I forgot the important part. The hum only appears when the plasma is on.
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Sounds to me like you are picking up some interference from the plasma, possibly from the cabling attached to it. Moving the rig out & in again, perhaps the video cables are in too close a proximity to the audio ones.. or same thing with the power cables attached to the components.
Try moving them farther apart from one another and rechecking their connections that they are snug.
Yeah..a lot of plasmas give off a low level hum or noise all by themselves. Some won't notice it. I didn't notice mine until I replaced it with an LCD..whisper quiet.
I think the power supply has something to do with it. Unshielded transformers are very susceptible to electromagnetic interference. My TVC preamp was like a antenna to this kind of hum.
Try removing the TV from the equation , no connections to anything . Let any capacitors discharge .
If that does not do it , check to see if you have two outputs from a source connected to the pre .
I had a problem like yours when I had two output connections from my cable box going to my pre/pro . One analog for the CD quality music channels and one toslink for the surround sound 5.1 broadcasts . Disconnect one of the two and the hum stopped !
I had to "lift the ground" on my tubed preamp to eliminate an annoying buzz. Here is a quote form IAR-80 that may help in understanding the problem.
In most installations, you the consumer utilize the grounding lug on the power cord, for all of the various components in your system chain. This is great for electrical safety, but unfortunately it means that your various components are grounded to each other via two distinct paths, via the power cords and also via the signal interconnect cables. This doubling of the ground connection creates ground loops. Ground loops are an undesirable no-no, for several reasons. Ground loops allow circulating ground currents, which create spurious hum and noise. Furthermore, these circulating currents, traveling through the non-zero impedances of the various ground paths, degrade what should be a single reference baseline for ground throughout each audio component and indeed throughout your system. In all audio components, the signal is literally defined by its (usually voltage) level measured with reference to ground as a reference baseline. If that ground reference level is different among different components, or among different parts of any single audio component, then the whole reference baseline is corrupted, and the audio signal is perforce similarly corrupted. Thus, ground loops effectively corrupt and degrade your audio signal. Sonically, the stereo imaging suffers, and the music can become more veiled and less pristine and pure.