Okay, more response.
Look, I'm sorry but this is your opinion. It is not a fact and it also depends on the system you are dealing with as to whether this is a good or bad thing. A low output impedance that can deliver the required current levels with low distortion is generally a good thing. However, that low impedance must be maintained all the way to the amplifier (or preamp, recorder or whatever else ...). Once you raise that impedance with anything, the quality is in very real danger of dissipating. A "passive preamp" (volume control in a box) is one of the best ways I know to degrade the sound quality. The only place for any type of volume control, be it a continuous control like a potentiometer or stepped resistors, is either in the control amplifier, or in the amplifier proper. Depending on the type of input circuit(s), you may in fact need to use a buffer circuit of some type. Nakamichi called this buffer "HTA", Marantz did this in 1968 when they designed the model 500 amplifier. I'm sure there are plenty other examples of this.
I'm sorry, I don't understand what you mean to say here.
I apologize, but once again I'm not sure what you are saying.
Actually, I disagree. By allowing the signal output to be a high impedance, you become open to higher noise levels and other problems. One of the ways that good equipment reduces noise is to have a low output impedance - especially in the preamplifier to amplifier link. Those signals can be at levels approaching what comes out from your MM phono cartridge. It deserves more care than what I normally see in the "high end" brands. Curiously, so called "mid-fi" products tend to get that mostly right.
Well, at least you thought about it. I disagree on purely technical grounds, and I also disagree from an experience point of view.
I wish him luck with this, but you must always acknowledge the rules of physics and electronics. Any time you have a signal "leaving the box" and have to drive a cable, you need to maintain a low impedance, as reasonably low as you can get without adversely affecting other parameters. You always have to look at the entire system when deciding on possible circuit changes.
Well, tubes (valves) are generally more noisy than solid state parts are. If he really wants to install a tube in the signal path, it should be installed before
the volume control so that at low volumes the output noise from the tube doesn't dominate. One example of doing this wrong can be found in the Counterpoint SA-1000 preamplifier. The tube is run "wide open" and is after the volume control. Guess what? That preamplifier has problems with signal to noise ratio (think - hissssss and microphonics).
I am sorry, but your meaning is unclear. But I will say again that the use of any device in the signal path that degrades the overall performance is a bad idea. This is the basic problem as I see it.
I'm afraid that you appear to be using the wrong ingredients for your salad there. For what you are doing, you do need cabbage.
I've commented on this idea earlier, but does my 30 + years in audio electronics count for anything? You are surrounded by members who are full engineers, and others that have a good feel for audio design work. I don't know Flavio, or his work, but from what I see here, I have strong reservations about what his beliefs are where audio is concerned. Sorry. But thats okay, he very obviously disagrees with me just as strongly.
I missed much of your intent here, but I suppose one solution you are referring to is the use of op amps (from National?). Understand that there are many ways to build a buffer or amplifier. Some involve individual parts.
That isn't surprising to me at all. Lead dress can take a fair amount of time to get right. Having those large capacitors hanging around on the input and feedback areas is not helping you. Keep in mind that wire can receive and retransmit noise from one area to another. Input signals should always be as short and direct as is reasonably possible without throwing it all away by using over-sized parts.
Well, it's really too bad there is a language problem between us. Some of your ideas don't look too bad as theory, but that board on the voltage amp PCB really needs to go.
I wanted to draw something else to your attention. It concerns your volume control. The input pair of transistors do pass a small current out of their bases to your common ground. Any change in impedance will interact with this base current and create a a signal that might sound like a very small "pop" as you change the volume setting. You may or may not hear this unless you have efficient speakers and a quiet room.