No global negative feedback

Antonio,

You did mention the old veil effect. Out of interest, did you use an SPL meter to level match the same piece of music before or after? Or did you try to set the same levels manually?

 

Thorsten,

Thanks for explaining what happened with my little tweak, I am not familiar with scientific data about amplifiers, but considering the specs of open loop, my speakers and the fact that I crossover at 80hz, I did an "educated" gamble with the chances on my side...

I think care was taken to mechanically damp the driver: "The MB10 has little holes in the membran and a cup of our violin Art oil/laquer"

Unfortunately thats what happens in most forums...

 

You dont want me to do a blind test, do you...

I listen every day to the same sources, I "know" how they sound, it took time to do the operation, and warm the amplifier, it takes 1,5 minutes to start, (talk about careful designer), of course I had to lower the volume to have similar level, but what improved was realism, transparency...

 

Rod Elliot is wrong, on this at least. Although when you read the article on his site he appears to be right as well.

The Peter Baxandall article in the December 1978 Wireless World covers it, and is probably the source of much of the misinformation, caused by people not reading to the end and seeing what they wanted to see....

Paul

 

Thanks Paul.

So the question comes down to whether there is evidence that the very small amount of residual high order HD is actually above the threshold of hearing at typical listening levels? Anything available on that other than the Geddes experiments?

 

Some comments from a friend in Portugal that builds his own tube amplifiers:

 

There's the rub. There are those who disagree and I haven't yet seen proof to the contrary.

A bit of a sweeping statement, given that the reason for removal of the NFB loop seems to be it's alledged propagation of higher order harmonics. The thinking is that some forms of distortion are a big deal - the question is what level of these is audible.

It's certainly not as simple as feedback being evil. Properly implemented feedback seems to be highly beneficial - and most amplifier designers seem to agree on that. I've yet to see conclusive proof that there are any audible downsides to properly implemented NFB.

 

Of course they agree, saves them a lot of money, just add NFB and any crap amplifier measures fantastic, good to fool objectivists, not different from many cables designers...

 

You see that's where we will disagree. You now seem to think all feedback is the work of Satan whereas had you simply increased the output impedence you might well have got the same results for all we know. I can think of too many wonderful sounding amplifiers that use NFB to take the cultists views seriously - and I still wait for someone to provide me with any proof whatsoever that properly implemented NFB is anything but beneficial.

 

Unfortunatelly no one invented the wire with gain yet...

Every thing introduces some distortion, NFB is just one of the evil choices, sure not the universal panacea...

Same for output transformers, crossovers, capacitors, etc...

 

The (abridged) Baxandall conclusions...

Even fets* used without feedback generate high order harmonics - and therefore on programme high order intermodulation products.

A small amount of negative feedback (eg. 6dB) in a single-ended stage, though reducing the second harmonic distortion, and also the total (unweighted) distortion, by about 6dB, will increase the higher order distortion and the quality of reproduction may well become worse as judged subjectively.

If enough negative feedback is applied all significant harmonics ( and corresponding intermodulation products) can be reduced to a far lower level than without feedback, though the amount of feedback required to achieve this becomes larger the higher the order of the harmonic considered.

The magnitude of harmonics of extremely high order will be increased by the application of negative feedback, no matter what practical amount of feedback is employed, but this is of no consequence if, when thus increased, they are, say, 120dB below the fundamental.

(paraphrasing) As signal levels drop higher order harmonics drop faster than lower order.

Paul

*I think it's safe to assume that tubes are at least as bad as fets in the raw linearity stakes. The analysis in the article is based on 'square law' transfer characteristics which tend to generate 2nd harmonic.

 

Not really Antonio because NFB actually reduces distortion - unlike all of the other components you mention.

And it's still not proven that it introduces any audible distortion at all, so it's quite possibly a panacea of kinds, if implemented properly.

 

Like Anthony de Mello would say:

Wake up !

 

Ah OK so it's a spiritual approach to music replay!

Sorry I don't do holistic audio Antonio - preferring at least a little real evidence to back up my convictions on internet forums these days. I would actually say that I have "woken up" funnily enough.

I for one am not saying (and haven't said) that your butchering your amp was wrong per se. I do however look for the logic behind any perceived improvements.

 

I also haven't said that feedback is useless, or that butchering my amp was right...

You have a preconceived idea, and wont change your mind, even with very solid arguments from respected people, not including myself, of course, I used to think like you, but am open to experiment, and let my ears be the judge...

 

Which is what exactly Antonio?

 

I take it you are trying to be argumentive for the sake of it?

Come on, answer the question. What is my preconceived idea?

 

Argumentive me...

 

What a great shame you didn't have something to say.

By the way, if you'd been really clever you could have built in a variable feedback loop to the Cayin. That would have enabled you to have got the best possible results and matched the amp to the speakers perfectly.

 

I am not clever, at least I admit it... :crazy:

But even I can understand that NFB has side effects, the worst blind is the one that doesnt want to see...