High Power Class-AB Differential Amplifier

A gain of 90 is very high for a power amplifier. Distribute more of the gain to a preamp. I usually don't make my power amps with a gain more than 10.

Well, I guess the differential part is the preamp.

Anyway, what's wrong with giving it a high gain?

Higj voltage gain usually increases distortion. When handling a heavy load, distortion becomes more prominent. In amplifiers, most distortion is generated in the power stage. Also imagin this. Your preamp gives 0.1% distortion, which is a very typical value, for normal preamps. Your amp has a gain of 90. This means that it'll amplify the distortion 90%, becoming 9%, which for me is unacceptable, and the problem is, not matter what global feedback you introduce to the power amplifier, as long as the distortion comes at the input, there's nothing you can do to decrease it. My advice if you're building this to test it - decrease the gain, add a good preamp, and add a frequency dependant negative feedback.

High gain poweramplifier also have a high chance of unstability and oscillation occurring

Still like your circuit ;-)

I think that's why he's using negative feedback, solves most of the problems but oscillations, I'd recommend adding a main pole that lows down the gain in frequency a little

Stability might be a reason, but big amp do need some gain and must handel that proper. Anyway, Its not because of distortion (professor buggzy and its farytails). Its because of termal noise. Audio chains do there gain as soon as possible and not at the end of all processing stages. Suppose the following chain: microphone (with integrated amp), mic amp, mixer, equaliser, final poweramp. All stages adding termal noise, you cant do nothing against it, except gaining distance between noise and signal as soon as possible. Or better said try to keep S/N ratio high, exact (S+N)/N. Anyway, noise is a effect which adds something to your signal, and if you do that adding again and again S/(N1+N2+N3+N4...) the quality drops more and more. But if you have a high signal S from beginning and increase the signal S also with every stage the noise will be less importante. One word to professor buggzy's farytail math from another planet: 1V+1mV distortion is 0.1%, right (what a shit amp) but (1V+1mV)*90 is 90mV distortion and still 0.1%! Foul mouth buggzy just shutup!

Look how elusively he avoids using the word distortion. Increased noise leads to increased distortion you fucktard :D

Here we are again. You have to separate the word NOISE from DISTORTION. And if you do not see/know the difference, its conplete waste of time to read anything you talk about amplifier performance. Start to understand termal noise and what distortion is. Foulmouth idiot.

Why do you always write THD+N, and not just distortion? Because distortion is harmonics that are based on the input signal, and caused by bad amplification of the signal, while noise is completely separate from distortion, as the signal isn't being distorted by noise, but the noise is being added to the signal. Noise isn't a feature of amps that distorts the signal, it just adds unwanted nonsense to the signal. If you freeze an amp in liquid nitrogen, its distortion doesn't go away, but the thermal noise probably does, as there's a lot less thermal energy around to pollute the signal. DISTORTION IS SEPARATE FROM NOISE. I used to like you better than @hurz, because @hurz isn't so nice, but now I'm starting to think he's better, despite his personality.

Also, I agree about amplification not increasing the distortion percentage, because 1/0.001 equals 0.001, or 0.1%, and 90/0.09 also equals 0.001, or 0.1%.

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@thebugger how would signal independent thermal noise increase signal dependent distortion?

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Wow, its so silence, probably he is reading a book.

Exactly jason, didn't use the right term, the THD+N rating increases. Yes they are two separate things, given that distortion occurs during amplification, and noise is induced even at idle, BUT both lead to the unwanted reshape of the input signal, that's why the combined measurement THD+N is used. Hurz is also right, the noise factor doesn't increase with gain of a single stage, the distortion does. Got that wrong ok, I noticed, but he'd already opened his mouth :D

After years of your foulmouth comments you gave me on this topic, and today you tell us you see whats was wrong in your understanding, I think, I have the right to be pedantic and correct you a last time on this topic to close it. THD+N as number is quite useless, cuz its not a ratio and does not compare anything. It does give an outstanding person no idea how good this product is. What you need is a ratio which compares two things. Suppose "S" in our wanted signal, S/(THD+N) or more common but not fully correct S/N. But THD appears only with a signal applied, measurements express (S+THD+N)/N and is as ratio mostly given in dB. Cuz THD+N is quite small for a good product, most engineers just talk about S/N. They set the THD+N to zero. For most cases thats fine. Topic closed.