How to cure ground loop caused by cable TV?
Peter A. Korp
I recently setup a new amp, preamp and speakers in my AV setup and
when connecting the TV and VCR to the preamp got a terrible humming
noise(ground loop). I was able to isolate it to the cable inputs to the
devices. What is the easiest way to get rid of this problem?
Thanks,
--
Peter A. Korp
Assistant Scientist
Argonne National Laboratory
ko...@eid.anl.gov
Avery Son
Ty Bower
>
> I recently setup a new amp, preamp and speakers in my AV setup and
>when connecting the TV and VCR to the preamp got a terrible humming
>noise(ground loop). I was able to isolate it to the cable inputs to the
>devices. What is the easiest way to get rid of this problem?
>
Get yourself a 75 ohm to 300 ohm matching transformer (looks like a
little barrel with a female 75 ohm coax jack on one end and a 300 ohm
flat wire on the other.) Mate it to a 300 ohm to 75 ohm matching
transformer (looks like a little box with a push on f-type 75 ohm
connector on one side and a pair of screw terminals on one end.)
You want to connect the flat wire to the screw terminals.
Now, take this little doohickey, and push the f-connector onto the back
of the television. Screw the cable feed onto the remaining connector.
The transformers effectively block the DC component of the cable
which is causing the hum, but pass the RF signal through.
Monty Solomon
> when connecting the TV and VCR to the preamp got a terrible humming
> noise(ground loop). I was able to isolate it to the cable inputs to the
> devices. What is the easiest way to get rid of this problem?
The Magic Box for $69.95 from Audio Outlet may fix your problem.
They offer a money-back guarantee.
Audio Outlet Inc.
914 666 0550
--
# Monty Solomon / PO Box 2486 / Framingham, MA 01701-0405
# mo...@roscom.com
Steve Martin
> >
> > I recently setup a new amp, preamp and speakers in my AV setup and
> > when connecting the TV and VCR to the preamp got a terrible humming
> > noise(ground loop). I was able to isolate it to the cable inputs to the
> > devices. What is the easiest way to get rid of this problem?
A pair of matched transformers did it for me. You know the little
dohickies that let you hook the cable TV to the antenna connectors on an
older TV? Get two of them, connect the 300 ohm ends together and the cable
TV coax to one end, and the cable to your "devices" to the other. I didn't
notice any loss in picture quality.
Worked for me.
--
Steve Martin
Internet: mar...@metronet.com Compuserve: 72727,1471
AppleLink: SteveMartin America Online: Steve Mart
Chuck Forsberg WA7KGX
>> In article <CJ9x3...@mcs.anl.gov> ko...@eid.anl.gov (Peter A. Korp) writes:
>> >
>> > I recently setup a new amp, preamp and speakers in my AV setup and
>> > when connecting the TV and VCR to the preamp got a terrible humming
>> > noise(ground loop). I was able to isolate it to the cable inputs to the
>> > devices. What is the easiest way to get rid of this problem?
>
>A pair of matched transformers did it for me. You know the little
>dohickies that let you hook the cable TV to the antenna connectors on an
>older TV? Get two of them, connect the 300 ohm ends together and the cable
>TV coax to one end, and the cable to your "devices" to the other. I didn't
>notice any loss in picture quality.
You want at least one of those transformers to have no DC path.
Many matching transformers have a DC path, useless for ground
loop problems.
--
Chuck Forsberg WA7KGX c...@omen.COM 503-621-3406
Author of YMODEM, ZMODEM, Professional-YAM, ZCOMM, and DSZ
Omen Technology Inc "The High Reliability Software"
TeleGodzilla BBS:621-3746 FAX:621-3735 CIS:70007,2304
Bob Niland
: I recently setup a new amp, preamp and speakers in my AV setup and
: when connecting the TV and VCR to the preamp got a terrible humming
: noise(ground loop). I was able to isolate it to the cable inputs to the
: devices. What is the easiest way to get rid of this problem?
If you don't use the baluns listed below, you run a HIGH risk of
getting single-ended (i.e. common ground) units that will fail to
solve your problem.
__________________________________________________________________________
From: j...@cmkrnl.com
Newsgroups: sci.electronics,rec.audio,rec.video
Subject: Re: Revisited: CATV baluns for eliminating AC hum
Date: 3 Nov 93 15:59:13 PST
Organization: Kernel Mode Systems, San Diego, CA
Someone wrote to me and asked for details of how baluns can be used to
eliminate hum in audio systems caused by cable TV-inspired ground loops.
Here's the procedure. Go to Radio Shack and buy one each of:
- 15-1253, "300-ohm TV-VCR Matching Transformer"
This looks like a little box with two screw terminals and a push-on male F
(coax) connector.
- 15-1140, "75-ohm coax/300-ohm twin lead indoor/outdoor macthing transformer"
This is a longish box or tube, with a female F connector on one end and a bit
of twin-lead coming from the other. The twin-lead ends in a pair of what are
called "spade lugs" (shaped like U's). (I've never in my life seen a "spade",
either a shovel or a card suit, shaped like these, but that's what they're
called!)
