IDX-PRO+VDI

[dfbowers]

What are you using for U3? an LM324 or LF347?
If using an LM324, I have found that the offset can be a problem, try an LF347 instead.

[gustavodigitales]

I`m using LF347
Recently I switched u 3 to LM324 but it was less sensitive than LF347. also I tested 5 LF347 ics.
Turning gain pot make a big diference whit these mod, at minimun only detect bigger al things close the coil, and at maximun becomes erratic.
The problem may be in the coil ?
In air test condition 1 e coin at 25 cm but over the soil many beeps and it can`t detect the coin .

[dfbowers]

Sounds a coil problem but hard to tell. Is it a homebuilt coil? Phasing may be off.

[Davor]

preamp output is 660 mv at minimun posible and the coil output is 17 mv at the best null point, i think it will be less.

You most definitely have a coil balance problem.

[eduardo1979]

Hi to all
I`m very stucked whit my project.
The sensivity isn`t god than other project of idx i`ve read here.
1 e coin at 25 cm, small gold ring at 30 cm maximum and a uhu all plast at 50 cm.
values for air test.
I`m using a home made 27 cm dd coil
TX 34 t 0.70 mm 1 mf cap at 6.56 khz
rx 255 t 0.20 mm 0.027 cap at 4950 hz
shield, very thin al foil from scrap capacitor and nulled for test whitout pot yet.
GEB works very well cos ferrite rod is complety invisible. also disc works as well too.
preamp output is 660 mv at minimun posible and the coil output is 17 mv at the best null point, i think it will be less.
other mod was remove the 1m resistor r 24 to improve the gain at maximun.
i worked a couple of days and that is the best results.
what`s wrong????
thanks Gustavo

Your problem is the coil. The cancellation of the coil must be below 4mV. Mine is THE FOLLOWING: TX = 34 turns with wire of 0.60, more 1uF capacitor in parallel. RX = 260 turns of wire of 0.20, more 33nF capacitor in parallel. These measures are for a plate of 27DD. Measurement with AC voltmeter in parallel with the RX coil is 2.55 mV. The cable you are using, too much influence. I use a 4-wire cable plus a shield.

[eduardo1979]

TX Coil = 0,580mH = 1,7 Ohms.
RX Coil = 34mH = 92,1 Ohms.

[gustavodigitales]

Hi I measured my coils and found a 80 deg out phase so i decided to make another coil but i don`t known some basic.
i`m gonna try with co planar like dfbowers one.
again sorry for the basic cuestion.....
tx tank is + tx coil + bucking coil +
+ tunning cap +
or

tx tank is + tx coil + bucking coil +
+ tunning cap+

and.... which side is conncted to the hot tx ocilator, tx end or bucking end ?

[gustavodigitales]

tx tank is , tx coil plus bucking coil and tuning cap or tx coil tuning cap plus bucking coil?

[reiv]

"2" Second version

dfbowers: Nice coil, what is the voltage Vpp and the phase shift is near 90 or 180 degree? And connections loooks like this?

[dfbowers]

Number 2.

[dfbowers]

The wiring connections look correct. Now, some say that the phasing should be 180 degrees, but mine is 90. Also, my BM600 coil built by White's is 90 degrees.
Vpp is 15V. Make sure that your Tx wire size is correct or Tx amplitude will not be correct.

I used 34 AWG magnet wire for the Rx and resistance is a bit too high, but seems to be forgiving with this as all works just as good as White's stock coils.
Anyone know what the down side is of a coil with a lower Q factor?

[reiv]

Ok thanks for answer

[Davor]

Anyone know what the down side is of a coil with a lower Q factor?

In Rx it affects noise. However, IDX is not a champion in low noise input, so you'll hardly notice anything.
Example...
Your coil is, say, 100ohm (1,2nV/sqrt(Hz)). It is configured as a parallel tank, and by virtue of near resonance the coil voltage is doubled. With doubled signal the thermal noise is doubled as well, thus providing a better noise matching of the pre-amplifier. However, this noise (~2.4nV/sqrt(Hz)) is still lower than the input noise of mid-range op amps (~4nV/sqrt(Hz) and more) ... hence you can't notice any difference.

[dfbowers]

Coil update:
I finally got a chance to get out with this coil today. Without a doubt, best coil I ever made! Air tests were not that impressive, but it ends there. I have 3 lead balls (.570" diameter) buried in my test garden @ 7", 8" and 9". Doesn't seem like much, but in MY soil, ALL my detectors struggle to find any of them. My Shadow X5 is ablsolutely quiet. This coil with Silverdogs IDX kit hits all of them loud and clear in DISC mode and is more stable and deeper than my 9" and 10" homebuilt DD coils.. Ha! there is really something to be said about smaller coils. Again, this is only a 7.5" coil inside an 8" shell. (My TGSL MIGHT hit the first one on a good day with a 10" DD coil). Now, with that said, this is all old technology.. but exciting for a home hobby builder.

Also, I found an absolutely great source of graphite spray, which I used to finish my paper shields. Under $5.00 at you local Home depot in the U.S. Now, as I have read in other threads in the forum, some warn that covering large areas with graphite spray can spoil the null of your coil. I found this to be true, but insignificant.. maybe a 25% increase in the null voltage.

Don

[ApBerg]

Hello Don,

Thanks for the info !!

Best regards.

Ap