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-   -   Distortion spectrum vs feedback (https://www.diyaudio.com/forums/tubes-valves/191161-distortion-spectrum-vs-feedback.html)

artosalo 20th June 2011 08:00 AM

Here is the graph which is a bit more illustrative than just a table.

https://kuva.termiitti.com/image/17765.gif

knutn 20th June 2011 08:51 AM

Quote:

Originally Posted by artosalo (https://www.diyaudio.com/forums/tubes-valves/191161-distortion-spectrum-vs-feedback-post2611825.html#post2611825)
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.

artosalo 20th June 2011 12:15 PM

Quote:

Since you already have applied local feedback before you are starting your measurements, the comparision fails.
Meaning what ???

The GNFB was ofcourse disconnected completely when the first "0 dB" measurement was done.

knutn 20th June 2011 12:49 PM

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....

artosalo 20th June 2011 02:01 PM

Quote:

You are starting with local feedback and are then applying global feedback. so my point is: a comparison is not relevant.
Quote:

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 ?

SY 20th June 2011 02:07 PM

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.

Yvesm 20th June 2011 02:37 PM

Quote:

Originally Posted by artosalo (https://www.diyaudio.com/forums/tubes-valves/191161-distortion-spectrum-vs-feedback-post2612580.html#post2612580)
Meaning what ???

The GNFB was ofcourse disconnected completely when the first "0 dB" measurement was done.

Perhaps knunt refers to UL configuration :confused:
<edit> Ooops, Stuart already pointed that </edit>

artosalo 20th June 2011 03:10 PM

I see that the main information and value of my experiment - at least to me - is that there is no reason to avoid small GNFB levels (3...6 dB) at tube amplifiers.
This will not generate or increase high order harmonic levels as has been believed as a fact.

artosalo 20th June 2011 03:56 PM

Quote:

He may mean the feedback to the output tube screens.
Quote:

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.

SY 20th June 2011 04:24 PM

Quote:

Originally Posted by artosalo (https://www.diyaudio.com/forums/tubes-valves/191161-distortion-spectrum-vs-feedback-post2612806.html#post2612806)
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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