FKK coils, free beach movement :)

[Davor]

Interesting - yes. There are some conflicting requirements to reconcile before committing to some design, especially if the path was never trodden before. Of course, I'll have to test it before, which I'm not quite able on my li'l island as yet, but in a week or two I will.
If you care to try it yourself, and have the necessary supplies - be my guest. Of all people you surely do understand a good preamp in a well thought of project. There are great similarities with your TEM solution pre.
Here it goes, with values as for IGSL with Musketeer coils, operating frequency is ~8kHz and the Rx coil is 15mH. The upper circuit is a standard IGSL pre designed as a differential amp and in semi-resonant operation. The lower one is a current state of my design, CFIA with garden variety transistors. Please note that you may use much worse opamps than the 5532s shown here with marginally worse results. When normalised for gain, I get ~1.4nV/sqrt(Hz) input equivalent. It could get lower with a coil that has lower resistance. I pumped up the gain to compare gains at the similar noise levels. CFIA supplies ~11.5dB more gain, and a bit less noise. In LTspice, of course. Must try the real thing.

Try playing with this model. V1 is a differential exciter, V2 is a common mode exciter. The IGSL equivalent is provided with values ready for worst case analysis, just remove the asterisk in front of the .step param run...

Just ask

[Tinkerer]

Now a bit about the balanced differential operation. In professional sound equipment it is used to discourage hum from the mains and various crackling sounds entering the system by virtue of cancelling the common mode signal. It is justified by long cables going everywhere and meeting different pieces of equipment connected to the ground taps that are on different potentials and so fort.

So, how can metal detectors benefit from this approach? Well, there are several effects that are introducing some interferences by electric field that are normally tackled with the coil shielding, and that would put a cork to it, unless ... you want to go FKK. In fact there may be some side benefits as well, but let's just uncover one thing at a time.

My first and main opposition to shielding is the effort on applying it.

The second order reason would be the eddy currents in the very shielding that kinda spoil my worldview of the equipment measuring the eddy currents - that just doesn't compute. Especially not the tin foil. Like going fur coat on a FKK beach. Kind of like the observer effect. You know the one where the measuring equipment - the coil - adds some of its own resistance to the targets, thus influencing their discrimination.

Anyway, to go FKK I must fix the common mode signal component, and key to that is balanced operation. There are a few approaches to that.

The obvious, but not the only solution is a center tap. It divides a coil into a self-transformer with both coils having L/4 inductance and near perfect coupling. So for a differential signal the voltage is added and coils' inductances are added and doubled by their mutual inductance, while for the common mode signal it is a short with no inductance. In a perfect world it would be just perfect because any unbalance further on would be ironed out by perfect transformation ... which is slightly spoiled by the coil resistance of the real world.

A perfectly balanced Rx frontend could do that as well even without the center tap, but having the center tap is still a bonus. Such frontend would obliterate common mode signal (high CMMR) and provide low noise gain for the differential signal. Simultaneously it would provide equal impedance on both inputs for differential mode, and high or equal impedance for common mode - I prefer low but equal. Such frontend acts as if there is a center tap even without it.

Differential amplifier assembled from a single op amp and having matching values of R1=R3 and R2=R4 is not balanced. It has high CMMR and high and equal input impedance for common mode, but for differential mode it's inputs impedances are very unequal. Without a center tap this configuration causes the coil to float wildly with the output swing. And it is noisy too. It doesn't make much sense as a frontend.

An instrumentation amplifier is an option, but usually a noisy one, and it costs extra. Using shunt resistors the input impedance can be low and equal for differential, and either low or high for common mode, low being a better choice. We've seen some implementations of instrumentation amplifier on this forum, and apart from noise there is nothing wrong with this approach. Say AD620 with 9nV/sqrt(Hz).

A better option is an integrated "audio preamplifier" which is very similar to an instrumentation amplifier, yet with much lower voltage noise and optimised for low input impedance. Think of SSM2019, and if picky THAT1510. They both go down to 1nV/sqrt(Hz) and both have true differential inputs.

My choice would be a semi-discrete solution as per the Graeme Cohen's preamp that employs current-feedback instrumentation amplifier (CFIA). Much of it is in THAT1510, yet I can find all the components I need in just about any shop, while THAT1510 is complicated to come by. I have it running in a LTspice already, but I'd like to make a few more touches before posting it here. It easily reaches below 2nV/sqrt(Hz) in a configuration with a typical MD coil, and provides perfectly balanced input. Not bad for ~4 bucks of parts.

