High-End Tube preamp with ECC88

[Shoog]

I suppose this is easier than providing actual, you know, data.

Maybe you and DF96 would furnish us with a detailed analysis of why the F5 and nelson Passes other top end offerings are not as good as your high feedback designs.

I have personally compared a RLD to a F5 and there was no contest, the F5 is fantastic, the RLD sounds like a generic feedback amp. Of course thats just my personal experience. However my friends RLD went into storage and the F5 is his standard amp after that. The builder never scrimps on parts choice in whatever he builds, so I am certain the RLD was giving of its best.
After this experience I started treating your opinion with a large pinch of salt.

Shoog

 

[DF96]

You, like everyone else, may safely ignore my opinions if you don't like them. You ignore truth at your own cost. I don't trust my own ears, so I certainly don't trust yours.

 

[SY]

Indeed, not everyone wants an amp that's neutral.

 

[popilin]

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;

The Numbers Game

Shoog

Looks like I got a little late.

BTW. The higher the harmonics, the lower times involved, so NFB is less effective on higher order harmonics because of the impossibility of the travel to the past.

NFB Math don't include relativistic corrections.

Not to mention that conductors aren't perfect, and we work with valves, capacitors...

I escape to NFB like the plague.

 

[Shoog]

You, like everyone else, may safely ignore my opinions if you don't like them. You ignore truth at your own cost. I don't trust my own ears, so I certainly don't trust yours.

Its a dangerous place to be when you are prepared to stand against everyone in expressing a theoretical position over their personal experience. No one here is advocating shoddy high distortion designs and they go to great lengths to achieve this without resorting to gNFB.

I personally use input transformers, DC coupling, Low Mu triode, balanced designs operating in pure class A with plate to grid feedback. That's a lot of careful design decisions which have gone into my position. Have you built such an amp to compare to one of yours ? Do you have enough experience to make an informed comment on what can be achieved in zero global feedback designs ?

I suspect you are a well trained engineer who "knows what works", allow me to differ with that position.

Shoog

 

[Wavebourn]

We're talking about preamps here. Overload in power amps driven to clipping is a different story, thus the rather elaborate measures I used in my power amp to minimize recovery time, or your soft clipping circuits.

Williamson was just an example. I don't hesitate to squeeze the maximum from 2-stage designs when I permit myself to. But with 3-stage design (if I decide that it is needed) it is very interesting game to fond the optimum. Sometimes it takes an year before I give up.
But I always tend to use nested feedbacks with more than 2 stages. Nested feedback is more than a sum of feedbacks. It is like a duck's triangle migrating, compared to a single duck. And please don't tell me the theory that synapses in duck's heads in triangles are wired all together!

Precisely. As with any other aspect of circuit design, if you don't understand it, you're likely to get a bad result. Some attribute it to the design element rather than their own inability to understand and use the tool properly.

It depends, "properly". Properly for the end result you wish, or properly for the accepted standards of measurements?

 

[DF96]

I am not standing against "everyone", just those who claim that gNFB is a bad thing. As Wavebourn says, the real issue is understanding what it can do and how to use it properly.

Most music is, and almost always has, been recorded using systems which use huge amounts of feedback around op-amps. Fortunately, most of this kit was designed by people who knew what they were doing. Note that any feedback around an op-amp is global, not local, because the op-amp has several stages. If this adds something nasty to the sound then nothing you can do at home will remove it. Therefore it is likely that it does not add nasties.

 

[Wavebourn]

Indeed, not everyone wants an amp that's neutral.

I feed the people on the last BAF by a neutral PA. It seems to me they liked it. I mean, they liked that they heard people well, instead of hearing the PA.
Let's try the next year to make more neutral PA and see what people say.

 

[SY]

Properly for the end result you wish, or properly for the accepted standards of measurements?

These are not mutually exclusive unless you have an unusually crabbed definition of "accepted standards of measurements."

 

[Wavebourn]

These are not mutually exclusive unless you have an unusually crabbed definition of "accepted standards of measurements."

No, my personal tastes are far from them.

 

[Merlinb]

Its a dangerous place to be when you are prepared to stand against everyone in expressing a theoretical position over their personal experience.

It is OK to state your personal experiences of listening to equipment, but it is dangerous to attribute your various subjective experiences to highly technical issues like 'feedback in general', especially if you do not understand those technical issues at a high enough level.

The subject of feedback is so broad and intricate, that it is exceedingly difficult to make a fair test that can truly identify feedback as being the cause of the 'niceness' (or 'badness'!) of a listening experience.

You might think it would be a simple matter of building an amp, and then adding some variable feedback, and listening to it while varying the feedback. But is that a fair test? By adding feedback you're altering the transfer curve and moving the poles around, so really you're comparing two very different amps. Apples with oranges. Keep adding feedback and you may border on instability. Is that the feedback's 'fault'? No, it is the designer's fault.

A fairer test might be to build two amplifiers, one with feedback, one without, but arrange both circuits to have the same transfer curve and the same poles/zeros.
This is not an easy test to carry out, and until someone does, it is very bad science simply to brand gNFB as 'bad for music' and bury your head in the sand thereafter.

