Trying to get more depth

[msysta]

I have C.Scope 990XD non-motion machine which is otherwise wonderful but lacks in depth. Four inches is the quite maximum it is capable when talking of interesting sized objects like coins.

I guess there's a way to tweak the circuitry for more sensitivity but how about a really nasty and cheap way first; simply adding an amplifier into coil lead? There's two ways, either pushing more to TX or getting more out from the RX.

What do you think? Is there something principal issue that kills this idea or should it work in theory at least? Actually I made one test already, I had old phono preamp connected but it did not do anything when put to TX and started to oscillate when put to RX.

[Götz von Berlichingen]

A phono preamp is really bad for this.
As far as i know, the Frequency response is not linear:
http://en.wikipedia.org/wiki/RIAA_equalization
Also, its not designed to have any output power that could drive a Coil.

[msysta]

Yes I think so too. I used it because it was the only one available. Now I shall build and try with RX preamp that has straight frequency response. Let's see. I was checking the schematics for VLF machines that I could find and the critical thing is only that the phase stays. So this actually could work.

[headshed22]

Four inches max for coin size objects ?

The CS990XD is a very capable metal detector, you should be picking up these objects easily at 8 to 10 inches.

I suspect a faulty unit.

Russ

[msysta]

Hi,

Well, I guess I have to be more precise with the size. There are COINS and coins I made a mistake when saying max.

For UK Penny the air test may very well give the figures you are talking about, but let's try with Roman A3 (Nummus) size. Air test gives me 5 inches but I reduce one inch because at the field that's about the height that coil runs on top of the surface. My friend air tested his GoldMaxxPower for A3 and got 17cm or about 6,5 inches. I'd be quite happy with that kind of improvement with my preamp. I think I am able to test the amp today. Let's see.

[Skippy]

Would it not make more sense to try increasing the gain of the internal amplifiers, by 50% to 150% initially? This is the kind of gain you may be able to successfuly realise, and it will provide some of the depth increase you want.