John Curl's Blowtorch preamplifier part II - Page 1338 - diyAudio
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Old 5th July 2011, 02:04 AM   #13371
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I am looking forward to the measured results, too! As far as 'feedback' is concerned, it has been a major staple of audio design for more than 60 years. No point in 'quibbling' whether a triode has a feedback mechanism, that is not the intent being shown here.
Everyone please remember, since the late '40's with the Williamson Amplifier, global negative feedback was put forth as the greatest thing since 'canned milk', and as much global negative feedback was used as practical, usually 20 dB in power amps, by Marantz, McIntosh, Fisher, Dyna, etc, etc, for example.
Only when direct coupling was made allowable by solid state, did we normally use more feedback in audio designs. Then, about 45 years ago, IC op amps came into being and extreme global negative feedback became possible, or at least, practical. However, for audio, it seems to create as many problems as it fixes.
Let's hope that Ed Simon's measurements show something new.
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Old 5th July 2011, 02:38 AM   #13372
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Quote:
Originally Posted by jcx View Post
once again - that would be more exactly stated as "without external linear feedback" - the triode has built in negative feedback

as the tube experts explained in the thread I linked the internal feedback from the anode V being visible to the grid has a similar nonlinear power law as the gm, giving a low distortion, internally feedback fixed mu
It is also worth noting that the Ultra Linear output stage effectively implements negative feedback to the screen grids. The legendary McIntosh designs also make use of negative feedback in the output stage.

Cheers,
Bob
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Old 5th July 2011, 02:47 AM   #13373
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Quote:
Originally Posted by john curl View Post
I am looking forward to the measured results, too! As far as 'feedback' is concerned, it has been a major staple of audio design for more than 60 years. No point in 'quibbling' whether a triode has a feedback mechanism, that is not the intent being shown here.
Everyone please remember, since the late '40's with the Williamson Amplifier, global negative feedback was put forth as the greatest thing since 'canned milk', and as much global negative feedback was used as practical, usually 20 dB in power amps, by Marantz, McIntosh, Fisher, Dyna, etc, etc, for example.
Only when direct coupling was made allowable by solid state, did we normally use more feedback in audio designs. Then, about 45 years ago, IC op amps came into being and extreme global negative feedback became possible, or at least, practical. However, for audio, it seems to create as many problems as it fixes.
Let's hope that Ed Simon's measurements show something new.

Negative feedback is a powerful and benevolent tool when properly used. You have shown that to be the case in many of your own designs, such as the JC-1, which has about 36 dB of global negative feedback the last time I looked. You know how to use negative feedback, and so do a great many other good audio designers. It is time to stop looking back 40 years at the miserable 741 to make the case that NFB is bad. If there ever was a straw man...

However, it is true that a certain amount of skill is needed to use negative feedback. I imagine there are some amplifiers out there that occasionally break into bursts of parasitic oscillations under some conditions. Of course, there are also idiots that will cut their finger off using a table saw.

Cheers,
Bob
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Old 5th July 2011, 03:01 AM   #13374
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Nothing new, but worth a reminder; graceful handling of clipping is important. I've known designers who never looked inside the loop to see what happened during clipping. It can get pretty dramatic inside during open loop conditions.
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Old 5th July 2011, 03:08 AM   #13375
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Hi John,

Quote:
Originally Posted by john curl View Post
I am looking forward to the measured results, too! As far as 'feedback' is concerned, it has been a major staple of audio design for more than 60 years.
There global looped negative feedback in the WE 91 Amplifier, frequency dependent as well (read, equalising).

BTW, I personally have come to see local loop feedback, local degeneration and global looped (e.g. spanning several stages) Feedback as distinct individual cases and find it unhelpful to understanding the subject matter to lump them into one "negative feedback".

All else being equal, the biggest problems with the non-ideal properties of real circuits and negative feedback arise in multi stage looped feedback.

One may argue that instead of having one big feedback loop with tons of loop gain applying local degeneration and local feedback loops (like the miller feedback loop used to stabilise global loop feedback amplifiers) is the same "negative feedback", however I do not normally find this the case.

An even more interesting case is positive feedback.

I am employing in a number of my designs, either frequency dependent for purposes of equalisation, or general for purposes of realising high gain without needing to apply bypass capacitors to cathode/source/emitter resistors that stabilise operating points.

Quote:
Originally Posted by john curl View Post
No point in 'quibbling' whether a triode has a feedback mechanism, that is not the intent being shown here.
Arguably yes. What I prefer to say is that in triodes the direct moderation of electron flow due to the variable anode voltage acts in a manner that is analogous to local loop negative feedback and may be in a first order approximation be modelled as such, however in is not in the actual case local looped feedback.

Quote:
Originally Posted by john curl View Post
Let's hope that Ed Simon's measurements show something new.
Yes, lets. The literature I have from the 50's only ever considered and tested two distorting stages, so we have to interpolate from that three stages, however, perhaps such interpolation is wrong?

Of course, one of the real problems I have in the case of Simon's test is that he uses real devices of a type that are not quite unlinear by nature, but he also uses them in a manner that maximises this AND that also maximises device to variations, plus the bias methode employed is very sensitive to device parameter variations.

As a result these non-ideal behaviours may swamp out the processes we would expect.

