I have the Miyajima Mono ZERO and had hum issues but I was told to expect this since, I believe, the L/R channel utilize the same neg. pole internally. As far as I understand a "true" mono cartridge will do this when connected to a stereo preamp. I have both a mono switch on my preamp and phono preamp so I didn’t need to utilize the "Y" connector option though I had one just in case. From all my reading about people using a mono cartridge, it is very preamp and cart. dependent. I had a Dlr. advise me to stay away from a Shelter mono for this reason alone, and he sold them.
The thing I found interesting was that while playing mono LP’s with a stereo cart., it always sounded better when the "mono" switch was used on my phono preamp vs. the preamp. But the opposite was true when using the ZERO. Overall, I am a bit underwhelmed by the ZERO vs. my LP-S on mono LP’s. While the ZERO shows some improvement it’s small, though I only have about 50hrs. on it so it may still need some added break-in to sound better. I think most of the differences I hear could be attributed to the sonic qualities of the ZERO vs. an LP-S more so than the purely mono vs. stereo cartridge differences. I find the ZERO to be a bit "brighter" sounding vs. my LP-S which wasn’t what I was expecting at all. Again, maybe this is due to more break-in needed. Time will tell.