A note on the experimental breadboard setup in the initial post on this subject, that is a one channel circuit, not a working stereo one.
I thought I would clarify the intentions for this phono preamp design. First it is to be a fully balanced circuit that would allow the cartridge to be floated off of signal ground - or not. There will be a tube differential amplifier first stage that may or may not be augmented with some FETs below it for lower noise. Following the input stage will be a passive RIAA equalizer that may be the full network or just the 2120 Hz rolloff part. If a partial equalization network, the output stage will have active feedback to generate the 500 -50 Hz bass boost. If the equalizer is the full network, the output stage is flat in frequency response. This describes the main signal circuitry and is setup for MM gain. For MC inputs, a stepup transformer of suitable quility will be used.
There are a lot of considerations in all of this. First is the issue of input noise. I am most familiar with the work of John Curl and James Bongiorno in achieving super low noise MC input stages. Personally, I am not all that worried about a little input noise if the sound is better. I once made a tube MM/MC phono preamp using very low noise 7788 tubes. The MM 2120 Hz HF rolloff was with a series 47K resistor and a suitable capacitor at the input of the tube. Heresy! All of those 10’s of K of noise resistance ! This also provided the essentially 47K loading for the cartridge up in the 20 kHz range. It was quiet enough to not be audible at the listening position at normal playing gain. I also did the same thing for MC use by a much lower impedance RC input network. That also worked pretty dammed good with the same circuit. So bottom line on this, this preamp circuit may not end up as the lowest noise circuit on the block but it going to sound dammed good and serve the music.
Aside from considerations of shielding a power transformer from hum radiation in the same chassis and issues of power supply regulation, the main issue for me in this initial circuit experimentation is the design of the second gain or output stage of the circuit. First is the matter of whether it is a bass boost equalizer or flat amplifier. The output devices in this proposed output amplifier are P channel MOSFETs that would be operated in the common source mode with attendant high output impedance. Overall output circuit feedback will reduce this to a sufficiently low value. Now if it is the flat response version where the RIAA equalization is all done in the interstage network between the input and output amplifier, all is well. However, if it is configured as the 500-50 Hz bass boost part of the RIAA equalization, the output impedance is going to rise as frequency goes down below 500 Hz because the amount of feedback is going to decrease. This will have the effect of the very low bass frequencies will rolloff more with low impedance loading on the output of the preamp. Also there will be different amount of input signals available to the output stage in each case. Less signal for the full interstage EQ and more for the 2120 HF rolloff case.
More on the circuit for the output amplifier. It is going to have a differential tube input stage. Unlike the first stage of this phono preamp and the tube stages of the BHK preamp and power amplifiers where the tube stages stand alone outside of any including feedback loops, this first stage of the output amplifier will be included in the circuit’s overall feedback loop. Servo circuits will keep the output phase to phase and each phase to ground close to 0 VDC.
This is enough for now and I realize it is too technical for some but not others. If there are good ideas to consider in this and perhaps alternative circuits to consider, chime in to contribute!