Detectormods Forum
Members Only Section => Information on the new developments => Topic started by: woody on February 15, 2017, 06:08:09 pm
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Well guy's, I've been busy soldering components as big as ants and testing a lot of different electronic system to get more smaller gold and also larger gold at depth. Many of the standard detectors have great difficulty finding simulated 3 oz nuggets at 22 inches, even with a large coil. I am sure that when we see a lot of big holes and people say they got a 5 gram at 20 inches it is not possible, my hunch is that the nugget was always in the wall higher up and falls in when the hole is widened out. Anyway I digress, I have come up with a way to get an honest 30% depth increase on large gold, you know the coke can sized nuggets that everyone just walks over. I think it will marry up perfectly with the new 24 inch evolution coil that is coming out. Anyway, if you want to hunt for the next hand of faith at greater depth let me know.
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Ok Woody I'm game. I've got a gpx 5000 now, what would the mods consist of and what would the price U.S. be?
Wes.
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And to add Wes's question. What cost are we looking at for the already modded 4500 and 5000's?
Peter
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Hi guy's
Well I have to add to the list of another innovation that allowed even greater gain, lower noise and perfect ground balance.
I was having an uphill battle trying to get fast response time (slew rates) while knocking out the wide band interference the AD797 cops up to 100 Mhz.
I found an exisiting design issue in the GPX series that was working against obtaining lower noise, it ended up being the way the dual gate mosfets switch the input stage to ground, I know why they did it and it was to remove the constant resistor bias and switch the 1k resistors on and off on the input. This just current pumped my input filter to cause a R/C filter to cause a chop chop sound when i was attempting to increase the capacitance to roll off the lower frequency interference component.
To cut a long story short, I can use a 12 mono in the house and detect 0.1 gram at a few inches and an aerosol can at around 4 feet... I have always said that these detectors suffer huge depth loss if noise gets into the system and upsets the integrators. The one thing that really amazed me is the way the detector ground balances to zero offset at full gain using nasty Kalgoorlie hot rocks..3 up and downs and it is balanced. So if it works this good on the work bench i cannot wait to get out in the field and run it through its paces.
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Look forward to seeing the results in the next vid Woody.
Peter
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Some more refinements, I have a GPX-4500 sitting on the work bench at full gain running a mono coil and the detector is just about dead silent except for a very small amount of 50 hz EMI from mains electric cables. small gold performance is exceptional and large deep target response is way beyond anything that i have seen before. I was thinking that removing the high frequency signal component would screw up the ground balance but this is not the case, ground balance is exceptional. this detector upgrade will be the one to beat. As a side note to anyone experimenting with detector input circuit design, any type of diode in the receiver path used as a a clamp or speed up setup on an opamp can cause serious intermodulation from mixing unwanted signals with the dv/dt of the input switching, once it occurs it is near impossible to remove, some soft knee fast recovery diodes can be substituted but its not the be all end all cure, dc servo steering of response time and also keeping dc offset and drift to tight specifications using chopper stabilised servo control has merits, just need to be careful in filtering out any feed through clock noise such that can be observed in the LT choppers in some detectors that i have inspected, a simple 10 ohm resistor and 3300pf capacitor removes most of the noise at some small expense to THD and common mode rejection. When dealing with nanovolt signal levels there is now scope for greater improvement in the signal to noise levels, the limiting factor is still in the input series Fets that can gate modulate the the input signal and resistive and capacitive losses and stray coupling on the input circuit. All these improvements will get more gold, that is 100% for sure.
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And to add Wes's question. What cost are we looking at for the already modded 4500 and 5000's?
Peter
Can we get an idea of dollar cost for these newer upgrades to existing modded detectors Woody? You did my GPX5000 in 2015..what a great machine it's been afterwards with the elite & evo coils :)
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The extra add on's are $450 for extra long pulse and VF if not on the detector, a input preamp upgrade can be carried out at the same time and that takes it to $650.
