Newsgroups: rec.audio.tech
Subject: Re: Subwoofer Feedback??
References: <[email protected]> 
"Bill Whitlock"  writes:

> I really doubt that this is feedback. It has the classic symptoms of a
> so-called "ground loop." Go to www.jensen-transformers.com, click on ISO-MAX
> audio isolators, and then on "download troubleshooting guide."  This will
> explain how ground loops happen, how to pinpoint their location, and how to
> *safely* fix them. For a more technical explanation, go to "white papers"
> and download AN004.

Those are very good documents. I can also recommend reading them.
Other good place to visit which also contains lots of information
on this groundloop field is my groundloop pages at
http://www.hut.fi/Misc/Electronics/docs/groundloop/

> If you're using unshielded interconnect wire, it may help to replace it with
> shielded type, especially if the wire is more than a few feet long. But poor
> shielding (even no shield at all) is not usually the cause of such hum. 

You are right.

> The hum isn't picked up out of the air like so many people think!

Hum can be picked up out of air. The signal can couple to the cable
in two ways: capacitively and inductively. Capacitive coupling
usually occirs form nearby power or speaker wire to ver baddly
shielded audio cables. Usually the problem with capacitive coupling
is at higher frequencies than mains 50 Hz or 60 Hz. Good shielding
does help to get rid of this.

Inductive coupling occurs most often if you have your audio cable
near a power transformer or high current carrying cables. The 
normal shielding on audio cables has very little to do in helping to 
get rid of this noise at mains frequencies. Putting the audio cable
to a wiring tube made of iron or similar materila helps. Or using
balances interconnections with shielded twisted pair cable 
(or even better star quad cable).

Third kind of noise is radio frequency interference (RFI), which
can couple magnetically and capacitively. This can add noise,
but very rarely anythign at mains frequency.


More information of different sources or noise entering the cable
can be found at
http://www.hut.fi/Misc/Electronics/docs/groundloop/otherhumming.html


But you are right in many things. The most common way for the 50 Hz
or 60 Hz noise is the ground loop.


>  wrote in message news:[email protected]...
> > I recently got a subwoofer amp for my formally
> > passive 15" Pioneer DVC Subwoofer.  It works
> > great execept the constant humming feedback that
> > it is getting.  I have only tried to hook it up
> > through the RCA low level inpuits and have tried
> > this with my old receiver (Sony 1020) and through
> > my new receiver (Pioneer Elite AV21) through the
> > sub  low level output but it still is getting
> > that humming feedback??  Any suggestions? the
> > humming seems to change as you move the RCA wire
> > around near the amp so I'm wondering if I need a
> > sheilded RCA cable?  Anyone every have this
> > happen?  THanks in advance for any help...
> >
> >
> > Sent via Deja.com http://www.deja.com/
> > Before you buy.
> 
> 

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