That 12AY7 cascode was what a mate had. It was best to all other tube phonos he had made, they were several well known schematics. But he sold it soon after he had made the Itch.
47nF is correct. Gives opportunity for NOS small Teflon. There is high enough impedance to couple to. Can go up to 0.1uF with somewhat less VLF roll off control.
While I’m waiting for parts to arrive, I wondered what could be done to get more gain, for those of us with LOMC? I was never a big fan of 6SN7 (but I have not heard a 1578melz), but I know I like the sound of 6N1P (when fired up) and 6N23P. Both of these have a higher gain and many give me the gain I need, plus the 6N23P would have lower noise. I know the 6N1P has a plate voltage limitation, so maybe the 6N23P would be a better choice? Would anyone like to try to redesign the circuit for it?
Make it as is and use a proper SUT. Changing valves and boosting gain without disturbing the SNR is impossible. Known tonal qualities of other valves in other gain builds lose relevance when the signal is competed by input noise in a phono preamp. The total noise is governed by the input stage in the Itch. You can use some adequate tube in the second stage for some more gain since the signal is already linear there after the passive RIAA but if you reach more than 50dB total then the 2nd stage will start contributing. And that is not enough for LMC. Listen to it as recommended first is my 2C.
Yea, I kinda wondered if the 6N23P would change the design to much, and sacrifice the SNR and sonic "color" as designed. I'll just have to wait for parts to show up and build as is. I do have a set of RCA red plate 6SN7, that I plan to use.
I'm using an Audioquest Onyx LOMC, and currently running it through a set of vintage Shure SUT 1:10 (which I like a lot). Arm is a Sumiko Premier MMT on a modified VPI HW 19 MKIII w/outboard motor. So far I have been happy with a Siren Song, SB cascode 12AY7, and a Salas NJFet RIAA w/SSHV reg (thanks Salas for the great designs). Both the Siren Song and NJFet are really quiet.
I'm using an Audioquest Onyx LOMC, and currently running it through a set of vintage Shure SUT 1:10 (which I like a lot). Arm is a Sumiko Premier MMT on a modified VPI HW 19 MKIII w/outboard motor. So far I have been happy with a Siren Song, SB cascode 12AY7, and a Salas NJFet RIAA w/SSHV reg (thanks Salas for the great designs). Both the Siren Song and NJFet are really quiet.