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Distortion spectrum vs feedback

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Bear in mind that -6dB is far too little feedback. You either need none, or enough. With none, you just get the valve non-linearity (mainly low order). With enough (-20dB?) you get significant distortion reduction. At -6dB you get lots of high order products arising from the feedback, but little suppression of them. Worst of all worlds.

I saw this comment at an other tread. This is not the only I have seen claiming that low amount of (global ?) NFB will increase the level of higher order harmonics.

One documented article can be found here: http://www.passdiy.com/pdf/distortion_feedback.pdf

There is results (at page 10) about an experiment done with class A SE Mosfet amplifier showing the increase of higher order harmonics with low amount of NFB.

I made my own test with a 15 W UL-connected tube amplifier to see if I can see this phenomenon.

I made the test with constant -3 dB (7,5 W) output level. Global NFB was adjusted from 0 to 22 dB and the level of harmonics up to 7th was read on each NFB level.

The result is that my experiment did not show such behaviour. Instead the level of all harmonics decreased when the NFB was increased.

The results are here:

An externally hosted image should be here but it was not working when we last tested it.



http://kuva.termiitti.com/image/17762.gif

I also have the spectrum plots at each NFB if somebody want to see.

Why this difference ?
 
Using just a 1kHZ sinewave as a test signal may not show a few things. At supersonic frequencies where the feedback is likely to roll off, the distortion spectrum may get ugly. Now that so many program sources are digital, imperfectly turned back into analog, and may contain a small amount of this supersonic or even Rf energy, it could be important to look at. Also there's intermodulation distortion that gets generated by any negative feedback "corrector" circuit (usually the front end). But the person who found that 6dB of neg feedback is worse than 0 or 20dB may have done a sloppy job of setting up the measurement process. Not everybody realizes all the variables at play.

Personally I'd rather have as much as 0.2% harmonic distortion than to have high I.M., any chance of slewing related distortions exasperated by high feedback, any chance of overload related problems or phase margin issues... My own tests showed higher order distortion product went up when neg feedback was increased. With no feedback only the 2nd and 3rd harmonics were above the noise floor of a $30,000 HP spectrum analyzer. I'm sure this varies with topology and a few other things.

With tubes, the only place I ever use neg feedback is in the output stage when it's a push-pull topology, and I use as small a loop as is practical. Part to minimize crossover distortion, and part because the output stage is driving the substantially reactive load of the speaker. It may be better to have a lower output impedance for driving such a reactive load, or not, depending on all the variables.
 
Hello

There is this graphic about nfb vs thd.

Bye

Gaetan
 

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  • thd vs gnfb.gif
    thd vs gnfb.gif
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wow, a very interesting measurement. It reminds me that I read somewhere else another person had looked at a solid state Class AB output ad also found that any amount of gnf was good and the more of it the better. In other words, there are real life situations where measurements don't match the theory of simple topologies.

I also notice that despite Nelson's article on gnf he consistently uses gnf in designs that I've seen (e.g. Zen, F5) so again, there's more to gnf than theory.
 
I already got some questions as Private Message about the amplifier used at the tests.

Here is the schematic:

17764.gif



Kuva 17764 - Kuva.termiitti.com

And one photo, (where the opt is Edcor, not the Hammond used at the tests).

An externally hosted image should be here but it was not working when we last tested it.


The reason for low distortion values - even without GNFB - is that all stages were optimized before any tests were done. At the voltage amplifying stage (6J8) the screen voltage is adjusted to the "sweet spot", similarly the bias and the bias balance of output tubes were optimized. The optimimum total current of 6P9 was 20 mA measured at the cathode.
 
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I saw this comment at an other tread. This is not the only I have seen claiming that low amount of (global ?) NFB will increase the level of higher order harmonics.

One documented article can be found here: http://www.passdiy.com/pdf/distortion_feedback.pdf

There is results (at page 10) about an experiment done with class A SE Mosfet amplifier showing the increase of higher order harmonics with low amount of NFB.

I made my own test with a 15 W UL-connected tube amplifier to see if I can see this phenomenon.

I made the test with constant -3 dB (7,5 W) output level. Global NFB was adjusted from 0 to 22 dB and the level of harmonics up to 7th was read on each NFB level.

The result is that my experiment did not show such behaviour. Instead the level of all harmonics decreased when the NFB was increased.

Why this difference ?

Since you already have applied local feedback before you are starting your measurements, the comparision fails.
 
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Don't misunderstand; I am not saying your work is pointless. I say that the reference in post #1 uses no feedback at all as a starting point, then applies more and more feedback to see the effect on the distortion spectrum. You are starting with local feedback and are then applying global feedback. so my point is: a comparison is not relevant.

If you used no local feedback at all, and then applied more and more global feedback, I would expect you would find a result more like what could be expected (?).

But, as other quite right say, the distortion spectrum is different for valves, FETs and BJTs....
 
You are starting with local feedback and are then applying global feedback. so my point is: a comparison is not relevant.

If you used no local feedback at all, and then applied more and more global feedback, I would expect you would find a result more like what could be expected (?).

Completely irrelevant point !

The local feedback due to un-bypassed 100 ohms resistor at the voltage amplifying stage is 0,7 dB and due to un-bypassed cathode resistors at the output stage is 0,4 dB. Totally 1,1 dB.


At the Linsley-Hood graph the increase of harmonics continued up to - say - 10 dB NFB and after that begun to decrease.

Do you seriously think that this 1,1 dB can turn my result upside down ?
 
He may mean the feedback to the output tube screens.

Nonetheless, your point remains correct- if the transfer function of a box of gain is not pure square law (the starting assumption for the Baxandall calculations), adding a feedback loop will not necessarily increase the magnitude of higher order harmonics.
 
He may mean the feedback to the output tube screens.

Perhaps knunt refers to UL configuration

Well, Knutn did not say what he meant yet.

Instead, do you SY and Yvesm see that the UL-connection of output tubes makes such local NFB that my experiment gave false result compared with Lisley-Hood graph ?

If so, then the information of Linsley-Hood documents can be applied to Pentodes only since triode connection or real triodes have even more "internal" feedback than UL-connection does.

This would not seem very meaningful or logical.
 
Instead, do you SY and Yvesm see that the UL-connection of output tubes makes such local NFB that my experiment gave false result compared with Lisley-Hood graph ?

No, your result is not false, as I said before, it's accurate. The Baxandall calculations (WW, Dec 1978, figure 7) so beloved of feedback-phobes, only apply to pure square law devices. Tubes are not, so even if you didn't have local feedback, you still wouldn't get the results predicted by Baxandall. And that's OK, you're not trying to disprove his calculations, you are demonstrating that for real world amps, small amounts of feedback are useful and don't always increase their higher order harmonics (at least out to as far as you checked). That's a useful demonstration!
 
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