High-End Tube preamp with ECC88

[Shoog]

Originally Posted by artosalo

I can not find this phenomenom, but I do not say that somebody else can not hear the difference.
What you think is the reason to this ?

I understood that you think local NFB does not spoil the sound.
What you think makes the difference ?

I have not really considered the underlying reasons for why more gNFB sounds worse, I am simply reporting my personal experience.

One of the oft quoted reasons is that as low order harmonics go down, high order harmonics go up, and gNFB causes this. The human ear is exquisitely fine tuned to these high order harmonics. Phase shift issues seem to play a large part also.
Fundamentally though it is important to remember that THD measures are fairly inadequate at characterizing what is really going on in any given amp as there are multiple other influences on the sound.

This is an interesting article which expresses what seem like reasonable opinions. It highlights the advent of the Williamson design as the cause of the popularity of gNFB which absolutely needed copious gNFB to work;

http://www.dolphin-hsl.com/articles/distortion.html

Shoog

[artosalo]

Originally Posted by Shoog

...One of the oft quoted reasons is that as low order harmonics go down, high order harmonics go up, and gNFB causes this...

Shoog

This is not the case with tubes. GNFB will reduce all harmonic components almost linerly. I have once studied this, but unfortunately I could not find my files at the moment.

I think that those people who can hear the difference with local and global NFB should make more deeper analysis about the possible reasons.
We, who do not hear it are not right persons to study this.

[Shoog]

Ultimately DIY audio is art as much as it is science. We do it for the fun and stress relief.
Pulling together the equipment and skills to analyze what has been done already, and still left undone, by trained acoustic engineers is not what some are here for.

Shoog

[Merlinb]

Originally Posted by Shoog

I have not really considered the underlying reasons for why more gNFB sounds worse, I am simply reporting my personal experience.

One of the oft quoted reasons is that as low order harmonics go down, high order harmonics go up, and gNFB causes this.

This is true for both local or global NFB- it's not particular to global.

Mathematically there is no difference between the two- precisely the same trasfer function can be obtain with local, global, or any combination of feedback (loops within loops...).
The only significant difference between local/global NFB in practice, is that a global loop necessarily encloses a more complicated circuit, with more poles/zeros and possibly a more ugly open-loop transfer function. In theory you could still make this behave in precisely the same way as a series of local NFB stages, but actually doing it is tedious / difficult.

In practice then, global NFB is more likely to result in a 'peculiar' transfer function than local NFB. Not because there's something inherently bad about global NFB, but because its so bloody difficult to compensate for all the stuff inside the loop.

[DF96]

Originally Posted by Shoog

Overall it shows that you are listening to gNFB rather than a valve amp.

That is precisely what NFB is supposed to do! Remove the effect of any active devices, so all you hear is the feedback network distortion which will always be much smaller. Even a poor resistor will be more linear than any valve or transistor.

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

Much smaller signals so much less distortion so much less need for any linearising technique. You are comparing apples with oranges. Also, remember that some people deliberately choose to use preamps with significant levels of distortion because they prefer the 'sound'.

[Shoog]

Originally Posted by Merlinb

This is true for both local or global NFB- it's not particular to global.

Mathematically there is no difference between the two- precisely the same trasfer function can be obtain with local, global, or any combination of feedback (loops within loops...).
The only significant difference between local/global NFB in practice, is that a global loop necessarily encloses a more complicated circuit, with more poles/zeros and possibly a more ugly open-loop transfer function. In theory you could still make this behave in precisely the same way as a series of local NFB stages, but actually doing it is tedious / difficult.

In practice then, global NFB is more likely to result in a 'peculiar' transfer function than local NFB. Not because there's something inherently bad about global NFB, but because its so bloody difficult to compensate for all the stuff inside the loop.

I would agree entirely with this.
Even compensating for a 4 element (valves, caps & transformers) supply chain is difficult and failure is a dead/poor/wrong sounding amp.

Shoog

[Shoog]

Originally Posted by DF96

That is precisely what NFB is supposed to do! Remove the effect of any active devices, so all you hear is the feedback network distortion which will always be much smaller. Even a poor resistor will be more linear than any valve or transistor.


Much smaller signals so much less distortion so much less need for any linearising technique. You are comparing apples with oranges. Also, remember that some people deliberately choose to use preamps with significant levels of distortion because they prefer the 'sound'.

There is far more going on here than a simple substitution of one set of distortions for a smaller set.

Why will people still consistently prefer the zero gNFB preamp to the one using gNFB. The feedback has changed the quality of the sound in some significant way which is not revealed by looking at the THD figures.

This gets down to the who why do caps sound different when they measure the same issue. Measurements are inadequate to assess the quality nof a complete amp, and using gNFB to produce good measurements will not guarantee the amp is pleasant to live with.



Shoog

[SY]

Originally Posted by Shoog

Why will people still consistently prefer the zero gNFB preamp to the one using gNFB.

Depending on the circumstances, the allure of an effects box (for high distortion preamps) or pure fashion and preconception (for low distortion preamps).

[Shoog]

Originally Posted by SY

Depending on the circumstances, the allure of an effects box (for high distortion preamps) or pure fashion and preconception (for low distortion preamps).

I would agree with the first point - but entirely disagree with the second.

Unfortunately since you would maintain there is no difference between a plain CF and a SLCF, we really can't take this discussion further without ending up disparaging each others opinions.

Shoog

[Shoog]

Nelson Pass ( a designer i respect greatly) has some relevant thoughts on the issue of harmonic distortion and feedback.
He concludes that THD measurements are no way adequate to analyze the complex nature of a real world signal passing through a distorting device (amplifier), and that Global feedback in particularly has a way of introducing a hugely complex compound distortion which bares absolutely no relationship to its THD measurement;

https://passlabs.com/articles/audio-...n-and-feedback

Needless to say, Nelson Pass strives to use the minimum amount of Global Negative Feedback possible. Put that down to fashion if you like, but I think there are thousands who would disagree with you.

Shoog