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In a recent post of John Broskie I noted this graph (taken from an old paper of the British audio engineer Peter Baxandall), that is:
"design for minimum open loop distortion and limit GNFB to get a barely sufficient dumping factor".
ecc230
Follow Ups:
Is that eliminating the three high gain small signal tubes (12AX7s) for low output-Z medium Mu and more linear tubes, allows you to significantly lower total NFB, without any shift upwards in OPT-Z into spkrs.Loop NFB around the OPT stage also proves to be a good idea. Moves opt-Z even lower, and it still sounds nicer.
Noting of course that the OEM St20 is highly regarded on the niceness front.
Given that by the 1980s most OEM St20s would have been in need of a re-cap, I figure I am very lucky to have got them as a swap for a St60 which I was given.
'Not a LEAK any more?' WGAS! I don't.
Here's a 'systems' thought - 'It depends."
Also, please comment on this thread re strapping my two modded St20s - as mono-blocks - into QUAD 63s.
8 ohm taps would give me a 4 ohm 'mono' tap. Using the 4 0hm taps would give a '2ohm' tap.
Should I simply try both ways?
The mods we did are covered in 'Reviews'. Both amps will be in pentode mode and both now have 4 x 4 470uf 400V 105deg. caps, bypassed.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 05/14/17 05/14/17
This diagram shows distortion of a (presumably) single-ended triode amp at full power. It's important to understand that distortion falls as the power goes down, and that higher harmonics fall faster than lower ones. That adds a third dimension, not shown in the diagram but important to music listening.
Talking of listening though, there were many studies going back to the thirties attempting to understand what distortion people could hear. It was widely accepted and backed up by many such experiments that higher harmonics were more audible at even very low levels.
" The problem with using THD as a yardstick of quality is the order of the distortion term has a far more audible effect than its absolute magnitude. "
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Local NFB can deliver some quite good results. Going balanced in the NFB circuit offers even better results.
The Plate-to-Grid, either implemented via Schade's circuit, or through use of taps along the primary( E-Linear ) both deliver a lowered source impedance, and increased linearity of the amplifying element w/o a long path plagued with phase shifts.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Naz
The plot shows a log scale on the y-axis and a linear scale on the X-axis, which is kind of misleading, since the amount of HD actually goes down dramatically with incremental increases in NFB. Can we really hear 4th, 5th, and 6th order HD? I would guess not so much. And maybe 4th and 6th order are not so bad anyway. Therefore if you confine your concerns to 2nd and 3rd order HD, you could use these data to argue FOR the use of GNFB. For example, at around 18-20db of NFB, the 2nd is down 10-fold, and the 3rd is not worse than the initial (at 0 db NFB) amount of 3rd.Having said this, I am no advocate of using lots of NFB, either.
Edits: 05/13/17
Unless I'm misinterpreting this graph, it's based on an amplifier that produces 10% 2nd harmonic distortion with no GNFB at all. How is that relevant to modern amplifiers? If someone intends to make a legitimate argument against feedback, maybe they should start with a 1% amplifier. I suspect the modest increase in higher harmonic energy that occurs when 10-20dB NFB is applied will then be at levels no human can be proven to hear.
--------------------------
Buy Chinese. Bury freedom.
Edits: 05/13/17
That graph is an interesting example that illustrates what can happen in an extreme and idealised case. (Almost 10% 2nd harmonic distortion, with comparatively negligible higher harmonics, open loop.) It has been widely latched on to by a certain subset of the audio community, as a "proof" that negative feedback is evil. I've not seen, or tried for myself, calculations or measurements in more realistic situations, but I suspect that the results would be much more favourable to the benefits of negative feedback.
It has, probably inadvertently, led to an amusing illustration of the way in which the "trust your ears" brigade will usually scorn any measurement or calculation, unless it happens to support their case.
Baxandall was, of course, a great proponent of high fidelity sound reproduction, and his name is attached to a very nice active tone control design. I suspect he would not have approved of the way some of his efforts were later mis-appropriated!
Chris
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