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Major discovery applicable to all SD GP and GPX series detectors.

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woody:
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.

spot:

--- Quote from: debongos on February 18, 2017, 11:23:57 pm ---And to add Wes's question. What cost are we looking at for the already modded 4500 and 5000's?

Peter

--- End quote ---

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 :)

woody:
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.

filternozzle17:
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.

woody:
We can increase the gain of the detector that gives more depth and sensitivity at the same time...

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