How are you testing this, with headphones (if so, what kind [1])? Is input monitoring enabled - if so, could you turn that off? Which model soundcard are we talking about anyway? What sort of computer is it connected to?
[1] I am asking because I have been able to clearly hear hum caused by mains leakage (from power supply mains filter Y caps) in whatever equipment e.g. some Soundmagic E10 earphones were connected to. The same outputs were fine in battery operation.
Hi, you hit the nail on the head. This is a pair of Lundahl step-up transformer buit in a home-made aluminium box, see picture. The phono preamplifier is actually a sound card with switchable RIAA eq, and it can be operated standalone without PC, but can do an A/D conversion and send to the PC via USB.
Now, the phono preamp has no hum with shorted or open input. The step-up transformer has some hum pickup with shorted inputs, and it gets worse in the proximity of my tuner or when I move it close to the wall. It seems the stray magnetic field caused the hum. I need to make a magnetic shield for the SUT.
There is another source: the turntable motor. If I turn it on, the hum increases, especially in the right channel. I connected the turntable chassis to the mains earth, and the arm ground to the phone preamp (without these connections the hum is horrible). I am still searching for the reason.
In any case, it is not caused by the common L-R GND.
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