"RWG" writes:

This could be a little tricky to explain, so I'll try to be as clear as
possible. I'm getting problems with hum, when connecting my audio system to
my computer system.

I have a conventional stereo system and a PC system (pretty much the usual
Wintel stuff, with PCI sound card). The PC has a three-prong cord, with
ground. The stereo equipment has the typical polarized 2-prong plugs. (BTW
This is North America, 110V.) The PC components all plug into a surge
suppressor. I run the tape output from the stereo into the sound card of the
PC, and I run the sound card output into the aux input of the stereo
receiver. I find that, if I connect all three prongs of the PC equipment
(through the surge suppressor) to the wall outlet, I get hum when I listen
to the sound card.

This seems to happen wherever I plug in the stereo: I can plug it into the
surge suppressor, or the same wall receptacle, or a completely different
receptacle. I still get hum.

The thing that DOES get rid of the hum is to plug the PC system into the
wall, separately from the audio stuff, through a 2-prong adapter, thus
interrupting the ground connection. That's what I've done in the past. But
I've been advised that this offers the PC less protection againsty surges
and other line anomalies than using the ground, which is obviously there for
a reason anyway.

The PC shoudl be grounded. Do not use those 2-prong adapters,
beacause it cuts thr ground. Without the ground the surge suppressor
does not work well and the whole ungrounded PC is potential
electrical safety risk.

Should I ground the PC case to the stereo case?

Extra grounding wire from PC to stereo case can help sometimes
soemwhat, but usually it does not remove the problem.

The problem is elsewhere.
http://www.epanorama.net/documents/g...oop/index.html

Will a UPS provide line
isolation and thus eliminate the problem?

Generally UPS does not help in this.
The ground connection is continuous through the UPS.
It does not change the grounding of the system.

(I've been planning to get one
anyway for other reasons.)

UPS might be OK for those other reasons and a good idea
to get it.

Or should I just cut out that pecky ground connection, the way I did in the
past?

No.

Read the document http://www.epanorama.net/documents/g...oop/index.html
instead. When you understand what is happening, pick a safe
method from there to solve this problem.

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