Re: LF: VLF/LF-Preamp

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

Sounds good Alan.

 

I don't think I have anything to add. I hope Peter3LP sorts his problems using a twisted pair DC feed through a (modified Coax cable , I like that idea) screen.

 

Oh and I don't like to spend money either so good advice. 

 

Working at audio (8Kc/s Dreamers - 20kc/s SAQ / Mil) with an active antenna I could see there may be an issue with the long power feed to the amplifier picking up EMC which may be getting through to the RX (soundcard). The screen to conductor capacity varing owing to screening sheath to conductor spacing altering because of  applied cyclic mechanical force did not seem an unlikely cause of microphony. Basically a very long capacitive microphone. However if no mechanical force (air born pressure wave or otherwise) is applied then no microphony should occur.

 

So your explaination is the likely one..... probably.

 

73 es GL Pete M0FMT IO91UX

From: Alan Melia <alan.melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
Sent: Monday, 28 May 2012, 19:18
Subject: Re: LF: VLF/LF-Preamp

Hi Peter just a quick butt-in...... dont waste money on noiseless microphone
cable! :-)) This is prevent noise on low-level audio signal WHEN THE CABLE
IS FLEXED AND TWISTED. It has no advantage is the cable is not moved. There
what is called a "tribo-electric effect in cable which is akin to rubbing a
plastic rod (or an amber rod if you are old or rich enough :-)) ) which a
cats pelt. (or possibly silk) The flexing of the coax (or STP) generates a
static charge which varies with movement so generates an audio
noise/crackle. Use a tightly twisted pair like you suggested and you can
actually pull the centre insulator out of old UR67 or RG213 and pull the
twisted pair though that.  Cheaper than solid plumbing ! I was first shown
that by telephone engineers :-))

Alan
G3NYK

----- Original Message -----
From: "M0FMT" <m0fmt@yahoo.co.uk>
To: <rsgb_lf_group@blacksheep.org>
Sent: Monday, May 28, 2012 6:40 PM
Subject: Re: LF: VLF/LF-Preamp


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