What are the pros and the cons of those 3 vas ? - Page 8 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 6th December 2010, 04:59 AM   #71
AKSA is offline AKSA  Australia
diyAudio Member
Join Date: Sep 2001
Location: Melbourne, Australia

It might be better to put the limiter on the collector rather than the emitter side, still measuring current, but set to a ridiculous current well short of the max collector current but much higher than the VAS operating current.

If you place it on the drive (base) side, there will be a transition zone of activation which could affect the sound quality.

Aspen Amplifiers P/L (Australia)
  Reply With Quote
Old 6th December 2010, 05:48 AM   #72
diyAudio Member
Join Date: Apr 2007
Hello Hugh

I was not sure about this VAS, usually I don't use a current limiter in a VAS, so I did asked for the pros and cons of it.

Btw, I've just look in all my amps schematics and this VAS was coming from a Leach amp, the image I've post was the VAS alone (no amp schema) from a web site.




Last edited by gaetan8888; 6th December 2010 at 06:09 AM.
  Reply With Quote
Old 6th December 2010, 01:34 PM   #73
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
The Leach and similar amps that have current and/or IV limiting on the output stage can overload the VAS when the limiting activates.

The extra transistor is there to prevent the VAS being driven to destruction by the LTP trying to force the VAS to drive the current limited output stage when it is already overloaded.

The sum of the two resistor is the required degeneration that the VAS needs.

The BE resistor should be sized to drop ~2/3 Vbe (~400mV) when the VAS is passing ~ two times it's bias current.

If VAS Ib=10mA then VAS can go from 0mA to 20mA and not be overloaded. When running at upto this range of currents the Protection transistor must not interfere with the VAS feed from the LTP. If the lower resistor is 20r then 20mA of maximum VAS peak output current produces a Vbe of 400mV. The protection transistor will be fully active when the VAS current rises to ~30mA (~3times Iq).
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote


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
what are the pros and cons of valve amps? pkgum Tubes / Valves 24 26th January 2014 06:34 AM
Cons / Pros Borat Tubes / Valves 34 1st September 2009 01:52 PM
2 way + active sub in same cabinet pros and cons? FE3T Multi-Way 6 12th September 2008 11:34 PM
Pros and Cons of this technique? G Tubes / Valves 35 23rd October 2003 06:37 PM
tweeter offset: pros and cons gary f Multi-Way 6 15th August 2003 08:57 PM

New To Site? Need Help?

All times are GMT. The time now is 08:37 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2