Reply 20 of 22, by johnnycontrario
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dionb wrote on 2020-07-11, 10:51:Thing is, the PC is the problem device (at least, once the SGI monitor is on) and I want at least 3 maybe 4 connections into it: 1 out, 2 or 3 in (although the third 'in' would be digital, so using Toslink to avoid an electrical connection would keep that out of the equation). Also the audio in would come from two different mixers (I have two RX1602 and want to capture from both). Other devices only have a single audio out (or maybe 2 or 3 if I have a lot of sound cards- but always the same mixer) but no other device in the setup causes problems.
I'll start with a general safety disclaimer here.
The earth ground connection on most devices exists for safety reasons. In antique equipment, it was an innovation that prevented people from being electrocuted if the 'hot' and 'neutral' mains wires were reversed. In most equipment, it's primarily there to protect the humans or other interconnected equipment in the event of a catastrophic power supply failure. Uncontrolled electricity will take the shortest path to earth ground: the case/shielding > power cable > mains earth. When disconnecting earth ground, the electricity would have nowhere to go other than an audio cable whose ground is connected to another device with an earth connection, or an unfortunate human who is touching the metal case.
So, disconnecting the earth connection on a device to solve a ground loop or noise issue truly is a dirty trick and not a good idea. The proper solution is to use a ground loop filter, however I present the info below as a troubleshooting tool to identify where the loop exists and to help better understand them.
When I used to record audio with desktop PC's, I would sometimes run into ground loops that included the PC. The first thing I would check is to make sure that the devices involved were plugged into the same power strip or mains outlet. If that didn't fix it, and I didn't have a ground loop filter, then I would disconnect earth on the involved device plugged into the PC. The concept that guided me was to ensure there is only 1 path to earth ground: through the audio cables into the PC and out the PC's power connector. It's an ill advised dirty trick, but the majority of devices plugged into the PC had small power supplies, and I only operated this way for a recording session and then disconnected things. I was at peace with the potential risks, however unreasonable that may be.
Connecting other PCs as the sound source to your recording PC brings me pause. If you're experiencing a ground loop problem between 2 PCs (especially vintage PCs with their typically questionable PSUs), a ground loop filter is the safest option. This way, each PC can have its own earth connection. Again, in this scenario, the filter ensures each device has only 1 path to earth ground.
dionb wrote on 2020-07-11, 10:51:As for the SGI power supplies - yes, they are highly suspect, but I've replaced one already after it blew up (bad caps inside), and that didn't change this experience one bit. Also a bit lost as to how ungrounded devices could cause a ground loop, as it's definitely the two SGI things that are to blame - if I hook up a different monitor, the PC is fine.
Still, I'll try the various suggestions and see what works best. Thanks!
Did you replace the power supply with a good quality modern equivalent? Good power supplies will have EMI/EMC filtering on the mains input to reduce emitted electromagnetic noise and noise conducted back to the mains circuit that can leak into other power supplies.
The SGI devices definitely seem to be the culprit. If it's not the power supplies, then the devices themselves are conducting noise to the PC via the signal cable.
We know the noise is conducted, not electromagnetic, because the filter eliminated it. Perhaps the path to earth from video card connector to PSU earth is of higher resistance than the path to earth from video card connector to the mixer's earth connection (via the audio cable).
I suppose you could connect a thick wire, as short as possible, to the video card connector's ground and connect the other end as close as possible to the earth connection of the PSU. The mains earth wire is usually screwed to the PSU case internally, so as close as you can get to this point. Another possibility is the SGI equipment may need its own earth connection. It may be too noisy for the PC's grounding configuration. You could connect a thick wire from the SGI case ground and connect it directly to mains earth.
...or it could be as simple as some dirty/loose connections. Maybe reseating/cleaning the video card, video card bracket, connectors, video cables, and disassembling and cleaning the SGI gear could do it.
