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High-End Tube preamp with ECC88

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Yipes.


I should perhaps react here in lower case print; there seems to be quite a variation in what members want ... and when that from each is elevated to "should be" status, .....

Everybody has right to his kind of sound, and when it comes to 'warmth', I doubt whether there will be much agreement - OK, to be shure audible 2nd harmonic, and as I understood decades ago, some 3rd harmonic to give string instruments "that edge" - that seems to have been established.

My modest contribution is that I have thought for the past 50 years odd that "high fidelity" means "as close as possible to the original" or some such. Otherwise one is adding another musical signature, and as said, the right of everybody . . . but 'High-end'; 'Hi-Fi'? What exactly is it then that is high?

But regarding my concept of hi-fi, if that was part of the mix here at all, I did not see a circuit of a simple two-triode amplifier with enough negative feedback to give the desired gain, low distortion, output impedance, h.t. independance etc. to at least equal if not surpass several of the tube-rich designs presented here. Perhaps tubes are not as expensive elsewhere as in RSA ......

I had also hoped that the lame idea that "feedback-is-bad" has gone its way by now, plus a few others. And - I also did not read about the requirement of headroom for pre-amplifiers, although admittedly in tube circuits that is not so serious. Perhaps I am behind the times here 'down south', my aim being something like Zout <200 ohms, D<0,05% (with NO interfering high-order harmonics), S/N about -90dB, achievable with a double triode or triode-pentode.

But I see that will not be the horse-for-the-course of some.
 
High Fidelity, Hifi has been determined as you wrote, and that happened a long time ago.
At 60's first european standard (DIN...) was created to specify the minimum performance of Hifi-equipment. All essential feature was specified.

When looking the circuits presented here today It is obvious that very many of these designs do not fulfil even this first Hifi standard. Most of such are different SE-amplifiers and preamplifiers with no feedback. Actually I think that no real Hifi amplifier can be built without any sort of NFB. Now I mean with Hifi such equipment that has good test results done by measurements.

I have also noticed that the first thing that a starting tube hobbyist "knows" is that he does not want to use NFB because it ruins the sound.
 
High Fidelity, Hifi has been determined as you wrote, and that happened a long time ago.
At 60's first european standard (DIN...) was created to specify the minimum performance of Hifi-equipment. All essential feature was specified.

That was DIN45500 !
I remember it well because my dad is proud that his little early 70's Telefunken intergrated adheres to that standard!

And I agree about the rest you said. Also, hi-end is not an officially defined term, just a marketing department term to indicate "very expensive"
 
Global Negative feedback is the bogy man here. Listening to an amplifier with and without gNFB and it is obvious that the feedback constrains the sound and robs it of its life. Local feedback can be acceptable if done correctly.
The very reason why most people are here building valve amps is because of their intrinsic linearity the amount of feedback needed is vastly reduced. This is the very essence of why valves win over transistors. Apply lots of gNFB and a valve amps sound moves towards that of a transistor amp.

The FVP amps by Allen Wright are extremely low in distortion without resorting to Global Feedback - and they are still relatively simply, only requiring four triodes.

Shoog
 
Shoog,

You are thus saying that nfb is bad?

There is no reason why local nfb should "sound" or measure any different from global nfb, quantity being the same and stability factors excluded. The basic laws of electronics preclude this.

I respect your right to your opinion, but there seems to be disagreement about "obvious"! I would however strongly agree with "done correctly", in whatever way! Some examples of nfb are high-order harmonic generators; that is not the fault of the nfb per se.

Not to go OT by re-opening that argument here; I believe it has been widely discussed on this site in the past. Only to say that such a stance/experience is shrouded in controversy because of its subjective nature.
 
Hi,
What do you say to this schematics (tube/valve preamp with ECC88):scratch2:, that give a comment.
thank you!
 

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artosalo said:
I have also noticed that the first thing that a starting tube hobbyist "knows" is that he does not want to use NFB because it ruins the sound.
Yes. I blame postmodern culture and poor science teaching in schools. People enter adult life thinking that truth is just a matter of opinion. Then they discover that NFB and noNFB sound different (as they should), and prefer noNFB because that is what 'free-thinkers' (i.e. non-thinkers, in some cases!) seem to do. Then they get upset when people like me suggest that perhaps they like the sound of low-order distortion and restricted bandwidth. Of course, any amp where you can just 'dial-in' your preferred level of feedback could be poorly designed anyway and almost certainly has the wrong amount of feedback.

