Leaving class A how to make this amp leave class A to AB when volume is increased? - 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 20th December 2012, 07:56 PM   #1
diyAudio Member
 
Join Date: Aug 2012
Default Leaving class A how to make this amp leave class A to AB when volume is increased?

Gain-X2 - LME49811 15-150Watt Gainclone amplifier

I just had a thought to add a switch to make the amp bias in either class A or class AB but I believe there is a way where the amp will leave class A once it reaches certain volume level to class AB and then falls back if its using low power.

Please tell me how to achieve this automatically without a switch...
  Reply With Quote
Old 20th December 2012, 08:12 PM   #2
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

A very poor article regarding standard amplifier biasing.

All class aB amplifiers operate in class A at small powers and switch
to class B at higher powers automatically, you just set the bias.
Bias is set to optimise distortion at high powers.

Class AB amplifiers have more bias and operate in class A to higher
powers than class aB (they still switch to class B at high power).
Due to so called "gm doubling" they have more distortion at high
powers than optimised aB but less distortion below the class switch.

Even with low voltage rails and "class A" biasing the above amplifier
will enter class B automatically with any very low impedance loads.

Bias levels are severely affected by the heatsinking available,
as the higher the bias the higher the standing heat dissipation.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 20th December 2012 at 08:37 PM.
  Reply With Quote
Old 15th June 2013, 07:02 AM   #3
mjona is offline mjona  New Zealand
diyAudio Member
 
Join Date: Jul 2007
Location: Upper Hutt
Quote:
Originally Posted by rhythmsandy View Post
Gain-X2 - LME49811 15-150Watt Gainclone amplifier

I just had a thought to add a switch to make the amp bias in either class A or class AB but I believe there is a way where the amp will leave class A once it reaches certain volume level to class AB and then falls back if its using low power.

Please tell me how to achieve this automatically without a switch...
If you are using MOSFET output stages crossover distortion is not a problem if you run them with enough standing current. 200 m.a. would be enough.

The Aleph series of amplifiers by Nelson Pass use a MOSFET constant current source in the output stage and a circuit that increases the current that can be drawn from it and thence into a loudspeaker load.

You can try looking for information about these single-ended Class A amplifiers in Pass' forum. If you are prepared to go this far there will be clone versions you could consider.

g.m. doubling is associated with bipolar transistor stages.

Michael J
  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
Class-AB meets Class-D: Yamaha's EEEngine Topology - where are this Diy Projects? tiefbassuebertr Solid State 29 14th April 2014 04:12 PM
Contest: Linear Power Amp in a mint tin (class Aa, class AB, or class B) danielwritesbac Chip Amps 206 19th January 2013 08:52 PM
does class A amp feel more powerful with same ratings compared to class AB? rhythmsandy Solid State 7 10th December 2012 05:21 PM
Configuring an AB class amp to work in pure A class jrme Tubes / Valves 5 23rd February 2010 06:31 PM
Can a Class AB PP amp be said to be operating in Class A at low signal levels? ray_moth Tubes / Valves 19 23rd January 2009 07:52 PM


New To Site? Need Help?

All times are GMT. The time now is 03:55 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