moded gp extreme

[ronjeanosborne]

Hi All, Peter, GP Extreme Detector arrived on Friday,Running Excellent the volume is a lot louder, My mate steven Has had 2 half days detecting on a old race course that closed 80 odd years ago,  with 11 inch dd Minelab with discriminate,  Found 18 pre dismal  coins and a heap of lead, a cuff link, P/S peter check in post on Monday,  I have no photoes  yet,  Ron.

[woody]

Good to hear that the detector is running good... it has been getting very busy here since me moved to Ballarat, did some coil tests and will put up some video later.

[Joeboy]

Gday woody, I've got an opportunity to pick up a gp3500 pretty cheap,
I was wondering if the new developments with mods would work well on the 3500. I was thinking of selling my 45 for basically the same price i bought it for and getting the 3500 modded which would free up some cash to get it done sooner. I'm not a fan of fine gold or enhance. Will the 35 accommodate the latest mods ok.
Cheers joeboy.

[woody]

Hi Joeboy,

We can add the current upgrades into the GP series, if you are only interested in getting deep gold the GP is a good detector.

[Joeboy]

Hi Peter, yeah i would like it to hit a bit deeper. There's a few of area's I'd like to go back too that produced some nice lumps, Where the ground was deep and extremely noisy even running the 45 in enhance. Getting my 35 to handle ground better with extra depth would be main objective i think. My main coils for use will be dd's. There are so many different mods developed by you over the years, i dont really know what mods to ask for that will best suit my needs on the gp. The variable gain sub board is a must, i assume some old components in the gp would have to be brought up to speed aswell. So I'm thinking not going the full upgrade, somewhere around the midway point. If you could Pm me, an idea which upgrade would suit my needs and price that'll be much appreciated.
Cheers Joeboy.

[Joeboy]

Hi Peter,

This will probably sound like a silly question, is the signal control on the 3500 is it a gain adjustment. Does your variable front gain mod provide more gain over standard and how does it effect the signal control if at all.
Cheers Joeboy.

[woody]

Hi Joeboy,  Well i could go into explaining about low frequency shot noise or 1/f noise or taking advantage of a low resistance signal impedance input stage to reduce overall noise and improve the signal to noise ratio but it could be convoluted and tedious. Firstly the the coil resistance is 0.5 ohm, in a normal minelab the N channel input Fet is around 2 ohm, the secondary N or P Fet (depending on model) is around 3 ohms.  2+3 = 5 ohm, to have complete power transfer the 0.5 ohm of the coil should  see 0.5 ohm or less series resistance. Firstly I replace the input Fets with a 0.750 Ohm device and a 0.2 ohm device.  The gate capacitance is too high to effectively with the Fet Gate capacitance and does not sustain a steady gate current and thus the fluctuations in gate drive modulate the series resistors of the Fets causing noise. This can be seen in the non steady state of the integrators with shorted inputs.
So we overcome some of the inherent gate modulation and resistive channel noise by using V type Fets that actually have lower intrinsic noise. The input Operational amplifiers are switched out (due to them having fixed 12k ohm resistors internal on their input n channel transistors and going to transistor input with very low ohm base spreading resistance with a moderate Hfe to get lower noise. Then the output from dual differential input stages is then after moderate fixed gain amplification is buffered by two low noise fast opamps that are then fed to a dual channel matched tracking variable gain amplifier.  All components critical to operation are 0.1% tolerance and metal film to lower circuit noise at all critical amplification or signal shaping stages. So utilising a low noise front end you can raise the gain up or down while retaining a very low noise figure with low noise gain control. There are some other secrets in the design but everything is done to get as much low level signal and get it out of the noise to get signals that the detector would lose in its own noise and resistive losses. There is a trade off using a given amount of fixed gain as the greater the gain in the first stage dominates all noise in the system down the amplifier chain. With all detectors, noise is the thing stopping prospectors getting the gold.

[Joeboy]

Hi Peter,
Thanks for your detailed explanation.
Joeboy.