VLF MD with digital signal processing : Bee-Buzz 1

[pito]

Hello friends,

it is now time for the new 3 V pre-amp.
If I take an RX coil with 1 Ohm series resistor
Cheers

and the coil resistance Rx is?

[Aziz]

and the coil resistance Rx is?

I am using currently 2 Ohm RX coil. But this isn't much important. You can use up to 10 Ohms. The pre-amp gain changes a bit. Best noise performance is achieved with low RX coil resistances (1 Ohm).

[pito]

i intend to check circuit from post #480 to see it performances

[Aziz]

i intend to check circuit from post #480 to see it performances

You can leave it. It's waste of time. The circuit will produce too much noise. It isn't useful for our project.

[Aziz]

Hi all,

before the next prototype is finished, it gets obsolete. I have found a bug during a circuit simulation.

We must decouple the power supply for the op-amp from the floating voltage rail over the battery. Otherwise we will have high distortion ouput if the input signal goes beyond a critical level.
I have changed my battery to 9 V block. The next version should work as expected (see below). I have to change my new bread board now...


Cheers,
Aziz

[Olly]

Isn't that starting to get a bit messy. Why not stick with the original moving coil amplifier arrangement with a totally floating battery. Then if further op-amp buffering/gain is required, add a non-inverting gain stage but with it's own power supply which is not referenced to the 1st stage battery..

[Aziz]

Isn't that starting to get a bit messy. Why not stick with the original moving coil amplifier arrangement with a totally floating battery. Then if further op-amp buffering/gain is required, add a non-inverting gain stage but with it's own power supply which is not referenced to the 1st stage battery..

Yep, it isn't getting a KISS project anymore. I was quite surprised, how much the voltage rails can really vary. So much, that the op-amp can't regulate anymore and thus distorting the output.

That's funny. I can leave in the original 3 V design the gain setting resistor R5 and let the pre-amp gain define by the input impedance of the sound card.
I will do some measurements with the 3 V battery design before I finish the improved op-amp version.

[moodz]

...read the spec sheet for the NE5534 too ....you are not supposed to operate the bipolar supplies below +/- 5 volts.

[moodz]

It's not a rail to rail amp.

[Aziz]

It's not a rail to rail amp.

Yes, it's not a good idea.

I have plugged the 3V version and observed absolutely no benefit. It is even worse.
Using the other pre-amp gives absolutely no benefit too.

Back to the KISS principle. No pre-amp. No battery (power supply). Only passive circuit elements (both TX and RX side).
Cheers,
Aziz

[Aziz]

Hi all,

my USB Sound BlasterX G6 gets quite hot after 5-10 min operation at maximum TX-power. This is a good indication, that the power headphone amplifier is not efficient.
Anyway, I should wrap my choke L1 next time to get the dual frequency TX VLF/LF operation working.

Cheers,
Aziz

[Aziz]

Hi all,

I know the reason for the bad performance with the old pre-amp. It's the fkn sound card, which had been operated at maximum power output (head phone output). No good.
It can get into oscillations, bad behaviour and very much noisy. And it will get hot and suck the battery of the Tablet PC rapidly empty.

Again, low power to the TX coil (even at line level output) and a good pre-amp with gain of max. 50 (this is absolutely max) is a good choice to start with. The sensitivity of the response is very good. And we are saving battery power. Everything is good.
So I'll leave the Richard's pre-amp and develop the previous pre-amp using ZTX-transistor pair with NE5534.

I have looked at three decoded bins around the resonant frequency using a narrow band chirping TX-signal. It is working but it isn't convincing much so far. I will look at the dual frequency response, which should give us the total benefit.
Cheers,
Aziz

[Detectorist#1]

Hi Aziz,
To leave exotic ideas is good idea. Yes, you was obligate to try this solution! Using of NE5534 according me is possible, but this is not pointed to the future - now we have possibilities to use more good OpAmps with lower current consumption, lower noise and wider frequency band. All we, old (good) engineers have to switch for more new solutions.

[JoyJo]

Common-base is generally the better approach to low noise. Here is a CB preamp I was messing with several years ago. It does not require a floating voltage source. I don't recall where I found this but probably an audio application. The values shown are what was used in my last sim.

Is it possible to use this scheme with unipolar power supply 3,3v
as an input stage of a metal detector located in the coil?

[Aziz]

Hi Aziz,
To leave exotic ideas is good idea. Yes, you was obligate to try this solution! Using of NE5534 according me is possible, but this is not pointed to the future - now we have possibilities to use more good OpAmps with lower current consumption, lower noise and wider frequency band. All we, old (good) engineers have to switch for more new solutions.

NE5534A or LT1028A/AD797A? This is the art of ultra low noise design.
I say, you don't need the expensive ones. Take a cheap NE5534A and you can achieve much better results.
If the cheap op-amp NE5534 gets into trouble due to drive capability for instance (max 500-600 Ohm loads), then make a workaround (see below picture for the power driver).

For instance:
I can achieve at >10 kHz operating frequency with a RG1=1 Ohm, RG2=20 Ohm approx. (! super low drive resistance), RS=1 Ohm at Gain=20 (!) an input voltage noise density of 0.26 nV/rtHz. The 200 Hz bandwidth noise @10kHz is less than 0.1µV rms. This with a cheap NE5534 and a cheap medium power transistor BD179 to drive the feed-back-loop. But the draw back is, that the pre-amp draws almost 400 mW at 9 V.

Look at the below picture:


This is really impressive.