LCR phono stages we know about

Wise statement Lewm. I think the same but can't break my addiction. Oh well.

Lewm --

I certainly do not have the technical background to comment intelligently on your question...all I know is it sounds good! :-)

If you load the input stage cascode with an active current source you can effectively achieve an infinite output impedance. This allows you switch the RIAA series resistor from a series connection to a shunt connection. No issue with RIAA response changing as the tube ages. The excess current from the plate load CCS is shunted across this resistor to ground and sets the plate voltage. Neat trick. To maintain the set plate voltage the cascode pair needs to be biased with a cathode/source (tube or tube/SS hybrid cascode) CCS.

There is also the Wavac LCR-X2 phonostage which uses LCR type RIAA equalization. Apparently, Wavac's founder is considered the originator of the LCR concept as detailed in some internet pages (don't flame me if you think otherwise, I am just passing along info). I have the Wavac and also the DIYHiFi Supply's Cole LCR phonostage which was my first phonostage. Have always found them to be musical and palpable but not sure if it's because of the LCR equalization or other system design contributions.

John, The input stages of the Allen Wright RTP (shown in the schemtic posted by Hiho) and of the Atma-sphere MP1 are both dual-differential cascodes , i.e., balanced topologies. Thus in both cases, a CCS is used optimally at the junction of the cathodes of the two "bottom" tubes in the cascodes and ground. (Take a look at the RTP schematic; I think AW used an LM317, which is definitely not my choice, but it's a CCS nevertheless.) Thus there is no place for more CCSs on the plate side. However, your point is relevant to a single-ended topology. But in a single-ended topology, the output Z would be the parallel sum of the impedances of the CCS and of the cascode stage below. The impedance of the CCS is ideally infinite (but is always finite in reality, albeit very high). However, would not the impedance of the cascode below be a very finite number, thus dominant in the calculation of output Z? And also subject to change as the tube(s) in the cascode age? Please correct me if I'm wrong; you probably know more about this stuff than I.

Now I re-read your post, you seem to be talking about a dual-differential cascode with CCSs between cathode and ground AND between plate and B+. AW discusses that possibility in his TPCB. Have you ever tried that?