Correct wiring of an MC step-up transformer

[lcsaszar]

This is a general question. I built a pair of step-up transformers in a metal box. The input and output RCA sockets are mounted isolated from the box. I connected the L channel RCA input GND to the output GND, similarly the R channel RCA input GND to the output GND. Then I connected both transformers' shield to a GND screw on the metal box. So the L channel GND is isolated from the R channel GND, and both are isolated from the metal box.

My turntable has a built-in RCA cable and a separate GND cable that is connected to the chassis of the TT. If I leave this GND cable unconnected, there is big hum. I connected this GND cable to the GND screw on the MC transformers' metal box, but there is still some hum. The hum is much reduced if I tie the GND screw to either the L or R channel RCA connector's GND.

I wonder if this is the correct way of GND connection? Where is the turntable chassis GND normally connected to (if there is an MC step-up transformer between the cartridge and the phono preamplifier)?

 

[lcsaszar]

I found this diagram:

But I don't like the idea to connect chassis GND to both the L and R output RCA connectors' GND. It could create a ground loop, doesn't it?
(Mods, please remove the link if it is against forum rules)

 

[directdriver]

I would not use the input center tap to ground at all. It's not necessary.

 

[as_audio]

Connect the metal case ground to amp ground also. In
that case cut conection from RCA output to metal case.

Agree with post 3 if your drawing shows in fact center
taps labeled as no. 4.

Edit - I just see that you show a drawing of somebody else.

 

[lcsaszar]

Referring to the above diagram, I have R 2-5 connected, L 2-5 connected, and GND connected to L 5. Center tap is not connected. It is silent.