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jan.didden 20th June 2011 04:39 PM

It's a Baxandall graph, not Linsley-Hood as SY noted.
The graph is a result of feeding back a signal through a square-law device. Any harmonics in the feedback get again distorted by the square law, go through the feedback again, get distorted etc etc. So initially, when feeding back, you get a whole slew of additional harmonics before the feedback gets so powerfull that they all get supressed.
In your case, if you don't have the square law, if your amp is already reasonably linear, the feedback products don't get distorted again so much because your amp is reasonably linear.
The original Baxandall graph was for a single non-degenerated FET device which, as we know, has an almost perfectly square-law transfer curve.

BTW a pure square-law would be like Vout = A*Vin^2. After that, it's just mathematics ;)

jan didden

knutn 20th June 2011 05:36 PM


Originally Posted by artosalo (
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.

You are right (but remember: you have a phase splitter with a lot of local feedback). However it was not my intension to make this an academic discussion. I also find your experiment interesting.

knutn 20th June 2011 05:38 PM


Originally Posted by SY (
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!


artosalo 20th June 2011 05:42 PM


You are right (but remember: you have a phase splitter with a lot of local feedback).
I can not see how the high local feedback at the phase splitter could have effect to whole amplifier.

My explanation to the results I got is that tubes (I used and how those are biased) form a very linear amplifier chain and therefor differs very much from square-law components like fets and transistors.

Wavebourn 20th June 2011 05:56 PM

I always prefer nested feedbacks.

costis_n 20th June 2011 06:41 PM

Basically, I am impressed that you chose to use all metal tubes, which are considered "lesser". Well done!

john curl 20th June 2011 07:08 PM

I am concerned with the coupling caps. Ceramic, by any chance?

jan.didden 20th June 2011 07:10 PM


Originally Posted by artosalo (
I can not see how the high local feedback at the phase splitter could have effect to whole amplifier.[snip]

The phase splitter is one of the stages that together determine the transfer curve of the amp. If you make one very linear, the whole amp gets a bit more linear.

jan didden

Bob Richards 20th June 2011 07:59 PM

Global Negative feedback looks great on paper, with a 1kHZ test signal. In the real world there are many variables; topology, device choice, local or degenerative feedbacks, slewrates and miller effects of each device, etc. The increased higher order harmonics that I saw with increased GFB may be due to the limitations of the feedback loop in trying to deal with supersonic frequency energies generated by the non perfect front end differential amplifier (or equiv.) (I.M. products included), or the rolloff caused by the output devices, thereby undoing the corrective efforts at those higher frequencies. It may be a combination of several things. You never want to ask a feedback loop to correct something it can't correct, for whatever reason, or it may well create some new distortions; spurious oscillation, I.M. products, slewing, overdrive/clipping of internal stages, or excessive smoke and funny popping sounds. Oh, was that your $5000 speaker? Sorry.

jan.didden 20th June 2011 08:10 PM

Well I do agree with you but you make it sound as if it is some kind of black magic. It isn't. It is all fully understood for many decades and competent designers know how to design a feedback amp to stay clear of the problems that could pop up if you don't follow the well-known rules.

jan didden

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