Newsgroups: rec.audio.tech
Subject: Re: Common Ground
References: 
"RX-7 & SC 400"  writes:

> I have an audio line from my stereo to my computer, and I recently got a UPS
> for the computer, and I put in a 30A isolated circuit for it.
> Great....nasty hum from the speakers.  I suspected it was a ground problem,
> but then again maybe it was a frequency or voltage differential because the
> computer was getting nice clean power (its a badass industrial strength
> line-interactive UPS) so I plugged the stereo into the UPS...no change.
> Then I say well...must be a grounding problem after all.  Okay....go down to
> rat shack and get some alligator clips and some old wire (16 AWG), and
> string up between bare metal on the chassis of the UPS, and a nice shiny
> copper screw on the back of my Harmon Kardon AVR-70.  Great....less hum.
> But its still there!  I tried various attachment points on each (including
> the ground lug on the HK, as well as the shield portion of the coax antenna
> input), with no improvement.  

Does removing the antenna connection on the stereo receiver
solve the problem ? 
If that solved the problem, then go and buy an antenna
isolator for your radio antenna connection (and all
other antenna connections in your system).
This will solve the problem.

This kind of antenna ground and mains ground problems
are quite common problems. You cna read more about them at
http://www.epanorama.net/documents/groundloop/index.html

> Is there an optimal spot in say the amplifier
> or the transformer that I should attach to, or would bigger wire help?  

Isokation transfoerm for antenna connections will do the job
most propably.

Other possibility is to use isolation transformer in the audio
cable connection from your computer to the stereo system.

> Is 
> this just something one has to live with when going from computer to
> reciever?  

No. This problem can be solved quite easily with proper
isolator placed in proper place in your system.

> Anybody make grounded recievers or are they all floating ground?

Almost all consumer stereo receivers are designed as 
as floating devices. There are very few which are
grounded (some higher power models between consumer
and professional markets).

A floating stereo receiver gets generally grounded to
the antenna neteork ground when you connect it to
the antenna outlet on your wall (your own outside antenna
properly grounded or cable TV antenna connection which 
is grounded). 

Generally when you have some devices which are grounded
(=grounded to mains power ground) and devices which 
are connected to antenna network (=connected to
some groudn with somewhat different potential) 
you will get hummign problems. Generally the fact if you
have grounded or ungroudned stereo receiver does
not change the situation.. you will get humm...
even thick grounding wires between equipment do not
completely remove it. To remove the hummign noise,
you need to isolate the antenna connection from
your systen (do not connect the antenna wire at
all or connect it through antenna isolator).



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