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-   -   Leaving class A how to make this amp leave class A to AB when volume is increased? (http://www.diyaudio.com/forums/solid-state/226128-leaving-class-how-make-amp-leave-class-ab-when-volume-increased.html)

rhythmsandy 20th December 2012 07:56 PM

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...

sreten 20th December 2012 08:12 PM

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.

mjona 15th June 2013 07:02 AM

Quote:

Originally Posted by rhythmsandy (Post 3293074)
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


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