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...

The "lack" of electrical damping as a result can be replaced by mechanical damping in either enclosure or driver. Such a solution is at any extent preferable as mechanical problems ideally should be solved mechanically, as evidenced by the worlds first high fidelity coaxial speaker, the legendary Eckmiller designed in the late 30's....
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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"

I noticed however that after Antonio presented his experience ("I removed NFB and liked the results") load of people jumped on to declare that thuis should never be done and cannot work and pretty much that Antonio should put it back and not speak about it again.

That is an approach I have noticed here on several occasions when views and experiences are presented that seem unpalatable to some.
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Unfortunately thats what happens in most forums...
 
Stereo Mic said:
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?
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You dont want me to do a blind test, do you... :confused:

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...
 
I'd love to know who's right!
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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:

Difficult loads are NOT the ones that go to low impedance, but those who go into low impedance and have a severe phase shift, like f.i. the well known B&W 802D. In these cases you can force the output devices to sustain more than 5 times the normal dissipation, and leed them to clipping, operate in non linear way. If you have some output impedance, that will reduce the efect of the phase shift...

If an amp is linear enough, it will sound better without feedback, anytime. I gave up feedback since I studied it at university, specially when using opamps (you could have a bandwith as short as 100Hz and expect it to be linear by increasing the input (via the feedback) above the 100Hz till 100kHz. All you did was saturate the amp with the feedback signal, but a steady safe sinusoide would look great...

Feedback is a must in a poorly designed amp, but an evil in a properly designed one.

Distortion can mean a lot, but just consider this, our own ears introduce a lot of harmonic distortion (above 2%), but our brain can correct it so you listen clear. Phone signal is band limited, chopped, distorted, etc.. but you recognise most of the times who is calling just by the voice.

Our brain has the ability to overcome and compensate a lot of the sound problems, so if you don't feed your ears with problems your brain cannot compensate, distortion is not big issue (although I agree that no distotion is better than some distortion, any time)
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melorib said:
Some comments from a friend in Portugal that builds his own tube amplifiers:
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Feedback is a must in a poorly designed amp, but an evil in a properly designed one.
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There's the rub. There are those who disagree and I haven't yet seen proof to the contrary.

distortion is not big issue
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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.
 
Stereo Mic said:
Properly implemented feedback seems to be highly beneficial - and most amplifier designers seem to agree on that.
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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... :confused:
 
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.
 
melorib said:
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...
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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.
 
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...
 
I take it you are trying to be argumentive for the sake of it?

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

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