Newsgroups: rec.audio.tech
Subject: Re: Cable grounding potential problem
References: 
[email protected] writes:

> This is a problem that I have seen many times here, but I need some advice
> on
> my specific issue.   
> 
> When I hooked up my new CD player into my entertainment system, I noticed
> an irritating hum.   When I remove the cable television coax from the overall
> system, the hum goes away. According to numerous posts in this newsgroup 
> and others, the problem seems to be a mismatch on grounding between the 
> house and the cable. 

You are righta that this is a common problem. 
For this reason I have written a document on this type of problems. 
It is available at http://www.epanorama.net/documents/groundloop/index.html
Read it.

> So, the cable guy came out.   Initially, they said that the problem 
> was not theirs.    But, I finally convinced the tech to look at the ground. 
> The ground wire ran from the cable outside to a screw on the electric
> meter.    The tech said that this was not how it should be and re-ran the
> ground inside to a cold-water pipe.    
> 
> Hum still there.   :-( 

Propably the cable is right now. 

You are facing a problem called "ground loop". 
And with normal home hifi equipment with unbalanced connections 
(RCA cables) this kind of problems cannot be solved by cable company. 
The problem is caused by quite small grounding potential differences 
on your electrical outlet ground and cable TV cable grounding. 
Those smal ground potential differences are practically unavoidable 
in real world wiring, which is designed for safety in mind, not 
minimum noise. So in real world you need to accept small 
ground potential differences and solve the problems in other 
ways.

One way is to get professional audio system components for whole 
system. Equipment with proplerly designed balanced audio interconnections 
do hot be bothered of some small ground potential differences 
(with some not so good equipment the balanced interconnections 
do not help much, because they have design flaws in their balanced 
connection implementation).

In systems where you use unbalanced interconnections you 
have to accept the fact that their noise shielding against 
this kind of grounding potential differences is very poor. 
A system with unbalanced interconnections works acceptably 
when there is no grounding (all ungrounded equipment) or 
there is only one ground connection (only antenna connection or 
no other connections and one grounded equipment). 
If there is more than one ground connection, there is 
a great potential of humming problems. Most typical  
humming problems are caused with connections to both 
mains power outlet ground and cable TV ground. 
In this kind of case, you need to cut one of the grounds. 
The right place to cut the ground here is to buy an 
antenna signal isolation to the wire that goes to cable TV 
outlet. 

> I have since noticed that the house electrical box is grounded to a hot
> water pipe.    Could this still be a problem?   Should I have the cable
> company reground it to the hot, should I have an electrician ground the 
> house to the cold, or something else? 

Most propably no matter how many times the cable company 
changes the grounding point, the problem does not go away 
completely!

> More info:   If I do the trick of wiring a coax to flat lead back to coax
> adaptor the hum goes away. Unfortunately, this cuts enough of the 
> signal that three of the upper digital channels become unavailable. 
> If all paths from the cable line to the receiver are removed, the 
> hum goes away. 

You have here effectively built an antenna isolator. 
And it was proven to solve the problem.

Unfortunately that isolator seemed to attenuate your antenna 
signal to much (especially those upper channels). 
The possibilities to solve this problem is to get either 
an isolator which attenuates the signals less or 
install an antenna amplifier that compensated the attenuation 
caused by antenna isolator.
 
> I know that I can buy devices for 100 dollars or so that block the 
> ground problem but I don't know if I would lose signal strength. 

All isolators will loose some signal strength. Some more, some less. 
If the lost signal strength is a problem, dpeends on what level you 
begin with. If you have a strong antenna signal, some attenuation 
does not cause any problem as long as the attenuated signal is 
still higher than minimum signal level needed for good picture. 
If the signal level is originally just above the the minimum 
needed signal level needed, even a small attenuation caused 
by anything (many meters of antenna cable, antenna isolator etc.) 
can cause the picture to go bad and digital signals become unusable. 

There is one simple and cheap solution to antenna signal strength 
problems: antenna amplifier. A simple and cheap amplifier 
(10-20 US dollars) can easily amplify your antenna signal 
15-20 decibels, which means that after amplification, there 
is lots of room for signal attenuation on the way to the 
receiver before the signal attenuates to unusable low level.

> Also, if the cable company is supposed to ground to the house
> ground, I would rather they fix it. 

Cable company is supposed to ground the cable ground to 
house ground. And according what you described they have 
done that. And done that as well they should do that.

> Anybody have any good advice?   Should I still be harassing the 
> cable company or should I drop the 100 dollars or should I call 
> an electrician?

With a combination of rewiring the house electricity very 
carefully, then reconnecting cable grounding well to the system 
etc. If could get rid of the most noise, or not. This will 
cost lots of money and the results are not guaranteed! 
The grounding system in house is designed originally for safety 
in mind. Later there have been some extra demands on that  
it is good enough that for example equipment on different places 
can be interconnected without problems. 
But it was never designed to give "hifi quality" gound reference 
for audio equipment systems! This kind of grounding would be 
very expensive to build to a typical house, and making such 
grounding is not sensible (this kind of special very good 
grounding might be built to some audio studios and electronics 
laboratories, and yet still it does not solve all problems, 
because no grounding is ideal, they all have resistance and 
possibility for induced currents).

So I would say that problem will not be solved sensibly 
with an electrician or with your cable company. 
The situation is most propably already as good that is 
practically  sensible what comes to the electrician and cable company
(you might still want electrician to check out your electrical 
outlets on your room, and wiring to it, but most propably 
they are OK, so this might not help anything).

The humming problem needs to be solved within your audio system. 
The company who built your hifi equipment most propably did not 
take into account this kind of problems when desiging the equipment.
The reason for ths could be that either manufacturer engineer did 
not recognize the potential problem at all, or considered this so 
rare at it needs not to be taken into  account because it would have 
increased the price of equipment too much (balanced interconnections 
and built in antenna input isolation are known solutions for this 
kind pf problems, just too rarely used in consumer hifi equipment 
because they cost some money to built into equipment). 
because your equipment did not take into account all the enviroment 
issues, you need to take this into account when building your system. 
This kind of situation can be take account by selection of system 
components (interface types, grounding practices) and using 
suitable ground isolation accessories where needed. This is what 
the builder of any larger audio/video system needs to take into 
acount in the design of the system.


My advice in your case is to get an antenna amplifier 
(preferably one with adjustable amplification) and try 
your original isolator. If this works nicely, then OK. 
If this did not work, get a better antenna isolator 
and use it with or withoyt the amplifier (whatever works better). 
It is your choise in what order you try things and throw in 
the money. A cheap antenna amplifier can be bought for 
10-20 US dollars/euros. A cheap and well working antenna 
isolatiors are also available in the same price range 
(at least in Finland, the country I live in, if you 
know where to shop). 

To me 100 US dollars seems to be excessive price for an 
antenna isolator. But that's usually the extra you need to 
pay for a special items that are not mass produced. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/