Newsgroups: rec.audio.tech
Subject: Re: VCR/preamp hum
References: <[email protected]>
	
	<[email protected]>
	
From: Tomi Holger Engdahl 
Date: 18 Jul 1999 14:33:50 +0300
Message-ID: 
Organization: Helsinki University of Technology, Finland
Lines: 44
X-Newsreader: Gnus v5.3/Emacs 19.34
[email protected] () writes:
> The cable tv's ground and the ground at your house are at different
> voltage potentials, causing a current flow through your signal cables
> introducing the hum.  This is a  classic ground fault. 

Very classical situation. I have described it and how to solve it at
http://www.hut.fi/Misc/Electronics/docs/groundloop/
 
> Solutions:   
> 1) have the CTV company fix their !@#$%^&!! equipment.  Probably of
> success: about zero.  

Propably there is nothing terribly wrong in the cable TV equipments.
Even if they would fix everyhting to be very well, there is still big
change that you will not be completely rid of the problem. The
voltages which cause the humming are so low that they can be easily
caused by the voltag drops in you mains wiring or being induced to
the wiring. 

> You can also try grounding the CTV ground to your
> house's ground with heavy wire, say 8-10 guage wire. 

It should be grounded where it ends to your house. COnnecting exterla
grounding wires inside the room usually creates more problems and dangers
than what it solves.

> 2) use two 75->300 ohm transformers back to back.  Make sure at least one
> of them is a true isolation transformer. 

The ways to do the connection are shown at
http://www.hut.fi/Misc/Electronics/docs/groundloop/home_solving.html

I have shown a few ways how to make such isolator at my document at
http://www.hut.fi/Misc/Electronics/docs/groundloop/antenna_isolator_building.html

> 3) mondial audio's 'magic box'.  This is kinda pricey but works great. 
> I paid $125 for one;  I dunno if the price has droped since.

Some other products for this are listed at
http://www.hut.fi/Misc/Electronics/docs/groundloop/antenna_isolator.html

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web pages at http://www.hut.fi/Misc/Electronics/