John Curl's Blowtorch preamplifier part II

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ThorstenL said:
Anyway, these are very credible efforts for a beginner, as you build more you will learn more, as you learn more, you will change your designs. Maybe you will care more about HF noise and less about THD, and about interactions between base current and non-linear wiper impedance in potentiometers. Or maybe you simply will convince yourself that is all B$ and continue making stuff like this.

:D

maybe Bonsai already convinced himself after doing ABX LISTENING TEST, that all designs sound the same?
 
The only thing that comes to mind is 'parallel expectations' and how they sometimes don't converge.
Let us say that one of us decided that we wanted a small business, and decided to sell 'hotdogs' from a simple stand, somewhere where people passed by fairly frequently.
Now this person, likes food in general, but let us say that he is not obsessed with differences in food, and tends to criticize those who do so, as gullible or paid 'shills' who work for local newspapers.
This person feels that a clean and neat stand is the key to getting the public to buy his 'hotdogs' and what actually goes into these 'hotdogs' is fairly unimportant, so long as it is safe, fills the stomach, and doesn't fall apart (especially the bun) when consumed.
Therefore, this person shops for the cheapest hotdogs and buns that meet this criteria, and still LOOK GOOD, not greenish, off smelling, or burned, or something slightly objectionable.
Then this person opens up for business. Now, what about his pricing? Well, others charge a few dollars for their 'hotdogs', so he will as well. After all, it still took money to make the stand, get and maintain the cooking equipment, and make up the order.
Well, he sets off in his new business, makes sure that he has change for larger bills, gives a few samples away to curious associates, and even snags a positive evaluation in a local newspaper.
YET, for all that he has done, very few people come to buy his 'hotdogs'. WHY?
Could it be that his 'hotdogs' are much more expensive than several chain 'fast food' shops, and people continue to say that his 'hotdogs' are unexceptional compared to 'Nathans' or some other 'uptown' hotdog palace, and they prefer to go out of their way to get a 'premium hot dog' rather than his.
The secret must rely in 'marketing', bribes, or gullibility. What do the rest of you think?
 
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There is nothing wrong with the AD797, if that is what you are getting at. It is somewhat more expensive than some similar products, and it inherently has some 'theoretical' limitations, but it sounds pretty good for a op amp IC. Dick Sequerra used it in his $25K phono preamp, but he did something unusual, he ran it OPEN LOOP. (What a concept!) But he was very happy with the results.
 
I wonder if he meant open loop in the same strict sense that you and I would. Assuming that he loaded the VAS with an RIAA equalization and let the followers follow along without ant loop feedback, how heavy would he need to load the VAS for, say, 60dB midband gain?

My concern is that the VAS has a limited current sourcing ability and that the AD797's effective Gm must be pretty big. Interesting idea, fersure.
 
Verrry interesting. Quite a fine juggling act to keep all the balls in the air using an existing device that he didn't have any control over - I guess it helps to start by being Dick Sequerra!

Been working on a new phono eq for myself using a compromise version of this idea. Mine has a transconductance output driving the RIAA network, followed by a follower,
but differs by bringing the RIAA back to inverting input rather than ground. The RIAA network sets open loop gain at a more-or-less fixed amount above closed loop. Not strict enough for Mr. Sequerra, but does have better overload margins. Compromises abound.

Much thanks!,
Chris
 
You fail to show any link between your metric (which I understand to be "Distortion" - is that THD btw or something more useful?) and fitness for the purpose.

You were answering to :

By all possible measurements, GNFB amps have better caracteristics
in respect of accuracy of the amplified signal.

To say it otherwise , any parameter that define an amp caracteristics,
be it THD , IMD , PSRR, bandwith , phase response , damping factor
and any other such parameter will have values closest to perfection
with an amp using GNFB than with tube based or SS feedbackless amps.

As already pointed, neither tubes based amps nor SS FBless amps
are accurate enough to reproduce CD audio formats..
 
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