What do you mean, 'enduring' ..?
It's a joy to read such a coherent and lucid, even sceptical diary of 'project work in progress.'
I look forward to more; and thankyou!
LOL, I have a couple of those supplies that I use for the same reason as you.
They're actually not bad - designed & built during the time of "thorough and robust" analog work, complete with nicely spec'd parts for what was available in those days. Back then, you got what you paid for ...
Youre right about speaker systems being better tools for monitoring imho. Whenever i try to AB something with headphknes the psycho acoustics set in quicker and i behin to doubt my objectivity. Not so much so if i use the same circuit as a preamp to my ss amp. I think this happens due to thw simple fact that cables are easier load than transducers and that whatever difference needed to sucessfully AB test gets amplified 20 30 times by the amplifier. Plus there can be other factors such as stereo imaging easier to distinguish on speakers if the song was mastered on speakers (which is most of the time) ect.Someone replied to me personally just now about the sound of components. To echo what I said a while back, my attempt to characterize the sound quality differences between the DCG3, A1, A2, and A30 Pro left me with more questions than answers. I'm pretty confident of my impressions of the DCG3 versus the rest. But the seemingly large differences I thought I heard among the others have diminished to point where I'm no longer sure about them. I don't know if it's listener fatigue, or maybe it's true that all properly-operating high-feedback amplifiers sound basically the same and I was just imagining things at first.
I still have the sense that the EIZZ attenuator has taken away some of the "magic" from the A2. It's not so easy to change that out, but I do plan to try something else in the not-too-distant future.
I also have the consistent feeling that the A30 Pro has stronger bass than the A2. My friend in SLC commented that the A1 is a little light in the bass. These amps measure flat down into the infrasonic range, so why the bass would sound different remains a mystery. I remember, decades ago, comparing a power amp I built to a commercial Threshold unit. I was "blown away" by how much more authority the Threshold had in the bottom end. It's not surprising given the relative numbers of output devices and the sizes of the power supplies. But you wouldn't know based on simplistic small-signal response measurements.
In my relatively limited subjective testing experience, I've been surprised how "huge" differences can surprise you when you thought what you were listening to before was already as good as it can get. It's very satisfying when an improvement is so obvious that you don't even need to try to hear it. When you have to strain to identify the difference, it's just a nuisance.
I have the feeling that it's easier to compare equipment using loudspeakers than with headphones, because the spatial qualities are more evident. But I don't have a good enough listening room or speaker-based system for that. I was going to buy some high-end cans (though more money doesn't always get you better performance) but that plan got sidelines by unexpected household expenses at the end of the year.
I will take this one step at a time. I'm not going to worry about whether Vishay resistors are a good choice before I've even had a chance to listen to the amplifier.