Phono Cart Impedance Question

@lewm 

Yes, I was referring to the internal resistance of the cart itself.  I'm running the cart through a SUT set at 30 ohms.  The " white noise"-like hiss is mainly through the tweeter and only becomes audible as volume is increased near the peak of my listening range.  It doesn't change much if I adjust the gain over a 22dB range nor does it change with different loadings.  When I bypass the SUT and use the MC input on my phono stage I still get the same hiss.  I used to have a HOMC (2.0mV) cart and it didn't produce any hiss at all through the same phono stage.  I'm wondering if a ~0.5mV cart would pretty much eliminate the issue.  It just seems to be related to the gain needed on the 0.28mV cart in my system.

@hagtech 

Good suggestion.  I disconnected the leads from the cart and I still get the same white noise type hiss.  I have a Herron VTPH-2A.  I'm wondering if it could be a bad tube issue.  I think I'll send Keith a note and see what he thinks.

Other ideas?

@mijostyn   Thanks for the link on tubeaudiostore.  I've never ordered tubes from him but those Super Low Noise tubes sound compelling.  Although super low noise certainly comes at a price!

typo...should have said " I realize moving to a slightly HIGHER output cart could very well reduce the hiss."....since I could probably use less gain to obtain same volume.

@almarg 

I'd like to hear your XA25 through those high efficiency speakers.  I'm running a XA30.8 through 90db speakers and suspect we are going after a bit of the same sonic profile.  Mostly acoustic jazz from the 50s/60s emanating from my speakers.

@uberwaltz   Sorry for the confusion here.  I meant that I have the lower output configuration of the VTPH-2A which is set at 64dB for MC (vs the 69dB configuration).  It's only 43dB in MM mode which is of course how I use it when running through the SUT.  I use the SUT with a gain setting to match the 64dB gain that I would get running through MC mode of the VTPH-2A.

@uberwaltz 

Agreed on the phono stage.  I sent Keith a note and I'll see what he thinks. 

@almarg

Thanks for weighing in. I too use the 64dB gain setup on the VTPH-2A. I suspect this is a noisy tube issue. I don’t think it’s a ground loop and there is certainly no 60hz hum. Just white noise at high volume. If I move the single volume knob on my line stage down by 35-45 degrees I’m pretty close to dead silent (no noise unless ear is almost on the speaker). I get no white noise using other components on my line stage and I even tried swapping the inputs around. I think we can rule out a line stage issue. Also I agree that it is doubtful that the 2A is noisier than the 2! I’m curious what Keith says but if I were a betting man I’d guess that I’m going to try swapping out some tubes. 

Btw - how much gain do you have on your line stage?  Mine is 10dB.

@uberwaltz   I don't have any extra tubes to try at the moment.  It takes 5 tubes - a combo of (2) 12AX7 with (3) 12AT7 or you can use (4) 12AX7 and just one 12AT7 to get the extra 5dB of gain.  Keith uses all EH tubes for this model so at least they are pretty cheap.

@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.

@lewm
@almarg
@chakster
@jmolsberg I appreciate all of your really helpful feedback/input.

@uberwaltz  Agreed.

What do you guys think...if I wanted to perhaps try just a couple of the SLN tubes to see if they start to lower noise where would be the best place to start?  As follows:

1st gain stage (pair)
2nd gain stage (pair)
output stage (single)

It would be nice to just dip my toe in the water to check for improvement w/o making a financial commitment for all 5 SLN tubes at $95/ea.  Where would the biggest improvement most likely be heard?

@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.

In case anyone is curious, Keith confirmed what I was hearing was likely normal.  He indicated :

"There will be some white noise if you turn the level up beyond where you would normally listen as the VTPH-2A has a lot of gain and you would not want to play music at those levels.If you notice a significant difference between the two channels left and right then you may have a noisy tube (most likely a 12AX7)." 

@uberwaltz Yep!

Although I will say I've learned a lot over the past few days interacting with both Keith and Roger on this topic.  I'd be really curious to A/B a 6922-based phono stage in my system someday and see it how it performs.  Maybe something like the Manley.  But honestly, the music coming out of my Herron is joyful and a true bargain at its price point.  My money is being allocated towards vinyl over gear for the next 12 months at least ;-)