AB output stage biasing question

[Lucky-Luka]

Hi all
I've tried the output stage of an amplifier with +-14V output swing and 10ohm resistive load.
For project reasons I have to use those two transistor as output transistor.
I thought that the current source value just needed to be greater than the maximum base current at emitter current=1.4A (peak current value).
The beta gain of those transistor are 66 and 73 (from LTSpice parameters) so I did Ie_peak/66=Ib_peak=21mA.
I left 4mA for the Vbe multiplier and I thought I was ok.
With a 25 mA current source I get a lot of distortion.
Rising current source value to 75mA improves the distortion a lot!
I don't understand how I can choose that value. Just trying to increasing it and looking at the result?
Is there a way to improve this circuit working the biasing?

 

[KeepItSimpleStupid]

Can you post a readable version too, like PDF so others can help?

 

[Lucky-Luka]

Can you post a readable version too, like PDF so others can help?

Sure.

 

[KeepItSimpleStupid]

I'm just going to make a comment or "food for thought" so to speak.

Q3 is turned on too much. You do need some adjustability and you want
about 1.2V between E and C of Q3 and Q3 should be thermally coupled to the output 3055's.

The simple version I have used have been two diodes and two resistors.

Search for Vbe multiplier

You should be able to short E and C of Q3 and get crossover distortion.

 

[Ylli]

LTspice shows Bf = 73 in the .model for the 2N3055, but that is not at 1.4 amps. Drops considerably at that current. 30 is more likely.

And Lucky-Luka, can you keep everything in one thread so I don't end up answering something twice.