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In Reply to: RE: It is not a tricky issue at all posted by 6AS7_6SN7 on October 13, 2010 at 01:14:20
Luca, The 3rd order harmonic distortion is clearly reduced by the presence of the 100-ohm resistors (what I use in my amps), but is it not true also that higher order distortion products are increased, WITH the resistors, based on what I am seeing in your data? Thanks.
Follow Ups:
That's feedback.
Moderate feedback (like in this case) reduces low order harmonics (about 12 dB), but creates higher order ones.
In real world you can not have, at the same time, a completely filled in cask and a drunk wife (this is my best translation of an Italian proverb).
I suppose this is the reason why excessive feedback has such a bad reputation and why it is generally accepted that a good basic design (with inherent low distortion) is needed before ever thinking of applying NFB.
I realized this fact by extensively playing with simulations and this is the main driver for adopting frequency selective feedback instead.
If feeedback is vanishing away (starting from a few KHz), the creation of potential high order harmonics, that would be otherwise created by flat frequency feedback, is automatically prevented.
In conclusion, I think that, based on a careful case by case assessment, frequency selective feedback can be effective in improving audio performance.
Best Regards
Luca
ecc230
In conclusion, I think that, based on a careful case by case assessment, frequency selective feedback can be effective in improving audio performance.
Luca, I hope you see that in this quote we are in complete agreement, and that in fact that has been the point of my initial link, as well as the succeeding arguments.
I read about the GE study in a book, but so far I have not seen that anyone has published a report of it online, FWIW.
To be clear though, GE was saying that the 3rd, as well as the 2nd and 4th, are not regarded by the ear as irritating; it is the 5th, 7th and 9th where the problem lies. If you spend any time on the SET Asylum, you will find that that is a commonly-held viewpoint that is not unique to me. As your simulations show, these troublesome distortions seem to arise as result of the use of feedback.
It is for this reason that the cathode resistors as mentioned in the differential amplifier are not desirable- sure, they reduce lower orders (although the even orders will be canceled, so we are really only talking about the 3rd harmonic), but at the price of increasing the higher orders.
Agree with You Ralph about GE harmonic distortion study and their conclusion.Same is for SET example You give,everybody know why in SET Amps GNFB is more Problematic than PP Amps:Basicly beacose there is no automatic distortion canceletation like PP amp doo,SET Amps generate allmost all harmonic distortion products,so GNFB for SET Amps is `must` and value of inserted GNFB is allways compromise beetwen real `Ear` Audio performance and acceptable measurment(Osciloscope) performance,and this is allways valid for SET higher Audio spectrum reproduction.
Any way beside that from above,the Bigger problem for most SET Amps is actually OPT(price/sound limitation/ degradation isue),think the acceptable and real good `Ear` sounding SET Amp is relative low output power(~10W)A/A2 class SE OTL Amp with moderate GNFB.
Best Regards
__
Enlightened Evolution-Astral Projection
Edits: 10/21/10
Best performance corresponds to 6-13 dB feedback.
http://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=190082
This is my last project and I have recently added a top panel switch to select 9dB or 14 dB NFB.
Even though the amplifier is very natural with zero feedback too when using 14 dB setting the absolute performance is (believe me or not) undistinguishable from my OTL!
In my opinion high plate voltage, almost zero PS impedance and high bias current (i.e. true class A operation) allow tetrode amplifiers to match OTL triode performance.
Best Regards
Luca
ecc230
And all inmates are now in the position to fully developing their own (informed) opinion.
As a final remark, I think that we all also agree that the ear is the final judge.
If somebody prefers zero feedback this is good for him.
If somebody else prefers some moderate feedback this is also good for him.
And those who still like the old Leak (30dB NFB) or McIntosh (about 20 dB NFB) tube amplifiers, again, it is good for them too.
Best Regards
Luca
ecc230
So, should I get those resistors out of there, in my amps? Wish I could find that Glass Audio article which gave me the idea to use them in the first place. Without the FB resistors, my amps would have a very high input sensitivity (meaning a very low signal voltage would drive them to full output). I am using type 6900 triodes in the dual-differential cascode. These are super-5687s with tremendous Gm and low Rp.
Hi LucaHere is some old(1966) Philips AG9007 60W OTL Circlotron HI- FI Tube amplifier design.
Found interesting in this design that Philips designers used 2x2 symetrical Negative Feedback.
First NFB loop is symetrical from output stage to driver stage only,and second GNFB loop is symetrical from output stage to input stage.
Best regards
Check this link:
__
Enlightened Evolution-Astral Projection
Edits: 10/14/10
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