FKK coils, free beach movement :)

[Davor]

Here is schematic of a Garrett that used a transformer in the receiver.
Not sure if there were any additional components in the coil?

Mouser carries a full line miniture of audio matching transformers for under $2.

Yes, that's a fine example. My local problem here is that I don't have any source of dedicated audio transformers at hand, but as mains transformers are wildly growing just about everywhere, I'd check those first. In case a plain old mains transformer can do the trick - hurray!

The main reason why transformers left the scene was a poor low frequency response, and that is not a problem at all with MDs.

BTW, a garrett example is actually a wee bit missing the point on this topic, as one of the Rx coil's sides is grounded, hence the common mode is converted to differential mode, and the coil's resilience depends solely upon the shield. In this example only impedance transforming feature is used, while common mode suppression is not. A FKK coil would surely use the common mode suppression capability.

[Davor]

Maybe one could be used "back to front" to step up the coil signal.

That's precisely the idea with mains transformers. In case of a 220:15+15 I have in fact 220:30 with ground tap, and it is an impedance ratio of 7.333^2=54 and it is precisely in a butter zone for a coil with R~20ohm and a preamp at 4nV/sqrt(Hz) ... it just places it there.

The best of all is that it also combats the common mode, so it is perfect for a FKK coil, with or without a centre tap.

[Davor]

I rounded up values a bit to obtain square numbers, and made a comparative simulation of a normal IGSL front end and a front end with a 1:7 transformer. Both use 16mH search coils designed as centre tapped, hence two 4mH tightly coupled coils. The transformer front end is simulated with a voltage dependent voltage source that acts as an ideal amplifier. It gives me more freedom in playing with input parameters, e.g. input noise source.
It is designed to simulate both differential mode (target) response using V1, and common mode response (wet grass) using V3 and 10p capacitors. Simulation also includes a worst case calculation option that sweeps the resistors' values in IGSL front end. It is disabled by commenting a ".step param run 1 30 1" line. If uncommented it does 30 worst case iterations. It may be useful in observing common mode rejection of an op amp circuit.

Here goes the noise performance (optimised for 8kHz) of a transformer front-end. Spice file is included. Other configurations are possible by disconnecting ground here or there

[Davor]

I revisited a balanced preamp idea, and came up with a possible poor man's solution to a fine low impedance and low noise balanced preamp solution.

For some time now the RF front end development goes into one interesting direction, a LNA based on a capacitor cross-coupled common gate (or base) configuration. Because of cross coupling the sources with opposite phase gates, this configuration reduces noise, impedance, distortion, common mode, and power consumption ... simultaneously. With impedance at 30-50ohm it is a perfect match for RF systems ... but a bit lowish for a serious MD front end.

I rearranged this configuration a bit to use the best features, and came up to a configuration that has it all, and also has input impedance in kohm range. The best part is that no special transistor matching and no bipolar capacitors are needed, and noise performance better than 1.5nV/sqrt(Hz) is achievable with garden variety transistors at modest currents.

I did not test this configuration in real life yet. Due to easy to obtain components it is high on my to-do list. If it works in real life as well as it does in simulation, it will go into my IGSL front end.

Presented is a configuration with series resonance balanced coil at 16mH and a centre tap.