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Zero negative feedback tube preamplifier - a feasible DIY project?

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SoaDMTGguy

SoaDMTGguy

Member
Joined 2021
#1
I'm just dipping my toe into the world of diy vacuum tube equipment, so I'm not yet sure what questions I should be asking. But, intrinsically it appeals to me to build a zero feedback single-ended tube preamp or line stage. What I don't know is the complexity or challenges of such a project. I'm seeking direction more than anything. If it's not possible, or if it's much more realistic to build a low-feedback design, etc.
 
#2
I have made a preamp, such as riaa, zero feedback, and the channels were within .1 dB.
And a line stage SE with a large triode, no FB, similar.
Also a WE92 (6SJ7/300B @ 4K) schema with or without 3 dB FB . . Ditto (although listening to the latter I want to recheck the balance yesterday, today the balance is perfect again)
And I did not even have to select tubes . .

My takeaway: it is easier with tubes than sillicon, as long as you stay well inside the limits.
 
#5
triode_al said:
I have made a preamp, such as riaa, zero feedback, and the channels were within .1 dB.
And a line stage SE with a large triode, no FB, similar.
Also a WE92 (6SJ7/300B @ 4K) schema with or without 3 dB FB . . Ditto (although listening to the latter I want to recheck the balance yesterday, today the balance is perfect again)
And I did not even have to select tubes . .

My takeaway: it is easier with tubes than sillicon, as long as you stay well inside the limits.
Click to expand...
Would you post your schematic?
I'm looking for a dht phono preamp...
 
#9
Feedback makes the world work. Without feedback you couldn't live; if your heart rate increases and your blood vessels don't dilate...
Your endocrine system relies on feedback, driving a vehicle, raising a cup to your lips etc.
A bias against feedback is mindless.
 
#11
Feedback like “what is the state of thing A so thing B can react” isn’t the same as “the output of thing A is directly routed back to thing B to modify its output”.

If we can achieve the same thing with less signal manipulation, so much the better.
 
#13
peteki said:
Feedback makes the world work. Without feedback you couldn't live; if your heart rate increases and your blood vessels don't dilate...
Your endocrine system relies on feedback, driving a vehicle, raising a cup to your lips etc.
A bias against feedback is mindless.
Click to expand...
A MINUSCULE part of the Universe uses feedback, a zillion million trillion things do NOT use feedback at all.

The Sun does not adjust its output depending on how deep we tan, nor even on how our crops grow, or Earth temperature, Planets follow their orbits precisely without feedback, Zeners, water boiling point, length of the meter standard held in Paris, anything which is *reference* value does not.

If anything,"they" are the reference around which others adjust, using feedback.

Back to the OP: don´t worry, 95% of Tube circuits shown in datasheets do NOT use feedback.

Build any of them and be happy.

Example:

VFzbfuT.png
 
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#14
No gNFB in my preamps -- including cathode caps... Lots of gNFB in my power amps though. Not that they need it for linearity, they just need it for gain reduction (ok and lowering Zout etc) 🙂 Or 1V in would be 50mV in or something.
 
#15
kodabmx said:
We're talking about a preamp here, not the universe... No feedback is needed in a normal tube line amp.
Click to expand...

Amplifiers are part of the universe. Your comment about no feedback is needed ignores the distortions present in a preamp or any other audio circuit. One remembers back to designs like the Mullard two valve preamp with its passive tone controls; it sounded awful.
 
#17
Any sufficiently complex process must either be moderated by feedback, or run away towards another stable state. The sun, for example, would collapse into itself without the feedback of its internal pressure. It effectively samples its internal pressure, by heating, then feeds this information back by PV=RT, which then forces it to expand or allows it to contract in the right amounts to sustain it for billions of years.



Living things only survive by means of elaborate feedback mechanisms. In humans, failures of the feedback mechanisms are called diabetes, high blood pressure, cancer, etc.


Folks who worry about the F-word somehow don't mind using cathode degeneration, triode connected pentodes, cathode followers, "UltraLinear", long tail pairs, split load inverters, etc. etc. There's feedback everywhere. Don't be afraid to say the word.


All good fortune,
Chris
 
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