Is the cable's other end floating (unconnected) or attached to the preamp? If the latter, is the preamp on? Also if the latter, what happens if you move the preamp closer and use a short (e.g. 3') cable instead?
I won't be moving anything, at least until it comes time for a major cleanup... The cable has a couple of feet of slack, runs under the rug on top of the carpet along with a powered repeater USB cable (to the DAC), optical cable, another USB cable (no repeater, to the miniDSP), and maybe another unconnected analog cable.
Wow, looking now, there are 8 cables running under the rug PC to rack, I can only remember what 4 of them do, some must be superceded/disconnected.
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The first "measurement" is made by selecting "Stereo Mix"
in REW as the input with nothing playing to or from or in the PC - whatever that really means. Amirm might remember. If I play a video on YouTube through the PC speakers the audio shows up on this internal "input".
The second measurement is with REW set to monitor "Line in at rear panel(Orange)"
into which is inserted the cable below from the Zone 2 output of the preamp with everything that is normally on, turned on, lacking only a spinning disc in the CD player, which would make an analog signal to the Source 2 input of the preamp, which would route that input to the Zone 2 preamp out. Also "powered on" are the HDRadio, DAC2, Behringer DEQ2496, Cable DVR Box, amplifers, Oppo (standby), niniDSP Open DRC-DI, an ethernet switch, and Audio Authority 1177A switch at the rack side, and PC, external drive caddy, idle Canon USB scanner, idle powered speakers, APC UPS, two flourescent LG monitors, Cable Modem, wifi router, Ooma telephone base, and an idle Brother Laser printer,
The third measurement is the same as 2, but with the TV turned on showing pictures of Big Brother glaring at me, no, wait, it was Election stuff.
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The cable is a cheap (but heavy) twin RCA to stereo 1/8 that claimed shielding and cost $10 or so delivered...
Looks like this:
Do you think that the added noise from the plasma TV comes from noise injected into the power supply (and thereby into your motherboard), or do you think it comes from poor shielding of the 25 foot cable, which is running too close to the TV or power lines? Out of curiosity, do you get less noise if you were to move the cable so that it is further away from the TV?
The TV plugs into the Equitech (isolation, balanced AC power) along with everything else at the rack side, the PC is across the room and plugged into a UPS. It all comes off the same breaker, which is about 80 cable feet (25m) away on the wall in the Used Car Department.
As to what is interfering with what, since it's a big mess of wires and a big lump of gear, is pretty much anybody's guess. I just thought I'd show a picture of three idle measurements. There's no audible consequence that I can detect.