lobo improved

[porkluvr]

I read Tesoro com. No need this believes.
Frequency Tesoro Gold(en) Sabre there 12 Khz
But indeed 15kHz.

What does that have to do with LOBO?

It's nonsense.

I provided a link to the LOBO manual which plainly states 20kHz, not 12kHz, not 15kHz.

Operating FrequencyApprox. 20 kHz
If either 15kHz or 17500 is approximately 20kHz, then I am approximately the Duke of Windsor.

[maxbiri]

Hi All

On my pcb with 2.5% tollerance capacitor around 555 i have 20 khz with pot set near half. May be problem is a capacitor with large tollerance +-20%.

By Max

[porkluvr]

Max, Foma,
Thanks Max It's hard to keep up with this project because everybody seems to have different ideas about what works. That's OK, but without proper explanation it becomes a hodge-podge of confusion, especially when there are as many unknown quantities as this started with.

Max, A while back you offered to post some scope shots. If that offer still holds, we'd all appreciate it - or, what about a schematic, updated with your parts choices?

My detector will get finished before summertime, or I'm not the Duke of Windsor (approximately).

[maxbiri]

I porkluvr

I stopped the lobo project because poor deep end no time for hobby;
i can resume project in the next days, let me know where i must connect probe referred at my last schematic and i will take pictures from oscope.
In my opinion tx capacitor must be selected for resonance around 20 khz, the problem is rx capacitor, it affect the phase of signal and these is more important respect amplitude.
If i remember right best conditions, on my pcb, is with a 1.3-1.5 nf rx capacitor.
I suspect that all my problems of low deep are a conseguenze of non perfect rx phase.
Circuit is very stable even at full power with no false signal even if i have pcb on desk without schield.
Sorry for my bad english.

By Max

[FOMA]

Hy ALL.
Has Selected the resistor R10.

[tchoci]

Hello to vsichki.sazhalyavam, chenyamah time to participate in outings with diskusiyata.Byah of Lobo and was very pleased with it nego.Ostavih frequency = 17,2 KHz, because I want to catch different types of fine bronze coins in Bulgaria they have a lot also catch large coins dalbochina.Ima good to gold, so I think some frequency = 17-18KHz priemliva.Po is a higher incidence in low sensitivity to the subjects in nadalboko.Osnoven problem generator transistors is "tried a lot, but at my sound into a 2T914A.
български




> английски




размяна
Превод

[tchoci]

To put NE555 timer or SE555 but in any event LM555-I know from experience.


Tchoci!

[porkluvr]

If i remember right best conditions, on my pcb, is with a 1.3-1.5 nf rx .
By Max

If i remember right best conditions, on my pcb, is with a 1.3-1.5 nf rx capacitor.
I suspect that all my problems of low deep are a conseguenze of non perfect rx phase.

Bad news you have stopped project. You have 10uH receive coil, correct?
Using +/- 5V could explain some lack of depth, but also rx phase... very important.

The component values in receiver amplifier are extremely sensitive and work hand-in-hand with rx coil components to set demodulator phase.

It would be good to see dual trace scope of transmit signal and receiver amplifier output signal at the same time. Maybe somebody who has more knowledge than I can see from scope presentation if you should make changes. Perhaps you should repost your schematic and show component values you have used. Ever since the first schematic was shown here with many "?" there have been many variations proposed. It's hard to know what is what.

I quit LOBO to work on PI but I've come back and want to finish. PI will wait.
(Your English is OK.)

[porkluvr]

Hy ALL.
Has Selected the resistor R10.

Thank you, FOMA.

That 470k R10 should give a little bit wider range on the discriminate control, but not too much.

[porkluvr]

PDF operator's manual

The Tesoro website currently has only an HTML version of the Lobo operator's manual. I don't remember when I got this, but here is a PDF.

The file size was over the 2MB maximum upload limit until I found the way to reduce file size in Acrobat. Now it squeaks by. Compatible with Acrobat 6 and up.

[simonbaker]

The Tesoro website currently has only an HTML version of the Lobo operator's manual. I don't remember when I got this, but here is a PDF.

The file size was over the 2MB maximum upload limit until I found the way to reduce file size in Acrobat. Now it squeaks by. Compatible with Acrobat 6 and up.

Thanks for link. That's a great machine.

-sb

[tchoci]

Hi all kolegi.Dnes LOBO again went with him to do tests with a colleague with metaldetektor Sovereign.Selishteto was quite dirty from (the Ottoman yoke). If you believe metaldetektora me very dobre.Izvyadih relics as a colleague Sovereign.Razlikata only that a signal of some rusty forged zheleza.Za me this is not a problem because so come and peaks of streli.Prosto am delighted.




Tchoci!

[mikebg]

Let we improve the preamp circuit

Different designed preamp.
Lobo is designed somewhere at end of 80-ties. I think the desiner is not Jack Gifford because the preamp circuit of Lobo differs significant from other primitive preamp circuits used by Tesoro. The differences are:
1. The preamp is formed as three stage amplifier. Other Tesoro preamps use only a single stage.
2. The resistors of first stage are designed for low noise. The resistors in other Tesoro preamp circuits have much larger resistances, ie they generate more noise.
3. RX coil is connected to form second order band pass filter. In other Tesoro circuits, RX coil forms second order low pass filter. That means they can not suppress enough the audio frequency (330 - 550Hz) and
third harmonic of mains frequency (150 or 180Hz).
However:
1. Despite its three stages, the preamp operates with low gain. I see something is written for gain in the circuit diagram attached below.
2. Resistors and RX coil have low resistances, but this is useless because opamp generates spectral noise density more than 1kohm resistor. That means the designer can decrease the weight and price of RX coil without increasing input noise..
3. The tuned circuit (band pass filter) is damped with 6kohm resistor. That means increased noise because a) the damping resistor also generates noise and b) damping forms wide band input (noises are proportional to bandwidth)
Let we make analysis, revision and redesign of this circuit.

[Davor]

If the coil allows it, I'd go for a true differential input, and leave filtering to the subsequent opamps. True differential setup has a moderate input impedance that is symmetrically distributed between each input and a ground. The only problem with this design is that resistors values are not easy to pick. However, I calculated values for 6k8 total input impedance that has amplification 48.5 and uses only E6 values. These are 6k8, 330k, 68ohm, 3k3. I use this setup in my IGSL.

[mikebg]

REVISION AND REDESIGN OF PREAMP

Calculation of cutoff frequencies.

Every RC timeconstant forms a cutoff frequency fc at which the phase is shifted with 45 deg and magnitude is changed with -3dB. The formula for calculation of cutoff frequency is fc=0.16/(RC). Below are shown the results from calculation.
C1 and R1 form a high pass filter with Fc=1.17kHz. That means it will suppress significant a) the audio frequency and
b) the third harmonic of mains frequency. Both frequencies are additional suppressed because R1 and C1 form with L1 a second order bandpass filter.
Timeconstant R2*C2 is not correct calculated because this is high pass filter and TX frequency appears below its cutoff frequency.
Timeconstant R5*C5 is also incorrect because this is low pass filter and TX frequency appears above its cutoff frequency. That means the gain of second stage is below G=R5/R4=48.7/8.45=5.8.
The total gain of the whole preamp is less than 21*5.8=122 or <42dB.
The SPICE analisis will show how the circuit operates.