Hooking SB card with stereo. PROBLEM !

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ASKER

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 ?

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.

ASKER

May I connect ground-wire of cable with AC outlet's ground ? Will it
solve the problem, or I'll burn all my hardware ?

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.

Ian,
Are you using sheilded cable for the audio connections?
Ralph

ASKER

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 ?).

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

ASKER

Thanks for ideas. Give me coupla days to check them.

BTW, my stereo doesn't have ground lead in AC plug. ;(

>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.

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

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.

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

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.

ASKER

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

ASKER

Plugged PC and stereo to the same AC. No go :(
What I'm gonna do ???? Is "hum buster" my only option left ?

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

ASKER

I can solder without problems. Will try that soon.