[note, each of these part numbers may have a "B" or other letter at the end.
These indicate slightly different details of functionally equivalent parts.
Don't worry about it.]
Just connect the two spade lugs to the two screw terminals on the other unit.
(And make sure they don't touch each other; this shouldn't be difficult to get
right.) This gives you the "isolator", with a female coax connector on one end
and a male coax connector on the other.
Just insert the isolator "in line" in the incoming cable lead. ie treat it as
you would a (very short) extension cord. You can do this right at the back of
the VCR (or whatever the cable is hooked to).
The hum of course may persist after this. If you have any equipment in the
system with three-prong plugs, isolate them by plugging them in via three- to
two-prong adapters, and do NOT connect the ground pigtail on the adapters to
anything. In general you want to get down to at most one connection to an
earth ground. NO connections to earth ground will often be better still. If
the safety aspects of this bother you, install a Ground Fault Circuit
Interrupter (with its ground lead unconnected) and power the equipment through
that.
Another source for AC hum is leaky insulation in the power transformer of some
component in the system. The leak allows small amounts of AC line voltage to
get into the system's frame and signal grounds. Since no insulation is
perfect, ALL ac-powered devices will show a tiny bit of this. You run into
hum problems if two components have more than a tiny bit of leakage; one will
have more leakage than the other, current will flow between them via the
shields of the various patch cords, and the amplifiers will boost it to the
point where it's audible. The best approach is to pull stuff out of the system
one unit at a time until the hum goes away.
--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
Internet: j...@cmkrnl.com (JH645) Uucp: uunet!cmkrnl!jeh CIS: 74140,2055
Jamie Hanrahan, Kernel Mode Systems
> In article <martin-090194093644@DialupNewsWatcher> mar...@metronet.com (Steve Martin) writes:
>>> In article <CJ9x3...@mcs.anl.gov> ko...@eid.anl.gov (Peter A. Korp) writes:
>>> > I recently setup a new amp, preamp and speakers in my AV setup and
>>> > when connecting the TV and VCR to the preamp got a terrible humming
>>> > noise(ground loop). I was able to isolate it to the cable inputs to the
>>> > devices. What is the easiest way to get rid of this problem?
>>
"matching", not "matched". :-)
> You want at least one of those transformers to have no DC path.
> Many matching transformers have a DC path, useless for ground
> loop problems.
Yes. The distinction here is between a "transformer" and a "balun".
A transformer has two separate windings and therefore no DC path; a simple
balun does have a DC connection between input and output. But this assumes
that the terms are used correctly -- Radio Shack sells at least one item that
is marked as a "transformer" but has a DC path.
I'm attaching an article I posted recently on this subject, including the Radio
Shack part numbers for a transformer which I tested and found no DC path.
--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
Internet: j...@cmkrnl.com (JH645) Uucp: uunet!cmkrnl!jeh CIS: 74140,2055
(included text follows)
Go to Radio Shack and buy one each of:
- 15-1253, "300-ohm TV-VCR Matching Transformer"
This looks like a little box with two screw terminals and a push-on male F
(coax) connector. (this is really a balun, by itself or even in pairs, it will
provide no isolation.)
- 15-1140, "75-ohm coax/300-ohm twin lead indoor/outdoor macthing transformer"
This is a longish box or tube, with a female F connector on one end and a bit
of twin-lead coming from the other. The twin-lead ends in a pair of what are
called "spade lugs" (shaped like U's). (This really is a transformer, with
two separate windings.)
(note, each of these part numbers may have a "B" or other letter at the end.
These indicate slightly different details of functionally equivalent parts.
Don't worry about it.)
Just connect the two spade lugs to the two screw terminals on the other unit.
(And make sure they don't touch each other; this shouldn't be difficult to get
right.) This gives you the "isolator", with a female coax connector on one end
and a male coax connector on the other.
Just insert the isolator "in line" in the incoming cable lead. ie treat it as
you would a (very short) extension cord. You can do this right at the back of
the VCR (or whatever the cable is hooked to).
Note that you could also use two of the 15-1140 parts, with their twin-lead
ends connected to each other. This would give you an "isolator" with two
female "F" connectors instead of one male and one female. By contrast, you
cannot connect two of the 15-1253 parts to each other and get isolation.
(end included text)
PE. Smee
> I recently setup a new amp, preamp and speakers in my AV setup and
>when connecting the TV and VCR to the preamp got a terrible humming
>noise(ground loop). I was able to isolate it to the cable inputs to the
>devices. What is the easiest way to get rid of this problem?
You sure it's a ground loop, and not a signal loop (feedback)? Reason
for asking is that I could get our setup to generate incredible hums if
I set the amp input to the TV, and told the TV to watch the VCR, while
having the VCR in 'simulcast' mode (where it takes, as its audio input,
the output from the preamp's tape out). Due to signal-limiting
somewhere in the chain, that never grew into the full-fledged
ever-growing-until-something-breaks feedback scream, but just settled
down to a loud growly hum. Might be worth thinking about your signal
paths to see if that might be a possibility. (Even if it's not your
problem, might help someone.)
