LINEUP Input-VAS, idea - Page 6 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 14th March 2011, 08:11 PM   #51
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
My FB is AC coupled??? When did that happen?

You mean #37?

--------------

As long as NJFET pair matches and drifts together, and PJFET pair matches and drifts together,
NJFETs do not require close match to PJFETs.

Last edited by kenpeter; 14th March 2011 at 08:27 PM.
  Reply With Quote
Old 14th March 2011, 08:59 PM   #52
diyAudio Member
 
Join Date: Jun 2004
Location: Copenhagen
Hi Lineup!
Please comment:
Hi Lineup!
As far as I can see, you wont get double gain by using complement pairs. Look at it this way: One transistor will need a certain drive to pass a certain current. The other transistor will need an equal amount of drive just to pass the same current. ( which it will have to , since they are in series. ) So you will get the same gain as with only one transistor ( your example one ).
You might get some linearization of the FETs though.
Thorsten Larsen

Last edited by thorstenlarsen; 14th March 2011 at 09:07 PM.
  Reply With Quote
Old 14th March 2011, 09:18 PM   #53
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
True, drive voltage double is canceled when transconductance is halved.
I still think this plan is superior for PSRR. Differentially driven with most
power noise cancelled in the common mode. In the VAS itself, a high Z
drain or collector points at rail noise, rather than low Z source or emitter.

I don't see how the JFET is linearised? (current or voltage linearity?)
Expending gain to linearize current would be working directly against
the objective of a linear voltage amplifier. Only care transconductance,
and the direct coupled local loop makes voltage linearity.

Take away all feedback that "linearizes" the FET in the current domain,
and the voltage domain distortion of the loop goes down -10db.

Try to have it both ways at once, you get a tube amp that might sound
nice, but isn't necessarily linear in either domain.

Last edited by kenpeter; 14th March 2011 at 09:42 PM.
  Reply With Quote
Old 14th March 2011, 09:40 PM   #54
artu is offline artu  Chile
diyAudio Member
 
artu's Avatar
 
Join Date: Nov 2010
Location: Santiago, Chile
Quote:
Kenpeter :

My FB is AC coupled??? When did that happen?

You mean #37?
Yes #37, C1 (100uF) that's AC coupling, R13 connected to 0V directly is DC coupling, not the case shown in #37

Cheers
Arturo

Last edited by artu; 14th March 2011 at 09:46 PM.
  Reply With Quote
Old 14th March 2011, 09:53 PM   #55
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
#37 has plenty other problems not worth the low offset.
Not relevant here, except how the biasing trick proved graftable to Lineup's circuit.
  Reply With Quote
Old 14th March 2011, 10:15 PM   #56
artu is offline artu  Chile
diyAudio Member
 
artu's Avatar
 
Join Date: Nov 2010
Location: Santiago, Chile
Anyway, with or without the biasing trick, if direct coupled (as it is lineup's circuit) it will have much more thermal drift than AC coupled FB loop, maybe you will need a DC servo ... new problems, among others is the clipping asymmetry at low freq. <100HZ, and the stored charge at the integrator = slow response.
  Reply With Quote
Old 14th March 2011, 10:47 PM   #57
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
I am still not seeing where this thermally drifted DC offset happens?

The input compares to the output, and matched parts thermally bonded have the same drift...
Again, N's don't have to match or drift with the P's. But the input side does have to match
and drift with the feedback side. Why is this circuit causing you to worry so much more on
drift than another with the same matched and bonded component pairs at the input?

Last edited by kenpeter; 14th March 2011 at 10:50 PM.
  Reply With Quote
Old 14th March 2011, 11:33 PM   #58
artu is offline artu  Chile
diyAudio Member
 
artu's Avatar
 
Join Date: Nov 2010
Location: Santiago, Chile
Things in reality are much more complex, the theory takes account on what 'should' happen given a hypothetical math. perfect model and other unrealistic assumptions (the ideal OP amp, infinite input impedance, zero output impedance, infinite OLG) , physical real devices departs slightly but enough from theory, to make arise all this effects, that's why nowadays chip makers uses some built-in mechanism to address this issue. I have experienced at simm's and in practice (there are some topologies more or less prone to drift, other discussion). Do some temp. (30-100) simm's and you'll verify that in a DC FB loop this drift could be as high +/-100mV or more. Maybe a +/- 30mV drift is OK for a power amp, but not for a headphone amp, that's my point.

Cheers
Arturo

Last edited by artu; 14th March 2011 at 11:35 PM.
  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 Regulator for VAS and Input Stage Havenwood Power Supplies 14 16th September 2010 04:28 PM
input stage and VAS testing gain Solid State 9 25th October 2008 09:27 PM
Idea for linearizing a cascoded VAS ionomolo Solid State 9 27th June 2008 08:44 PM
Help designing Input and Vas stage's DoomPixie Solid State 28 13th January 2006 09:28 PM


New To Site? Need Help?

All times are GMT. The time now is 11:38 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