Electronics > Beginners
AB-class amplifier schematic analysis & optimization
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Yansi:
I am not sure if two diodes are enough for any bias to flow through the output stage, are they?  Such current will be mostly ill-defined.
David Hess:

--- Quote from: Yansi on May 22, 2019, 11:48:33 am ---I am not sure if two diodes are enough for any bias to flow through the output stage, are they?  Such current will be mostly ill-defined.
--- End quote ---

They are, especially if the right diodes are selected, but you are right, the current will be ill-defined.  A 60 millivolt difference yields a 10 fold change in current.  An alternative for better matching is to use an extra set of output transistors as the diodes and if you do that, you might as well configure the output circuit as the full diamond buffer.

Diode connected transistors:

https://www.edn.com/design/analog/4363991/High-speed-buffer-comprises-discrete-transistors

Diamond buffer:

https://www.tubecad.com/2012/09/blog0244.htm
dietert1:
The tubecad log is interesting but who wants to make a completely discrete circuit nowadays? I think the proposal to bypass the two 10 Ohm output resistors with a capacitor is valid, but it's application isn't really practical with SMD only.
I think the Rod Elliott circuit shown in the beginning is a little more rational. Depending on the OpAmp it will certainly outperform all those diamond circuits.
I would use a modern audio OpAmp like OPA2134 instead of a TL082. By the way, the OPA2134 is specified for a 600 Ohm load (+/- 35 mA output current). So for headphones you may not even need an extra output stage.

Regards, Dieter
ratatax:
I agree I'm don't like very much to design a discrete circuit, as it's a bit like reinventing the wheel and taking so much extra risk in design flaws, but I'm stuck with all the confusion in the amplifier world... So much nonsense informations everywhere, I can't believe finding a modern headphones amplifier schematic is almost impossible, all searches always brings very old designs without any explanations about performances except "it sounds good"  ::)

I've looked at the OPA2134 and the fact the datasheet mentions 600ohm load and +/-18V VCC is very interesting.

However it mentions the voltage output to be max +/-2.5V at 600 ohm, which is I=U/R = 4mA. 4mA @ 2.5V is about 10mW, which seems low for those purposes. In real world I don't know, a typical headphones with high impedance would be about 90dB/mW so it seems OK, but without much margin
Alex Nikitin:
The best IC for a headphone amp I've tried is the LM7171 , however it is not an easy opamp to deal with, requires a good layout and P/S decoupling, plus some careful filtering on the input and the output.

Cheers

Alex
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