LM3875 / LM3886 quasi-AB ?

[ash_dac]

Hi,

Are these chips class B or slightly biased into class A ? (In the datasheet LM3875 is referred to as quasi-AB amplifier http://www.himeaudio.ol.com.br/LM3875.pdf , p15)

If they are biased slightly into class A how many watts are class A ?

Can you bias the output more into class a or will this cause the chip to explode ?

I've read a review of a 'gainclone' type amplifier in a recent hifi magazine, and they mentioned that under tests there was crossover distortion present.

[darkfenriz]

'Quasi' probably refers to output stage topology (two npn power trasistors).
They say that total questient power supply current is typically 30mA, assume optimistically 20mA idle current through output stage, makes 40mA of class A region, so around 7mW of class A work at 8 ohm load.
Don't take seriously the class B from AB division.
Most 'class AB' amps are really class B, I mean several mW in classA while dozens of W in class B. It just would be commertial suicide to call something 'B' when 'AB' is also correct, because definitions are impractical (how low current is no current?).
Also nearly none says how much power there is in A-region. The numbers just look bad.

If you want more class A region than add current source from output to -rail. 100-200mA seem reasonable to me, but best to experiment. Watch the temperature! It can get hot without good heatsinking!

regards

[Macaba]

Perhaps a noobish question to ask, but my curiosity is boundless:

Why do we need to make our -ve rails as beefy as our +ve rails, IF the output is mostly Class B?

[ash_dac]

Originally posted by darkfenriz
'Quasi' probably refers to output stage topology (two npn power trasistors).
They say that total questient power supply current is typically 30mA, assume optimistically 20mA idle current through output stage, makes 40mA of class A region, so around 7mW of class A work at 8 ohm load.
Don't take seriously the class B from AB division.
Most 'class AB' amps are really class B, I mean several mW in classA while dozens of W in class B. It just would be commertial suicide to call something 'B' when 'AB' is also correct, because definitions are impractical (how low current is no current?).
Also nearly none says how much power there is in A-region. The numbers just look bad.

If you want more class A region than add current source from output to -rail. 100-200mA seem reasonable to me, but best to experiment. Watch the temperature! It can get hot without good heatsinking!

regards

Thanks for the information !

There is a kit based upon the tda1521 IC that I could experiment with pre LM3875 /LM3886! (http://www.semiconductors.philips.co...21_Q_CNV_2.pdf)