16X ) when you flick that switch that is 4X gives you a Total of 64 X the original signal strength,

To prove this to your self, If you turn on the machine and turn it up slowly you can hear it make a series of clicks and each click is 1x, 2X, 4X, 8X and 16x, So then multiply the setting you are using, say at the preset mark = 8X and flick the Boost switch so now you are operating at 32X the original signal strength,  

Now to Prove that the Gain control is a listening device when you turn it up to high the EMI gets worse so you have to back it off until the machine is stable, BUT if it was an output Gain control EMI would not effect it because it would be sending more power not listening to EMI because EMI is made up of signals from outside Source's.  Ok

hope this helps. RR

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« Last Edit: March 23, 2016, 06:00:20 am by Ridge Runner »
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« Reply #16 on: March 23, 2016, 10:12:32 am »
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GD and RR,  Thank you both for excellent posts and great explanations. I have pondered these questions for some time but have never been able to exactly formulate how to ask them. Your answers make clear the engineering aspects and thus the operational functions. Greatly appreciated.

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« Reply #17 on: March 23, 2016, 10:43:17 am »
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  Maybe I missed something but cranking up your Sensitivity can cause Target Loss because the Target Value is less then the Ground Value.  Hunting Gold in serious mineralised grounds  I found turning it down more then a few notices gave me better depth even on coins.  

This is why I never us Discrimination.     It Blinds you.   If you want it all you dig it all.   

Posted on: March 23, 2016, 10:37:41 am
Quote:Posted by Christian
The sensitivity will determine the depth of your detector. The more you turn it up, the deeper it will penetrate and pick up targets. However turning it to the max will cause your machine to get noisy and unstable giving lots of false signals. Set it to a point where you machine will not produce false signals. Like this will be at 7 - 8.

Regards,

Christian



Not So.   It can blind your detector.   Like looking in the Sun.

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« Reply #18 on: March 23, 2016, 11:34:46 am »
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Quote:Posted by homefire
 Maybe I missed something but cranking up your Sensitivity can cause Target Loss because the Target Value is less then the Ground Value.  Hunting Gold in serious mineralised grounds  I found turning it down more then a few notices gave me better depth even on coins.  


That's the Blinding effect I was talking about, Reducing the received signal cuts down the Ground noise, And on machines like the GMT @48khz, Goldbug 2 @ 71khz and the Eureka Gold @60 khz and now the New Makro Gold Racer @56 kHz  suffer because of the high frequency and in such places you would be better off with either the MXT, MXT Pro and now the MXT All Pro and the Makro Racer 2, by using a lower frequency of around 14khz they do not suffer from hot ground half as much as the mid KHz machines Oh and the Nokta fors core not the fors Gold because the fors Core has the Beach setting which is a real bonus if you are going detecting out in the salt pans where as the fors Gold does not have the Beach mode and it will suffer out there,, Watch out for that 56khz Gold Racer because I now believe it to be the machine to have/Beat

These new Makro machines are a full on prospecting machine and they have fully Blown Discrimination which no other gold machine has and they are lightning fast too, The perfect setup would be to Own the Makro Gold Racer @ 56khz and the Nokta Fors Core then you would be fixed up as good as it Gets and there are over 16 coils on the market made for them and the have 3 forms of Ground balancing from AUTO, MANUAL and Ground Grab just like the GMT has but the GMT only has about 6 coils that will fit it, So Makro and Nokta is where people should be thinking and the Company will Listen to you if you have constructive input and they are very fast to respond to customers Ideas, So the Top 4 Companies better Look out because they will soon be knocked off the top spot.

Oh and Homie the Nokta and Makro give you full Disc readout in the All Metal Mode so you won't loose depth but they will tell what it is, while still maintaining full power.

RR.

Posted on: March 23, 2016, 10:58:12 am
Quote:Posted by homefire
 Maybe I missed something but cranking up your Sensitivity can cause Target Loss because the Target Value is less then the Ground Value.  Hunting Gold in serious mineralised grounds  I found turning it down more then a few notices gave me better depth even on coins.  

This is why I never us Discrimination.     It Blinds you.   If you want it all you dig it all.  

Posted on: March 23, 2016, 10:37:41 am

Not So.   It can blind your detector.   Like looking in the Sun.


Yes I did say it blinds the machine two posts back and with  the MXT in All Metal Mode (Prospecting) it still gives you the VDI Readout but you have not control to disc out things but it tells you what they are as does the Makro and the Nokta's, If I switch in to the Relic mode it will disc out stuff but you must never set it above 2 on the dial and  and the depth loss is minimal if any because the MXT has a Third tone that tells you something is down there and it can Add up to 40 to 50% more depth using that method and it is about as good as the all metal mode, In an Air test although they are of little Value it can hit a Quarter between 15 to 18 inches depending on how good your hearing is.

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