 

[Bigun]

I don't trust my own ears

If you are designing and building for other people then fair enough, but if it's DIY and you don't trust your own ears - what's the point of it ?

 

[DF96]

The point is listening to music, with my amplifier doing as little as reasonably possible apart from amplifying the signal. If I make a change and it sounds better/worse, then I need to convince myself from science that it really is better/worse and not the reverse.

Lets say I make a change and it sounds better. Then I find by some combination of measurement or calculation that it is actually worse (e.g. higher distortion, less flat frequency response). I therefore conclude that my ears are deceiving me. I do not conclude that my change has introduced some unmeasured/unmeasurable advantage.

Sometimes my ears and calculations tell me the same thing, such as when I found I had miscalculated the deemphasis capacitor for my FM tuner. I tried different caps, then afterwards found that the one which sounded 'right' happened to calculate right too. I don't assume this happy coincidence will always happen. The only reason I did this 'tuning by ear' was that I knew I could calculate the right value and I was curious to see how much audible change the wrong cap did.

 

[Hierfi]

There are thousands who disagree with me about alien abductions and the existence of fairies. So what? Data are everything. No data, you're just waving your hands.

I first became aware of an alien presence when my parents revealed themselves to me. As it was explained, only fairies passing themselves off as aliens were engaged in the alien abductions being alleged and misreported.

I have always treated dielectric materials as critical. To be brief, dielectric absorptions are occurring when electrons move in and out of materials in depth. This is a function of voltage and released in delayed time. Under conditions of a sine wave being imposed this can create delayed sine wave fundamental reflections being released.

This suggests that the nature of dielectric effects can be created by imposed a sinusoidal signal by a distortion analyzer. What I am trying to suggest is that the fundamental frequency being treated as pure and clean in a distortion analyzer cannot be taken as true. The fundamental frequency can also contains artifacts of sinusoidal dielectric effects buried within the fundamental. Being phase shifted doesn't change the fundamental frequency.

If one is going to emphasize the worth of a device by the distortion measurements the question I would have is to what degree the fundamental is a true reference to distortions, those that could be greater and hidden within the fundamental itself. It isn't clear to me the quantitative differences ,nor the degree, of harmonics vs. dielectrics.

 

[Shoog]

If all this was just a personal experience then I wouldn't make any fuss over it - but its a very general observation that I am commenting on, shared with some of the best and most respected designers on the planet. I think I am justified in drawing some broad general conclusions about the effects of the application of gNFB, and not just take our "experts" word for it.

Shoog

 

[DF96]

shared with some of the best and most respected designers on the planet.

Is this a fact or an opinion? I have sometimes seen the 'most respected' people talk nonsense, and sometimes talk wisdom. In either case, the test is not "who said it?" but "is it true?".

 

[Shoog]

Lynn Olsen, Allen Wright, Gary Pimm and Nelson Pass. I would take any one of their opinions over yours DF96.

Shoog

 

[DF96]

Yes, you have already made that clear. It is up to you whose opinions you accept.

 

[Merlinb]

Lynn Olsen, Allen Wright, Gary Pimm and Nelson Pass. I would take any one of their opinions over yours DF96.

Shoog

 

[Johan Potgieter]

I hesitate to comment on the 'many people agreeing with me' kind of argument. Yes, it probably is true - but has a poll ever been taken? OK, before folks me for becoming ridiculous/hair-splitting .... but isn't the truth that as many disagree? ... meaning that one probably tends to remember the favourable better than the opposite. All in all, I would certainly not venture to take such statements for unequivocally establishing what is true and what not. Leaving that for the moment - because truthfully Shoog said that it has the danger of steering toward escalation.

Rather: I am not any great audio engineer (certainly not in terms of being well-known), but I would challenge certain statements on grounds of what is revealed (our friendly internet) by probably acceptable tests done under as controlled conditions as one could wish for, including statisticaaly valid conclusions. One is that there are audible things out there that science still cannot measure. Sorry; it would appear that present technology is up to detection comfortably deeper than audible discernment. Again, with full respect for disagree-ers: Science is not democratic. (I sometimes wished it was, but having spent my professional life in research, that convenience was never an option.)

Then, I would agree that simply applying nfb in varying amounts per se and using the audible results as gospel, cannot float as such. (By the way, nobody said thd is an acceptable measure of anything; well, I certainly do not.) I have analyses on a modest amplifier that I designed showing that up to 30dB of global nfb reduces higher order harmonics up the no. 11 by almost equal quantities. But, on the way there I also found that there are ways of using nfb that started 'backfiring' after some 13 dB - as witnessed to previously. (It depended on the topology; not having maximum linearity to begin with, very easily gave that result.)

I must concede that these tests were for a power amplifier; I do not have the equipment to measure in the 0,00x% required for pre-amps. Still, I do believe that the tendency holds.

Then, I still read about how the result sounded. What did the original sound like? Proper amplifier measurements regard the addition of anything by the amplifier, not the end result per se. (In that respect such yardsticks are often simpler than obtaining a final "niceness" experience.)

Finally, and certainly without any intention to offend: When last did you have your ears tested? (Not necessarily asked of honourable members posting here ). Still, that question is relevant, at least for some people proclaiming their experiences of niceness! As said before, any test that does not exclude taste can be questioned.