I have an external sound card with built-in phono preamplifier. There is some 50Hz hum on the output, even with shorted inputs. I suspect the L signal ground and L signal ground meets at the GND of the headphone socket. Could it cause a ground loop?
Where is the nearest power transformer, power cord? Does the hum change as you move the preamp around?
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.
Your headphones will not cause a ground loop. Many times phono stages will output hum if the turntable is not grounded. I had a Technics turntable that had a separate ground wire. If yours does as well, connect that to the ground of the sound card.
Tom
Tom
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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How can I make this step-up transformer immune to stray magnetic field? Should I build it in a steel box? What is the minimum wall width?