Phono Preamp Tube Rush

Thanks. I can’t argue with your logic, and won’t.I might wonder whether one can model a phono cartridge based solely on its internal resistance, in that calculation.  But I have no better idea.

I might wonder whether one can model a phono cartridge based solely on its internal resistance, in that calculation.

@lewm, that’s a good point. But the inductance of my cartridge is spec’d as 25 uH at 1 kHz. If we assume its inductance is also around 25 uH at 20 kHz, its inductive reactance at 20 kHz would only be about 3 ohms. And it would be progressively less than that at lower frequencies, of course. So it would appear that the cartridge’s impedance is primarily resistive throughout the audible frequency range.

Best regards,
-- Al

@lewm - Sorry I wasn't more clear.  Yes, I meant thermal noise from input resistance. 

The SUT in question had 3900 ohms on the secondary (thin wire), which generated quite a bit of noise, much more than I get from my JFET-based headamp.

And yes, you can use the ESR of a cartridge to perform a baseline minimum noise floor calculation.  You then have added noise (aka noise factor in the RF world) from the amplification (which you can break down by stage).  The first stage is by far the most important.  That's why I use special low noise matched JFETs for MC front-end.

Another good test for "tube rush" is to compare noise level between open and shorted inputs.  Most of the noise you hear open is from the 47k loading resistor.  Shorted, you hear the amplifier and tube rush.  

if executed in the balanced domain you get a maximum of 6dB less noise

Actually, that's not true.  With balanced amplification you get 6dB more signal gain, but you also get 6dB more differential noise gain.  Overall SNR is the same.  

The benefit of balanced amplification is rejection of common mode noise such as power supply crud, and crap injected on both + and - inputs together (sometimes hum).  You get a better amplifier with higher overall performance, but noise floor from a differential source such as the resistance of a cartridge is the same.  

Lewm, according to the meter I have yes with peaks just slightly over 110 dB. (I had ear protection on at the time:) God knows how accurate the meter is. I think more remarkable is that this is with ESLs (using subwoofers from 125 Hz down). From 10 row center the last NIN concert I went to hit 120 dB (I also had hearing protection on) Getting the meter into the concert was a bit rough. Had to take the battery cover off and show that it was working while everyone behind was getting pissed. Anyway, the loudest I ever play it on rare occasions is 105 dB usually with concert videos. Most of my listening with Rock and Jazz is at 95 dB. Don't forget to warm up your ears first! 
I love your Steelhead, almost bought one but right now I am looking at Phono amps with balanced outputs and inputs. Coming from where Manley does you would think they would do that. 
We live in a house I have been building since 1993. 
In regard to the SLN tubes the only thing I can say is that with the system set at 95 db I can not here anything with my ear right against the speaker over the background noise of the house. With the volume maxed out I can just make out a hum, again ear against the speaker. The Tubes are now 6 years old. I have no idea if this is noisier than when they were new but even if they do get noisier with age I suspect a SLN tube will always be quieter than a LN or standard tube of the same age and make.