Comparison of VAS using two transistors - 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 24th October 2010, 04:18 PM   #1
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Default Comparison of VAS using two transistors

We could use two transistors for the VAS stage.
One is an emitter follower stage after the input stage feeding an ( ac) grounded emitter VAS transistor. The second would be a darlington VAS stage.
There is a difference in the distortion spectrum between the two . Is there any audible sonic difference between the two ? Any one prefered over the other ?
Attached Images
File Type: gif VAS.GIF (5.1 KB, 379 views)
__________________
AM
  Reply With Quote
Old 24th October 2010, 04:38 PM   #2
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
Darlington introduces additional "Miller multiplied" Zcb distortion that is avoided in the 1st circuit with no countervailing advantage - no one would use Darlington for VAS

buffering the VAS input also enables larger distortion reduction from diff pair current mirror instead of the collector R shown
  Reply With Quote
Old 25th October 2010, 02:20 AM   #3
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
Thanks JCX . You've clarified again what the problems are .
However I asked about the audible differences because when I was trying out some schemes some time ago I 'expected' the buffered VAS to sound better but it was the reverse and I couldn't understand it. It just sounded cleaner. Strange ! Anyway never mind and thanks for taking time to reply.
__________________
AM
  Reply With Quote
Old 25th October 2010, 03:48 AM   #4
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
Default Difference when clipping

The first circuit, when it clips, can supply nearly unlimited base current to the last transistor. That saturates the heck out of it...and it takes a long time to recover.

The second circuit, with the darlington, the collector voltage sags, limiting the base current, and the saturation of the darlington pair...as a result, it clips too, but comes out of clipping a bit sooner.

In the limit, the first circuit, when clipping to the negative rail, the input transistor can kill the transistor it drives...

Typically, the first circuit may need some kind of limiter of the drive current.
  Reply With Quote
Old 28th October 2010, 05:19 PM   #5
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
You are right that the first circuit can kill the following transistor when driven hard. Long ago I found out that the hard way. I have used that circuit earlier with a limiting resistor in the collector . It's been a long time since I did that !
Cheers.
__________________
AM
  Reply With Quote
Old 29th October 2010, 01:51 AM   #6
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
After reading this thread - I happened to notice on several commercial amplifier circuits (One was Sansui, don't recollect which one the other was) that they used the first style (collector on the first emitter follower to ground) with a moderately large (10K in one case) resistor in the collector circuit. My guess was originally to limit collector voltage, but realized it may be a form of current limiting for the transistors. (The circuit I saw had a 2.2 k resistor going from emitter to the supply rail and a 10k resistor with nothing attached to it going collector to ground. Second circuit used a 6.8k emitter resistor and a 10k resistor.
__________________
Steve
  Reply With Quote
Old 29th October 2010, 06:43 AM   #7
diyAudio Member
 
ashok's Avatar
 
Join Date: Jun 2002
Location: 3RS
2.2K on the emitter sounds OK but 6.8K looks rather large ( very low Iq !). But then on ehas to look at the full circuit to determine proper operating currents. 10K on the collector will limit the current but I had used 1 K to minimise distortion in the emitter follower stage cause by a varying Vce.

Cheers.
__________________
AM
  Reply With Quote
Old 29th October 2010, 02:29 PM   #8
sregor is offline sregor  United States
diyAudio Member
 
Join Date: Jan 2006
Location: massachusetts
The amp with the 6.8k emitter resistor (Sansui BA-3000) also had an emitter resistor (maybe 120 0r 220 ohm - don't have schematic in front of me) on the VAS transistor and fairly high rail (around 70 volts).
__________________
Steve
  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
About VAS and drivers transistors gaetan8888 Solid State 80 23rd September 2010 02:38 AM
VAS transistors ChocoHolic Solid State 44 9th July 2010 08:00 PM
Double transistors in the VAS Why? GEirin Solid State 7 25th February 2010 09:08 PM
Comparison between Bipolar transistors and MOSFET as output devices raveenvijendren Solid State 4 17th November 2005 04:28 PM


New To Site? Need Help?

All times are GMT. The time now is 02:10 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