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Old 19th September 2012, 08:48 PM   #1031
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gpapag, please don't 'tickle me' because I am not allowed to respond to anything exotic.
"Condemnation without Examination is Prejudice"
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Old 19th September 2012, 09:14 PM   #1032
gpapag is offline gpapag  Greece
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Ha, ha.
Well I haven’t tried to drag you toward anything exotic.
On the contrary, I am trying to make you demystify your thoughts (I mean the ones you present here ).
If you do this, it will be welcomed by anyone.

["Second Law is a bitch." - SY] ["BTY my shop's large door is 14 feet wide so I can get my ego in each morning. Some evenings it can leave via the mail slot". -Ed Simon]

Last edited by gpapag; 19th September 2012 at 09:17 PM.
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Old 19th September 2012, 09:31 PM   #1033
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...went on with simulation of the symmetrized LinuxguruBuffer.
The attached schematic is showing some additional resistors to allow proper biasing.
Overall the simulated performance is among the better ones I saw in simulation so far and worth a real life trial (..my LTSpice never pleases me with -120db THD...)
But I have to state again, I do not trust the absolute values of the simulation of harmonics in the sub -80db range.

In fact the choice of Q3 and Q4 is not my favorite (heat handling and linearity of Ic/Uce characteristic), but was available in LTSpice.
In reality I would propose 2SA1930 and 2SC5170.
Q3 and Q4 are biased around 30mA.
The schematic shows degeneration resistors for Q3 and Q4.
They are not necessary for biasing, but I would guess that reality performs better in THD and TIM with them, while simulation does not show much effect (up to now only single tone simulated).
To my understanding Q3 and Q4 cannot be rated as inactive support functions, they are actively handling music signal in a similar way we are used
to it from Sziklai darlingtons. And we get the same sort of oscillations that need taming, which I did with a brute force dominant pole - not really nice, but acceptable as a starting point.

Furtheron in reality it might be better to use red LEDs for D1 and D2.

Last but not least the buffer is showing a strong increase of distortion when driven from high impedances, similar as it is known from diamond buffers.

The attached pictures show
- Schematic
- Harmonics at 20kHz/1V into 100 Ohms (R6)
- Harmonics at 20kHz/1V into 33 Ohms (R6)
- Harmonics at 20kHz/1V into 33 Ohms (R6), but 100k driving impedance (R5)
Attached Images
File Type: jpg LinuxBuffer_schem.JPG (44.0 KB, 283 views)
File Type: jpg 1V_100R.JPG (32.0 KB, 271 views)
File Type: jpg 1V_33R.JPG (32.4 KB, 250 views)
File Type: jpg 1V_100R_100k.JPG (32.9 KB, 245 views)
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Old 19th September 2012, 09:44 PM   #1034
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Sure, Q3-Q4 swing voltage. That was the problem in my experimental proposal. I had the Led idea too but it could also be a string of transdiodes. You could simulate the input impedance and do a low pass filter at the input to avoid that range.
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Old 19th September 2012, 09:46 PM   #1035
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100kOhm driving impedance will not happen in real life and 33 Ohms is taxing.
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Old 19th September 2012, 09:48 PM   #1036
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R3, R4 could be constant current sources. Now it gets complicated.
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Old 19th September 2012, 09:49 PM   #1037
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Q3, Q4 can be folded cascodes without criss crossing.
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Old 19th September 2012, 09:58 PM   #1038
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I agree, nothing really wrong with a string of 1N4148.
It is more my taste to use red LEDs, because of less component count and red LEDs are hard to beat in terms of noise (not much of relevance here, agreed..).

From my understanding the influence of the drive impedance is caused by the fact that Q1 and Q2 do operate on mostly constant Ic, but not perfectly constant. You will find the modulated base currents of Q3 and Q4 there (similar as in a diamond buffer). This then translates into modulation of the input current of the buffer - reflecting the non linearities of the current gain of all the BJTs. BJTs show these non linearities also at low frequencies and DC. I would assume that a filter may shape the frequency dependency of these distortions, but I am wondering how a filter should cure it.
Nevertheless, I a curious to your real life findings - in case you decide to go for experiments.
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Old 19th September 2012, 09:59 PM   #1039
bcarso is offline bcarso  United States
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Originally Posted by ChocoHolic View Post
...went on with simulation of the symmetrized Linuxguru Buffer.
I may start a new thread for in-depth explorations of linuxguru's approach. It deserves some elaboration and further development, I think, and I'd also like to make things public to prevent someone from patenting them.

I have a good many variants already, while loosely justifying the time spent because of the circuit's applicability to many things. Although it can be incorporated into global loops, I'm seeing such good performance of modified versions that it's tempting to let them stand alone, with possibly a d.c. servo in some offset-sensitive cases.

It's interesting how close the circuit is to merely a complementary feedback pair. But not quite.
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Old 19th September 2012, 10:18 PM   #1040
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That is the reason i propose Q3, Q4 to be folded cascodes.
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