Re: LF: VLF/LF-Preamp

[M0FMT <m0fmt@yahoo.co.uk>]

Hi Peter

 

If you are talking about the dc supply cable being both an electromagnetic antenna but also a microphone then I am sure you already know that A/ using a twisted balanced arrangement may cut the first,  and B/ spending a load of cash on a specialist microphone cable by say ATM may cut the second  http://www.mtraudio.com/atmcables.htm  ............. They say

 

" The secret of noiseless microphone cable is to combat both electromagnetic interference and mechanical vibration (microphony). ATM uses a twist technique and high grade lapp shield to reject electro-magnetic interference. The outer jacket material is chosen for resilience and pliability but also has vibration absorbing properties. Internal cotton fillers further reduce microphony. These features combine to give ATM accurate, noise free signal path performance."

 

But a more practical approach may be to twist your own wire in a hand drill (like I do) and feed it through a thin copper pumbing tube then filling the whole thing with that sealing expanding foam (which I don't do). It may work, never tried it myself. Also look at the screen grounding, have the far end un-grounded or visa versa.

 

Thinking about it some more I use microphone cable for my Rig to PC connections stolen from my sons Guitar Kit Its very thick and squashy. That never gives any RF or Microphony issues but that may be just luck.

 

Suggest you migrate to the Yahoo RSGB LF site for a wider discussion group there are more informed people there than me.

  

Any way good luck and let us know how you get on, this cannot be a new issue I am sure.

73 es GL Pete M0FMT IO91UX

From: pws <pws@legal-medicine.de>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Sent: Monday, 28 May 2012, 16:42
Subject: LF: VLF/LF-Preamp

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