VLF MD with digital signal processing : Bee-Buzz 1

[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

[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.

[pito]

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

[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]

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?

[pito]

T coil resistance.

I mentioned such a circuit, can measure microvolts from low resistance coils

The highest magnitude of current that may appear across the test probes on the board which is being tested will be 200 pA, which looks too modest to cause any sort of issues to the PCB under test. The test result indication is through a LED.

[Aziz]

...so now we just wait for Aziz to finish building it.

I will make it and test it for you all. Just for fun.
Then comparing to the other pre-amps (it will have high input impedance, so not loading the RX coil).
I will test 3 different pre-amps. Maybe a simple NE5534 pre-amp too.

[Aziz]

That's correct. Gain is determined by R5 and the coil resistance.

You have understood it.

[moodz]

...so now we just wait for Aziz to finish building it.

[Aziz]

...though the original write up says the design is dependant on the source DC resistance of the coil pickup....affects gain.

Yep. R5 and source resistance defines the gain in the circuit above. The battery voltage if affecting the collector current.
In my modification there is an input protection diodes (charging the capacitors in high voltage pulses seen by the RX coil) and load balancing/discharging of the capacitors to ground.

It isn't a good solution. But it is really very tricky and simple. The signals are riding (floating) at the battery connectors.

[Olly]

That's correct. Gain is determined by R5 and the coil resistance.

[moodz]

I believe that the original idea behind the floating battery was to eliminate any possibility of DC current through the cartridge coil..

...though the original write up says the design is dependant on the source DC resistance of the coil pickup....affects gain.

[Olly]

I believe that the original idea behind the floating battery was to eliminate any possibility of DC current through the cartridge coil..

[moodz]

Parallel to battery there are 2 capacitors 100uF in series = 50uF for 10kHz that will be less than 1 ohm shorting AC signal

Batteries are short circuits already (thevenin) but in this case the battery / caps are unreferenced to ground ...it's more unusual than clever .... High side MOSFET driver circuits do similar things to stop nasty voltages getting to ground. The challenge is to keep the floating supply away from noises ...in this case the use of a battery for the floating supply. The coil noise is referenced to ground however but environmental noise will be some orders above any noise consideration of the amplifier. If I was building an ECG this circuit would be useful.

[Aziz]

Oh man!,

I'm really tired. Get the spice simulation file in the previous post and make the simulations. If you don't have LTspice, download it and install into your computer. And run the fkn simulation.
This piece of circuit is really a KISS masterpiece. Thanks to Richard Lee and others making this possible.
The power consumption is not more than 11 to 12 mA at 6 Volt battery voltage. A short circuit would suck the battery to death. This is not the case. Why battery? No additional noise sources.

It's a moving coil (MC) pre-amp. What do you think what we are doing? We are moving the coil above the ground.
Aziz