obeerbongkenobi (clever screen name)
One thing to check for sure is that your preamp (or any other audio gear) isn't on the same circuit as a high-load cycling appliance.
A refrigerator or AC unit for example can draw huge amounts of power when starting up the compressor and also cause issues when they power off. The reason you would only hear this thump when you have the phono selected versus other inputs is that the gain is so much higher than the line-level sources.
This would be my guess as to what's going on.
Have a look and report back
Greg
One thing to check for sure is that your preamp (or any other audio gear) isn't on the same circuit as a high-load cycling appliance.
A refrigerator or AC unit for example can draw huge amounts of power when starting up the compressor and also cause issues when they power off. The reason you would only hear this thump when you have the phono selected versus other inputs is that the gain is so much higher than the line-level sources.
This would be my guess as to what's going on.
Have a look and report back
Greg