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 4th September 2009, 03:51 AM #1 Borat   Banned   Join Date: Aug 2009 Location: Silicon Valley & NYC dumb question about class AB what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power ?
ostripper
diyAudio Member

Join Date: May 2008
Location: Smoky Mountains , Tennessee
Nothing , no difference. A class B can be a class A, just grossly overbias it.
This would be unrealistic for even a NJW21193/4 (SOA considerations), so IRFP240/9240's would be essential.
(attachment is a happily overbiased class B amp)
OS
Attached Images
 T3T_A.gif (14.3 KB, 470 views)

 4th September 2009, 12:51 PM #3 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Hi, there is a design situation where the ClassA amplifier is exactly the same as the ClassAB amplifier. Take a 25W into 8r0 ClassA amplifier as the reference. The peak output voltage for a continuous sinewave output signal is 20Vpk. The maximum output current when driving a resistive test load is 2.5A. The bias of the ClassA output stage is just >1.25A, say ~1.3A. Now look at a ClassAB amplifier with a 4pair output stage using 130W To247 devices and Re = 0r068 The optimum bias for ClassAB is Vre <26mV. Take Vre=22mV as the optimum determined by distortion/spectra analysis. Ib~324mA/device. The total bias current is 1.294A, it is biased identically to the reference amplifier. Both amplifiers are 25W into 8r0 and both deliver 25W of ClassA into 8ohm. Both will deliver substantially more ClassAB power into lower impedance loads. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 4th September 2009, 02:09 PM #4 stoc005   diyAudio Member     Join Date: Feb 2007 Location: Midwest in the USA what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power They will sound different. The sonic signature or summation of all the distortions and noise of each is different. Other than that, they are very similar.
Andre Visser
diyAudio Member

Join Date: Sep 2007
Quote:
 Originally Posted by ostripper (attachment is a happily overbiased class B amp) OS
With very bad 2nd harmonic distortion.

AndrewT
diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally Posted by AndrewT The optimum bias for ClassAB is Vre <26mV. Take Vre=22mV as the optimum determined by distortion/spectra analysis. Ib~324mA/device. The total bias current is 1.294A, it is biased identically to the reference amplifier. Both amplifiers are 25W into 8r0 and both deliver 25W of ClassA into 8ohm. Both will deliver substantially more ClassAB power into lower impedance loads.
Quote:
 Originally Posted by stoc005 They will sound different. The sonic signature or summation of all the distortions and noise of each is different.
why will they sound different?
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regards Andrew T.
Sent from my desktop computer using a keyboard

Nelson Pass
The one and only

Join Date: Mar 2001
Quote:
 Originally Posted by Borat what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power ?
It is difficult to offer a general answer, as much depends on
the devices and the conditions.

jan.didden
diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
 Originally Posted by stoc005 what is the difference between a low power class A amp and a moderate power class AB amp with a very high bias dissipating the same quiescent power [snip].
My 2 eurocents worth: Assuming that the AB bias is the same as the class-A bias, this implies that the class AB has higher supply/higher max output.
They will both work in class A up to that bias point. After that, the class A will start to clip, while the AB goes into, well, AB. As long as they stay below the class A clip point, difference depends on topology and devices.

jd
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 4th September 2009, 11:16 PM #9 Borat   Banned   Join Date: Aug 2009 Location: Silicon Valley & NYC if the two amps are mostly the same except one clips transients and another doesn't - then why not build all class A amps as heavily overbiased class AB amps ? have transition from A to AB occur lets say 10db below clipping. if used to actively drive a tweeter such an amp would spend 99% of the time in class A. ? Last edited by Borat; 4th September 2009 at 11:19 PM.
 5th September 2009, 02:10 AM #10 eyoung   diyAudio Member     Join Date: Aug 2007 Location: Sunny SC,USA 15 min south of Charlotte NC Hola Compadres, Is not the point of class a bias to get the transistor to a more linear part of the curve and to avoid crossover distortion. Taking into consideration my simple mind and how things work once you get past the working area of class a most designs go to ab as a sort of soft clipping. Just a disclaimer I am not an engineer and I defer to those that I revere. Regards, Elwood __________________ "Lead me not into temptation...I can find it myself."

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