Electronics > Beginners

why is class A audio amp saturating?

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mikerj:

--- Quote from: d4n13l on August 14, 2019, 06:52:14 am ---Audioguru, thanks a lot for that explanation, I learned yet again new things about op amps. Also I totally forgot to account the dc bias  when I calculated the power for the speaker, however I didn't noticed it getting hot, probably because I only did brief tests. I switched to a bigger one I have that can handle 25W. I adjusted the schematic a bit a now it works much better although it is still a little off the mid point but I think I can move on now.

--- End quote ---

Why aren't you heeding the advice given for the resistor values?  Why have you changed R2 but not R3?

Reduce the feedback resistor values by at least factor of ten, 10k input resistor, 100k feedback.  Reduce the rail splitter resistors R2 and R3 to ~10k each.  The intention is to put this point at half the supply voltage so the resistors need to be equal.

d4n13l:
Hi mikerj,

I think I did heed the advice, as audioguru and you said the op amp won't go all the way to the supply voltage and I have to account also for the voltage drop in the transistor, so I don't want the rail splitter resistor to be the same anymore. As for the resistor values I didn't change them as you mention simply because it didn't affect the output I think, I'm doing this just an exercise, it's not a circuit I'm going to keep.

Anyway I did try the changes you mention (fig 1) and on fig 2 you can see the output with the changes I made. I couldn't get to the mid point because I don't have appropiate resistor values but close enough I think.

mikerj:

--- Quote from: d4n13l on August 14, 2019, 03:27:53 pm ---Hi mikerj,

I think I did heed the advice, as audioguru and you said the op amp won't go all the way to the supply voltage and I have to account also for the voltage drop in the transistor, so I don't want the rail splitter resistor to be the same anymore.

--- End quote ---

I take your point about centering the operating point in the useable output range of the op-amp, but the Vbe voltage drop through the transistor will be taken care of with negative feedback.  The reason your output bias is not what your were expecting is because your feedback and rail splitter resistor values are all too large.  The old LM358 draws significant bias currents on it's inputs, and it is these currents flowing through your large value resistors that cause this offset error.

d4n13l:
ok, got it, thanks. I'm getting used to do the analysis as how the component behave in the real world instead of their ideal behavior.

cvanc:
Shouldn't the speaker be capacitively coupled to the amp output?

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