I would suggest to employ at the least fixed bias with a lowish value bias resistor, maybe 50K (no problem if each stages gain is padded back to zero, if it is in later shown to be problematic, the outputs of each stage may be buffered) and to adjust each tube to identical anode voltage (thus also anode current), to control the parameter variations of real devices as much as possible.

Ciao T
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Old 5th July 2011, 04:21 AM   #13376
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It would seem to me that it would complicate the design too much to add bias, unless it was from a separate DC source. This is not a REAL design, but a design example of what REALLY HAPPENS as you add stages and feedback. This is often lost, and not in the mind of most engineers.
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Old 5th July 2011, 09:43 AM   #13377
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Hi,

Something I posted to another thread, that may have a passing bearing on the debate here...

Des Technokraten Gruselkabinett


Ciao T

PS, the German heading is not really translatable, it is related to the concept of the classic Horror-Show, but not identical. Maybe the closes is George Orwells "Room 101" from 1984, so a room 101 for Technocrates and Objectivists, where all their worst fears are brought to face them...
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Old 5th July 2011, 09:43 AM   #13378
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Quote:
Originally Posted by Bob Cordell View Post
Negative feedback is a powerful and benevolent tool when properly used. You have shown that to be the case in many of your own designs, such as the JC-1, which has about 36 dB of global negative feedback the last time I looked. You know how to use negative feedback, and so do a great many other good audio designers. It is time to stop looking back 40 years at the miserable 741 to make the case that NFB is bad. If there ever was a straw man...

However, it is true that a certain amount of skill is needed to use negative feedback. I imagine there are some amplifiers out there that occasionally break into bursts of parasitic oscillations under some conditions. Of course, there are also idiots that will cut their finger off using a table saw.

Cheers,
Bob
Hi Bob,

Let's try to look forward, then.
You all remember the (in)famous 'Baxandall curves' showing increase in distortion with feedback in some area's. This was extensively discussed and explained in Bruno Putzeys' article in Linear Audio Vol 1.
Menno van der Veen, another successful (tube) designer, commented on that and surprisingly, Bruno's reply also contains an interesting conclusion about the 'law' behind those 'Baxandall curves' once you get over the 'hump'.

The letter exchange is here:

http://www.linearaudio.net/userfiles...1_LTE_MvdV.pdf

Any thoughts, anyone?

The figure Bruno uses in his conclusion is attached.

jan didden
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File Type: jpg bp fig 2 web.jpg (93.0 KB, 204 views)
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Old 5th July 2011, 09:56 AM   #13379
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John,

Quote:
Originally Posted by john curl View Post
It would seem to me that it would complicate the design too much to add bias, unless it was from a separate DC source.
That is my suggestion. As shown the circuit uses gridleak biasing.

Now as it so happens, I am great fan and advocate of using it in certain situations, however I am also very much familiar with it's practical challenges and less than ideal properties.

It is best used at very low levels, if there is a lot of signal the Bias will wander with signal AND harmonic content in the signal and throw off the operating point, exactly how will depends on the precise tube used, as different tubes react differently (in an extreme there was > 20dB THD difference between sections in a dual triode with highish levels and gridleak biasing)....

Quote:
Originally Posted by john curl View Post
This is not a REAL design, but a design example of what REALLY HAPPENS as you add stages and feedback. This is often lost, and not in the mind of most engineers.
John, I understand that (ever since I read Crowhurst, Shorter, Baxandall and Scroggie (it is an old hat, honestly).

My point is that there is little point adding several more variables that affect distortion into such a rig, if we wish to examine the results of daisy-chaining multiple distorting stages.

Ciao T
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Old 5th July 2011, 10:22 AM   #13380
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Jan,

Quote:
Originally Posted by janneman View Post
Any thoughts, anyone?
This is quite interesting...

Here another take on the same stuff...

harmonics

I am still thinking my way through things (for years now)...

First, if we had infinite feedback and infinite bandwidth for the feedback, there would be no distortion, no matter how much open loop distortion is present.

If we had zero distortion and infinite bandwidth open loop, no matter how much feedback we apply, there would be no change in distortion and feedback would optional to use and only serve to set gain.

(these two extremes may seem obvious and should need mathematical proof, nor are they useful in an engineering sense, they are to remind us of the extremes)

As I remarked elsewhere, amplifiers (especially AB) generally show increasing non-linearity as the frequency rises (Self's book does a good job covering the various underlying causes).

They also show falling amounts of available loop gain with rising frequency and thus falling amounts of negative feedback.

This mechanism also increases the circuits susceptibility to common mode effects as frequency rises and reduces the power supply rejection as frequency rises.

Further, as Renardson reminds us, we need to worry about the Intermodulation products, which routinely fall into frequencies below that of the actual signal and/or distortion/noise etc. components.

Based on this it would seem we need to "look up" at the behaviour of looped systems at rather high frequencies and perhaps (due to the case for thermal memory affecting distortion) at rather low ones.

In other words, that last thing I would be worried about or interrested in is THD1, even THD20 has limited use, now THD100 or even THD500 would seem an entierly different kettle of fish.

Of course, no-one likes measuring so high, first of all the gear to do it is in limited supply (and AP2 cannot go anywhere near that high) and secondly the measurements of all these "ppm THD" designs at so high frequencies are disappointing.

However, I would suggest that these very mechanisms may at least be contributory factors (together with "thermal memory") in the difficulties of applying negative feedback.

Ciao T
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