Discrete Opamp Open Design - Page 104 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 19th September 2012, 08:48 PM   #1031
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
gpapag, please don't 'tickle me' because I am not allowed to respond to anything exotic.
  Reply With Quote
Old 19th September 2012, 09:14 PM   #1032
gpapag is offline gpapag  Greece
diyAudio Member
 
gpapag's Avatar
 
Join Date: Nov 2002
Location: Athens-Greece
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.

George
__________________
["Second Law is a bitch." - SY] ["The Road To Heaven:Specify the performance & accept the design. The Road To Hell:Specify the design & accept the performance"-Bruno Putzeys]

Last edited by gpapag; 19th September 2012 at 09:17 PM.
  Reply With Quote
Old 19th September 2012, 09:31 PM   #1033
diyAudio Member
 
Join Date: Dec 2003
Location: Munich
...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, 223 views)
File Type: jpg 1V_100R.JPG (32.0 KB, 216 views)
File Type: jpg 1V_33R.JPG (32.4 KB, 199 views)
File Type: jpg 1V_100R_100k.JPG (32.9 KB, 196 views)
  Reply With Quote
Old 19th September 2012, 09:44 PM   #1034
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
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.
  Reply With Quote
Old 19th September 2012, 09:46 PM   #1035
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
100kOhm driving impedance will not happen in real life and 33 Ohms is taxing.
  Reply With Quote
Old 19th September 2012, 09:48 PM   #1036
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
R3, R4 could be constant current sources. Now it gets complicated.
  Reply With Quote
Old 19th September 2012, 09:49 PM   #1037
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
Q3, Q4 can be folded cascodes without criss crossing.
  Reply With Quote
Old 19th September 2012, 09:58 PM   #1038
diyAudio Member
 
Join Date: Dec 2003
Location: Munich
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.
  Reply With Quote
Old 19th September 2012, 09:59 PM   #1039
bcarso is offline bcarso  United States
diyAudio Member
 
Join Date: May 2005
Location: Canoga Park, California
Quote:
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.
  Reply With Quote
Old 19th September 2012, 10:18 PM   #1040
diyAudio Member
 
Joachim Gerhard's Avatar
 
Join Date: Mar 2009
That is the reason i propose Q3, Q4 to be folded cascodes.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Discrete OPAMP audio-gd Vendor's Bazaar 27 20th September 2012 04:02 PM
discrete opamp help blackpowderaudio Parts 0 16th December 2009 03:46 PM
THAT transistor headphone amp (250ma discrete opamp) design sanity check. Russ White Headphone Systems 19 13th December 2007 12:52 PM


New To Site? Need Help?

All times are GMT. The time now is 01:42 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2