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Hooking SB card with stereo. PROBLEM !

Posted on 1998-02-12
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Last Modified: 2010-08-05
I'm trying to hook Line Out of SB32 PnP card with stereo to
hear MP3s through big speakers. OK, so I picked long cable
and did it so. I get sound, but also a constant background
noise on low frequencies (stereo's equalizer shows 10 Hz and
100 Hz). Noise continues even computer is turned off. When I
pull out from SB cable, noise stops (so this is not shielding
problem), and sound is good even if I connect cable to Sony
Walkman. Computer is grounded OK (I believe).
What I'm gonna do ?
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Question by:busuka
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jhance earned 200 total points
ID: 1128031
If you can reverse the AC power plug on the amplifier going into the wall, you will likely solve this problem.  It's 60Hz hum caused by a "ground loop" between the PC and the amplifier.  Since the two "grounds" are at different levels, you get a signal flowing through the ground connection between the two devices.  If you can't reverse the plug on the amplifier because it's polarized or has a ground prong, try one of more of the following:

1) Plug both the PC and maplifier into the same AC outlet.

2) Cut the ground connection on your long cable so that it is connected at only one end.

3) Get a "hum buster" from a local audio shop.  This is essentially a isolation transformer which separates the DC connection between two devices.

4) Use a shorter cable, the longer the cable, the more pronounced the problem is.
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by:busuka
ID: 1128032
Hmmm, I don't completely understand what I should do, but will try.
- what do you mean by "reverse plug" on amplifier ?
 On side of stereo I have two jacks "rose"-type (sorry, for non-
 technical) Maybe its RJ-45, for right and left channel.

BTW, I'm afraid that following can cause a problem:
on AC outlets at home I have 3-contacts: phase, zero and ground.
PC connected to UPS, and UPS connected to wall though 3-wire.
BUT, stereo connected to wall with 2-wire (phase and zero). Can
be hum caused by lack of ground contact ?

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by:jhance
ID: 1128033
By reversing the plug on the amplifier (i.e. the stereo) I mean take the two prong plug that goes into the wall and flip it over.  Hum can be caused by many things, both by lack of ground contact and also by ground contact.  Remember, you are dealing with very low level signals here and it takes only a tiny mismatch between the two systems and you get hum.
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by:busuka
ID: 1128034
May I connect ground-wire of cable with AC outlet's ground ? Will it
solve the problem, or I'll burn all my hardware ?
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by:jhance
ID: 1128035
You can do that but I don't recommend it.  The two system are already connected together via the ground lead/shield of the connection wire.  In many cases, this is the source of the problem.  Please read my original posting and try those suggestions in that order.
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by:rmarotta
ID: 1128036
Ian,
Are you using sheilded cable for the audio connections?
Ralph

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by:busuka
ID: 1128037
To Ralph: I think answer is yes. Ground wire from pin-type of jack
divided and going shield-type around each channel's wire. Read my
question: hum is stopped, once I pull out cable from PC and leave
it unplugged or plug it into Walkman.
Tried to flip AC: hum persists (maybe slightly lower, maybe not).
... I don't really want to cut cable, so I tried to pull out plugs on amplifier side
so central pin inserted, but ground "rose" is isolated: no
signal (or central pin just doesn't contact ?).

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by:rmarotta
ID: 1128038
Ian, I believe that your equipment grounds are somehow isolated from each other by the UPS.  Number 1) in jhance's proposed answer should help.  You might also try either plugging the computer into the wall outlet directly, or put the amp on the UPS.  (If the amplifier won't overload the UPS.)
I have seen many motherboards mounted with no grounding screw fastening them to the chassis.  Many mount with plastic standoffs and are isolated.  You might check yours to see if the board is in fact grounded or not.  Perhaps the card bracket isn't screwed down to the chassis?  The audio out jack on the card may be shielded but floating.
Hope this helps,
Ralph

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by:busuka
ID: 1128039
Thanks for ideas. Give me coupla days to check them.

BTW, my stereo doesn't have ground lead in AC plug. ;(
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by:jhance
ID: 1128040
>BTW, my stereo doesn't have ground lead in AC plug. ;(

Usually they don't and for very good reason.  If you tie the stereo ground to the house ground, you end up generating a lot of additional noise.  The "ground" in a stereo is usually floating w.r.t. the AC ground.  This also allows you to reverse the AC plug in and fix many hum problems.
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by:rmarotta
ID: 1128041
jhance,
Just wondering....
Is there the possibility of a "hot chassis" in the stereo?  I know that it was done on older audio equipment, but I haven't worked on any in recent years.  This could present a short accross the AC line if plugged in incorrectly.  i.e. the polarized plug was designed to prevent this from happening.
regards


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by:jhance
ID: 1128042
Not usually in a stereo.  This was (and still is) commonly used in televisions.  TVs normally only have on connection to the outside that being the antenna, which is isolated by default.  Stereo's have multiple inputs and outputs and it takes a lot of extra stuff (translate that as costs to build it) to isolate each input or output.  Remember, in a hot chassis device, no part of the hot chassis can be touchable by any user when the case is on.  Even if it is a hot chassis, none of the exposed connection points wil ever be hot (barring an internal failure or course).  

Remember, that it takes only a few mV (1/1000 volts) of hum to be audible if it is present at the input of an ampifier.  Reducing hum is one of those things that is more of an art than a science.  It's a pesky beast that plagues even the most seasoned sound equipment professional.  Ever been to a concert or other live performance and heard hum?  It happens and when it does it's not usually intended.
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by:rmarotta
ID: 1128043
Ian,
Sorry for taking up your space here.
jhance,
Thanks for the info.  I have mostly seen hot chassis on TVs, as you said.  I've heard hum on lots of P.A systems too.
When busuka said the hum was present even with the computer turned off, would that indicate the offending hum being generated by the UPS? (square wave?)...or the house ground.
Getting time for me to go to work so I'll see you guys later.
Ralph

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by:jhance
ID: 1128044
I guess I missed the comment about the UPS as it wasn't in the original question.  It's possible that the UPS is generating noise BUT most of the noise generated by the UPS will be at a frequency greater than 60HZ.  Remember the output of an UPS is a pseudo-sine wave, which is a fancy way of saying it's a messy waveform.  Generaly, noise from an UPS is most pronounced at 3 and 5 times the fundamental so that would be 180 and 300 Hz.  
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by:busuka
ID: 1128045
Ralph, OK. I like even to see good joke here.
On UPS I have ONLY computer, when I turn it off, I turn off UPS as
well then (but hum stays).

Ian
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by:busuka
ID: 1128046
Plugged PC and stereo to the same AC. No go :(
What I'm gonna do ???? Is "hum buster" my only option left ?
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by:rmarotta
ID: 1128047
Ian,
Try this:
It's not complicated if you can solder.
Go to a Radio Shack or some electronics parts house and get a replacement mini stereo phone plug.  (I'm guessing that this is what your soundcard uses for audio output.)
Cut the molded plug off your audio cable at the soundcard end.
Use the replacement plug without connecting the shield wires.
Just use the tip and ring lugs for the left and right audio signals, leaving the shield isolated.
I hope it helps,
Ralph

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by:busuka
ID: 1128048
I can solder without problems. Will try that soon.
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by:busuka
ID: 1128049
OK. I believe that you put me on right track, so I decided to grade
you. Still have not time to cut the damn thing and check it, but
whadda heck ...
:)

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