Unbalanced inputs, ground noise, skipping jumper?

jstrom

Hi,

I've got two old 9088(ii) devices. I have two line inputs connected to a PC stereo line out. This have been connected with + (tip&ring) to Hot, and - (sleeve) to Cold with a jumper to Shield as per the manual.
The PC & the Soundweb are both properly grounded in the same power distributor (the PC has a few more meters of cable but thats it).


I've had issues with some kind of PC switching noise leaking through ground/signal before (on multiple PCs & inputs, not only this one), but it has been so low I could ignore it. Now I've installed a high performance graphics card, which seems to output the same kind of noise but at a much higher amplitude, making it impossible to ignore: https://www.youtube.com/watch?v=1ZdIM3HGZs8



I've found one solution which seems to work perfectly fine, eliminating all noise: Connecting + to hot, - to cold, but leaving Shield disconnected.

Now, I did a search for this, and found a post (http://www2.bssaudio.com/phpBB2/viewtopic.php?t=326&highlight=unbalanced) which very verbosely tells me NOT to do this..

Could someone with some more knowledge please elaborate on why? My guess is that it has to do with bad grounding and floating voltages, which shoudn't (?) be a problem when both ends are properly grounded at the same point

Thanks!

jstrom

Two things I have noticed so far:

1) If i unplug the cable at the PC end, I get an effective antenna picking up very audible static. This is mitigated by adding a shield-to-cold jumper.

2) The PC seems to power down (?) the audio output stage after ~30 seconds of silence. When this happens, I get static noise similar to if I disconnected the line. As soon as a sound is played, the static disappears immediately. Didn't test jumper in this case, but I suspect it would work as well.

This static becomes extra apparent as I now have +12dB on the input to get more headroom (don't know why I haven't done that before...).



The first thing, disconnected cable, isn't a huge problem. The second can hopefully be solved with some software setting.


Is there any good hardware approach to solving this? I read something (non soundweb-specific) about having a shielded two-wire cable, connecting the shield to the soundweb shield (only), and the two wires to +/- on the source. Would that help?


EDIT:
I'm now using a cable with two twisted pairs, individually foil-shielded with a common drain-wire.
At the soundweb input, the shield is connected to the Shield terminal, and then one pair for each channel: one wire for + and one for -.
At the PC end, the shield is unconnected & isolated. Both the - wires are both connected to the mini-tele sleeve, and the other wire from each pair to tip and ring.

I can now unplug the cable at the PC end without any noticable noise (unless I turn the volume way up at the speaker amp). Of course, touching any pin on the mini-tele introduces hum. Not unexpected, proper shielding is good...:)

The other issue, PC static/digital noise when sound card is in power-saving mode, is still there though. And if it really leaves the output floating, I guess it isn't really strange that noise is picked up via the leads on the PC motherboard.
I can probably work around this by making sure it doesn't power down.