Yes - this suggests that in your SUT the chassis is not grounded unless you connect a wire to the preamp from the earth post of the SUT. If the SUT chassis is not grounded then all the soldered connections inside are unshielded and prone to RF and this is why the noise reduces further when you connect the SUT ground to the preamp.
SUT Hum
I bought a Hashimoto HM3 SUT.
It has a ground connection, but when the ground cable from the turntable is connected to the SUT, there is an audible hum. However, when the ground cable is connected to the ground connection of the preamp, there is no hum.
Is it a common problem? I'm OK as long as the SUT does not generate hums. But I wonder whether it is due to faults in the SUT.
It has a ground connection, but when the ground cable from the turntable is connected to the SUT, there is an audible hum. However, when the ground cable is connected to the ground connection of the preamp, there is no hum.
Is it a common problem? I'm OK as long as the SUT does not generate hums. But I wonder whether it is due to faults in the SUT.