Hiss/ground problems on a LTA MZ2/40

Hi all,

I bought a MZ2 secondhand about 6 months ago, I used it some time as integrated and everything was good.

A couple of months I also purchased secondhand a ZOTL40, and when I hooked up the Microzotl as preamp I start experiencing some ground noise issues, which eventually by manufacturer recommendation were solved by unplugging the ground cable spade connector on the Linear power supply. That solved it.

I'd be curious to know if other owners have experienced a similar issue, and wether this is a safe solution or not, and what to do otherwise.

The system ran silent for quite some time, until I started noticing a noticeable hiss coming from the tweeter of my speakers (Tekton Double Impacts), nothing accentuated but eventually hearable from a few feet away.
Initially I thought was on the power amp, and sent it to the manufacturer for a check, but they couldn't replicate the issue.

The hiss can only be hard when plugged to the poweramp, otherwise MZ2 operations on headphone or integrated are dead quiet.

I have realised recently that swapping tubes from 12at7 to 12au7 has reduced the hiss. When hooking up other devices to ZOTL 40  (a phono amp, or a chord mojo) i haven't experienced any ground or hiss issues at all.
I recently have purchased an extra linear power supply, but the hiss is still there.

Just want to say that LTA has been extremely supportive, and helped me a lot trying to figuring out this problem on a long email exchange.

I'd be curious to know from other owners if they ran into something similar, and if eventually have sorted out a solution.

Many thanks.