--
Paul Smee, Computing Service, University of Bristol, Bristol BS8 1UD, UK
P.S...@bristol.ac.uk - Tel +44 272 303132 - FAX +44 272 291576
Douglas Baker
I had a simmilar problem. The solution was to get a 1:1 RF
transfomer isolator and put it in series with the cable (cable TV).
Crap Shack didn't have one I was in a hurry so I wound one my self.
If you can't find one check with someone thats into Ham Radio.
Doug
Les Reeves
This is the right way to do it. However, there are two ways of building
the balun transformer inside these devices. One way passes DC, the other
does not. There seems to be no rhyme or reason as to whether or not the
manufacturer used on or the other. About half the ones I have tested
pass DC.
To test, just use an ohmeter. If you get no continuity between the
75 ohm and the 300 ohm side, that's the one to use.
Be glad you don't need a *video* isolation xfmr.
A company called North Hills Electronics makes them and the cheapest
is about $100!
Robert Moore
shield to the VCR ground post and inducing hum (w/o plugging in the signal conductor).
I tried the back-to-back baluns, but I found the signal was too lossy, so I went to
my local hi-end hi-fi dealer and bought a cable ground-breaker (< $10 ). Works fine.
Bill Hengeveld
What if you need to pass DC? Are you out of luck? For example, with
Satellite Dish Receivers, the same coax that delivers the video signal
from the dish also provides a DC voltage to the dish to power the low
noise amplifiers via the shield. There are NO RF (coax) outputs from most
Satellite Receivers (most use RCA connectors for video out) to put
isolation transformers onto
Dr. Moze <Steve Marsh>
to my receiver. I used the 75-ohm-to-300-ohm-to 75-ohm converters
'trick' on the cable tv coax and it didn't work at first. I then
attached the converters to the 75-ohm FM antenna coax going into my
receiver, and the hum disappeared. Excellent! Thanks to the poster of
this info... $-)
- Dr. Moze <ma...@anvil.nrl.navy.mil>
Byron Yani
I am thinking of buying the Sony MDP-800 laserdisk player
and would appreciate any comments. I know that is capable of
playing both sides of a disk and has digital memory for freeze
frames. But I have not seen any reviews with any specs. If you
have any information please respond to this post or email me at
men...@solbourne.com
thanks in advance
Peter A. Korp
accident. I was in the market for a line conditioner/surge supressor and
ended up buying the Panamax Max 1000. It happens to provide 8 outlets
total with a common ground as well as a common ground for CATV.
Thanks for all the suggestions and responses,
Peter
Iskandar Taib
>You sure it's a ground loop, and not a signal loop (feedback)? Reason
>for asking is that I could get our setup to generate incredible hums if
>I set the amp input to the TV, and told the TV to watch the VCR, while
>having the VCR in 'simulcast' mode (where it takes, as its audio input,
>the output from the preamp's tape out). Due to signal-limiting
>somewhere in the chain, that never grew into the full-fledged
>ever-growing-until-something-breaks feedback scream, but just settled
>down to a loud growly hum. Might be worth thinking about your signal
>paths to see if that might be a possibility. (Even if it's not your
>problem, might help someone.)
Interesting. On my Samsung HiFi VCR I get a discontinuous string of
faint beeps, but with the JVC is a really loud scream.
Now I've got them daisy-chained (radio goes to Samsung, Samsung goes
to JVC, JVC goes to radio) it screams if I get the switches wrong.
The Samsung is by far the better VCR for recording music off the radio
at SLP. The JVC has all kinds of noise when I try it.
--
-------------------------------------------------------------------------------
Iskandar Taib | The only thing worse than Peach ala
Internet: NT...@SILVER.UCS.INDIANA.EDU | Frog is Frog ala Peach
Bitnet: NTAIB@IUBACS !
Stephen G. Searle
I, too, am thinking about purchasing this model.
I have one criticism, however. Product literature.
I like to compare the specs and features of different models before
I test them. Most of the electronics stores are useless for printed
information. And the various "Buyer's Guides" on the magazine stand
are typically too inaccurate. So, in the past, I have called the
manufacturer, who will mail it to me.
It seems someone at Sony came up with a "brilliant" marketing tactic.
They figured, "Hey, lets produce one large catalog, put in some fancy
photos so we can call it a magazine, and since its a magazine we can
charge for it and make it pay for itself. Then we don't need to
produce individual product literature sheets. This will save us lots
of money."
Thus we have "Sony Style" magazine. $5 + $1.50 s/h.
Now you too can >purchase< product literature. And the dealers
can't give literature even if they wanted because it is no longer
provided to them.
Why should prospective purchasers of Sony equipment have to pay
for product literature? Does Sony think that they have such market
share that they no longer have to provide free literature?
And what about new models? This "magazine" is currently dated October,
1993. I'll bet it doesn't have any information on the latest models.
I would encourage all prospective purchasers of Sony products to
write Sony (or call 800-222-SONY) and ask that they provide >free<
product literature.
-stephen
---
Stephen G. Searle (sea...@mot.com)
Senior Software Engineer
Motorola, Inc., Paging Products Group, Boynton Beach, FL USA