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AB-class amplifier schematic analysis & optimization
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Zero999:

--- Quote from: Alex Nikitin on May 23, 2019, 09:29:58 am ---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

--- End quote ---
That's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble.  :palm:

Speed is good, but having a lot of it just makes life harder, with no benefit.

It will also be very noisy, especially at bass frequencies around 50nV/√Hz and 60pA/√Hz, at 30Hz! The NE5532 with a suitable BJT follower on the output will be much less noisy and less prone to oscillation.
magic:

--- Quote from: Alex Nikitin on May 20, 2019, 01:35:53 pm ---If the NE5532 is used, as suggested, the polarity of C2 is wrong, it will be biased by the opamp input current (about -0.3uA for NE5532) x 22K feedback resistor or about -6.6mV on pins 2(6). It would be better to reverse the C2 polarity with NE5532.

--- End quote ---
Not a big deal given that elcos can safely take up to 500mV of reverse voltage (IIRC) but I am actually autistic enough that I would do the same ;)


--- Quote from: Kleinstein on May 21, 2019, 01:31:17 pm ---AFAIK it is not that uncommon to have some series resistor (100 Ohms range) at the output, so that a 600 Ohms and 32 Ohms speak could produce a similar sound level.

--- End quote ---
Certainly not common on modern low voltage portable devices driving modern low impedance headphones as it would unacceptably limit power output. And people say that headphones these days are designed to be driven from zero impedance, so there is that.

And it is not without side effects, because dynamic headphones and speakers are quite reactive due to diaphragm inertia and suspension compliance. Think of it as a DC motor: voltage controls speed (and air pressure), current is drawn in proportion to force required to move the diaphragm. At resonant frequency, force/current is close to nothing and impedance shoots to the moon.
Therefore voltage division between the driver and the series resistor varies with frequency and boom, unequal frequency response.

And yes, you are totally right that it is a problem with certain headphone amplifiers.
Zero999:

--- Quote from: magic on May 23, 2019, 10:25:41 am ---
--- Quote from: Alex Nikitin on May 20, 2019, 01:35:53 pm ---If the NE5532 is used, as suggested, the polarity of C2 is wrong, it will be biased by the opamp input current (about -0.3uA for NE5532) x 22K feedback resistor or about -6.6mV on pins 2(6). It would be better to reverse the C2 polarity with NE5532.

--- End quote ---
Not a big deal given that elcos can safely take up to 500mV of reverse voltage (IIRC) but I am actually autistic enough that I would do the same ;)
--- End quote ---
The problem is increased distortion, even if the capacitor isn't harmed, although it's probably negligible in this case.
Alex Nikitin:

--- Quote from: Zero999 on May 23, 2019, 10:09:20 am ---
That's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble.  :palm:

Speed is good, but having a lot of it just makes life harder, with no benefit.

It will also be very noisy, especially at bass frequencies around 50nV/√Hz and 60pA/√Hz, at 30Hz! The NE5532 with a suitable BJT follower on the output will be much less noisy and less prone to oscillation.

--- End quote ---

The LM7171 is the best integrated solution I've heard. On the noise side - it is not a problem in a line level amp, it is not a phono stage! The current noise is about 7pA/√Hz at 30Hz, not 60, and 2pA/√Hz at 1kHz, so a 10K resistor on the input will only increase the noise by about 3dB on top of the voltage noise, which is fine. I've designed a respected headphone amp using this chip about 20 years ago, nobody ever complained about the noise level  ;) .

Cheers

Alex

Zero999:

--- Quote from: Alex Nikitin on May 23, 2019, 10:46:03 am ---
--- Quote from: Zero999 on May 23, 2019, 10:09:20 am ---
That's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble.  :palm:

Speed is good, but having a lot of it just makes life harder, with no benefit.

It will also be very noisy, especially at bass frequencies around 50nV/√Hz and 60pA/√Hz, at 30Hz! The NE5532 with a suitable BJT follower on the output will be much less noisy and less prone to oscillation.

--- End quote ---

The LM7171 is the best integrated solution I've heard. On the noise side - it is not a problem in a line level amp, it is not a phono stage! The current noise is about 7pA/√Hz at 30Hz, not 60, and 2pA/√Hz at 1kHz, so a 10K resistor on the input will only increase the noise by about 3dB on top of the voltage noise, which is fine. I've designed a respected headphone amp using this chip about 20 years ago, nobody ever complained about the noise level  ;) .

Cheers

Alex

--- End quote ---
Yes, I got the decimal point wrong when looking at the current noise graph. I must've been looking at the scale for the voltage noise!

I agree, if it's not providing much voltage gain, then the noise is negligible. Still I think using a video amplifier IC for a headphone amplifier is asking for problems with oscillation. No doubt it can be done, but why bother? There are far more suitable solutions which are far more stable and are better suited to the application.
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