Message-ID: <5024439B.1000201@iup.uni-heidelberg.de>
Date: Fri, 10 Aug 2012 01:11:23 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: VLF/LF-Preamp
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Hello Peter, LF,

It took a bit longer due to all the MF projects but now i finally tried 
your symmetric preamp. One hour ago i finished the board, see 
https://dl.dropbox.com/u/19882028/LF/DSC04409.JPG

However it does not work really well in the first attempt. The probes 
are just 2 short wires from the resistors. If i come closer to one of 
them with my hand i am getting some V (!) output signal from my own 136 
kHz transmission which take place in the moment.
The circuit is battery fed but the output is connected to the scope to 
show the signal. The signal is much stronger one one wire when i come 
closer to it. I'm using CAT5 LAN cable, one pair for the 12V supply, 
another one for the output signal. The screen is not connected so far.

My hope is to improve my receive situation here, at least a bit, because 
i have no separate LF earth because the RX antenna is on a 35m high 
building, not in a garden... Another hope is to have a active antenna 
that covers VLF to MF...

So far i have no TLE2426 on hand, so i used 2x 100 Ohm / 2W resistors 
and 47uF in parallel to each one to form a virtual ground. The AF 
transformer you mentioned does not work on 137 and 475 kHz of course. So 
i took a FT50-77 with 2x 20 turns in the first attempt.

What is your final configuration and where did you connect the shield of 
the CAT5 cable? Have you found any improvements on the schematic? Or is 
the project lost in the junk box? :-)

Comments? (Yes Mal, i know, i should use a 100 ft inv-L instead ;-) ).

73, Stefan7DK7FC

Am 28.05.2012 17:42, schrieb pws:
> Hi,
>
> After all those meta-discussions here's a more serious problem:
>
> I'm playing around with a new e-field probe based on the INA111.
> http://www.ti.com/product/ina111
>
> Schematic: http://www.df3lp.de/misc/rx/INA111_preamp.png
>
> It's working fine and the local interferences are lower compared to
> my old non-balanced design running here on balcony. That was the goal.
>
> Amplification is quite enough to drive a M-Audio audiophile 192 
> soundcard.
> There's a second transformer (1:4) at the af-inputs. The coupling
> capacitance of those transformers are below 10pF each.
>
> I'm using the shields of a thin double coaxial line for bringing up
> the supply voltage. The voltage splitter is located at the far end 
> inside the small amplifier box.
>
> The problem is that I'm observing to a lot of interferences/microphonics
> "from behind" when touching the battery located inside the house.
>
> ???
>
> Peter, df3lp