Phono Cart Impedance Question

... 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)

While I'm not familiar with the internal design of the VTPH-2 or 2A, and I've never taken the cover off of my VTPH-2 (having seen a number of past comments which like Jmolsberg's comment above have described finding tube rolling to not provide much if any benefit to Keith's phono stages), my guess is that if you want to give it a try the first gain stage would be where to start. Especially if that is where the 12AX7s are used.  (I don't know if the 12AX7s are used in the first stage or the second stage).  

Since signal level is presumably lowest in that first stage any noise generated by that stage will degrade signal-to-noise ratio more than it would be degraded by a similar amount of noise generated further downstream. Also, it may be relevant that the 12AX7 has a higher intrinsic gain than the 12AT7. 

Good luck. Regards,
-- Al

how much gain do you have on your line stage?

My line stage/preamp is a DEQX HDP-5, which provides almost no gain. Actually less than 1 db with the internal jumper settings I use, and it would even provide negative gain if those jumpers were set differently. It is designed that way in order to allow headroom for the various digital signal processing functions it provides.

Also, my power amp (a Pass XA-25) has a lower than average gain of 20 db. However my speakers (Daedalus Ulysses) are very efficient, at 97.5 db/1 watt/1 meter.

On the DEQX volume is set by means of up and down buttons, rather than by a rotary control. When I listen to LP’s I typically have it set at around 60% to 70% of full scale.

I agree with your suspicion that the culprit in your case is most likely a tube that is a bit noisy.

Best regards,
-- Al

@three_easy_payments,

I have a Herron VTPH-2 (not the -A version), in the low gain configuration that is most commonly used (64 db gain for LOMCs). I use it with a 0.5 mv cartridge, an Audio Technica AT-ART9. I find it to be incredibly quiet.

0.5 mv is only about 5 db greater than 0.28 mv. So assuming those specs are accurate and are defined on a consistent basis, if I were to go to a 0.28 mv cartridge I would be setting the volume control on my preamp 5 db higher than I presently set it. Based on experiments I have done I can say with total confidence that were I to do so there would be zero perceivable hiss, even if I were to listen with my Stax electrostatic headphones firing directly into my ears.

So either:

(a) the VTPH-2A is a bit more noisy than my VTPH-2 (which I doubt), or
(b) something in your VTPH-2A is not performing optimally, or
(c) noise is being introduced somewhere in the signal path between the output of the phono stage and the volume control in your preamp or integrated amp. Perhaps by the circuitry at the front end of the preamp or integrated amp, or perhaps as a result of a ground loop between the phono stage and the preamp or integrated amp.

Also, on a typical single-turn rotary volume control a 5 db change probably corresponds to something like 35 to 45 degrees of rotation, assuming the control is not being used at relatively low settings (where volume changes occur more rapidly as a function of rotation). So regarding ...

I’m wondering if a ~0.5mV cart would pretty much eliminate the issue.

... If your preamp or integrated amp has a single-turn rotary volume control just turn it down from the settings you usually use by around 35 to 45 degrees (or less if you typically use the control below say the 10 o’clock position), and that will give you an idea of how much the hiss would be reduced if you were to go to a 0.5 mv cartridge. Again, assuming the specs are accurate.

Finally, regarding ...

I disconnected the leads from the cart and I still get the same white noise type hiss.

This is not really a fair test of the noise performance of the phono stage. A fair test would require either shorting the input of the phono stage, or at least terminating the input with a low impedance. (That may be particularly relevant in the case of the Herron phono stages because of the ultra-high input impedance of their LOMC inputs, if loading plugs are not used). See the following writeup, although it involves completely different equipment:

https://www.sounddevices.com/microphone-preamp-noise/

Best regards,
-- Al