There are lots of complicated mods but I only do them on specific requests if you need the detector to get specific gold in specific hot ground.
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Would your modifications to my GPX4500 give me any worthwhile advantage. I hunt for hammered silver coins in the UK. Ground is seldom highly mineralised, soil is often very damp and not very often bone dry. Coin weight in silver from 0.6g upwards. The land is either pasture or ploughed.
I was going to purchase a previously modified GPX4500 but it is no longer for sale so I am looking at the possibility of having the latest full upgrade done.
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We can increase the gain of the detector that gives more depth and sensitivity at the same time...
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We can increase the gain of the detector that gives more depth and sensitivity at the same time...
I assume stability will be okay?
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Would your modifications to my GPX4500 give me any worthwhile advantage. I hunt for hammered silver coins in the UK. Ground is seldom highly mineralised, soil is often very damp and not very often bone dry. Coin weight in silver from 0.6g upwards. The land is either pasture or ploughed.
I was going to purchase a previously modified GPX4500 but it is no longer for sale so I am looking at the possibility of having the latest full upgrade done.
With you modification, based on the above, what would be the approximate setting for both front and rear gain? And what soil timing do you believe would be best, I am assuming Normal but I may well be off track.
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It depends on the ground as to how to set the gain, bur as a general rule I crank up the front gain and tame the ground in the back menu gain setting. I use normal timings if the ground allows.
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It depends on the ground as to how to set the gain, bur as a general rule I crank up the front gain and tame the ground in the back menu gain setting. I use normal timings if the ground allows.
I understand it depends on the ground but initially where would you have the rear end gain in order to crank up the front gain?
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On my 4500 i run my back gain on 5 but on quiet ground i crank it to 8
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Would you clarify the difference between front end gain and rear LCD gain. Thanks.
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Sensitivity and low noise are dictated by where the low noise components and gain is set in the signal gain block. Firstly you get maximum depth and sensitivity the most important thing is to remove resistive loss before the first amplifier, this includes the resistive characteristics of the input mosfets. With a coil resistance of 0.5 ohm the resistance of the input fets at around 5 ohms eats up a fair amount of the signal that is available to be amplified. Other losses are the capacitance of the coil, the cable and capacitance of the input components that reduce the initial slew rate of the detector to be able to recover fast decay target signals. To recover all available signals the initial input stage should have low noise and adjustable gain to suit the ground conditions. The set gain of 33 gpx5000 or 47 gp series and gpx4500 is too much or too little depending on ground decay characteristics. The back gain takes place after multiplexing and as far as i can tell with the existing block gain there is signal loss and trying to boost it greatly increases the overall system noise and target signals can be swamped by the noise. The front end gain sets the input signal at a level that produces optimal signal levels in respect to ground noise the get an improved signal to noise ratio and get the most out of the detector. Many variables need to worked out properly and not just increase the gain, sometimes less is more and sometimes it is the opposite. I also found excess capacitance can cause an offset in the ground balance and thus needs to be greatly reduced as to get better ground balance.
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Thanks for that explanation Woody.
I don't know if you are familiar with the old Nautilus DMC-IIb. A 14kHz VLF detector, this has settings on it for balancing the coil to the machine. There are two controls Resistance and Capacitance (marked R and C) - Maybe this is why even to this day it is the deepest seeking VLF I have used.
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Around 10 years ago I wrote an article on geotech about installing variable damping resistance into the SD2000 or any P.I detector, setting the damping to match the coil and to a degree the ground reactance can make a substantial improvement to the early time detection of small signals. It would not be too dificult to incorporate an automatic variable damping system into a P.I to critically match the coil characteristics when using different modes of timing to cover various decay profiles. It would be even better to use dual damping on different pulse lengths to optimize the decay on both the high and low voltage tx pulses to better match the the system to the coils that all differ in decay due to manufacturing variations.