It will be interesting to see your circuit.
Environmental noise is ever present. Any way to reduce it is a good way.

Coil building and shielding is an art. It can get extremely complicated, but does not necessarily need to be. Good results can be achieved with simple methods if one understands the basic criteria.

Tinkerer

[Davor]

Now a bit about the balanced differential operation. In professional sound equipment it is used to discourage hum from the mains and various crackling sounds entering the system by virtue of cancelling the common mode signal. It is justified by long cables going everywhere and meeting different pieces of equipment connected to the ground taps that are on different potentials and so fort.

So, how can metal detectors benefit from this approach? Well, there are several effects that are introducing some interferences by electric field that are normally tackled with the coil shielding, and that would put a cork to it, unless ... you want to go FKK. In fact there may be some side benefits as well, but let's just uncover one thing at a time.

My first and main opposition to shielding is the effort on applying it.

The second order reason would be the eddy currents in the very shielding that kinda spoil my worldview of the equipment measuring the eddy currents - that just doesn't compute. Especially not the tin foil. Like going fur coat on a FKK beach. Kind of like the observer effect. You know the one where the measuring equipment - the coil - adds some of its own resistance to the targets, thus influencing their discrimination.

Anyway, to go FKK I must fix the common mode signal component, and key to that is balanced operation. There are a few approaches to that.

The obvious, but not the only solution is a center tap. It divides a coil into a self-transformer with both coils having L/4 inductance and near perfect coupling. So for a differential signal the voltage is added and coils' inductances are added and doubled by their mutual inductance, while for the common mode signal it is a short with no inductance. In a perfect world it would be just perfect because any unbalance further on would be ironed out by perfect transformation ... which is slightly spoiled by the coil resistance of the real world.

A perfectly balanced Rx frontend could do that as well even without the center tap, but having the center tap is still a bonus. Such frontend would obliterate common mode signal (high CMMR) and provide low noise gain for the differential signal. Simultaneously it would provide equal impedance on both inputs for differential mode, and high or equal impedance for common mode - I prefer low but equal. Such frontend acts as if there is a center tap even without it.

Differential amplifier assembled from a single op amp and having matching values of R1=R3 and R2=R4 is not balanced. It has high CMMR and high and equal input impedance for common mode, but for differential mode it's inputs impedances are very unequal. Without a center tap this configuration causes the coil to float wildly with the output swing. And it is noisy too. It doesn't make much sense as a frontend.

An instrumentation amplifier is an option, but usually a noisy one, and it costs extra. Using shunt resistors the input impedance can be low and equal for differential, and either low or high for common mode, low being a better choice. We've seen some implementations of instrumentation amplifier on this forum, and apart from noise there is nothing wrong with this approach. Say AD620 with 9nV/sqrt(Hz).

A better option is an integrated "audio preamplifier" which is very similar to an instrumentation amplifier, yet with much lower voltage noise and optimised for low input impedance. Think of SSM2019, and if picky THAT1510. They both go down to 1nV/sqrt(Hz) and both have true differential inputs.

My choice would be a semi-discrete solution as per the Graeme Cohen's preamp that employs current-feedback instrumentation amplifier (CFIA). Much of it is in THAT1510, yet I can find all the components I need in just about any shop, while THAT1510 is complicated to come by. I have it running in a LTspice already, but I'd like to make a few more touches before posting it here. It easily reaches below 2nV/sqrt(Hz) in a configuration with a typical MD coil, and provides perfectly balanced input. Not bad for ~4 bucks of parts.

[Davor]

First off, I'm not joking.

I've gathered some initial experience with an unshielded coil and there is nothing wrong with it. You might have seen my coil in action, links to youtube are on IGSL page, and so far I did not notice any annoyance that would convince me to dress it.

My coil is center tapped and supplied to not so perfect frontend, hence all common mode troubles are tackled with by center tap alone. I have some goals on my mind that I wish to share here with intention to gather other enthusiasts with same level of optimism. Let me summarise what I wish to achieve, so far with VLF:
- unshielded coil;
- aperiodic operation - to promote less than perfect coils made by amateurs;
- low noise - of course;
- differential operation with good CMMR.

My "true differential op amp" was a failure due to the common mode asymmetry, and in meantime I reminded myself on some real differential solutions that take care of common mode as well.

So, shall we go FKK?