Sorry for OT, but Shoog started it!
 
Yes. I blame postmodern culture and poor science teaching in schools. People enter adult life thinking that truth is just a matter of opinion. Then they discover that NFB and noNFB sound different (as they should), and prefer noNFB because that is what 'free-thinkers' (i.e. non-thinkers, in some cases!) seem to do. Then they get upset when people like me suggest that perhaps they like the sound of low-order distortion and restricted bandwidth. Of course, any amp where you can just 'dial-in' your preferred level of feedback could be poorly designed anyway and almost certainly has the wrong amount of feedback.

Sorry for OT, but Shoog started it!

The point been made is that Allen Wright manages to produces an extremely low distortion wide bandwidth amp without resorting to Global Negative feedback. It sounds excellent and in my experience better than a gNFB design.

Take it how you like - but thats reality - global negative feedback is not essential and has a generally negative outcome for many people who have made comparisons. Do not assume that I, or anyone else who comes to the same conclusion, is preferring the sound of increased distortion.

Tough if some disagree, but thats your right. You can blame it on a modern culture if you like, but then you would be wrong.

Shoog
 
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I have not felt compelled to use gNFB in most of my designs for over a decade, I haven't found it necessary and I am able to achieve quite good linearity, bandwidth and noise performance based on stated design goals.

I do use gNFB in my power supply designs, and in addition in large PP amplifiers using beam tubes.
 
It is possible to do a good tube amplifier without GNFB.
However, with GNFB it will be further improved.

By the way, what are the "extremely low distortion and wide bandwidth" of Allen Wright amplifiers ?

Spin over to his site and see what the measurements are - however let it be said that his designs are based upon HP oscilloscopes front ends which were expected to operate from DC to Mghz ranges with vanishingly small distortion.

Its also the primary reason why people go to all the trouble of using DHT in order to take advantage of their inate linearity.

My experience is that with each decibel of gNFB you can hear the life and presence drain out of the music. I choose valves because I can do without gNFB with valves.

You may be able to achieve 0.01% distortion measurements by slapping on the gNFB - but frankly it's not worth the cost.

Shoog
 
Note, FVP5A has about 9dB of NFB in the linestage due to the unbypassed cathode-resistor. As I understand it the CF is heavily feedbacked to get lower Zout. The only reason this underbiased E88CC has a chance to give us low distortion is due to the local feedback. Seems like a compromise just to get lower gain, as the E88CC has far to much gain for a lineamp.

Talking about lack of GNFB does not have so much relevance here, as there is only one gain stage. Allthough GNFB over more stages makes it all worse.
 
As I have said - local feedback done well works well.
Ultimately it doesn't matter to me - but don't disparage a reason choice of approach by suggesting its an illogical fashion statement - thats a personal insult.

Shoog
 
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just add a buffer ahead of amps that uses GNFB, and you get rid of the "stigma" of global negative feedback......there, easy and quick fix and it works......

recently i made a 6C33 set it is a non global negative feedback amp.....before that i made a pp 6V6 amps, this one uses gnfb....guess what, they both sound musical to my ears.....

making a sweeping statement that GNFB ruins the music is meaningless as well as useless to everyone else except the one making the claim....
 
Its not really meaningless to say gNFB kills the sound with every decibel added. Low gNFB will sound better than high gNFB. Many amps will sound terrible without gNFB, which is not necessarily a complement of gNFB.

Far better to design an amp which is innately linear and use's judicious amounts of local feedback mechanisms to improve on that. Only as a last resort would I resort to gNFB, and at that point I would be considering chucking the design out and starting again. As was pointed out, many of the large pentode amps will not work without gNFB - and that is why I don't build with them generally.

My point overall is that a high gNFB amp approaches the sound of a transistor amp, which to my ears is not a good thing. Overall it shows that you are listening to gNFB rather than a valve amp.

Ultimately the proof of the pudding is how many really outstanding valve preamplifier's employ gNFB ??

Its not off topic to be discussing this fundamental design decision when deciding on an approach to building a High-End preamp.

Shoog
 
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