Phono Cart Impedance Question

@intactaudio 

Thanks for your thoughts.  My own troubleshooting suggests the same culprit - noisy tube in the gain stage.  Upon setup I had addressed other issues you identified including placement of the SUT (which was sensitive to proximity to toroidal transformers) and by using a very well shielded cable between the SUT and the phono.  I tried several cables and I'm getting the best performance from an AQ Colorado with its active noise suppression.  It beat out some shielded Cardas cables I had.

I'm left now with no buzz or hum.  I also don't think the SUT is contributing to the "white noise"/tube rush.  It's exactly the same even when I remove the SUT from the chain and run the cart leads directly to the MC inputs on the phono.  Noise appears to be equal in level on both channels.

I'm just waiting to hear back from Keith before proceeding further.

I realized I had a spare pair of Mullard 12AX7's from a preamp I sold, so I started experimenting a bit this evening.  The long and short of it is that I've ruled a bad/noisy tube as the issue.  I swapped the Mullards into the 1st gain stage - exact same white noise at very high volume when stylus hasn't touched the vinyl; again, the noise goes away once it touches and this volume level is really louder than I'd ever listen.  I then replaced the original tubes into the 1st gain stage and used the Mullards in the 2nd gain stage (which actually increases the gain by 5dB as they're replacing a pair of 12AT7's).  Same exact white noise under same volume conditions, albeit a bit louder just because of the gain.  I then used one of the removed 12AT7's from the 2nd gain stage and swapped the output stage 12AT7.  Again, same result.  I then replaced all tubes back into original configuration - the sonics are actually best that way, unsurprisingly.

So I now suspect this is an RFI issue with the cart/cable relationship.  I see there are no shortage of threads on this forum about RFI and phono.  Again, this really isn't a huge problem and perhaps I should stop obsessing over white noise hiss that only is audible when the volume is cranked with a LOMC cart before the stylus ever touches the vinyl.

I doubt RFI three. All the original tubes in my ARC were noisy. The SLN tubes if I remember correctly are 1 in 100! But it could be just inherent noise in your phono section. It does not seem that bad according to your description. The best thing to do might be nothing rather than spend money chasing your tail. My own rough test for phono noise is if I can hear it between songs over the background noise of the blank groove I'll go after it. If not it won't matter.

@mijostyn  I'm thinking you're right and I do feel like I'm chasing my tail for no good reason.  I think this is likely normal noise in the phono section and perhaps I could lower it slightly buying a set of Roger's super low noise tubes but I'm wondering if the juice is worth the squeeze.  Noise isn't audible from listening position and goes away entirely when needle hits the record.  I suspect anyone with a LOMC and a tube pre will hear some white noise hiss if they crank up the volume before the needle hits the vinyl.

Op.
If you are talking levels that are only vaguely audible with ear pressed hard against speaker then I would say I might have the very slightest hiss at about 80% volume.
This is so slight though but I could even be imagining it.
Seated, needle in groove, anything at sane volume level..... Deathly Hallows quiet.

This is with a Chinook tube phono.
Running through a Denon au320 sut prior to it.

I would relax and sit back and enjoy.
Maybe think about the sln tubes at $90 each......